leaf development in angiosperms

P1: KKK/ary P2: KKK/SPD QC: PSA/anil T1: PSA

March 30, 1999 11:5 Annual Reviews AR082-16

Annu. Rev. Plant Physiol. Plant Mol. Biol. 1999. 50:419–46Copyright c© 1999 by Annual Reviews. All rights reserved

LEAF DEVELOPMENTIN ANGIOSPERMS

Neelima SinhaSection of Plant Biology, Division of Biological Sciences, University of Californiaat Davis, Davis, California 95616; e-mail: [email protected]

KEY WORDS: leaf morphogenesis, simple leaves, compound leaves, shoot meristems,homeobox genes

ABSTRACT

Leaves are produced in succession on the shoot apical meristem (SAM) of aplant. The three landmark stages in leaf morphogenesis include initiation, acqui-sition of suborgan identities, and tissue differentiation. The expression of variousgenes relative to these steps in leaf morphogenesis is described. KNOTTED-likehomeobox (KNOX) genes, FLO/LFY, and floral homeotic genes may be involvedin generation of leaf shape and complexity. The differences between compoundleaves and simple leaves in gene expression characteristics and morphogeneticpatterns are discussed.

CONTENTS

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 420

MORPHOGENETIC EVENTS AT THE SHOOT APICAL MERISTEM. . . . . . . . . . . . . . . . 420Leaf Initiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 421Gene Expression at the Time of Leaf Initiation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 421

LATER EVENTS IN LEAF MORPHOGENESIS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424Leaf Partitioning into Domains. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 425Acquisition of Leaf Shape, Size, and Complexity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 428Genetic Regulation of Leaf Shape, Size and Complexity. . . . . . . . . . . . . . . . . . . . . . . . . . . 428Acquisition of Tissue Identities in the Leaf. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 429

UNIQUE FEATURES OF COMPOUND LEAVES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 431Morphogenetic Patterns. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 432Genetic and Morphological Analyses. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 432Molecular Analyses. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 436The Leaf Shoot Continuum Hypothesis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 438

CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 439

4191040-2519/99/0601-0419$08.00

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INTRODUCTION

The flowering plant body consists of an above-ground shoot system and a below-ground root system. The shoot system produces several kinds of organs witha diversity of functions. Cell divisions at the shoot apical meristem (SAM)followed by enlargement and differentiation of the derivative cells give rise tothe shoot system. The central or axial part of the shoot consists of a generallyradially symmetrical stem that can be branched or unbranched and bears lateralorgans, the leaves. Leaves are typically bilaterally symmetrical and flattenedand are borne on the flanks of the shoot apical meristem in a pattern characteristicfor the species. SAMs have been discussed thoroughly elsewhere (32) as hasacquisition of organ and tissue identity (145) and the specification of leaf identityduring the life span of a shoot (68). Recent studies also seem to indicate thatgenes that play a role in meristem organization may also be involved (albeitsometimes by absence) in the formation or development of leaves.

Leaves are responsible for most of the fixed carbon in a plant and are criticalto plant productivity and survival. They are also fascinating in their mode ofinitiation at the shoot apex, their arrangements on the shoot, and the diverseshapes and sizes they can attain. A great variability in leaf shapes and sizes isseen in nature. Leaves also exhibit varying degrees of complexity and rangefrom simple to highly dissected. Leaves in most extant vascular plants have beentermed megaphylls by evolutionary biologists. This distinguishes them fromthe earliest simple leaves in vascular plants, microphylls, that were proposedto have arisen as enations from the stem, had a simple vein, and were notassociated with leaf gaps in the stem vasculature.

Within the seed plant lineage, both cycads and many angiosperm generahave compound leaves and the compound leaf of cycads possibly representsthe ancestral condition in seed plants (30). It is generally assumed that the an-cestral leaf form in angiosperms was simple (29, 148). The question naturallyarises: Did complexity in leaf form evolve once or multiple times within an-giosperms? Compound leaves occur in the palms, some aroids, andDioscorea(among monocots) and in many unrelated lineages in the dicots. The scatteredoccurrences of compound leaves in families such as Solanaceae and Aster-aceae, on the one hand, and Ranunculaceae on the other, point to numerousindependent origins of this feature in the dicots (42).

MORPHOGENETIC EVENTS AT THE SHOOTAPICAL MERISTEM

Developmental landmarks can be used to divide the process of leaf morphogen-esis into three stages (145). At the organogenesis stage (stage 1), cells on the

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LEAF DEVELOPMENT 421

flank of the shoot apical meristem are set aside as the founder cells of the initiat-ing leaf. Increased cell division rates characterize the region that will give riseto the leaf primordium. Stage 2 delimits the basic morphological domains forthe growth and development of leaf parts. Cell and tissue differentiation occursduring the final stage (stage 3) of leaf development by coordinated processes ofcell division, expansion, and differentiation (145). The latter two stages occurin the leaf primordium proper. Leaves can initiate at the SAM in one of severalpatterns. Numerous experimental studies indicate that preexisting primordiaand the SAM itself can influence the placement of initiating primordia andthat biophysical constraints may also play a role in primordium placement(36, 45, 137, 143, 162).

Leaf InitiationThe shoot apical meristem (SAM), once formed in the embryo or in an axillaryposition, initiates organ primordia throughout its life. The only exception maybe SAMs under seasonal dormancy. After generating a series of leaf primordia,the SAM often terminates by producing carpel primordia in its function as afloral meristem. Appropriate environmental signals inImpatienscan cause theSAM to revert back to a vegetative phase after production of several floral organwhorls (7, 107).

Alteration in cell division activity at the SAM leads to leaf initiation. Pericli-nal cell divisions in the presumptive primordium region of the SAM rather thanincrease in cell division frequency have been suggested to be a major factor inleaf initiation (88). When cell divisions are completely suppressed in wheatSAMs by gamma irradiation, a bulge in the outermost or L1 layer of the SAMappears at the predicted leaf initiation site, suggesting that these sites can be setup without cell divisions (47). Further, mutations with compromised epidermaldifferentiation (likecrinkly4 in maize) or altered cell division patterns in theepidermis and other cell layers (likepygmy-tangledin maize) are capable ofnormal leaf initiation and morphogenesis. Biophysical studies have also sug-gested a role for wall loosening and cellulose microfibril reorganization in theL1 layer prior to leaf initiation (46, 126). This appears to be borne out by anexperiment in which wall properties at the SAM were directly altered (36).

Gene Expression at the Time of Leaf InitiationWhat are the genes that regulate these biophysical parameters at the SAM?Most genes involved in organ initiation events at the SAM appear to fall intotwo classes: They encode either for transcription factors or for receptor/signalmolecules. Future research aimed at identifying downstream genes of thesetranscription factors should elucidate the signal transduction cascades that occurand connect gene expression to signaling events.

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KNOTTED-LIKE GENES The KNOTTED-like (KNOX) genes encode homeo-domain-containing proteins and have been subdivided into two classes (69, 158).Kn1 and other Class IKNOX genes have also been associated with gain-of-function mutations that affect the maize leaf (37, 122). Mutations at the ClassI KNOX geneSTM1 lead to absence of shoot meristems in Arabidopsis (6).The only known marker for leaf initiation appears to be the downregulationof Class IKNOX gene (likeKN1, KNAT1, andSTM1) expression in the P0primordium in maize and Arabidopsis (82, 85, 136). The maize SAM producesa series of repeated units called phytomers that comprise the internode, leaf,and axillary bud (38). Jackson and co-workers analyzed the expression of sev-eral KNOX genes in the SAM and the recently initiated phytomers in maize(53). The expression ofRough Sheath1(Rs1) and KNOX3 in a ring belowthe initiating leaf corresponds to the incipient internode and axillary bud andis thought to also predict the site of leaf initiation and the basal limit of thevegetative phytomer (53). The function of Class IIKNOX genes is as yetundetermined. 35S driven KN1 (or KNAT1) overexpression in Arabidopsisand tobacco leads to the production of lobed and puckered leaves and ectopicshoot apical meristems on leaves (18, 134). In contrast, not only are the ClassII genes more ubiquitously expressed, but transgenic plants that overexpressor underexpress theKNAT3 (KNOX II) gene display no obvious phenotypicabnormalities (127, 128). OtherKNOXgenes have been cloned from a numberof flowering plant species (89, 91).

NO APICAL MERISTEM The NAM family of transcription factors was identifiedby cloning genes causing mutations leading to fused organs and altered organnumber. These genes are expressed in the SAM at organ boundaries and maydelimit organs from each other and from the SAM. TheNAM gene encodes agene product of unknown function that is required for apical meristem forma-tion in embryogenesis. Mutant seedlings attain viability by producing escapeshoots (138). Arabidopsis has twoNAM genes calledCUC (CUP SHAPEDCOTYLEDONS) that have similar patterns of expression and function as thepetuniaNAMgene (1).

CLAVATA CLAVATAgenes control meristem size in Arabidopsis. Mutationsat CLV1 and CLV3 lead to enlarged meristems and accumulation of excessundifferentiated cells in the meristems (20, 21). TheCLAVATA1gene codesfor a receptor kinase, which suggests a role for signal transduction events inshoot meristem function. It has been proposed that these genes serve to balancecell proliferation and organogenesis events.CLV1 is expressed in the centralzone and is absent from the flanks of the SAM (22). Genetic analyses indicatethat theCLVgenes andSTMare each sensitive to dosage of the other and may

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play opposing roles in regulating the balance between cell division and celldifferentiation at the SAM (19).

MGOUN TheMGOUN(MGO) genes also have a role in patterning at the SAM.Mutations at theMGO loci in Arabidopsis lead to reduced leaf number whilelarger SAMs are produced that tend to fragment into multiple meristems. TheMGO genes act downstream ofSTM1, require meristematic tissue to act, andappear to be involved in primordium initiation (81).

TERMINAL EAR The maizeterminal earmutation shows aberrant leaf initia-tion and development, irregular phyllotaxy, and altered internode length. Themutation is caused by absence of an RNA-binding protein similar to the yeastMei2 protein.TE1RNA is expressed in semicircular bands just below the pointof insertion of leaf primordia, and the gap inTE1expression marks the site ofleaf initiation. It has been proposed that theTE1gene acts early in the processof leaf initiation by inhibiting cells that express it from becoming organizers ofleaf development (156).

PHANTASTICA Mutations at thePHANTASTICAlocus (PHAN) in Antirrhinumlead to loss of dorsiventrality in leaves and floral organs (159).PHANencodesa MYB domain protein. Although a role for thePHANgene in leaf initiationevents has not been determined, the gene is expressed in a pattern complemen-tary to that seen forSTM1(160). It has been proposed thatPHAN serves todownregulate Class IKNOXgene expression in leaf primordia (123, 147).

LEAFY/FLORICAULA TheLEAFY/FLORICAULA(FLO/LFY) gene encodes aprotein with a transcriptional activation domain (23, 163). Mutations inFLO/LFY result in replacement of flowers with leaf-bearing shoots and a reit-eration of the inflorescence phase of development. TheFLO/LFYgene productappears to be necessary for the production of determinate floral meristems. Al-thoughFLO/LFYexpression is absent from vegetative meristems in ArabidopsisandAntirrhinum, the gene is expressed in newly initiated leaf primordia (11). Intobacco meristems, theFLO/LFYhomologsNFL1 andNFL2 are expressed invegetative SAMs in cells that may be precursors to procambium, as well asin the peripheral zone of the shoot apex. It has been proposed that the roleof NFL may be to establish determinacy for recent derivatives of apical initialcells (66).

The expression patterns of these genes at the SAM and in the developingshoot are summarized in Figure 1. Other genes with roles in formative eventsin leaf initiation are expected to be identified. If biophysical properties of theSAM play a role in generating phyllotaxy one would expect to find some cy-toskeletal and cell wall proteins that regulate phyllotaxy. This would also be

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Figure 1 Gene expression related to simple leaf initiation at the shoot apex. The expressionpattern of Class IKNOXgenesKN1 (136),STM(85),KNAT1(82),CLAVATA1(22),LEAFY(11),PHANTASTICA(159),RS1andKNOX3(53),NAM/CUC (1, 138) andTERMINAL EAR(156) as itrelates to leaf initiation at the SAM and to the repeated phytomers on the shoot.

the prediction for leaf initiation events as early periclinal cell divisions areinvolved. It is also expected that plant hormones will be involved in this regu-lation. Hormone applications can cause shifts in phyllotaxy in ivy (114, 115).Application of auxin transport inhibitors such as triiodobenzoic acid (TIBA)not only leads to changes in leaf shape but also cause changes in phyllotaxy(124). Perhaps the most intriguing data that would connect gene expressionlevels to hormone signaling have come from measurement of plant hormone lev-els in transgenic plants overexpressing homeobox genes. These plants have re-duced GA1 content and suppression of GA 20-oxidase gene expression (72, 73).Thus, a complex network of events involving signaling at the SAM, hormoneperception and transport, rearrangements of cytoskeletal and cell wall ele-ments, and gene expression cascades occurs during the process of leaf initia-tion. The interconnecting threads between these various events remain to beidentified.

LATER EVENTS IN LEAF MORPHOGENESIS

After leaf initiation, stage 2 in leaf morphogenesis proceeds, and suborganidentities or domains in the leaf are delimited. Along the three axes of theleaf these are the abaxial-adaxial (dorsiventral), apical-basal (proximodistal),

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and margin-blade-midrib (lateral) domains. In the maize leaf there are twomajor developmental gradients, one extending from the distal (leaf tip) to theproximal (leaf base) region and the other extending laterally out from the midrib.Developmental gradients are usually visible as a gradation of cell or tissuedifferentiation (especially trichomes on the epidermis) on the leaf (49). Thehomologies between leaf suborgans in dicot and monocot leaves are unclear.Analysis of leaf development in monocots by Kaplan (59) indicated that thereis a distal unifacial upper leaf zone and a proximal bifacial sheath. Variation inleaf morphology results from a reciprocal elaboration and suppression of thesetwo zones. In a number of monocot leaves the upper leaf zone is reduced toa fore-runner tip, whereas the lower leaf zone (the leaf base equivalent in adicot leaf) gives rise to the rest of the leaf blade. Bharathan (10) examined leafdevelopment in selected monocot genera and determined that some monocotspecies (for example,Smilax, Aristolochia) elaborate blade from the upper leafzone and thus are more similar to dicots. Further detailed studies are needed todetermine homologies of structures like the grass leaf sheath or dicot stipules.

Leaf Partitioning into DomainsSimilar to the maize leaf, most other leaves also delimit domains along the threeaxes. The earliest reported acquisition of domain identity occurs in the maizeleaf where cells that will give rise to the margin region of the leaf are morpho-genetically distinct from other cells at the P0 or founder cell stage (118, 119).In thenarrow sheathmutation these cells fail to become incorporated into thefounder cell population and a narrow leaf results. Differentiation of the leafalong its three axes also gives the leaf a shape characteristic of the species.Various physiological and genetic manipulations can lead to altered leaf shapesbut their direct effects on differentiation along the three axes are unknown.

Dorsiventral patterning or acquisition of specific features along the adax-ial/abaxial axis occurs very early in leaf primordia. While the terms adaxialand abaxial have very specific meanings, the terms dorsal and ventral have seenconflicting usage in plants (49, 59, 159). In this review, the more precisely de-fined terms adaxial and abaxial are used throughout to avoid any confusion. Theabaxial surface of the leaf primordium grows faster (and shows earlier cellulardifferentiation) than the adaxial face and causes the primordium to arch over theSAM. Thephantasticamutation inAntirrhinumproduces leaves that are radi-ally symmetrical and almost completely abaxialized. It has been proposed thatPHAN plays a role in the acquisition of adaxial identity in lateral organs and thatleaf blades arise at the junction between the adaxial and abaxial domains of theleaf. In the absence of PHAN function adaxial identity is not acquired, leadingto loss of a boundary between the adaxial and abaxial domains and absence ofblade (159). ThePHAN transcript localizes to initiating leaf and floral organ

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primordia at the SAM and shows an expression pattern that is inverse of that forthe KNOX geneSTM(160). In the dominantphabulosa-1d(phab-1d) mutationin Arabidopsis, leaf polarity is also altered (92). However, in contrast tophan,adaxial features are present on the abaxial surface. These leaves fail to developblades, supporting the hypothesis put forward by Waites & Hudson (159). Themaizerough sheath2(rs2) mutant phenotype resembles that seen when KNOXgenes (e.g.Rs1, Kn1, Lg3) are ectopically expressed in the maize leaf. Analysisof gene expression inrs2shoot apices indicates ectopic expression of the maizeKNOX genesRs1andKn1 in the leaf and a failure to downregulate KNOX 1genes in the ring of founder cells. This leads to narrow and often bladelessleaves (123). The mutation is caused by alteration in aPHAN-like MYB gene(123, 147). Thus,PHAN-like genes may regulate the expression ofKNOXgenes.How this translates into the acquisition of dorsiventral identity remains to beelucidated. Mutations at theARGONAUTE(AGO) locus in Arabidopsis alsolead to radially symmetrical leaves (12). In maize, theleafbladeless(lbl ) muta-tion also produces radially symmetrical leaves that are abaxialized. It has beenproposed thatLBL may have role in directly or indirectly downregulatingKN1in leaf-like lateral organs (150). These results are summarized in Figure 2.

Partitioning in the proximo-distal dimension is more variable and dependson the gradient of differentiation in the leaf. In leaves with basipetal differenti-ation, the blade part of the leaf forms first and the petiole and base differentiate

Figure 2 Adaxial-abaxial patterning in the leaf.A. The SAM with initiating leaf primordia show-ing KN1 expression (53, 136) and PHAN expression (159). The P0 site has no KN1 expression.Adaxial-abaxial differentiation is apparent in initiating primordia and in the mature leaf in trans-verse section (B). C. Transverse section of completely abaxializedphanleaf with no blade (159).D. Transverse section of completely adaxializedphableaf with no blade (92).E. Transverse sectionof a rs2 leaf showing abnormal growth and ectopic KNOX expression with normal blade (123).

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later. However, acropetal differentiation is also seen in a number of genera. Ac-quisition of proximo-distal identity in the maize leaf has been most thoroughlyanalyzed. The leaf primordium at plastochron 2 shows both transverse andlongitudinal anticlinal divisions in the protoderm layer. During P3 blade cellsbecome differentiated from sheath cells and a preligular band forms betweenthe two. During P4 and P5 ligule outgrowth can be seen as a ridge across theadaxial leaf surface and during P5 the upper third of the blade completes its dif-ferentiation (144). TheLIGULELESS2gene in maize encodes a basic leucinezipper (bZIP) transcription factor that is expressed in the SAM and developingligule regions. Mutations in the gene lead to absence or incorrect positioningof both ligule and auricle tissue in the maize leaf (161). TheLIGULELESS1gene in maize also encodes a nuclear localized protein similar to SQUAMOSAPROMOTER BINDING proteins 1 and 2 from snapdragon. The gene is ex-pressed in leaf primordia at or prior to plastochron 6. Mutations at this locusalso lead to loss of ligule and auricle and the formation of an imprecise bladesheath boundary (98). It has been proposed thatLG1 andLG2 function in thesame pathway. EarlyLG2 function determines the precise positioning of theligule and auricle and lateLG2 function interacts (either directly or indirectly)with LG1 function to transmit and receive the make ligule/auricle signal (51).Analysis of chimeric leaves that ectopically express theLG3 KNOXgene insectors indicates that there are competency states that all leaf cells go through:sheath, followed by auricle, immature blade, and mature blade. Expressionof LG3 delays the acquisition of older cell fates in the leaf cells. Thus largesectors (early expression of LG3) remain sheath-like, whereas narrow smallsectors (late expression of LG3) are almost normal (99). This confirms thematuration schedule hypothesis for the maize leaf put forward by Freeling (37).

In dicot leaves, partitioning of the leaf in the lateral dimension occurs con-current with or a little delayed from proximodistal partitioning. Proximodistalleaf partitioning in tomato is initiated very early with the terminal leaflet tip dif-ferentiating at plastochron 1–2 (17). The first pair of lateral leaflets is producedbasipetally when the primordium is about 300 microns in length at plastochron3 (26). Procambium develops acropetally from the stem vascular cylinder andreaches the apex of the leaf primordium at approximately 300 micron lengthand is correlated with lamina development (24). This procambium will giverise to the leaf midvein. The marginal fimbriate vein, a characteristic featureof the tomato leaf margin, develops basipetally and continuously in the laminaof the terminal leaflet at about 500 micron leaf length and marks the end ofmarginal growth of the terminal leaflet (24). Later basipetally produced leafletsreiterate this pattern.

In contrast to the late development of marginal features in tomato leaves,recent studies indicate that maize leaves may be quite different. At plastochron0 the presumptive leaf primordium is represented by a crescent of cells in the

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tunica and subepidermal regions that are undergoing cell divisions (129), re-ferred to as the leaf founder cells. Development of the leaf midvein procambiummarks the differentiation of the midrib region of the leaf and this event occursearly in leaf ontogeny, at plastochron 2 in maize. Procambial strands for thelateral veins that demarcate the blade reach the tip of the blade at plastochron5 (129). This would suggest that differentiation in the lateral dimension togenerate the blade occurs concurrently with proximodistal partitioning. Themaize KN1 protein is absent from founder cells of the P0 leaf primordium atthe shoot apex. These cells form a complete ring that encircles the base ofthe SAM. Analysis of the narrow sheath mutation in maize indicates that thereis a patch of cells that retain KN1 expression in the P0 primordium, and arenever initialized to make the margin domain of the maize leaf (120). Clonalanalysis suggests that a region of thens meristem is not utilized to generatefounder cells in the margin domain (119). Therefore, partitioning in the lateraldimension (especially to generate the leaf margin) is determined in the SAMin maize and stage 1 and 2 are thus almost coincident. Whether this affects thepeculiar linear and canalized way in which the maize leaves (and indeed allgrass leaves) develop is not known. Perhaps very early lateral axis delimitationis the cause (or effect) of a very reduced upper leaf zone.

Acquisition of Leaf Shape, Size, and ComplexityLeaves can vary greatly in shape, size, and complexity along a single shoot. Thisvariation can be of two types, heteroblastic or heterophyllic (2, 28, 41, 57, 93, 9697, 121, 125, 139). Hormones like gibberellic acid (GA) or abscisic acid (ABA)appear to be involved in both heterophyllic and heteroblastic leaf development(3, 4, 27, 33, 40, 58, 113–115, 165, 166). The subject has been thoroughly re-viewed by Kerstetter & Poethig (68).

Genetic Regulation of Leaf Shape, Size and ComplexityHow leaf shape and size are regulated has also been investigated by usingspecific mutations that cause alterations in these parameters. Overexpressionof Class IKNOX genes in tobacco and Arabidopsis leads to leaf lobing inan otherwise unlobed simple leaf (18, 82, 134). Theasymmetricmutation inArabidopsis causes the later leaves to become deeply lobed and asymmetricabout the midrib. Based on a fewer number of hydathodes on theas1 leaf, itwas proposed that there was a loss of marginal segmentation in the mutation andthat this was a late effect due to change in the direction in which new cells weresupplied from the leaf base (155). Thecurly leaf mutation in Arabidopsis alsohas unusual leaf morphology including narrow, curled leaves of reduced length.TheCLFgene is a member of the polycomb gene family. Genetic and molecularevidence suggests thatCLF mRNA accumulates in leaf primordia and apical

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meristems where it serves to downregulate transcription of theAGAMOUSgenein leaves, inflorescence stems, and flowers (43, 130). Both the number and sizeof cells in the leaf are reduced inclf plants, which indicates a role forCLFin both the cell division and cell expansion phases of leaf development (71).Altered leaf shape and size are also seen in Arabidopsis plants transformed withan antisense construct of the Arabidopsis cDNA for methyltransferase (MET1).Ectopic expression of floral homeotic genesAGAMOUSandAPETALA3is seenin leaf tissue in these plants (34). These results suggest that altered expressionof homeobox genes or floral homeotic genes in leaves can lead to alterations inleaf shape, size, and growth patterns.

The role of cell division and cell expansion in organ shape generation has alsobeen studied using a variety of mutations. Arabidopsis plants lacking micro-tubule preprophase bands show irregular cell division but still generate organsin their correct positions and differentiate appropriate tissue types (151). Inthe maizewarty mutation abnormally large and improperly divided cells areproduced and the cell cycle speeds up. Analysis of the mutation suggests thatgrowth defects can be compensated for by alteration in cell cycle in neighboringcells and that cytokinesis may be linked to cell size ratios (110). Thediminuto(dim) mutation in Arabidopsis produces plants with very short organs. TheDIMgene encodes a protein with a putative nuclear localization signal, affects theexpression of aβ-tubulin gene,TUB1, and may play a critical role in plant cellelongation (146). TheANGUSTIFOLIAlocus in Arabidopsis specifically reg-ulates cell expansion in the transverse dimension, while theROTUNDIFOLIA3locus affects cell expansion in the longitudinal dimension; these two loci actindependently of each other (153). Mutations at these two loci produce leaves(and petals) that are narrower than normal or shorter than normal, respectively(153), indicating that defects in cell expansion may not always show com-pensatory mechanisms, as would be predicted by the organismal theory ofplant development (64). In contrast, compensatory mechanisms are seen in thepygmy/tangledmutation in maize, where alterations in cell division along oneaxis may be compensated for by divisions in other dimensions so leaf shapesremain essentially unchanged (135). This suggests that morphogenesis canbe uncoupled from the consequences of altered cell divisions but not alteredcellular expansion.

Acquisition of Tissue Identities in the LeafDuring stage 3 of leaf development the leaf acquires tissue identities. Develop-ment of photosynthetic capability, a vascular system, and epidermal tissue alloccur at this stage of leaf development. The various tissue identities are likelyacquired in a coordinate manner. Furthermore, acquisition of the various tissueidentities may not be hardwired as some tissues can develop even in the absence

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of others. An example is the acquisition of photosynthetic cell types even whenvascular differentiation is aberrant. The earliest differentiation events are vas-cular, and the procambium of the future midrib often begins differentiating intothe leaf primordium at its inception. The final events occur when the guard cellsdifferentiate in the epidermis (24).

VASCULAR AND PHOTOSYNTHETIC DEVELOPMENT Clonal analysis of maizeleaf development indicates that the L1 layer gives rise to the abaxial and adax-ial epidermises, while the L2 layer gives rise to all internal tissues in the leaf.This L2 layer divides once to form an abaxial and an adaxial layer. The adaxiallayer most frequently divides once periclinally to give rise to the innermosttissue of the leaf that includes the vascular bundles, bundle sheaths, and theinnermost mesophyll cells (76). As sector boundaries often lie in the middle ofa vein, it appears that the veins are of mosaic origin with each half coming froma different progenitor cell (76). Ability to fix carbon via the C4 pathway inmaize leaves seems closely associated with vascular development. RuBPCasedownregulation occurs in cells closest to veins and bundle sheath cells. Or-gans that have widely spaced veins (e.g. husk leaves), or mutations that fail todevelop proper bundle sheath cells, are C3/C4 intermediates with RuBPCASEexpression in mesophyll cells (77, 132). Thus photosynthetic differentiation inthe maize leaf appears to be intimately associated with vascular differentiation.This may not be true for dicot leaves as mutations that lead to a reduced vascularnetwork differentiate a normal complement of photosynthetic cell types. How-ever, chloroplast development affects differentiation of photosynthetic tissue intomato. In thedcl (defective chloroplast and leaf) mutation in tomato, chloro-plast development is aberrant and palisade cells fail to attain their characteristiccolumnar shape (65).

Several hypotheses have been put forward to explain vascular patterning inthe leaf (reviewed in 100). Sachs (116) proposed a differentiation-dependentmechanism to explain patterning. Canalization of auxin flow through certaincells causes vascular patterns to develop as these cells become better and bet-ter transporters of auxin and eventually differentiate into vascular tissue. Thediffusion-reaction prepattern hypothesis proposes that autocatalysis can lead tosmall random peaks in a homogeneous field, the peaks can increase by positivefeedback, and long-range inhibition can prevent spread of the peaks (95). Thesetwo hypotheses have been put forward to explain vascular development in ad-dition to stomatal and trichome patterning in the epidermis [for recent reviewssee (78, 100, 145)].

EPIDERMAL DIFFERENTIATION Epidermal identity is proposed to be estab-lished very early in embryonic development (13). Clonal analysis using revertant

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sectors generated in a mutablegl1 mutation in maize indicates that the regionof the epidermis between two adjacent lateral veins forms a compartment withan intermediate vein running in the middle. In this compartment, the foundercells are in the region closest to the midrib and undergo a very polarized modeof cell division in the protoderm of the maize leaf (14). Similar compartmentboundaries have been proposed for the internal tissues in the maize leaf, butpolarized cell divisions have not been described (76). Thecr4 mutation inmaize shows aberrant epidermal development, defective aleurone layers, andepidermal fusions. The gene has been cloned and shown to be a maize ho-molog for the human tumor necrosis factor receptor (TNFR), suggesting thatpeptide signaling is probabaly involved in epidermal development (8). Theadherentmutation in maize causes postgenital epidermal fusions without al-tering cellular identities within the leaf (131, 133). Recently, a large numberof such mutations with abnormal epidermal fusions have been described inArabidopsis and placed into nine complementation groups (84). In addition,a recently described mutation in maize,Xcl 1, causes extra cell layers to formin the leaf epidermis (70), and a multitude of both dominant and recessive lociappear to exist in maize that cause similar epidermal perturbations (S Kessler,personal communication). Taken together, these results indicate that multipleloci are involved not only in determining epidermal identity, but also in mainte-nance of this identity. Tissue patterning in the protoderm leads to the formationof specialized epidermal cells in a field of unspecialized or pavement cells.Cloning of the relevant genes from Arabidopsis indicates that MYB (GL1),basic helix-loop-helix (TTG), and homeobox (GL2) transcription factors areinvolved in regulation of patterning events, and that these factors may controltrichome patterning and development events through cytoskeletal proteins suchas kinesins (ZW1) (79, 80, 83, 101, 102, 109, 145). Stomatal patterning is lesswell understood but the future prospects in the field look bright (reviewed in78, 145).

UNIQUE FEATURES OF COMPOUND LEAVES

Organs that bear separate foliar units called leaflets have been termed com-pound by some researchers (9, 140), whereas others treat leaves as a continuumbetween simple and highly dissected (60). The recent genetic and moleculartreatments of leaves have used the compound terminology, which is also usedhere in the interest of homogeneity, with the caveat that the compound termi-nology is suggestive of such a leaf being a composite of many simple units. Onthe contrary, a highly dissected or compound leaf is thought by a majority ofmorphologists to be equivalent to a single simple leaf. However, compoundleaves possess certain unique features that set them apart from simple leaves.

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Morphogenetic PatternsIn dicots, compound leaves are very similar to simple leaves in initiation andgrowth patterns. Leaflets can be produced by one of three routes: acropetal,basipetal, or divergent (39). The most frequent pattern of leaflet initiationis basipetal. However, in bipinnately compound leaves the order in whichsecondary leaflets are produced is acropetal. Troll (152) proposed that in thedivergent form basipetally initiated leaflets are homologs of lateral leaflets ofthe first order, while acropetally initiated leaflets are second-order leaflets de-veloped from the terminal leaflet. According to Troll (152), acropetal leafletinitiation could be derived from the divergent form by suppression of basipetalleaflets.

The dicot simple leaf has been suggested to be derived phylogeneticallyfrom a compound leaf, and the basic leaf type is subdivided into segments(48). An opposing view suggested that that the simple leaf is the basic formthat is maintained in ontogeny, with leaflets developing as do lobes in a simpleleaf (31). It is likely that the ancestral state for seed plants as a whole was acompound leaf, whereas the ancestral state for the flowering plants was a simpleleaf. It is unclear if later acquisitions of compound leaves represent a reversionto the ancestral (i.e. the cycad-like) condition.

There is likely to be similarity between simple and compound leaves in thepatterning events at the shoot apex as no unique phyllotactic patterns are asso-ciated with compoundness. Similarly, morphological analyses of leaf initiationevents do not reveal any unique features associated with compound structuresin the early plastochrons. However, this may not indicate commonality ofmechanisms.

Genetic and Morphological AnalysesThe two species most thoroughly analyzed at both the genetic and molecu-lar level are pea and tomato. Leaves in both these species are unipinnatelycompound. The pea leaf produces a pair of basal stipules, several pairs of un-lobed lateral leaflets, two or more pairs of lateral tendrils, and a terminal tendrilstructure in acropetal succession. In contrast, the tomato leaf is unipinnatelycompound but leaflets are produced in basipetal succession on the rachis andare lobed.

GENETIC AND MORPHOLOGICAL ANALYSES OF PEA LEAVES A number of leafmutations and their interactions have been described by Marx (90). Three sub-divisions in the pea leaf were proposed by Marx in the proximodistal axis—aproximal region consisting of basal stipules, a middle region consisting ofpaired lateral leaflets, and a distal region consisting of paired lateral leaflets anda single terminal tendril. Marx also proposed a further subdivision of the distalregion into two compartments (characterized bytac-tendrilled acaciain which

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the upper leaf segment is divided into a terminal leaflet and with basal pairedtendrils).

The stipules have to be accounted for when discussing leaf complexity in pea.The homology of stipules is unclear, and these structures have been variouslytreated as an integral part of the leaf, as extrafoliar appendages associated withthe leaf, or as a unique organ (39). The presence of stipules per se does not makea leaf compound as many simple-leaved species also have stipules. However,in pea the stipules are markedly foliar. Combinations of mutations that affectthe stipules and also alter waxes on either the abaxial or adaxial surface ofthe leaflets indicate that the stipules are an integral part of the pea leaf, andthat stipule bases are similar to the abaxial leaflet surface, whereas stipule tipsresemble the adaxial leaflet surface (90). Further, based on mutant combinationsbetweensinuate leaf(sil) andcrispa, andsil, afila, andtendrilless, Marx (90)suggested that stipules, while unambiguously a part of the pea leaf, are notsimply homologous to leaflets but also have additional leaf-like features, asproposed by Jeune for the legumes (56).

In addition to making homeotic (90) or heterochronic (87, 157) conversionsbetween tendrils and leaflets, certain mutations can either singly, or in combina-tions with other mutations, increase or decrease the degree of complexity seenin the pea leaf. Theuni mutation leads to a leaf reduced to a terminal single, ortwo-leaflet structure with paired stipules still present at the base of the leaf.unialso produces abnormally proliferated floral meristems. Whenafila is placedin combination withtendrilless, a very complex parsley leaf–like phenotypecalledpleiofila is produced (87, 90). Lu and coworkers (87) suggested that thehighly dissected parsley-like leaf ofaf/af; tl/tl represents the basal leaf formfor pea. A heterochronic restriction of the developmental potential of this leafby AF and TL leads to a unipinnate structure. Leaf development in pea wasrecently reviewed by Hofer & Ellis (52a).

Young (164) proposed a model that correlated meristem size to leaf develop-ment; primordia of decreasing sizes would give rise in order, to rachis, leaflet,or tendrils. Meicenheimer and coworkers (94) analyzedaf, st, tl, and combi-nations of these mutations and concluded that what the primordium developedinto depended on whether and for how long various defined meristems are ac-tive. Based on their results, they proposed independent genetic control overthe formation of leaflet, stipule, and marginal meristems. These morphologicalstudies show that differentiation occurs early and that development proceedsacropetally. While leaflet initiation in pea leaves is in acropetal succession,Villani & DeMason (157) found that thepleiofila form (af/af; tl/tl ) showedbi-directional leaflet initiation in late postembryonic leaves.

GENETIC AND MORPHOLOGICAL ANALYSES OF TOMATO LEAVES Compoundleaves in the cultivated tomato have a terminal leaflet and three to four pairs of

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Figure 3 Wild-type tomato leaf.

lateral leaflets that are produced in a basipetal sequence. In addition, intercalaryleaflets are produced between the major lateral leaflets (Figure 3). The leafletsare often deeply lobed, and the lobes themselves develop largely acropetallyon each leaflet primordium (54). Thus, the tomato leaf shows two developmen-tal gradients, an early basipetal gradient in the leaf primordium that generatesleaflets, followed by a later acropetal gradient in each leaflet that leads tomarginal lobe formation (54).

Based on the phenotypes produced, four general categories of mutationsare seen in tomato (Figure 4). Type 1 mutations are defective in leaflet blade

Figure 4 The four classes of leaf mutations seen in tomato. Mutant phenotypes are describedagainst each class and typical leaves are diagrammed.

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expansion. Type 2 mutations change the leaf into a simple or nearly simpleleaf. Type 3 mutations alter the degree of leaflet lobing on a compound leaf.Type 4 mutations change the leaf into a more complex leaf with two or moreorders of pinnation (N Sinha, unpublished observations).

The Type 1 mutations may be responsible for generating the leaf/leaflet blade,and any reduction in leaf complexity seen in these mutations will likely reflectthe necessity of making a blade prior to leaflet initiation. This group of mu-tations is represented by thewiry series (142) and resembles thephantastica( phan) mutation inAntirrhinum(159). The several nonallelicwiry mutations intomato could well represent PHAN and other upstream or downstream factorsin the PHAN regulatory network.

The Type 2 mutations lead to reduced leaf dissection. Dengler (26) describedthe Lanceolate(La) andentire (e) mutants in tomato. In bothLa ande, lat-eral leaflet formation begins later than in wild type and is of shorter duration.Lamina expansion is faster in both these mutants compared to normal. BothLa andehave reduced lobes in the leaf margins. TheLanceolatemutation wasdiscovered in a primitive tomato cultivar in Peru and is an incomplete dominant.In the heterozygous stage small, simple, lanceolate leaves are produced. Thehomozygous mutant generally makes neither shoot apical meristem nor cotyle-dons and is a seedling lethal. Dosage analyses done by Stettler (141) show thatextra wild-type doses of the gene cannot rescue the mutant phenotype, whichsuggests that theLa mutation could be a dominant negative mutation.

Type 3 mutations have defective leaf margins and produce either unlobedleaflets or leaflets with very reduced marginal lobing. Chandra Shekhar &Sawhney (16) analyzed leaf development in thesolanifolia (sf) mutant. Inthis recessive mutation, potato-like leaves are produced that lack lobes in themargins. Major differences betweensf and wild-type leaves were seen at thetime of leaflet initiation. In wild-type leaves, the first pair of leaflet primordiawas produced at plastochron 3, whereas the first pair of leaflet primordia wasproduced at plastochron 5 insf. Some of the mutant effects of theSF genecan be overcome by GA treatments or temperature-shift experiments (15). Thepotato leaf(c) mutation is very similar to thesf mutation in having unlobedleaflets. Rick & Harrison (112) tested for allelism between the two mutationsand found thatc was not allelic tosf. The double mutationsf/sf; c/c producedvery reduced, unlobed and simple leaves.

Type 4 mutations increase the degree of dissection in the tomato leaf. ThedominantMouse earsandPetroseleniummutations, and the recessivebipin-nate, tripinnate, and clausa mutations all show increased orders of leafletproliferation leading to a bi- or tripinnate leaf. Since leaflets and marginallobes are equivalent structures and leaflet initiation is in a basipetal sequence,proliferation of leaflets from the basal region or conversion of leaflet lobes into

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leaflets would result in excessively dissected leaves in the Type 4 mutations.Tomato plants overexpressing the Class IKNOXmaizeKN1gene, or the tomatoLeT6gene, show leaves with increased dissection (50, 54). In mutations likeLanceolate, entire, andtrifoliate, all with reduction in the number of leaflets,KN1 overexpression merely reiterates the basic architecture of the leaf. Thus,the Lanceolateleaf is lobed but not compound, whiletrifoliate reiterates thethree-leaflet plan (50). This suggests that KN1-like function is insufficient torestore normal compound architecture in these leaves.

Molecular AnalysesSimple leaf development has been described in detail in maize and Arabidopsis(37, 106, 149, 154). It is unclear how common the morphogenetic mechanismsbetween simple and compound leaves will be. Indeed, whether the develop-mental principles derived from any of these model organisms will be generallyapplicable to all leaves remains to be seen. Determination of homologies be-tween the various kinds of leaves and various suborgans of the leaves will befacilitated by comparison of the expression of molecular markers specific todevelopmental domains or key steps in morphogenesis.

CLASS I KNOX GENES High levels of KNOX1 expression in the shoot apicalmeristems and downregulation or degradation of the gene product at presump-tive initiation sites indicate a role for theKNOX1genes in the initiation anddetermination of lateral organs such as leaves or flowers (53, 82, 85, 136).

KNOX genes have also been cloned from compound-leaved species (89).The homolog ofSTM1has also recently been cloned from pea (C Gourlay &N Ellis, personal communication). In tomato, two Class IKNOXgenes havebeen cloned and their orthology relationships determined.TKN1, andLET6(TKN2) are Class I genes, and a Class II gene,LET12, has also been identified(17, 50, 55, 103). Like the maize and Arabidopsis Class IKNOX genes, bothTKN1andLET6(TKN2) are expressed at high levels in the shoot apical meris-tem. However, these genes are also expressed at presumed leaf initiation sitesand in the leaf and leaflet primordia in tomato (17, 50, 55, 103). In contrast,in compound-leaved species in the Brassicaceae family, the leaf initiation sitesshow a downregulation of Class IKNOX gene expression. However, Class IKNOXexpression is turned back on in leaves at later plastochrons prior to leafletinitiation (T Goliber & N Sinha, unpublished observations). A different situa-tion occurs in pea leaves, where expression of the peaSBH1homolog is neverseen in leaf primordia or older leaves (C Gourlay & N Ellis, personal commu-nication). These results imply that the compound leaf in tomato, crucifers, andpea may not have arisen by similar mechanisms and are summarized in Figure 5.

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Figure 5 Gene expression during compound leaf development. Expression of Class I KNOXgenes in tomato (17, 50, 54, 103) is compared to that described for pea (C Gourlay & N Ellis,personal communication). The expression of LFY/FLO is also compared between tomato (105)and pea (52).

Thus in tomato, the compound leaf program may diverge from that of simpleleaves as early as stage 1 (leaf initiation), and the gene(s) controlling the dif-ference between simple and compound leaf programs probably work upstreamof TKN1 and/orLET6 (TKN2). This alteration in Class IKNOX expressionwithin the shoot apical meristems and early determinate organ primordia isalso seen in inflorescence meristems and in the Class II geneLET12, indicatingthat there may have been a global alteration in the regulation ofKNOXgenes intomato (55). Both simple and compound leaves can occur in different speciesof the same genus (simple leaves inLepidium africanumvs compound leaves inLepidium perfoliatum) or even on the same shoot (Neobeckia aquatica), imply-ing that determination of the degree of leaf dissection must involve only a smallnumber of genes. While changes in the expression domain of genes likeTKN1,LET12, andLET6 (TKN2) in the SAM indicate that in tomato these changesmay have involved regulatory genes, this cannot be the universal situation be-cause of differences between the expression patterns of Class IKNOX genesin tomato on one hand and pea andLepidiumon the other.

FIMBRIATA/UNUSUAL FLORAL ORGANS The pea homolog ofAntirrhinumFIMBRIATA (Arabidopsis UFO) called STAMINA PISTILLOIDA (STP) hasalso recently been cloned. Severe mutant alleles of STP affect heteroblastic

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development of the pea compound leaf. Instpmutants, production of the firsttrue leaf is delayed by a node compared to wild-type, and all leaves are moresimple (possess fewer lateral structures) than leaves at a comparable node onwild-type plants (S Taylor & Ian Murfet, personal communication).

LEAFY/FLORICAULA The pea leaf primordium generates second-order primor-dia in an acropetal series. In theunifoliatamutant, a pair of basal stipules subtenda terminal leaflet structure that may be single or subdivided into two units. Thebasic pinnate nature of the leaf is lost. Hofer and coworkers (52) have showntheuni mutation to be caused by deletions or alterations in thePEAFLOgene(the pea homolog ofLEAFY/FLORICAULA). Alterations in flower develop-ment accompany leaf abnormalities in theuni mutation.PEAFLOis expressedin initiating leaf primordia and becomes restricted to the more distal (leafletor tendril initiating) regions of the leaf in older primordia (summarized inFigure 5). While loss of FLO/LFY function leads to indeterminacy in inflores-cence and floral meristems, loss of PEAFLO function prevents the acquisitionof a transient phase of indeterminacy in pea leaves, preventing leaflet initiationand leading to production of a single lamina in theuni mutation (52). The roleof FLO/LFY in tomato leaf morphogenesis remains to be investigated. How-ever, expression studies indicate that the tomatoLFY/FLOhomologT-FLO isexpressed in vegetative apices and leaf primordia but expression is not seenin the central domain of the meristem (105). No known tomato mutations areassociated with this gene. Thus, genes responsible for regulating reproductivedevelopment also function in the regulation of vegetative development. How-ever, their roles in these different developmental phases of the plant may not beidentical or similar and may even differ across families.

The Leaf Shoot Continuum HypothesisBased on patterns of leaflet initiation from the leaf primordium, some re-searchers considered compound leaves to be equivalent to shoot systems(5, 117). InMurraya koenigiiandRhus typhina, leaf primordia show an acro-petal mode of leaflet initiation with leaflet primordia inserted transversely (withrespect to the rachis) at the leaf primordium tip, thereby resembling leaf initi-ation at the shoot apex (74, 75, 117). Thus, the general issue of homology ofcompound leaves appears to be in dispute. Homeobox gene expression in theleaf and leaflet primordia in tomato resembles the expression pattern seen inthe shoot apex itself (54, 55). Mutations that fail to maintain the shoot apicalmeristem and axillary meristems in tomato (111) initiate apical meristems atthe junctions between the petiolule and rachis, the pseudoaxils. The mutationcan be phenocopied by overexpressing homeobox genes (54), or pruning offall axillary and terminal meristems (CM Rick, personal communication). This

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suggests that the tomato compound leaf has some stem-like features and maybe an intermediate structure between simple leaves and stems, although thisfinding may not be generalizable, and compound-leaved species will have to beevaluated case by case. Compound leaves in pea have been likened to flower-like determinate shoots (52). This parallel is based on the recent cloning ofthe pea leaf mutationunifoliata. Since loss-of-function mutations at FLO/LFYin Arabidopsis and snapdragon do not show any abnormalities in leaf devel-opment, it can be hypothesized that any leaf component to FLO/LFY functionmay be restricted to species with compound leaves. This function could be thepresence of unique targets or activators present in compound leaves.

CONCLUSIONS

The utility of analyzing leaf development as a series of processes has beenproven by recent genetic and molecular studies. While most of the processesdiscussed overlap to a certain degree, this reductionist approach has allowedsimplified interpretations of what could otherwise be a complex series of events.However, in the final analysis, a synthesis of these various interpretations mustbe achieved so that a realistic picture emerges. Studies on simple leaves haveled to the identification and cloning of numerous genes. However, parallelsin morphogenetic processes between simple and compound leaves have beenharder to derive.

In the two cases examined at the molecular level, compound leaves appear toshow unique expression patterns of familiar genes. These are either the ClassI KNOXgenes (KN1, STM1, etc) that have a role in shoot meristem formationand maintenance, orFLO/LFY that has a role in suppressing indeterminacy inflowers. The “master regulator” of leaf dissection may be either one of thesegenes. However, alterations in some upstream regulator have more likely ledto the expression of these shoot-specific genes in the leaf (or prevention of theirturnover in leaf primordia), causing it to become compound. The coordinateupregulation of two Class IKNOX genes and one Class IIKNOX gene intomato leaf primordia is highly suggestive of the latter scenario. The fact thatoverexpression of Class IKNOX genes orFLO/LFY in species with simpleleaves is insufficient to cause the generation of dissected leaves suggests thatthe master regulator is another factor or that the expression of more than onegene has to be altered to generate compound leaves. In any event, the analysisof compound leaves has led to the discovery of unknown functions for familiargenes.

Studies on leaf morphogenesis are rapidly progressing into the realm ofgene cloning and functional analysis. Simultaneous investigation of modelorganisms such as maize, Arabidopsis,Antirrhinum, peas, and tomato should

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allow us not only to piece together the puzzle of how leaves are made, but also todiscover equivalence relationships between different parts of these leaves. Thisarea of investigation is poised to exploit the past genetic and morphologicalstudies of leaves, and by combining such studies with molecular and functionalanalyses, generalizable principles for leaf morphogenesis can be derived.

ACKNOWLEDGMENTS

I thank Tom Goliber, J-J Chen, Sharon Kessler, Campbell Gourlay, Noel Ellis,Scott Taylor, and Ian Murfet for generously sharing unpublished data; JudyJernstedt for sharing her knowledge of plant morphology and many helpfulcomments on the manuscript; members of my lab for their editorial suggestions;and Wynnelena Canio for help with processing the manuscript. Work in my labon leaf development in tomato is supported by the National Science Foundation(IBN 9632013).

Visit the Annual Reviews home pageathttp://www.AnnualReviews.org

Literature Cited

1. Aida M, Ishida T, Fukaki H, Fujisawa H,Tasaka M. 1997. Genes involved in organseparation in Arabidopsis: an analysis ofthe cup-shaped cotyledon mutant.PlantCell. 9:841–57

2. Allsopp A. 1965. Land and water forms:physiological aspects.Handb. Pflanzen-physiol.15:1236–55

3. Anderson LWJ. 1978. Abscisic acid in-duces formation of floating leaves in theheterophyllous aquatic angiosperm (Pota-mogeton nodosus). Science201:1135–38

4. Anderson LWJ. 1982. Effects of abscisicacid on growth and leaf development inAmerican pondweed (Potamogeton no-dosusPoir.).Aquat. Bot.13:29–44

5. Arber A. 1950.The Natural Philosophyof Plant Form. Cambridge: CambridgeUniv. Press

6. Barton MK, Poethig RS. 1993. Formationof the shoot apical meristem inArabidop-sis thaliana: an analysis of developmentin the wild type and in theshoot meristem-lessmutant.Development119:823–31

7. Battey NH, Lyndon RF. 1990. Reversionof flowering.Bot. Rev.56:162–89

8. Becraft PW, Stinard PS, McCarthy DR.1996. Crinkly4: a TNFR-like receptor ki-nase involved in maize epidermal differ-entiation.Science273:1406–9

9. Bell AD. 1991.Plant Form: An Illus-

trated Guide to Flowering Plant Mor-phology. Oxford/New York/Tokyo: Ox-ford Univ. Press

10. Bharathan G. 1996. Does the monocotmode of leaf development characterize allmonocots?Aliso14:271–79

11. Blazquez MA, Green R, Nilsson O, Suss-man MR, Weigel D. 1998. Gibberellinspromote flowering of Arabidopsis by ac-tivating the LEAFY promoter.Plant Cell10:791–800

12. Bohmert K, Camus I, Bellini C, BouchezD, Caboche M, Benning C. 1998. AGO1defines a novel locus of Arabidopsiscontrolling leaf development.EMBO J.17:170–80

13. Bruck DK, Walker DB. 1985. Cell deter-mination during embryogenesis inCitrusjambhiri: II Epidermal differentiation asa one time event.Am. J. Bot.72:1602–9

14. Cerioli S, Marocco A, Maddaloni M,Motto M, Salamini F. 1994. Early eventsin maize leaf epidermis formation as re-vealed by cell lineage studies.Develop-ment120:2113–20

15. Chandra Shekhar KN, Sawhney VK.1989. Regulation of the fusion of floral or-gans by temperature and gibberellic acidin the normal andsolanifolia mutant oftomato (Lycopersicon esculentum). Can.J. Bot.68:713–18

Ann

u. R

ev. P

lant

. Phy

siol

. Pla

nt. M

ol. B

iol.

1999

.50:

419-

446.

Dow

nloa

ded

from

ww

w.a

nnua

lrev

iew

s.or

gby

Bri

gham

You

ng U

nive

rsity

- I

daho

on

04/1

9/13

. For

per

sona

l use

onl

y.




16. Chandra Shekhar KN, Sawhney VK.1990. Leaf development in the normal andsolanifoliamutant of tomato (Lycopersi-con esculentum). Am. J. Bot.77:46–53

17. Chen J-J, Janssen B-J, Williams A, SinhaN. 1997. A gene fusion at a homeoboxlocus: alternations in leaf shape and im-plications for morphological evolution.Plant Cell9:1289–1304

18. Chuck G, Lincoln C, Hake S. 1996.KNAT1induces lobed leaves with ectopicmeristems when overexpressed in Ara-bidopsis.Plant Cell8:1277–89

19. Clark SE, Jacobsen SE, Levin JZ,Meyerowitz EM. 1996. The CLAVATAand SHOOT MERISTEMLESS loci com-petitively regulate meristem activity inArabidopsis. Development.122:1567–75

20. Clark SE, Running MP, Meyerowitz EM.1993. CLAVATA1, a regulator of meris-tem and flower development inArabidop-sis. Development121:2057–67

21. Clark SE, Running MP, Meyerowitz EM.1995. CLAVATA3 is a specific regulatorof shoot and floral meristem developmentaffecting the same process as CLAVATA1.Development121:2057–67

22. Clark SE, Williams RW, Meyerowitz EM.1997. The CLAVATA1 gene encodes a pu-tative receptor kinase that controls shootand floral meristem size in Arabidopsis.Cell 89:575–585

23. Coen ES, Romero JM, Doyle S, ElliottR, Murphy G, Carpenter R. 1990.flor-icaula: a homeotic gene required forflower development inAntirrhinum ma-jus. Cell 63:1311–22

24. Coleman WK, Greyson RI. 1976. Thegrowth and development of the leaf intomato (Lycopersicon esculentum). II.Leaf ontogeny.Can. J. Bot.54:2704–17

25. Collier DE, Grodzinski B. 1996. Growthand maintenance respiration of leaflet,stipule, tendril, rachis, and petiole tissuesthat make up the compound leaf of pea(Pisum sativum). Can. J. Bot.74:1331–37

26. Dengler NG. 1984. Comparison of leafdevelopment in normal (+/+), entire(e/e), and Lanceolate(La/+) plants oftomato,Lycopersicon esculentum‘AilsaCraig’. Bot. Gaz.145:66–77

27. Deschamp PA, Cooke TJ. 1983. Leaf di-morphism in aquatic angiosperms: sig-nificance of turgor pressure and cell ex-pansion.Science219:505–7

28. Deschamp PA, Cooke TJ. 1985. Leafdimorphism in the aquatic angiospermCallitriche heterophylla. Am. J. Bot.72:1377–87

29. Donoghue MJ, Doyle JA. 1989. Phylo-genetic analysis of angiosperms and therelationships of Hamamelidae. InEvolu-tion, Systematics, and Fossil History ofthe Hamamelidae, ed. PR Crane, S Black-more, 1:17–45. Oxford: Clarendon

30. Doyle JA, Donoghue MJ. 1986. Seedplant phylogeny and the origin of an-giosperms: an experimental cladistic ap-proach.Bot. Rev.52:321–431

31. Eames AJ. 1961.Morphology of An-giosperms. New York: McGraw Hill

32. Evans MMS, Barton MK. 1997. Genet-ics of angiosperm shoot apical meristemdevelopment.Annu. Rev. Plant Physiol.Plant Mol. Biol.48:673–701

33. Evans MMS, Poethig RS. 1995. Gibber-ellins promote vegetative phase changeand reproductive maturity in maize.PlantPhysiol.108:475–87

34. Finnegan EJ, Peacock WJ, Dennis ES.1996. Reduced DNA methylation inAra-bidopsis thaliana results in abnormalplant development.Proc. Natl. Acad. Sci.USA93:8449–54

35. Deleted in proof36. Fleming AJ, McQueen-Mason S, Man-

del T, Kuhlemeier C. 1997. Induction ofleaf primordia by the cell wall protein ex-pansin.Science276:1415–18

37. Freeling M. 1992. A conceptual frame-work for maize leaf development.Dev.Biol. 153:44–58

38. Galinat WC. 1959. The phytomer in rela-tion to the floral homologies in the Amer-ican Maydea. Bot. Mus. Lefl. HarvardUniv. 19:1–32

39. Gifford EM, Foster AS. 1989.Morphol-ogy and Evolution of Vascular Plants.New York: Freeman

40. Goliber TE, Feldman LJ. 1989. Osmoticstress, endogenous abscisic acid, and thecontrol of leaf morphology inHippurisvulgaris L. Plant Cell Environ.12:163–71

41. Goliber TE, Feldman LJ. 1990. Devel-opmental analysis of leaf plasticity inthe heterophyllous aquatic plantHippurisvulgaris. Am. J. Bot.77:399–412

42. Goliber TE, Kessler SA, Chen JJ, Bha-rathan G, Sinha N. 1998. Genetic, mole-cular and morphological analysis of com-pound leaf development.Curr. TopicsDev. Biol.41:259–90

43. Goodrich J, Puangsomlee P, Martin M,Long D, Meyerowitz EM, Coupland G.1997. A polycomb-group gene regulateshomeotic gene expression inArabidopsis.Nature386:44–51

44. Deleted in proof45. Green PB. 1987. Inheritance of pattern:

Ann

u. R

ev. P

lant

. Phy

siol

. Pla

nt. M

ol. B

iol.

1999

.50:

419-

446.

Dow

nloa

ded

from

ww

w.a

nnua

lrev

iew

s.or

gby

Bri

gham

You

ng U

nive

rsity

- I

daho

on

04/1

9/13

. For

per

sona

l use

onl

y.



442 SINHA

analysis from phenotype to gene.Am.Zool.27:657–73

46. Green PB, Lang JM. 1981. Towarda biophysical theory of organogenesis:birefringence observations on regener-ating leaves in the succulent,Grap-topetalum paraguayenseE. Walther.Planta151:413–26

47. Haber AH. 1962. Nonessentiality of con-current cell divisions for degree of polar-ization of leaf growth. I. Studies with ra-diation induced mitotic inhibition.Am. J.Bot.49:583–89

48. Hagemann W. 1984. Morphological as-pects of leaf development in ferns and an-giosperms. InContemporary Problems inPlant Anatomy, ed. RA White, WC Dick-ison, pp. 301–49. New York: Academic

49. Hagemann W, Gleissberg S. 1996.Organogenetic capacity of leaves: the sig-nificance of marginal blastozones in an-giosperms.Plant Syst. Evol.199:121–52

50. Hareven D, Gutfinger T, Parnis A, EshedY, Lifschitz E. 1996. The making of acompound leaf: genetic manipulation ofleaf architecture in tomato.Cell 84:735–44

51. Harper L, Freeling M. 1996. Interactionsof liguleless1 function during ligule in-duction in maize.Genetics144:1871–82

52. Hofer J, Turner L, Hellens R, AmbroseM, Matthews P, et al. 1997.UNIFOLIATAregulates leaf and flower morphogenesisin pea.Curr. Biol. 7:581–87

52a. Hofer JMI, Ellis THN. 1998. The geneticcontrol of patterning in pea leaves.TrendsPlant Sci.3:439–44

53. Jackson D, Veit B, Hake S. 1994. Expres-sion of maizeKNOTTED-1related home-obox genes in the shoot apical meristempredicts patterns of morphogenesis in thevegetative shoot.Development120:405–13

54. Janssen B-J, Lund L, Sinha N. 1998.Overexpression of a homeobox gene,LeT6, reveals indeterminate features inthe tomato compound leaf.Plant Physiol.117:771–86

55. Janssen B-J, Williams A, Chen J-J, Math-ern J, Hake S, Sinha N. 1998. Isolationand characterization of two knotted-likehomeobox genes from tomato.Plant Mol.Biol. 36:417–25

56. Jeune B. 1981. Mod`ele empirique dudeveloppement des feuilles de Dicotyle-dons.Bull. Mus. Natl. Hist. Nat. Sect. BAdansonia Bot. Phytochim.4:433–59

57. Jones CS. 1993. Heterochrony and het-eroblastic leaf development in two sub-species ofCucurbita argyrosperma(Cu-curbitaceae).Am. J. Bot.80:778–95

58. Kane ME, Albert LS. 1987. Abscisicacid induces aerial leaf morphology andvasculature in submergedHippuris vul-garisL. Aquat. Bot.28:81–88

59. Kaplan DR. 1973. The monocotyledons:their evolution and comparative biology.VII. The problem of leaf morphology andevolution in the monocotyledons.Q. Rev.Biol. 48:437–57

60. Kaplan DR. 1975. Comparative develop-mental evaluation of the morphology ofunifacial leaves in the monocotyledons.Bot. Jahrb. Syst.95:1–105

61. Deleted in proof62. Deleted in proof63. Deleted in proof64. Kaplan DR, Hagemann W. 1991 The rela-

tionship of cell and organism in vascularplants.BioScience41:693–703

65. Keddie JS, Carroll B, Jones JDG, Gruis-sem W. 1996. The DCL gene of a tomato isrequired for chloroplast development andpalisade morphogenesis in leaves.EMBOJ. 15:4208–17

66. Kelly AJ, Bonnlander MB, Meeks-Wag-ner DR. 1995. NFL, the tobacco homologof FLORICAULA and LEAFY, is tran-scriptionally expressed in both vegetativeand floral meristems.Plant Cell 7:225–34

67. Deleted in proof68. Kerstetter RA, Poethig RS. 1998. The

specification of leaf identity during shootdevelopment.Annu. Rev. Cell Dev. Biol.14:373–98

69. Kerstetter R, Vollbrecht E, Lowe B, Yam-aguchi J, Hake S. 1994. Sequence analysisand expression patterns divide the maizeknotted1-like homeobox genes into twoclasses.Plant Cell6:1877–87

70. Kessler S, Sinha N. 1997. Identity of extracell-layers produced in theglossy∗ muta-tion in maize.Maize Gen. Coop. Newslett.71:30–31 (cited by permission)

71. Kim GT, Tsukaya H, Uchimiya H. 1998.TheCURLY LEAFgene controls both di-vision and elongation of cells during theexpansion of the leaf blade inArabidopsisthaliana. Planta206:175–83

72. Kusaba S, Fukumoto M, Honda C, Ya-maguchi I, Sakamoto T, et al. 1998. De-creased GA1 content caused by the over-expression ofOSH1 is accompanied bysuppression of GA 20-oxidase gene ex-pression.Plant Physiol.117:1179–84

73. Kusaba S, Kano-Murakami Y, MatsuokaM, Tamaoki M, Sakamoto T, et al. 1998.Alteration of hormone levels in trans-genic tobacco plants overexpressing a ricehomeobox geneOSH1. Plant Physiol.116:471–76

Ann

u. R

ev. P

lant

. Phy

siol

. Pla

nt. M

ol. B

iol.

1999

.50:

419-

446.

Dow

nloa

ded

from

ww

w.a

nnua

lrev

iew

s.or

gby

Bri

gham

You

ng U

nive

rsity

- I

daho

on

04/1

9/13

. For

per

sona

l use

onl

y.




74. Lacroix CR. 1995. Changes in leaflet andleaf lobe form in developing compoundand finely divided leaves.Bot. Jahrb. Syst.117:317–31

75. Lacroix CR, Sattler R. 1994. Expressionof shoot features in early leaf develop-ment ofMurraya paniculata(Rutaceae).Can. J. Bot.72:678–87

76. Langdale JA, Lane B, Freeling M, NelsonT. 1989. Cell lineage analysis of maizebundle sheath and mesophyll cells.Dev.Biol. 133:128–39

77. Langdale JA, Zelitch I, Miller E, Nelson T.1988. Cell position and light influence C4versus C3 patterns of photosynthetic geneexpression in maize.EMBO J.7:3643–51

78. Larkin JC, Marks MD, Nadeau J, Sack F.1997. Epidermal cell fate and patterningin leaves.Plant Cell9:1109–20

79. Larkin JC, Oppenheimer DG, Lloyd AM,Paparozzi ET, Marks MD. 1994. Rolesof the Glabrous1andTransparent TestaGlabragenes inArabidopsistrichome de-velopment.Plant Cell5:1065–76

80. Larkin JC, Young N, Prigge M, MarksMD. 1996. The control of trichome spac-ing and number in Arabidopsis.Develop-ment122:997–1005

81. Laufs P, Dockx J, Kronenberger J, TraasJ. 1998. Mgoun1 and mgoun2: twogenes required for primordium initia-tion at the shoot apical and floral meris-tems in Arabidopsis thaliana. Develop-ment125:1253–60

82. Lincoln C, Long J, Yamaguchi J,Serikawa K, Hake S. 1994. AKnotted1-like homeobox gene in Arabidopsis is ex-pressed in the vegetative meristem anddramatically alters leaf morphology whenoverexpressed in transgenic plants.PlantCell 6:1859–76

83. Lloyd AM, Walbot V, Davis RW. 1992.Arabidopsisand Nicotiana anthocyaninproduction activated by maize regulatorsRandCl. Science258:1173–75

84. Lolle SJ, Hsu W, Pruitt RE. 1998. Geneticanalysis of organ fusion inArabidopsisthaliana. Genetics149:607–19

85. Long JA, Moan EI, Medford JI, BartonMK. 1996. A member of the KNOTTEDclass of homeodomain proteins encodedby theSHOOTMERISTEMLESSgene ofArabidopsis. Nature379:66–69

86. Deleted in proof87. Lu B, Villani PJ, Watson JC, Darllen

AD, Cooke TJ. 1996. The control ofpinna morphology in wild-type and mu-tant leaves of the garden pea (PisumsativumL.). Int. J. Plant Sci.157:659–73

88. Lyndon RF. 1982. Changes in polarity of

growth during leaf initiation in the pea,Pisum sativumL. Ann. Bot.49:281–90

89. Ma H, McMullen MD, Finer JJ. 1994.Identification of a homeobox-containinggenes with enhanced expression dur-ing soybean (Glycine maxL.) somaticembryo development.Plant Mol. Biol.24:465–73

90. Marx GA. 1987. A suite of mutants thatmodify pattern formation in pea leaves.Plant Mol. Biol. Rep.5:311–35

91. Matsuoka M, Ichikawa H, Saito A, TadaY, Fujimura T, Kano-Murakami Y. 1993.Expression of a rice homeobox genecauses altered morphology of transgenicplants.Plant Cell18:1039–48

92. McConnell JR, Barton MK. 1998. Leafpolarity and meristem formation inAra-bidopsis. Development125:2935–42

93. McLellan T. 1993. The roles of hete-rochrony and heteroblasty in the diversi-fication of leaf shapes inBegonia dregei(Begoniaceae).Am. J. Bot.80:796–804

94. Meicenheimer RD, Muehlbauer FJ, Hind-man JL, Gritton ET. 1983. Meristem char-acteristics of genetically modified pea(Pisum sativun) leaf primordia.Can. J.Bot.61:3430–37

95. Meinhardt H. 1995. Development ofhigher organisms: how to avoid errorpropagation and chaos.Physica86:96–103

96. Merrill EK. 1986. Heteroblastic seedlingsof green ash: I. Predictability of leafform and primordial length.Can. J. Bot.64:2645–49

97. Merrill EK. 1986. Heteroblastic seedlingsof green ash: II. Early development ofsimple and compound leaves.Can. J. Bot.64:2650–61

98. Moreno MA, Harper LC, Krueger RW,Dellaporta SL, Freeling M. 1997. Ligule-less1 encodes a nuclear-localized proteinrequired for induction of ligules and au-ricles during maize leaf organogenesis.Genes Dev.11:616–28

99. Muehlbauer G, Fowler JE, Freeling M.1997. Sectors expressing the home-obox gene liguleless3 implicate a time-dependent mechanism for cell fate acqui-sition along the proximal-distal axis of themaize leaf.Development124:5097–106

100. Nelson T, Dengler N. 1997. Leaf vascularpattern formation.Plant Cell9:1121–35

101. Oppenheimer DG, Herman PL, Sivaku-maran S, Esch J, Marks MD. 1991. Amybgene required for leaf trichome differenti-ation in Arabidopsis is expressed in stip-ules.Cell 67:483–93

102. Oppenheimer DG, Pollock MA, Vacik J,Szymanski DB, Ericson B, et al. 1997.

Ann

u. R

ev. P

lant

. Phy

siol

. Pla

nt. M

ol. B

iol.

1999

.50:

419-

446.

Dow

nloa

ded

from

ww

w.a

nnua

lrev

iew

s.or

gby

Bri

gham

You

ng U

nive

rsity

- I

daho

on

04/1

9/13

. For

per

sona

l use

onl

y.



444 SINHA

Essential role of a kinesin-like proteinin Arabidopsis trichome morphogenesis.Proc. Natl. Acad. Sci. USA94:6261–66

103. Parnis A, Cohen O, Gutfinger T, HarevenD, Zamir D, Lifschitz E. 1997. The dom-inant developmental mutants of tomato,Mouse-earandCurl, are associated withdistinct modes of abnormal transcrip-tional regulation of a Knotted gene.PlantCell 9:2143–58

104. Deleted in proof105. Pneuli L, Carmel-Goren L, Hareven D,

Gutfinger T, Alvarez J, et al. 1998.The Self-Pruninggene of tomato regu-lates vegetative to reproductive switch-ing of sympodial meristems and is the or-tholog of CEN and TFL1. Development125:1979–89

106. Poethig S. 1984. Cellular parameters ofleaf morphogenesis in maize and to-bacco. In Contemporary Problems inPlant Anatomy, ed. RA White, WC Dick-ison, pp. 235–59. Orlando: Academic

107. Pouteau S, Nicholls D, Tooke F, Coen E,Battey N. 1997. The induction of flower-ing in Impatiens.Development124:3343–51

108. Deleted in proof109. Rerie WG, Feldmann KA, Marks MD.

1994. The GLABRA2 gene encodes ahomeodomain protein required for nor-mal trichome development in Arabidop-sis.Genes Dev.8:1388–99

110. Reynolds JO, Eisses JF, Sylvester AW.1998. Balancing division and expansionduring maize leaf morphogenesis: analy-sis of the mutant,warty-1. Development125:259–68

111. Rick CM, Butler L. 1956. Cytogenetics oftomato.Adv. Genet.7:267–382

112. Rick CM, Harrison AL. 1959. Inheritanceof five new tomato seedling characters.J.Hered.50:91–98

113. Robbins WJ. 1957. Gibberellic acid andthe reversal of adult Hedera to a juvenilestate.Am. J. Bot.44:743–46

114. Rogler CE, Hackett WP. 1975. Phasechange inHedera helix: induction of themature to juvenile phase change by gib-berellin A3. Physiol. Plant.34:141–47

115. Rogler CE, Hackett WP. 1975. Phasechange inHedera helix: stabilizationof the mature form with abscisic acidand growth retardants.Physiol. Plant.34:148–52

116. Sachs T. 1991. Cell polarity and tissue pat-terning in plants.Dev. Suppl.1:83–93

117. Sattler R, Rutishauser R. 1992. Partial ho-mology of pinnate leaves and shoots: ori-entation of leaflet inception.Bot. Jahrb.Syst.114:61–79

118. Scanlon MJ, Freeling M. 1998. Thenar-row sheathleaf domain deletion: a ge-netic tool used to reveal developmentalhomologies among modified maize or-gans.Plant J.13:547–61

119. Scanlon MJ, Freeling M. 1997. Clonalsectors reveal that a specific meristematicdomain is not utilized in the maize mutantnarrow sheath. Dev. Biol.182:52–66

120. Scanlon MJ, Schneeberger RG, Freel-ing M. 1996. The maize mutantnarrowsheathfails to establish leaf margin iden-tity in a meristematic domain.Develop-ment122:1683–91

121. Schmidt BL, Millington WF. 1968. Regu-lation of leaf shape inProserpinaca palus-tris. Bull. Torrey Bot. Club95:264–86

122. Schneeberger RG, Becraft PW, Hake S,Freeling M. 1995. Ectopic expression oftheknoxhomeo box generough sheath 1alters cell fate in the maize leaf.GenesDev.9:2292–304

123. Schneeberger R, Tsiantis M, Freeling M,Langdale JA. 1998. Therough sheath2gene negatively regulates homeobox geneexpression during maize leaf develop-ment.Development125:2857–65

124. Schwabe WW. 1971. Chemical modifica-tion of phyllotaxis and its implications.Symp. Soc. Exp. Biol.25:301–22

125. Sculthorpe CD. 1967.The Biology ofAquatic Vascular Plants. London: Arnold

126. Selker JML, Green PB. 1984. Organo-genesis inGraptopetalum paraguayenseE. Walther: Shifts in orientation of corti-cal microtubule arrays are associated withpericlinal divisions.Planta160:289–97

127. Serikawa KA, Martinez-Laborda A, KimHS, Zambryski PC. 1997. Localization ofexpression ofKNAT3, a class 2knotted1-like gene.Plant J.11:853–61

128. Serikawa KA, Zymbryski PC. 1997. Do-main exchanges between KNAT3 andKNAT1 suggest specificity of the kn1-likehomeodomains requires sequences out-side the third helix and N-terminal armof the homeodomain.Plant J.11:863–69

129. Sharman BC. 1942. Developmentalanatomy of the shoot ofZea maysL. Ann.Bot. n.s.6:245–83

130. Sieburth LE, Meyerowitz EM. 1997.Molecular dissection of theAGAMOUScontrol region shows thatciselements forspatial regulation are located intrageni-cally. Plant Cell9:355–65

131. Sinha N. 1998. Organ and cell fusionsin the adherent1mutant in maize.Int. J.Plant Sci.159:702–15

132. Sinha N, Hake S. 1994. The Knotted leafblade is a mosaic of blade, sheath, andauricle identities.Dev. Genet.15:401–14

Ann

u. R

ev. P

lant

. Phy

siol

. Pla

nt. M

ol. B

iol.

1999

.50:

419-

446.

Dow

nloa

ded

from

ww

w.a

nnua

lrev

iew

s.or

gby

Bri

gham

You

ng U

nive

rsity

- I

daho

on

04/1

9/13

. For

per

sona

l use

onl

y.




133. Sinha N, Lynch M. 1998. Fused organsin theadherent1 mutation in maize showaltered epidermal walls with no perturba-tions in tissue identities.Planta206:184–95

134. Sinha N, Williams RE, Hake S. 1993.Overexpression of the maize homeoboxgene,KNOTTED-1, cause a switch fromdeterminate to indeterminate cell fates.Genes Dev.7:787–95

135. Smith LG, Hake S, Sylvester AW. 1996.The tangled-1mutation alters cell divi-sion orientations throughout maize leafdevelopment without altering leaf shape.Development.122:481–89

136. Smith LG, Greene B, Veit B, Hake S.1992. A dominant mutation in the maizehomeobox gene,Knotted-1, cause its ec-topic expression in leaf cells with alteredfates.Development116:21–30

137. Snow M, Snow R. 1931. Experiments onphyllotaxis. I. The effect of isolating a pri-mordium.Phil. Trans. R. Soc. London Ser.B 221:1–43

138. Souer E, Houwelingen AV, Kloos D, MolJ, Koes R. 1996. TheNo Apical Meristemgene of petunia is required for pattern for-mation in embryos and flowers and is ex-pressed at meristem and primordia bound-aries.Cell 85:159–70

139. Sparks PD, Postlethwait SN. 1967. Com-parative morphogenesis of the dimorphicleaves ofCyamopsis tetragonolobus. Am.J. Bot.54:281–85

140. Steeves TA, Sussex IM. 1989.Patternsin Plant Development. Cambridge/NewYork/New Rochelle/Melbourne/Sydney:Cambridge Univ. Press

141. Stettler RF. 1964. Dosage effects of thelanceolate gene in tomato.Am. J. Bot.51:253–64

142. Stevens AM, Rick CM. 1986. Geneticsand breeding. InThe Tomato Crop, ed.JG Atherton, J Rudick, pp. 35–109. NewYork: Chapman & Hall

143. Sussex IM. 1955. Morphogenesis inSolanum tuberosumL. Apical structureand developmental pattern of the juvenileshoot.Phytomorphology5:253–73

144. Sylvester AW, Cande WZ, Freeling M.1990. Division and differentiation duringnormal and liguleless-1 maize leaf devel-opment.Development110:985–1000

145. Sylvester AW, Smith L, Freeling M. 1996.Aquisition of identity in the developingleaf. Annu. Rev. Cell Dev. Biol.12:257–304

146. Takahashi T, Gasch A, Nishizawa N,Chua NH. 1995. TheDIMINUTO geneof Arabidopsis is involved in regulatingcell elongation.Genes Dev.9:97–107

147. Taylor CB. 1997.Knox-on effects on leafdevelopment.Plant Cell9:2101–5

148. Taylor DW, Hickey LJ. 1992. Phyloge-netic evidence for the herbaceous ori-gin of angiosperms.Plant Syst. Evol.180:137–56

149. Telfer A, Poethig RS. 1994. Leaf devel-opment in Arabidopsis. InArabidopsis,ed. EM Meyerowitz, CR Somerville, pp.379–401. New York: Cold Spring HarborLab. Press

150. Timmermans MCP, Schultes NP, Jan-kovsky JP, Nelson T. 1998.Leafblade-less1is required for dorsoventrality of lat-eral organs in maize.Development125:2813–23

151. Traas J, Bellini C, Nacry P, Kronen-berger J, Bouchez D, Caboche M. 1995.Normal differentiation patterns in lackingmicrotubular preprophase bands.Nature375:676–77

152. Troll W. 1935. Vergleichende Morpholo-gie der Fiederblatter.Nova Acta Leopold-ina 2:311–455

153. Tsuge T, Tsukaya H, Uchimiya H. 1996.Two independent and polarized processesof cell elongation regulate leaf bladeexpansion inArabidopsis thaliana(Il )Heynh.Development122:1589–600

154. Tsukaya H. 1995. Developmental genet-ics of leaf morphogenesis in dicotyle-donous plants.J. Plant Res.108:407–16

155. Tsukaya H, Uchimiya H. 1997. Geneticanalyses of the formation of the serratedmargin of leaf blades in Arabidopsis:combination of a mutational analysis ofleaf morphogenesis with the characteriza-tion of a specific marker gene expressedin hydathodes and stipules.Mol. Gen.Genet.256:231–38

156. Veit B, Briggs SP, Schmidt RJ, YanofskyMF, Hake S. 1998. Regulation of leaf ini-tiation by theterminal ear1gene of maize.Nature393:166–68

157. Villani PJ, DeMason DA. 1997. Roles oftheAfandTl genes in pea leaf morphogen-esis: characterization of the double mu-tant (afaftltl). Am. J. Bot.84:1323–36

158. Vollbrecht E, Veit B, Sinha N, Hake S.1990. The developmental geneKnotted-1is a member of a maize homeobox genefamily. Nature350:241–43

159. Waites R, Hudson A. 1995.phantastica:a gene required for dorsoventrality ofleaves inAntirrhinum majus. Develop-ment121:2143–54

160. Waites R, Selvadurai HRN, Oliver IR,Hudson A. 1998. The Phantastica geneencodes a MYB transcription factor in-volved in growth and dorsoventrality

Ann

u. R

ev. P

lant

. Phy

siol

. Pla

nt. M

ol. B

iol.

1999

.50:

419-

446.

Dow

nloa

ded

from

ww

w.a

nnua

lrev

iew

s.or

gby

Bri

gham

You

ng U

nive

rsity

- I

daho

on

04/1

9/13

. For

per

sona

l use

onl

y.



446 SINHA

of lateral organs inAntirrhinum. Cell93:779–89

161. Walsh J, Waters CA, Freeling M. 1998.The maize geneliguleless 2encodes abasic leucine zipper protein involved inthe establishment of the leaf blade-sheathboundary.Genes Dev.12:208–18

162. Wardlaw CW. 1949. Experiments onorganogenesis in ferns.Growth 13:93–131 (Suppl.)

163. Weigel D, Alvarez J, Smyth D, YanofskyMF, Meyerowitz EM. 1992.LEAFYcon-

trols floral meristem identity inArabidop-sis. Cell 69:843–59

164. Young JPW. 1983. Pea leaf morphogen-esis: a simple model.Ann. Bot.52:311–16

165. Young JP, Dengler NG, Horton RF. 1987.Heterophylly in Ranunculus flabellaris:the effect of abscisic acid on leaf anatomy.Ann. Bot.60:117–25

166. Young JP, Horton RF. 1985. Heterophyllyin Ranunculus flabellarisRaf: the effectof abscisic acid.Ann. Bot.55:899–902

Ann

u. R

ev. P

lant

. Phy

siol

. Pla

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ol. B

iol.

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.50:

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gham

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ng U

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04/1

9/13

. For

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l use

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Annual Review of Plant Physiology and Plant Molecular Biology Volume 50, 1999

CONTENTSEducator and Editor, Martin Gibbs 1Phosphate Translocators in Plastids, Ulf-Ingo Flügge 27The 1-Deoxy-D-xylulose-5-phosphate Pathway of Isoprenoid Biosynthesisin Plants, Hartmut K. Lichtenthaler 47

Chlorophyll Degradation, Philippe Matile, Stefan Hörtensteiner, Howard Thomas 67

Plant Protein Serine/Threonine Kinases: Classification and Functions, D. G. Hardie 97

Improving the Nutrient Composition of Plants to Enhance Human Nutrition and Health, Michael A. Grusak, Dean DellaPenna 133

Gametophyte Development in Ferns, Jo Ann Banks 163C4 Gene Expression, Jen Sheen 187Genetic Analysis of Hormone Signaling, Peter McCourt 219Cellulose Biosynthesis: Exciting Times for a Difficult Field of Study, Deborah P. Delmer 245

Nitrate Reductase Structure, Function, and Regulation: Bridging the Gap Between Biochemistry and Physiology, Wilbur H. Campbell 277

Crassulacean Acid Metabolism: Molecular Genetics, John C. Cushman, Hans J. Bohnert 305

Photoprotection Revisited: Genetic and Molecular Approaches, Krishna K. Niyogi 333

Molecular and Cellular Aspects of the Arbuscular Mycorrhizal Symbiosis, Maria J. Harrison 361

Enzymes and Other Agents that Enhance Cell Wall Extensibility, Daniel J. Cosgrove 391

Leaf Development in Angiosperms, Neelima Sinha 419The Pressure Probe: A Versatile Tool in Plant Cell Physiology, A. Deri Tomos, Roger A. Leigh 447

The Shikimate Pathway, Klaus M. Herrmann, Lisa M. Weaver 473Asymmetric Cell Division in Plants, Ben Scheres, Philip N. Benfey\ 505CO2-Concentrating Mechanisms in Photosynthetic Microorganisms, Aaron Kaplan, Leonora Reinhold 539

Plant Cold Acclimation: Freezing Tolerance Genes and Regulatory Mechanisms, Michael F. Thomashow 571

The Water-Water Cycle in Chloroplasts: Scavenging of Active Oxygens and Dissipation of Excess Photons, Kozi Asada 601

Silicon, Emanuel Epstein 641Phosphate Acquisition, K. G. Raghothama 665Roots in Soil: Unearthing the Complexities of Roots and Their Rhizospheres, Margaret E. McCully 695

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