look before you leap: memoirs of a “cell biological” plant pathologist
TRANSCRIPT
AR384-FM ARI 14 July 2009 23:48
Ann
u. R
ev. P
hyto
path
ol. 2
009.
47:1
-13.
Dow
nloa
ded
from
ww
w.a
nnua
lrev
iew
s.or
gby
Uni
vers
ity o
f A
laba
ma
- B
irm
ingh
am o
n 09
/26/
13. F
or p
erso
nal u
se o
nly.
ANRV384-PY47-01 ARI 2 July 2009 17:5
Look Before You Leap:Memoirs of a “Cell Biological”Plant PathologistMichele C. HeathDepartment of Cell and Systems Biology, University of Toronto, Toronto,Ontario M5S 1A1, Canada; email: [email protected]
Annu. Rev. Phytopathol. 2009. 47:1–13
The Annual Review of Phytopathology is online atphyto.annualreviews.org
This article’s doi:10.1146/annurev-phyto-080508-081857
Copyright c© 2009 by Annual Reviews.All rights reserved
0066-4286/09/0908/0001$20.00
Key Words
microscopy, rust fungi, host-parasite specificity, hypersensitiveresponse, nonhost resistance, specific elicitor, evolution
AbstractIn this article, I recount how I became a plant pathologist and usedclues derived from light and electron microscopy to direct my researchon the interactions between plants and biotrophic fungi. Examples ofthe value of microscopic examination are described for investigationsof host compatibility, the hypersensitive response, nonhost resistance,and the evolution of host-parasite specificity.
1
Ann
u. R
ev. P
hyto
path
ol. 2
009.
47:1
-13.
Dow
nloa
ded
from
ww
w.a
nnua
lrev
iew
s.or
gby
Uni
vers
ity o
f A
laba
ma
- B
irm
ingh
am o
n 09
/26/
13. F
or p
erso
nal u
se o
nly.
ANRV384-PY47-01 ARI 2 July 2009 17:5
INTRODUCTION
In the 2004 issue of the Annual Review of Phy-topathology, Anne Vidaver wrote that she washonored to be the first woman plant pathol-ogist to write a prefatory chapter for the se-ries (35). As the second woman to write sucha chapter, I, too, am honored and hope thatanother woman’s experiences and perspectivemay be of interest to readers. The following ispartly personal and partly a set of overlappingchronologies to show how my research was di-rected by visual clues provided by the plantsand fungi that I studied. With today’s pressureto win grants and get published in top journals,the freedom to follow these clues is not as greatas it once was, and basic microscopy may seeman old-fashioned technique to some. Therefore,I hope this chapter also demonstrates that awealth of information still exists in what youcan see at the cellular level.
HOW I BECAME A PLANTPATHOLOGIST
I have always been a visual person. I love therich colors and forms of nature and enjoyedpainting landscapes as a child and young adultin England. However, even as a child, I neverdoubted that I would become a scientist,although I had no professional scientists in myfamily. My father briefly taught science andthen became a mathematics teacher, both afteran unsuccessful attempt at making a career asa writer, and my mother was a high-rankingsecretary in the civil service. I developedasthma early in life and missed approximatelyone third of my schooling until I reached mylate teens, but I read every nature book I couldfind and even persuaded my parents to buyme a cheap microscope so I could look at themyriad of life forms that one could see in adrop of pond water. My father died when I wasfifteen, and my mother had the difficult job ofraising me and my baby brother at a time whenwomen were often treated as second-classcitizens, especially by financial institutions andmortgage companies. As a small, shy, easily
intimidated child, I am sure that I benefitedfrom the fact that most schools in England atthat time were single sex. Unisex schools tendto be unpopular these days, but in my opinion,they eliminate gender bias in the treatmentof students, and they encourage more youngwomen to enter the sciences. During my yearsas a professor, female students often told mehow they kept quiet in grade school scienceclasses because they did not want to be ridiculedor appear too smart in front of the boys.
I was lucky in that I had wonderful femalescience teachers who encouraged me in my lastfew years at school and were instrumental inhelping me to get a place at Westfield College,then an all female college within the Universityof London. There, in 1966, I received a first-class BSc honors degree in botany—a subjectthat I had never studied at school but which Ichose because I did not want to experiment onanimals. My independent research project onfungal sporulation convinced me that I lovedthe challenge of discovery through experimen-tation. I knew that I wanted to get a PhD, butdid not have any strong leanings toward a par-ticular discipline until I saw an advertisementfor a graduate student to study plant-parasite in-teractions. This sounded interesting, so I talkedto my mycology professor, who suggested thatI contact Professor R.K.S. Wood at ImperialCollege of Science and Technology, who wasone of the top researchers in the new field ofphysiological plant pathology. I did, he acceptedme as a graduate student, and my career in plantpathology began.
GRADUATE ANDPOSTDOCTORAL STUDIES
I like to say of Professor Wood that he “couldsee the wood for the trees,” meaning that hecould construct the large picture from all thesmall details, an ability that I have tried to em-ulate throughout my career. His approach tograduate students was to give them a broadtopic to study and then let them get on with it.My topic was to find out why leaf spot lesionswere self-limiting in size. I picked Ascochyta and
2 Heath
Ann
u. R
ev. P
hyto
path
ol. 2
009.
47:1
-13.
Dow
nloa
ded
from
ww
w.a
nnua
lrev
iew
s.or
gby
Uni
vers
ity o
f A
laba
ma
- B
irm
ingh
am o
n 09
/26/
13. F
or p
erso
nal u
se o
nly.
ANRV384-PY47-01 ARI 2 July 2009 17:5
Mycosphaerella leaf diseases of peas and, beinga visual person, started at what I perceived tobe the logical beginning of any study: to lookfor visual clues as to what was happening be-tween plant and fungus during the develop-ment of the disease. Cutting tissue sections,although useful in some situations, seemed tome to be an extremely inefficient and time-consuming way of examining the progress ofinfection, so I turned to cleared whole leaves orleaf pieces, which allowed me to examine largenumbers of infection sites in three dimensions.Not satisfied with what I could see with the lightmicroscope, I coerced my new boyfriend (andsoon-to-be husband, Brent Heath), a graduatestudent in the same building studying oomyceteultrastructure, into teaching me to use the elec-tron microscope (EM). Correlating what I sawwith studies on phenolic compounds, the newlydiscovered inducible antimicrobial compoundsknown as phytoalexins, and cell wall degradingenzymes clearly showed me the complexity ofplant-parasite interactions and the difficulty ofproving that a particular defense response is thecause of the cessation of fungal growth. It alsoimpressed on me the usefulness of microscopyas a foundation upon which physiological stud-ies can build (24).
In England in the late 1960s, there alreadywas a developing dichotomy between field-orientated and lab-orientated plant pathology,and still a heavy bias against women in the work-force, particularly in any area associated withagriculture. Having never been the subject ofgender bias during my previous education inall-female institutions, I took it for granted thatthere would be no problems as a graduate stu-dent, but now I realize how fortunate I was tohave a PhD supervisor who treated male andfemale students alike, and whose attitude didnot change after my marriage (although he didtell my husband not to get me pregnant beforeI finished my PhD!). As both my husband and Iwere interested in academic careers, we decidedto try to find postdoctoral fellowships togetherin the United States. My husband had neverseen America, and I was keen to return, havingspent a summer as an undergraduate traveling
Appressorium: aninfection structureformed by rust fungion the leaf surface,generally over stomata
Obligate biotroph:an organism that onlyobtains nutrients fromliving host cells andcommonly cannot becultured
the country. In those days, you could buy a “$99for 99 days” Greyhound bus ticket valid for anyroute—a real bargain even then. Having fin-ished our PhDs in 1969, my husband and I leftEngland with a total of $100 in our pocketsto go to the University of Georgia, where hehad a postdoctoral fellowship in the Depart-ment of Botany. I had a similar position in theDepartment of Plant Pathology and Plant Ge-netics in the lab of Dr. Willard K. Wynn to workon plant resistance to rust diseases, of which Iknew almost nothing. However, this again wasone of the many instances in my life when for-tune was on my side. At that time, Willard wasdemonstrating that appressorium formationby the bean rust fungus, Uromyces appendicula-tus, was triggered solely by topographical fea-tures of the stomata of host leaves (36). Hisenthusiasm for rust fungi was infectious.Working with him proved to be a wonderfullystimulating experience as he allowed me to gowherever the research suggested, while provid-ing critical encouragement along the way (healso loaned us money to keep us going until ourfirst paycheck). Because cowpeas were a signifi-cant crop in Georgia, I studied the cowpea rustfungus, Uromyces vignae (then U. phaseoli var.vignae), and soon was fascinated by the com-plex and sophisticated interaction that this ob-ligately biotrophic fungus has with the livingcells of its host. I have stayed with this systemthroughout my professional career.
Again as a visual person, I wanted to actu-ally see what happened between the resistantplant and its potential parasite. This seemed themost unbiased approach, given that all otherapproaches required a subjective decision onwhat metabolic process or defense compoundto study, the range of which depended on cur-rent theories and available techniques. So oncemore I turned to microscopy. Light microscopyof cleared leaves revealed significant variabilityin fungal growth and plant responses betweenindividual infection sites, leading me to take astatistical approach for comparisons betweenplants and treatments that was uncommon atthe time, and is still not ubiquitous today.This variability between infection sites, and the
www.annualreviews.org • Memoirs of a Plant Pathologist 3
Ann
u. R
ev. P
hyto
path
ol. 2
009.
47:1
-13.
Dow
nloa
ded
from
ww
w.a
nnua
lrev
iew
s.or
gby
Uni
vers
ity o
f A
laba
ma
- B
irm
ingh
am o
n 09
/26/
13. F
or p
erso
nal u
se o
nly.
ANRV384-PY47-01 ARI 2 July 2009 17:5
fact that rust fungi form both intercellular andintracellular structures, emphasized the valueof cytochemical techniques, which could revealplant responses at specific places in each indi-vidual infection site. To my mind, this infor-mation was far superior to that produced bymass extraction techniques that produced aver-age values and ignored the spatial componentsof interactions.
Rust fungi form feeding structures knownas haustoria within living host cells. The valueof electron microscopy in revealing the aspectsof this cellular interface between fungus andplant had been demonstrated by the pioneer-ing ultrastructural studies of wheat stem rust bythe Ehrlich husband and wife team in the early1960s (7). In the early 1970s, as plant fixationprocedures improved and cytochemical tech-niques applicable at the EM level were devel-oped, fascinating details emerged, such as thefact that the host plasma membrane was intactaround the invading haustorium (25), and that aunique structure, the neckband, [later shown byme to provide a seal to apoplastic flow along thehaustorial neck (12)] bridged the plasma mem-brane of both organisms. Coincidentally, one ofthe original papers on the neckband was pub-lished by a fellow graduate student who hadworked at the bench next to me at ImperialCollege (10).
Wanting to examine the cowpea rust systemin more detail than the light microscope couldprovide, I collaborated with my husband, whohad access to an EM, in the first of several jointpublications on rust fungal ultrastructure (22).Because my husband was used to cutting se-quential serial sections through each oomycetehyphal tip to obtain three-dimensional infor-mation on its ultrastructure, it seemed obviousto do the same in our studies of fungal infec-tion sites, rather than taking random sections,and this became a signature of our work. Thiswas a time when the EM became the instrumentthat every biology department had to have, andevery tissue section seemed to yield somethingnew and exciting, whatever the subject of inves-tigation. As more plant pathology labs carriedout ultrastructural studies, it became common
at plant pathology meetings to see groups ofparticipants huddled over handfuls of electronmicrographs—often to the light-hearted deri-sion of their nonmicroscopist colleagues.
HOW I GOT A JOB
The life of a postdoctoral fellow is a period freeof bureaucracy and responsibility that is likelynever to be achieved in one’s career again. How-ever, it eventually becomes unsatisfactory be-cause of its temporary nature and the frustra-tion of being in a nebulous position betweenstudent and faculty member. We decided thatmy husband should be the one to look for apermanent job first as two faculty positions atone institution seemed unlikely, especially withthe nepotism rules that then existed in manydepartments. There were few positions avail-able in the United Kingdom, so he acceptedan assistant professorship at York University inToronto, Canada, and we moved north in thesummer of 1971. Without a job or a work visa,I discovered for the first time how problematicit can be if both spouses wish to have a pro-fessional career in a related area. Rescue camefrom Dr. Verna J. Higgins in the Department ofBotany at the University of Toronto, a univer-sity that I had prophetically visited in my trav-els through North America as an undergradu-ate. Verna gave me a postdoctoral fellowship towork on the degradation of phytoalexins by thealfalfa fungal pathogen, Stemphylium botryosum.Within a year, and now with landed immigrant’sstatus that allowed me to take salaried work,my luck held yet again when a botany pro-fessor was unexpectedly appointed an assistantdean, resulting in an urgent need for someoneto teach first-year botany. On the basis of mybotany degree, I was hired first as a lecturer andsubsequently as a tenure-stream assistant pro-fessor when a position became available. As Irecall, I was told that I had received the perma-nent position while I was in hospital deliveringour daughter—carefully planned for late springwhen my teaching responsibilities were overuntil the fall. Those were the days when regu-lated faculty procedures were minimal. Female
4 Heath
Ann
u. R
ev. P
hyto
path
ol. 2
009.
47:1
-13.
Dow
nloa
ded
from
ww
w.a
nnua
lrev
iew
s.or
gby
Uni
vers
ity o
f A
laba
ma
- B
irm
ingh
am o
n 09
/26/
13. F
or p
erso
nal u
se o
nly.
ANRV384-PY47-01 ARI 2 July 2009 17:5
faculty members were rare and pregnant oneswere even rarer, so there was no official ma-ternity leave, and I was back at work within sixweeks. With today’s more structured employ-ment rules it may be a little easier, but trying tojuggle two academic careers in a family whileraising a child is always difficult. Many a timeour daughter slept in my office when we bothhad to work late, and she often accompaniedme, or my husband, to scientific meetings whenwe were going to different ones at the sametime. She did not find these, nor our constantscientific discourses at the dinner table, excit-ing. As a result, she swore that she would neverbe a scientist. Genes are powerful, however, andshe has ended up in a science-related profession.
I became an associate professor at theUniversity of Toronto in 1976, a full professorin 1981, and stayed at the university until I tookearly retirement in 2003. I consider myselflucky again that I ended up in a departmentthat was collegial and supportive. Canadianuniversities have a heavy emphasis on under-graduate teaching, which I enjoy as it developscommunication skills and ensures that onekeeps up to date with research outside of one’sspecialty. I have been involved in approxi-mately 11 different courses during my teachingcareer, mostly general botany, cell biology, andmycology. Not until 1986 did I get to regularlyteach plant pathology as well as other courses.On the whole, I think that this was a goodthing as it gave me a much broader contextfor my own research than if I had only taughtsubjects directly related to my research field.
When I was first appointed, the pressure toproduce large numbers of papers in top journalswas not as great as it is now, and one’s choice ofresearch topic was not so driven by the currentlypopular topics or techniques of the day. TheCanadian research grant system at that time pri-marily focused on the researcher rather than theproject and rewarded success even if the fundsresulted in publications not directly in the ar-eas mentioned in the grant proposal. For some-one like me, who preferred to follow whateverdirections the research suggested rather thanchoose projects based on current theories or
dogma, this system was ideal. Although some-times it meant that my work did not fit well intomainstream pathology research, it also allowedme to blaze a few trails.
THE EARLY YEARS AND THEDEVELOPMENT OF CONCEPTS
Through Dr. Wynn’s influence, I was invitedto be a symposium speaker at the second In-ternational Congress of Plant Pathology inMinnesota in 1973. I had managed to attendsome sessions at the first Congress in Londonin 1968 because Professor Wood had been theCongress secretary, and I had been involved indoing odd jobs to help. At that time, it hadbeen awe-inspiring to see the people behind thenames on the papers that I had avidly read as adoctoral student. Now, it was nerve-wracking topresent my research to some of these same peo-ple, including my PhD supervisor. I assumedthat I had done a good job when he afterwardasked a question from the audience that clearlyacknowledged that I had been one of his stu-dents. The research that I presented in thatsymposium was a comparative light and EMstudy of host resistance to the cowpea rust fun-gus expressed by resistant cultivars of cowpeaand nonhost resistance shown by plants con-sidered to be nonhost species for this pathogen(11). The results revealed that there was morethan one stage during the infection process atwhich a plant-parasite interaction could lead tothe cessation of fungal growth. It was also ap-parent that there was a difference between non-host plants and resistant host cultivars. Non-host plants tended to express resistance to rustfungi before the formation of the first haus-torium in a plant cell. In contrast, resistantcowpea plants, expressing what was assumedto be the gene-for-gene interactions originallydemonstrated for flax rust (9), more frequentlyexpressed resistance after haustorium forma-tion. These observations, in my opinion, wereof great conceptual significance and were theorigins of the concept of host-parasite speci-ficity that I published almost 10 years later(15).
www.annualreviews.org • Memoirs of a Plant Pathologist 5
Ann
u. R
ev. P
hyto
path
ol. 2
009.
47:1
-13.
Dow
nloa
ded
from
ww
w.a
nnua
lrev
iew
s.or
gby
Uni
vers
ity o
f A
laba
ma
- B
irm
ingh
am o
n 09
/26/
13. F
or p
erso
nal u
se o
nly.
ANRV384-PY47-01 ARI 2 July 2009 17:5
The common expression of host resistancewas the rapid death of the invaded cell, by thenknown as the hypersensitive response (HR), fol-lowing Stakman’s application of the term hyper-sensitive to this phenomenon in rust-resistantcereals (33). By the early 1970s, the HR hadbeen shown to be a common expression of re-sistance to a variety of pathogens, and whetherit was the cause or consequence of disease re-sistance became a matter of hot debate. Havinghad my work incorrectly cited as supporting oneside of the argument, I became irritated by thefact that the HR was being treated as a singlephenomenon even though my, and other ultra-structural studies, had shown significant differ-ences in this response between plant-pathogensystems. Moreover, I could see no reason whythe role of the HR had to be the same in everycase, given that the limited host range of eachpathogen suggested that many plant-pathogeninteractions must be species specific. As a re-sult, I wrote my first letter to the editor toPhytopathology, which was published (13), muchto my surprise.
By the mid-1970s, there was a wealthof ultrastructural studies on rust fungi, andDr. Larry Littlefield asked me if I would beinterested in coauthoring a highly illustrated,comprehensive book that brought all this in-formation together (26). Being young and en-thusiastic, I agreed, although I might not havedone so had I known the toll this would takeon my time and the stress it involved. Althoughneither of us made a fortune from royalties, itsold reasonably well for a specialized scientificbook, and I still consider it a valuable resourceof informative micrographs, whether or nottheir interpretation has changed with time.
My work, and the work of a few others,involving double inoculations of plants withcompatible and incompatible fungi stronglysupported the idea that, in susceptible plants,biotrophic fungi actively suppress the defensesthat would be induced if the same plant was act-ing as a nonhost to a different pathogen. Mydata also suggested that this suppression oc-curred even in resistant host cultivars, where
the absence of the prehaustorial, nonhost de-fenses allowed the rust fungus to form a haus-torium, triggering the subsequent expressionof the HR. The similarity in our thinking re-sulted in Dr. W.R. Bushnell and I agreeing towrite a companion set of letters to the editorto Phytopathology; his on the role of suppressorsin specificity (2) and mine on a general conceptof specificity that argued that the basic com-patibility (8) between a plant and its success-ful pathogen required a specific accommoda-tion of the latter that suppressed or otherwiserendered ineffective the nonhost defenses thatthe plant would otherwise express (15). Thismeant that cultivar (gene-for-gene) resistancewas superimposed on basic compatibility, andthat in some cases, selective pressure for suchresistance may develop only after the fungushad adapted to become a successful pathogen ofits host species. Both articles were supposed toappear in the same issue, with mine first. Unfor-tunately, they were published in different issues,and in the wrong order!
I believe my two letters to the editor, andthe fact that my research tried to address broadquestions related to plant-pathogen interac-tions, contributed to my election in 1982 asa Fellow of the American PhytopathologicalSociety—an honor that was totally unexpectedas I thought at the time that a requirement to bea Fellow was gray hair. Indeed, if I had aspired toany award, it was the Ruth Allen Award, as herpioneering light microscope studies of wheatstem rust that were published in the 1920s hadbeen an inspiration for me when I first startedworking with rust diseases. I’m not sure if it stillholds, but for many years, I was the youngestperson (almost 37 years old) ever to become aFellow. In the same year, I also was the firstwoman and first plant pathologist ever to re-ceive a prestigious E.W.R. Steacie MemorialFellowship from the Natural Sciences and En-gineering Research Council of Canada, whichprovided funds for me to buy a top-quality re-search microscope and gave me release timefrom my undergraduate teaching duties for twoyears. I have had additional awards since then,
6 Heath
Ann
u. R
ev. P
hyto
path
ol. 2
009.
47:1
-13.
Dow
nloa
ded
from
ww
w.a
nnua
lrev
iew
s.or
gby
Uni
vers
ity o
f A
laba
ma
- B
irm
ingh
am o
n 09
/26/
13. F
or p
erso
nal u
se o
nly.
ANRV384-PY47-01 ARI 2 July 2009 17:5
and I have always appreciated the altruism ofmy colleagues who felt moved to spend thetime writing a nomination for me. I was lessimpressed by my university, which, althoughcongratulating me on the honor and using myfellowship to boost its own prestige, initiallyclaimed that my two fellowship years did notcount as service to the university when I nextrequested sabbatical leave. It also was some-what discouraging that, through comparisonswith similar faculty members in the sciences, Ihad three anomalous salary adjustments duringmy career, at least one of which was the resultof a study comparing female faculty membersalaries to those of their male peers. Because Inever felt any gender discrimination within mydepartment, I attribute this, in part, to my dis-inclination to question my salary increases, incontrast to some of my male colleagues.
Canadian research grants for basic researchtend to be small by American standards, but in1977, I had a sufficiently large grant to affordto take on graduate students, and later, an oc-casional postdoc. However, for me, the fun ofresearch came from doing it myself and for thenext 20 years, I managed to get into the lab, orhave sufficient technical help, to publish paperson which I was the first, and sometimes only,author. Throughout this time, microscopy wasan integral part of all of the projects undertakenin my lab, and I have picked three interactingthemes to briefly illustrate how informative cy-tological data can be if it is part of a wider in-vestigation. These examples also amply illus-trate again how much luck was with me whenI picked the cowpea rust fungus as my researchorganism of choice.
RESEARCH THEMES: THECOMPATIBLE INTERACTION
As a postdoc in Willard Wynn’s lab, I had dis-covered that germinating urediospores of thecowpea rust fungus, like the bean rust fun-gus, will form appressoria in response to thetopography of oil-containing collodion mem-branes. Without any additional stimulus, these
appressoria develop infection structures thatappear identical to those formed during the firststages of leaf infection; however, no haustoriaform and fungal growth ceases within a coupleof days regardless of the presence or absenceof external nutrients. Unlike other rust fungi,the cowpea rust fungus not only formed infec-tion hyphae in high frequencies on these collo-dion membranes but also a terminal haustorialmother cell (HMC), which is a prerequisite forhaustorium formation. Over the years, I subse-quently used electron microscopy, differentialinterference contrast optics light microscopy,contrast-enhanced video microscopy, and cy-tochemistry to examine the cellular features ofinfection structure development. Among otherthings, this work showed that in the absence ofa host plant, the HMC underwent a premature,presumably programmed, senescence. Prevent-ing this senescence with certain carbohydratesat the right stage of HMC development ledto the first example of the induction of indis-putable haustoria in vitro (16). The data alsosuggested that HMC senescence in a suscepti-ble plant was prevented by the fungus-inducedrelease of degradation products of the plantwall. This work became part of my later ar-gument that the inability of the rust fungus togrow in culture was more related to its require-ment for a series of signals from its host plantthan to its need for some special nutrient thatits host might possess (17).
Most species of rust fungi have two para-sitic stages in their life cycles: the dikaryoticstage derived from abundantly produced andeasily stored urediospores, and the monokary-otic stage derived from ephemeral basid-iospores that are produced by often difficult-to-germinate teliospores. Understandably, mostrust researchers work on the dikaryotic stage,as did I for many years. Urediospore-derivedinfection structures usually penetrate intercel-lularly via the leaf stomata and form the firstdikaryotic haustorium in a mesophyll cell deepin the leaf tissue. These invaded cells are diffi-cult to see in living tissue, and I had long enviedresearchers who worked on powdery mildew
www.annualreviews.org • Memoirs of a Plant Pathologist 7
Ann
u. R
ev. P
hyto
path
ol. 2
009.
47:1
-13.
Dow
nloa
ded
from
ww
w.a
nnua
lrev
iew
s.or
gby
Uni
vers
ity o
f A
laba
ma
- B
irm
ingh
am o
n 09
/26/
13. F
or p
erso
nal u
se o
nly.
ANRV384-PY47-01 ARI 2 July 2009 17:5
fungi that formed haustoria in epidermal cellsbecause they could watch live cells respond tofungal invasion (e.g., 1). Eventually, this envyled me to turn to the monokaryotic stage ofthe rust fungus, during which germinating ba-sidiospores penetrate directly into epidermalcells, forming an intracellular invasion hypha enroute to establishing an intercellular myceliumfrom which monokaryotic haustoria are formedwithin adjacent plant cells.
Again, luck was with me as, unlike manyother rust fungal species, the cowpea rustfungus goes through its complete life cycleon a single host and was found to producebasidiospores relatively easily. Moreover,cowpea cultivars resistant or susceptible tothe dikaryotic stage of the fungus respondedsimilarly to basidiospore-derived infection.In resistant plants, the first invaded plant cellexpressed the HR in both situations, but inbasidiospore-derived infections, this first cellwas the epidermal cell, rather than a mesophyllcell. Exploiting the monokaryotic phase of thefungus opened up new research opportunities.My lab demonstrated that during the pene-tration of both resistant and susceptible hostcultivars, the rust fungus induces a transientand localized decrease in plasma membrane–cell wall adhesion. This seemingly disruptsthe communication between the fungus andthe plant’s cytoplasm that is necessary for theexpression of nonspecific defense responsesthat, in nonhost epidermal cells, are typicallyassociated with the high levels of penetrationfailure (27). We also used light microscopy, cy-tochemistry, electron microscopy, and cDNAsgenerated from cytoplasm extracted fromindividual epidermal cells to show that whilethe fungus was penetrating the plant wall, itinduced differential changes in the underlyingepidermal cells that predicted whether the cellwas going to express resistance in the formof the HR or susceptibility (30). All of thesestudies demonstrated that the establishment ofthe compatible interaction between the cowpearust fungus and susceptible host cultivars is justas complex as the induction of resistance.
RESEARCH THEMES: THEHYPERSENSITIVE RESPONSE
Before we had learned to manipulate themonokaryotic stage of the cowpea rust fungus,my wish to microscopically watch an HR wassatisfied in the late 1980s by investigating celldeath triggered by haustoria of the powderymildew fungus, Erysiphe cichoracearum, in epi-dermal cells of cowpea, which is a nonhost forthis fungus. This study showed that the EM im-ages were comfortingly similar to features seenin living cells, and that these features differedbetween the HR and chemically induced celldeath (29). A year or so later, we finally wereable to watch the invasion hypha of cowpea rustfungus trigger an HR in the epidermal cell of aresistant host cultivar. However, while we werestill developing the high-resolution contrast-enhanced video system needed to better ob-serve living infected epidermal cells, it becamemore widely appreciated that the HR was a formof programmed cell death (PCD) that may re-semble apoptosis, the most commonly studiedform of PCD in animals. Apoptosis is charac-terized by the cleavage of nuclear DNA intooligonucleosomal fragments that form a lad-der on agarose gels, and a set of striking mi-croscopic events such as condensation of thenucleus and chromatin, and fragmentation ofthe cell into apoptotic bodies. Plants, however,exhibit a number of forms of PCD during theirdevelopment, none of which resembles the HRin its association with cell browning, the accu-mulation of defense compounds and other de-fensive cellular changes. Nevertheless, as a cy-tochemical test had recently shown that nuclearDNA cleavage occurred during a developmen-tal form of plant PCD, we applied this test tofreshly cut sections of resistant or susceptiblecowpea leaves infected with the dikaryotic stageof the cowpea rust fungus. With accompanyingevidence that extracted DNA from resistant, in-fected leaves showed laddering on agarose gels,we demonstrated that this cleavage occurred inthe nuclei of haustorium-containing cells un-dergoing the HR in resistant plants (32). Thiswas the first example of DNA cleavage in a
8 Heath
Ann
u. R
ev. P
hyto
path
ol. 2
009.
47:1
-13.
Dow
nloa
ded
from
ww
w.a
nnua
lrev
iew
s.or
gby
Uni
vers
ity o
f A
laba
ma
- B
irm
ingh
am o
n 09
/26/
13. F
or p
erso
nal u
se o
nly.
ANRV384-PY47-01 ARI 2 July 2009 17:5
fungus-induced HR, and it differed in a num-ber of respects from the then recent studies of avirus-induced HR (see references in 32). Later,we showed that the cowpea HR also involvedthe activation of cysteine proteases, some ofwhich are similar to the caspases that charac-terize animal apoptosis (6).
Although this work suggested some homol-ogy between animal apoptosis and the partic-ular HR that we studied, the variability in thecharacteristics of the various forms of host andnonhost HRs that I had seen over the yearsstill made me doubt that every HR was iden-tical in its expression (19). This doubt wasstrengthened by our later demonstration us-ing the monokaryon of the fungus that epider-mal host and nonhost HRs exhibited signifi-cant differences in their timing, process, andresponse to pharmacological treatments, evenin the same plant. Such data raise the possi-bility that pathogen-specific resistance genesmay control a cell dismantling process that isdifferent, in some cases, from that triggeredmore nonspecifically in nonhost interactions(4). Therefore, the nonhost HR may be anancient form of defense that is not necessar-ily reinstated when fungus-specific gene-for-gene interactions evolve as a result of pathogenpressure after nonhost resistance had beenovercome.
From the beginning of my career as a plantpathologist, researchers had been trying todiscover the basis of the gene-for-gene rela-tionship that exists, particularly in biotrophicpathosystems, between genes for avirulence inthe pathogen and genes for resistance in cul-tivars of the host plant. The most favored in-terpretation in the 1970s was that the aviru-lence gene coded for a specific elicitor thattriggered an HR only in plants carrying thematching resistance gene. Although the 1990ssaw the cloning of a number of avirulenceand resistance genes, few were from fungalsystems and few fungal molecules were can-didates as specific elicitors. Therefore, it wasexciting when my lab discovered that basid-iospores could form invasion hyphae (normallyformed inside an epidermal cell) on collodion
membranes and that exudates from these differ-entiated basidiospore germlings, but not fromgerminated basidiospores lacking invasion hy-phae, induced necrosis in a cultivar-specificmanner when injected into cowpea leaves. Sucha result one might expect of a specific elici-tor that could be a resistance gene product orat least a determinant of host-cultivar speci-ficity. Washings from the intercellular spaces ofsusceptible cowpea leaves containing growingmonokaryotic colonies also acted as a specificelicitor, but not exudates from dikaryotic in-fection structures (without haustoria) on collo-dion membranes or intracellular washing fluidsfrom dikaryon-infected leaves. Assuming boththe monokaryon and the dikaryon produce thiselicitor, one explanation for this result is thatelicitor secretion is restricted to the haustoriumin the dikaryon and is prevented from being re-leased into the intercellular spaces of infectedleaves by the neckband along the haustorialneck (3). The presence of the elicitor in inter-cellular washing fluid from plants infected withthe monokaryon possibly relates to the lack ofa neckband in monokaryotic haustoria, allow-ing the escape of the elicitor into the apoplast.With great difficulty because of the incrediblysmall amounts of available material, two novelelicitor peptides were partially sequenced fromthe monokaryon secretions. Conceivably, thesecould be products of the two avirulence genespredicted to correspond to the two genes forresistance known to be carried by the resistantcultivar (5). This was the first example of char-acterized specific elicitors from a rust fungus,but isolating the elicitor was so technically dif-ficult and time-consuming that I could not per-suade a graduate student or postdoc to pursueit further.
RESEARCH THEMES:NONHOST RESISTANCE
Nonhost resistance is the norm in nature asevery plant is resistant to the vast majorityof potentially pathogenic microbes that it en-counters. The narrow host range of most plantpathogens, and the fact that extant pathogens
www.annualreviews.org • Memoirs of a Plant Pathologist 9
Ann
u. R
ev. P
hyto
path
ol. 2
009.
47:1
-13.
Dow
nloa
ded
from
ww
w.a
nnua
lrev
iew
s.or
gby
Uni
vers
ity o
f A
laba
ma
- B
irm
ingh
am o
n 09
/26/
13. F
or p
erso
nal u
se o
nly.
ANRV384-PY47-01 ARI 2 July 2009 17:5
rarely have changed their range in recordedhistory, indicates that nonhost resistance is notonly difficult to overcome but also that the par-asite’s features needed to do so must differ fordifferent plant species. Nonhost resistance wasnot a popular topic when I started working onit as a postdoc, and many researchers did notdistinguish it from cultivar resistance within anotherwise susceptible host species. There wasalso a general assumption that all forms of re-sistance followed the gene-for-gene model, al-though this could not be proven for nonhostresistance because of the inability to carry outclassical genetic studies with noninterbreedingspecies.
My interest in this topic probably wouldnot have been sparked had I not started work-ing with the dikaryotic phase of the cowpearust fungus. The linear, sequential developmentof distinct infection structures by this fungusis ideal for studying the spatial and temporalcytological manifestations of plant defense re-sponses. Indeed, it was this linear developmentthat revealed the clear distinction in timing be-tween resistant host and nonhost interactionsand the fact that nonhost resistance was mul-ticomponent, with different components beingeffective at different infection sites within thesame leaf. To demonstrate that fungal growthstopped within the plant because of active plantresponses, I showed that preinoculation treat-ments with heat shock, or agents that decreasedprotein or RNA synthesis, decreased detectableplant responses and increased fungal growth,the latter often dramatically and long after theeffects of the treatments had worn off (14). It isimportant to note that all of this work requiredstatistical comparisons of populations of infec-tion sites in control and infected leaves, andnone of the changes effected by the treatmentscould have been detected without microscopyas all leaves lacked symptoms to the nakedeye.
Given the multitude of defenses that plantspossess against microbial disease, unequivo-cally identifying which of them is responsiblefor a specific example of disease resistance hasbeen, and still is, remarkably difficult to do. My
lab’s closest success came from a study of theresistance of the nonhost French bean to thecowpea rust fungus. In this plant, inhibition ofearly infection hyphal growth is rare, and resis-tance is primarily expressed as the fungus triesunsuccessfully to form the first haustorium.Using cytochemistry, electron microscopy,and energy dispersive X-ray analysis, my labdemonstrated the deposition of callose, pheno-lic materials, and silica in the cell wall at the sitewhere the fungus initiated a penetration peg.Intuitively, silicification of the plant wall seemsto be a highly effective physical barrier to fun-gal penetration and haustoria formation, so wewere surprised when haustorium formation wasnot dramatically increased in silicon-depletedplants. However, we noticed that these plantsshowed a striking increase in phenolic materialsat the penetration sites. Moreover, careful EMstudies demonstrated that in normal plants,the penetration peg often stopped growingbefore it even reached the silicified plant wall,in concert with visible signs of senescence inthe HMC similar to that seen in the absenceof a plant. Interestingly, fungal developmentstopped even earlier in silicon-depleted plants(23). These observations, and the results ofselective inhibition of the different compo-nents of these wall deposits in infected, normalplants (31), suggest that silica deposition caneither prevent the fungus-controlled release ofmolecules from the plant cell wall that keep theHMC viable or act as a physical barrier if theHMC remains alive and forms a penetrationpeg long enough to reach the plant wall.However, in silicon-depleted plants, it seemslikely that the absence of the impermeablesilicified cell wall allows increased molecularcommunication between plant and fungus,resulting in increased deposition of phenoliccompounds at the infection site. The diffusionof these inhibitory compounds to the funguscould explain why it ceased its growth earlierthan in normal French bean plants. This abilityfor one defense to become effective whenanother is rendered inoperative is anothergood example of the multicomponent natureof nonhost resistance. Again, it is important
10 Heath
Ann
u. R
ev. P
hyto
path
ol. 2
009.
47:1
-13.
Dow
nloa
ded
from
ww
w.a
nnua
lrev
iew
s.or
gby
Uni
vers
ity o
f A
laba
ma
- B
irm
ingh
am o
n 09
/26/
13. F
or p
erso
nal u
se o
nly.
ANRV384-PY47-01 ARI 2 July 2009 17:5
to note that both normal and silicon-depletedplants were equally resistant, and the possibledifference in defense mechanisms was onlyrevealed by careful cytological studies.
Many will argue that only through genet-ics can the cause of resistance be unequivocallydetermined in any specific plant-parasite inter-action. By the year 2000, the weed Arabidop-sis thaliana had become well established as theplant of choice to study the molecular genet-ics of disease resistance. Although few availablemutants were deficient in known defense re-sponses, many were involved in signal trans-duction cascades, and these were starting to beused to investigate the role of signal transduc-tion in resistance to some pathogens (34). Asno rust fungus for Arabidopsis had been found,we exploited these signaling mutants to studynonhost resistance to the dikaryotic stage of avariety of rust fungus species (28). Althoughall mutants appeared resistant to the nakedeye, the light microscope revealed that sev-eral mutations affected prehaustorial growth,although the effects were different for differ-ent fungal species. Few mutants, however, al-lowed haustorium formation and posthausto-rial development. Strikingly, the most extensivefungal growth was seen in cowpea rust-infectedplants containing the NahG transgene codingfor a bacterial salicylate hydroxylase, in whichthe fungus produced large colonies resemblingthose seen in susceptible host plants, althoughthey did not sporulate. Other rust fungi didnot show such growth in NahG plants, and thecowpea rust fungus did not develop coloniesin transgenic tobacco and tomato containingthe NahG gene. These observations yet againdemonstrate the unique features of each plant-parasite combination.
EVOLUTION AS THE BASIS FORA CONCEPTUAL FRAMEWORKFOR HOST-PARASITESPECIFICITY
It has always seemed obvious to me that if wewant to explain the plant-parasite interactionsof today, we have to look at how they evolved.
To me, the unique details that I have seen ineach plant-parasite combination are a logicalresult of millions of years of evolution, driven ineach species by specific and overlapping selec-tive pressures of which, for the plant, pathogensare only a part. In the years after the publica-tion of my generalized concept of host-parasitespecificity (15), I was invited to write a numberof conceptual articles on the evolution of re-sistance, susceptibility, and host-parasite speci-ficity based on my own research observationsand that of others (e.g., 18). I was given evenmore free rein to my ideas when, in 1999, I be-came Editor-in-Chief of the journal Physiologi-cal and Molecular Plant Pathology, for which I hadbeen an Editorial Board member or Senior Edi-tor since 1989. Dr. Brian Deverall, when he wasa faculty member at Imperial College while Iwas there as a graduate student, had been one ofthe original founders of the journal (then calledPhysiological Plant Pathology). Being Editor-in-Chief gave me the luxury of writing editorialson topics such as multigenic disease resistanceand the basis of host genotype specificity (20)and the thorny issue of whether nonhost resis-tance was the result of nonspecific defense orthe result of specific recognition events (21).One of the principles that I tried to empha-size in all of these articles is that, because of theway each plant-parasite interaction has evolved,one cannot generalize from one system to an-other without hard data, despite the temptationto do so whenever some new, exciting infor-mation is obtained for a model plant-parasitesystem.
THE FUTURE
Hard data, instead of correlations and specula-tion, are now becoming more commonplace asmolecular biology provides the type of infor-mation not even dreamt of when I started mycareer. One of the challenges now is to makesense of the huge amount of data that can begenerated by today’s technology. Nevertheless,genes alone are not the ultimate purveyors ofresistance or susceptibility at the cellular level;molecular interactions involved in recognition
www.annualreviews.org • Memoirs of a Plant Pathologist 11
Ann
u. R
ev. P
hyto
path
ol. 2
009.
47:1
-13.
Dow
nloa
ded
from
ww
w.a
nnua
lrev
iew
s.or
gby
Uni
vers
ity o
f A
laba
ma
- B
irm
ingh
am o
n 09
/26/
13. F
or p
erso
nal u
se o
nly.
ANRV384-PY47-01 ARI 2 July 2009 17:5
events, cross talk between signaling pathways,and defense responses that can be spatially reg-ulated within the cell all play a part. Spatial con-siderations at the tissue level are also importantas different cells in a single infection site may becarrying out different processes. Plant pathol-ogy students interested in cell biology have beenin short supply in the past few years, but in myopinion, old and new ways of visualizing events
at the cellular level will be an important compo-nent of future research projects, and I hope thatinterest in this area will return. The marvelouscomplexity of plant-microbe interactions willkeep plant pathologists in business for a longtime, and I hope that the value of looking (witha microscope) before you leap into a researchproject will similarly be appreciated by futuregenerations.
DISCLOSURE STATEMENT
The author is not aware of any affiliations, memberships, funding, or financial holdings that mightbe perceived as affecting the objectivity of this review.
LITERATURE CITED
1. Bushnell WR. 1971. The haustorium of Erysiphe graminis. An experimental study by light microscopy. InMorphological and Biochemical Events in Plant-Parasite Interaction, ed. S Akai, S Ouchi, pp. 229–54. Tokyo:Phytopathol. Soc. Jpn.
2. Bushnell WR, Rowell JB. 1981. Suppressors of defense reaction: a model for roles in specificity.Phytopathology 71:1012–14
3. Chen CY, Heath MC. 1992. Effect of stage of development of the cowpea rust fungus on the release of acultivar-specific elicitor of necrosis. Physiol. Mol. Plant Pathol. 40:23–30
4. Christoper-Kozjan R, Heath MC. 2003. Cytological and pharmacological evidence that biotrophic fungitrigger different cell death execution processes in host and nonhost cells during the hypersensitiveresponse. Physiol. Mol. Plant Pathol. 62:265–75
5. D’Silva I, Heath MC. 1997. Purification and characterization of two novel hypersensitive response-inducing specific elicitors produced by the cowpea rust fungus. J. Biol. Chem. 272:3924–27
6. D’Silva I, Poirier GG, Heath MC. 1998. Activation of cysteine proteases in cowpea plants during thehypersensitive response—a form of programmed cell death. Exp. Cell Res. 245:389–99
7. Ehrlich HG, Ehrlich MA. 1963. Electron microscopy of the host-parasite relationships in stem rust ofwheat. Am. J. Bot. 50:123–30
8. Ellingboe AH. 1976. Genetics of host-parasite interactions. In Encyclopedia of Plant Physiology, Vol. 4,Physiological Plant Pathology, ed. R Heitefuss, PH Williams, pp. 761–78. Berlin/Heidelberg/New York:Springer
9. Flor HH. 1971. Current status of the gene-for-gene concept. Annu. Rev. Phytopathol. 9:275–9610. Hardwick NV, Greenwood AD, Wood RKS. 1970. The fine structure of the haustorium of Uromyces
appendiculatus in Phaseolus vulgaris. Can. J. Bot. 49:383–9011. Heath MC. 1974. Light and electron microscope studies of the interactions of host and nonhost plants
with cowpea rust—Uromyces phaseoli var. vignae. Physiol. Plant Pathol. 4:403–1412. Heath MC. 1976. Ultrastructural and functional similarity of the haustorial neckband of rust fungi and
the Casparian strip of vascular plants. Can. J. Bot. 54:2482–8913. Heath MC. 1976. Hypersensitivity, the cause or consequence of rust resistance? Phytopathology 66:935–3614. Heath MC. 1979. Effects of heat shock, actinomycin D, cycloheximide and blasticidin S on nonhost
interactions with rust fungi. Physiol. Plant Pathol. 15:211–1815. Heath MC. 1981. A generalized concept of host parasite specificity. Phytopathology 71:1121–2316. Heath MC. 1990. Influence of carbohydrates on the induction of haustoria of the cowpea rust fungus in
vitro. Exp. Mycol. 14:84–8817. Heath MC. 1995. Signal exchange between higher plants and rust fungi. Can. J. Bot. 75(Suppl. 1):S616–23
12 Heath
Ann
u. R
ev. P
hyto
path
ol. 2
009.
47:1
-13.
Dow
nloa
ded
from
ww
w.a
nnua
lrev
iew
s.or
gby
Uni
vers
ity o
f A
laba
ma
- B
irm
ingh
am o
n 09
/26/
13. F
or p
erso
nal u
se o
nly.
ANRV384-PY47-01 ARI 2 July 2009 17:5
18. Heath MC. 1997. Evolution of plant resistance and susceptibility to fungal parasites. In The Mycota,Vol. V, Plant Relationships, Part B, ed. G Carroll, P Tudzynski, pp. 257–76. Berlin/Heidelberg/New York: Springer
19. Heath MC. 1998. Apoptosis, programmed cell death and the hypersensitive response. Eur. J. Plant Pathol.104:117–24
20. Heath MC. 2000. In this issue: multigenic disease resistance and the basis of host genotype specificity.Physiol. Mol. Plant Pathol. 57:189–90
21. Heath MC. 2001. In this issue: nonhost resistance to plant pathogens: nonspecific defense or the result ofspecific recognition events? Physiol. Mol. Plant Pathol. 58:53–54
22. Heath MC, Heath IB. 1971. Ultrastructure of an immune and a susceptible reaction of cowpea leaves torust infection. Physiol. Plant Pathol. 1:277–87
23. Heath MC, Stumpf MA. 1986. Ultrastructural observation of penetration sites of the cowpea rust fungusin untreated and silicon-depleted French bean cells. Physiol. Mol. Plant Pathol. 29:27–39
24. Heath MC, Wood RKS. 1971. Role of inhibitors of fungal growth in the limitation of leaf spots causedby Ascochyta pisi and Mycosphaerella pinodes. Ann. Bot. 35:475–91
25. Littlefield LJ, Bracker CE. 1970. Continuity of host plasma membrane around haustoria of Melampsoralini. Mycologia 62:609–14
26. Littlefield LJ, Heath MC. 1979. Ultrastructure of Rust Fungi. New York: Academic. 277 pp.27. Mellersh DG, Heath MC. 2001. Plasma membrane-cell wall adhesion is required for expression of plant
defense responses during fungal penetration. Plant Cell 13:413–2428. Mellersh DG, Heath MC. 2003. An investigation into the involvement of defense signaling pathways in
components of the nonhost resistance of Arabidopsis thaliana to rust fungi also reveals a model system forstudying rust fungus compatibility. Mol. Plant-Microbe Interact. 16:398–404
29. Meyer SLF, Heath MC. 1988. A comparison of the death induced by fungal invasion or toxic chemicalsin cowpea epidermal cells. II. Responses induced by Erysiphe cichoracearum. Can. J. Bot. 66:624–34
30. Mold MJR, Xu T, Barbara M, Iscove NN, Heath MC. 2003. cDNAs generation from individual epidermalcells reveal that differential gene expression predicting subsequent resistance or susceptibility to rust fungalinfection occurs prior to the fungus entering the cell lumen. Mol. Plant-Microbe Interact. 16:835–45
31. Perumalla CJ, Heath MC. 1991. The effect of inhibitors of various cellular processes on the wall modifi-cations induced in bean leaves by the cowpea rust fungus. Physiol. Mol. Plant Pathol. 38:293–300
32. Ryerson DE, Heath MC. 1996. Cleavage of nuclear DNA into oligonucleosomal fragments during celldeath induced by fungal infection or by abiotic treatments. Plant Cell 8:393–402
33. Stakman EC. 1915. Relation between Puccinia graminis and plants highly resistant to its attack. J. Agric.Res. 4:193–200
34. Thomma BPHJ, Penninckx IAMA, Broekaert WF, Cammue BPA. 2001. The complexity of disease sig-naling in Arabidopsis. Curr. Opin. Immunol. 13:63–68
35. Vidaver AK. 2004. The accidental plant pathologist. Annu. Rev. Phytopathol. 42:1–1236. Wynn WK. 1976. Appressorium formation over stomates by the bean rust fungus: response to a surface
contact stimulus. Phytopathology 66:136–46
www.annualreviews.org • Memoirs of a Plant Pathologist 13
Ann
u. R
ev. P
hyto
path
ol. 2
009.
47:1
-13.
Dow
nloa
ded
from
ww
w.a
nnua
lrev
iew
s.or
gby
Uni
vers
ity o
f A
laba
ma
- B
irm
ingh
am o
n 09
/26/
13. F
or p
erso
nal u
se o
nly.
AR384-FM ARI 14 July 2009 23:48
Annual Review ofPhytopathology
Volume 47, 2009Contents
Look Before You Leap: Memoirs of a “Cell Biological” PlantPathologistMichele C. Heath � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 1
Plant Disease Diagnostic Capabilities and NetworksSally A. Miller, Fen D. Beed, and Carrie Lapaire Harmon � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �15
Diversity, Pathogenicity, and Management of Verticillium SpeciesSteven J. Klosterman, Zahi K. Atallah, Gary E. Vallad, and Krishna V. Subbarao � � � � � � �39
Bacterial/Fungal Interactions: From Pathogens to MutualisticEndosymbiontsDonald Y. Kobayashi and Jo Anne Crouch � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �63
Community Ecology of Fungal Pathogens Causing Wheat Head BlightXiangming Xu and Paul Nicholson � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �83
The Biology of Viroid-Host InteractionsBiao Ding � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 105
Recent Evolution of Bacterial Pathogens: The Gall-FormingPantoea agglomerans CaseIsaac Barash and Shulamit Manulis-Sasson � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 133
Fatty Acid–Derived Signals in Plant DefenseAardra Kachroo and Pradeep Kachroo � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 153
Salicylic Acid, a Multifaceted Hormone to Combat DiseaseA. Corina Vlot, D’Maris Amick Dempsey, and Daniel F. Klessig � � � � � � � � � � � � � � � � � � � � � � � � 177
RNAi and Functional Genomics in Plant Parasitic NematodesM.N. Rosso, J.T. Jones, and P. Abad � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 207
Fungal Effector ProteinsIoannis Stergiopoulos and Pierre J.G.M. de Wit � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 233
Durability of Resistance in Tomato and Pepper to XanthomonadsCausing Bacterial SpotRobert E. Stall, Jeffrey B. Jones, and Gerald V. Minsavage � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 265
v
Ann
u. R
ev. P
hyto
path
ol. 2
009.
47:1
-13.
Dow
nloa
ded
from
ww
w.a
nnua
lrev
iew
s.or
gby
Uni
vers
ity o
f A
laba
ma
- B
irm
ingh
am o
n 09
/26/
13. F
or p
erso
nal u
se o
nly.
AR384-FM ARI 14 July 2009 23:48
Seed Pathology Progress in Academia and IndustryGary P. Munkvold � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 285
Migratory Plant Endoparasitic Nematodes: A Group Rich in Contrastsand DivergenceMaurice Moens and Roland N. Perry � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 313
The Genomes of Root-Knot NematodesDavid McK. Bird, Valerie M. Williamson, Pierre Abad, James McCarter,
Etienne G.J. Danchin, Philippe Castagnone-Sereno, and Charles H. Opperman � � � � � 333
Viruses of Plant Pathogenic FungiSaid A. Ghabrial and Nobuhiro Suzuki � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 353
Hordeivirus Replication, Movement, and PathogenesisAndrew O. Jackson, Hyoun-Sub Lim, Jennifer Bragg, Uma Ganesan,
and Mi Yeon Lee � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 385
Ustilago maydis as a PathogenThomas Brefort, Gunther Doehlemann, Artemio Mendoza-Mendoza,
Stefanie Reissmann, Armin Djamei, and Regine Kahmann � � � � � � � � � � � � � � � � � � � � � � � � � � � � 423
Errata
An online log of corrections to Annual Review of Phytopathology articles may be found athttp://phyto.annualreviews.org/
vi Contents
Ann
u. R
ev. P
hyto
path
ol. 2
009.
47:1
-13.
Dow
nloa
ded
from
ww
w.a
nnua
lrev
iew
s.or
gby
Uni
vers
ity o
f A
laba
ma
- B
irm
ingh
am o
n 09
/26/
13. F
or p
erso
nal u
se o
nly.