ultrastructure of modified root-tip cells in ficus … · ultrastructure of modified root-tip cells...

J. Cell Sci. 4i, 193-208 (1980)Printed in Great Britain © Company of Biologists Limited 1980

ULTRASTRUCTURE OF MODIFIED ROOT-TIP

CELLS IN FICUS CARICA, INDUCED BY THE

ECTOPARASITIC NEMATODE

XIPHINEMA INDEX

U. WYSS,' H. LEHMANNf AND R. JANK-LADWIGf• Institut fUr Pflanzenkrankhciten und Pflanzenschutz der Universitat Hannover,Herrenhauser Str. 2, .D-3000 Hannover 21, FRG, andf Botanisches Institut der Tieraratlichen Hochschule Hannover,BOnteweg 17 d, D-3000 Hannover 71, FRG

SUMMARY

The migratory ectoparasitic root nematode Xiphinema index, added to Ficus carica seedlingsin sterile agar culture, fed exclusively on the tips of the roots. As a response the tips started toswell and became transformed into terminal galls as long as feeding was continued.

When the cytology of swollen root-tips was examined 24 h after the first nematode attack,necrotic cells, scattered singly or in small groups within the root apex, were found in ultrathinsections. These cells, whose protoplasts showed features of a hypersensitive reaction, were mostprobably those fed upon by the nematodes. Each necrotic cell was surrounded by severalenlarged, mostly binucleate cells with dense cytoplasm. One day later the binculeate cellswere multinucleate, containing 4 or even 8 nuclei. The clear-cut demarcation between necroticand modified cells indicated that only the stimulus for the induction of modified cells but notthe stimulus for cell necrosis passed into neighbouring cells.

Root-tip galls that provided the appropriate food for egg production in nematodes containedgreatly enlarged multinucleate cells between necrotic cells. The modified cells showed featuresof high metabolic activities, expressed in nuclear and nucleolar hypertrophy, invagination ofthe nuclear envelope, increased cytoplasmic density, abundance of mitochondria, plastids andrough endoplasmic reticulum. Wall ingrowths, typical of transfer cells, were rare and if presentoccurred only adjacent to necrotic cells. In older modified cells new cell plates, surroundedby phragmoplasts, were formed.

INTRODUCTION

Xiphinema index Thorne & Allen, 1950, with a world-wide distribution in soilswhere vines are grown, was the first nematode species proved to be the vector of aplant pathogenic virus (Hewitt, Raski & Goheen, 1958), and various aspects of itsbiology, ecology and pathogenicity have since been studied in more detail. The hostrange appears to be restricted to a few plants amongst which vine (Vitis vinifera L.)and fig (Ficus carica L.) support a rapid population increase under controlled con-ditions in glasshouses with temperatures above 20 °C. Attacked root-tips of bothplants become transformed into terminal gals. Sections through these galls revealmodified enlarged multinucleate cells (Radewald & Raski, 1962; Weischer & Wyss,1976; Wyss, 1978; Lehmann & Wyss, 1978; Rumpenhorst & Weischer, 1978) which

194 U. Wyss, H. Lehmann and R. Jank-Ladwig

are thought to play an important role in the successful development of the parasite(Weischer & Wyss, 1976; Wyss, 1978).

X. index is a migratory ectoparasitic nematode which frequently changes itsfeeding sites. The feeding behaviour on vine and fig roots has been studied (Fisher& Raski, 1967; Weischer & Wyss, 1976; Wyss, 1977 a, b) and recorded in a researchfilm (Wyss & Inst. wiss. Film, 1977). Roots of fig seedlings grown in agar are usuallyfirst attacked in the region of cell elongation or in the transition zone between theroot apex and cell elongation. The nematode inserts its odontostyle 3-4 cells deepbefore it starts feeding on a column of cells, with ingestion periods rarely exceeding10 min per individual cell during the initial attacks on root-tips. Root growth is soonretarded if feeding by a single female or late larval stage (L4) is continued for severalhours at different sites along the root-tip. The tip starts to swell and graduallybecomes transformed into a terminal gall as long as the nematode maintains itsattack for several days. Galled root-tips remain strongly attractive to feeding nema-todes and obviously provide the appropriate food source for egg production infemales. Meristematic activities within the root-tip swellings and galls are usuallyonly temporarily arrested (Wyss, 1978).

The main purpose of this study was to obtain a deeper insight into the developmentand ultrastnicture of modified root cells induced by a migratory ectoparasitic nema-tode and to compare these cells with the well-known cellular adaptations induced bysedentary endoparasitic nematodes.

MATERIALS AND METHODSDried Ficus carica fruits (Izmir figs) were soaked overnight in distilled water. On the

following morning the seeds were removed, washed and surface-sterilized for 20 min in afiltered 4% Ca(OCl)i. 4HtO solution. The seeds were then washed for 1 h in sterile waterand transferred on to o-8 % distilled water agar in plastic Petri dishes for germination underartificial light or in daylight at about 25 °C. When the seedlings were 2-3 weeks old they wereinserted singly into a layer 2 mm thick of o-6 % distilled water agar in plastic Petri dishes anda few drops of Hoagland's solution No. 1 were added. The plates were sealed with ParafUmand stored at 25 °C and about 3000 lux (16 h exposure/day) until they were inoculated withsterile nematodes.

Stock cultures of nematodes were kept in growth chambers (25 °C) on fig plants, culturedin sand with little organic matter, in which high population densities of up to 800 nematodes/100 ml soil developed. After extraction, batches of about 100 females and late larval stages(L4) were transferred into a 0-03 % NaNs solution in sterile staining blocks where the nematodeswere soon immobilized. After 1 h the sterilant was replaced twice in. quick succession bysterile distilled water and the nematodes were transferred on to agar in Petri dishes. One daylater the most active females and late larval stages were removed with a microneedle and placedsingly or in batches of 10 on to fig seedlings with roots that had just started to grow well.The cultures were then kept at 25 °C, but now the light intensity was reduced to 500-700 lux(16 h exposure/day). Nematode feeding was examined daily and attacked root-tips of differentdevelopmental stages from the first nematode attack were cut and processed for electron-microscopic studies.

Excised root-tips were fixed for 2 h in 3% glutaraldehyde in 0-05 M sodium cacodylatebuffer (pH 6-8), washed in 12 changes of buffer and postfixed for 2 h in 2 % osmium tetroxide,all at room temperature. After a few washings in buffer, the postfixed root-tips were dehydratedin an acetone series, followed by propylene oxide, and embedded in Durcupan (Fluka).Ultrathin sections were cut on a LKB Ultratome I with glass knives and mounted on Formvar-

Modified root-tip cells in Ficus

1

WS

1/V.S

, > , • • . - .

••'1 i / m

Fig. i. LS through a parasitized swollen root-tip, i day after the first nematodeattack. Necrotic cells (TIC) within the transition zone between root apex and cellelongation are surrounded by modified, mostly binucleate cells with lobed nuclei(arrows). The modified cells bulge into the necrotic cells whose contents have mostprobably been removed by nematode feeding. Nucleoli (mi) are occasionally left inthe necrotic cells. Binucleate cells with wall stubs (tos) attached to the mother cell walloccur at some distance from the necrotic cells, x 1700.


coated copper grids. The sections were stained for 10 min with 5 % aqueous uranyl acetateand then for 15 min with lead citrate. The ultrathin sections were examined in the EM 10A(Zeiss) or Elmiskop IA (Siemens).

RESULTS

Modified cells in swollen root-tips, 1-3 days after the first nematode attack

Sections through the apical region of unattacked, well growing root-tips of Ficuscarica seedlings displayed characteristic features of meristematic cells. All cells wereuninucleate and contained relatively large, more or less spherical nuclei embeddedin dense cytoplasm. However, when parasitized swollen root-tips were examined12-14 h after the first attack by a single nematode or several individuals, distinctcellular alterations were observed in the transition zone between the root apex andcell elongation. A general view of these changes is given in Fig. 1. A few necroticcells, either devoid of cytoplasm or still partially filled with degraded contents, weresurrounded by cells undergoing modification. These cells were now 2-3 times aslarge as corresponding cells in unattacked well growing root-tips, and they wereusually binucleate. Their nuclei were now considerably lobed and slightly enlargedand were embedded in dense cytoplasm. There was always a clear-cut demarcationbetween the necrotic and modified cells (Figs. 1, 2), a necrotic response was neverseen to spread into cells bordering the site of necrosis. The necrotic cells resultedmost probably from direct injury by the nematode's odontostyle and from thesubsequent injection of saliva and removal of cell contents during ingestion. In anearlier light-microscopic study on X. index-feeding sites (Wyss, 1978) necrotic cellswere occasionally found whose walls were perforated by holes of approximately thesame diameter as that of the nematode's odontostyle. Fig. 2 shows characteristicfeatures of necrotic cells, still partially filled with degraded contents. Disorganizednuclei, nucleoli and cytoplasm were more electron-dense in these than in adjacentmodified cells. The numerous vacuoles appeared to have fused, and myelin-likemultilamellar dense structures were common (Fig. 2, inset). Organelles such asmitochondria and plastids were no longer recognizable.

In spite of numerous examinations, no stages of cell-plate development could bedetected between daughter nuclei of binucleate cells adjacent or close to the necrotic

Figs. 2-4. Feeding of Xiphinema index on root-tips of Ficus carica seedlings. LSthrough parasitized swollen root-tips.

Fig. 2. Necrotic cells (nc) surrounded by modified cells, 1 day after the first nematodeattack. Note the clear-cut demarcation between the 2 cell types. The degraded cyto-plasmic contents and nuclei (n) of the necrotic cells are more electron-dense thanthose of the modified cells. Vacuoles in the necrotic cells have fused, and multi-lamellar dense structures are common. These structures (within white outline) areshown at higher magnification in the inset, x 4800; inset, x 17000.

Fig. 3. Two binucleate cells, a few cell layers distant to the necrotic cells, 2 daysafter the first nematode attack. Wall stubs (ws) are attached to the mother cell wall inthe region of the equatorial plane, x 3900.

Fig. 4. Magnification of a wall stub shown in Fig. 4. Plasmodesmata (pd) arepresent, and the end of the stub is bent and slightly swollen, x 11000.

Modified root-tip cells in Ficus

•i*


Figs. 5, 6. Feeding of Xiphinema index on root-tips of Ficus carica seedlings. LSthrough parasitized swollen root-tips.

Fig. 5. Necrotic cells (nc) with remaining nucleoli (nu) surrounded by enlargedmodified cells with up to 4 nuclei at the plane of section, 2 days after the first nematodeattack. Numerous small vacuoles are dispersed in the cytoplasm of the modified cells,x 2000.

Fig. 6. Illustration of 2 enlarged nuclei with invaginated envelopes, 3 days after thefirst nematode attack, x 6000.

cells. Wall stubs were, however, seen at some distance from necrotic cells (Fig. 1).These wall stubs were always attached to the mother cell wall, approximately in theregion of the equatorial plane. Fig. 3 shows 2 binucleate cells with such wall stubs ata higher magnification. In a number of cells, stubs opposite to each other were presenton the mother cell walls. Plasmodesmata were fully developed in the stubs (Fig. 4).Sections through 2- to 3-day-old root-tip swellings revealed modified cells that hadincreased in size and which were now multinucleate, possessing 4-8 enlarged nuclei(Fig. 5). Again, no cell wall stubs were present in the modified cells adjacent tonecrotic cells. The necrotic cells, devoid of cytoplasm, frequently contained severalwell preserved but rather compact nucleoli which indicated that these cells weremultinucleate before they were disorganized by nematode attack.

Modified cells in root-tip galls, 6-12 days after the first nematode attack

When single or several nematodes continued feeding on the same root-tip, theterminal swelling became gradually transformed into a gall which in many casesremained strongly attractive to the feeding nematodes. Five to six days after the firstattack some galls were fed upon by females which in the meantime started to produce

Modified root-tip cells in Ficus 199

eggs. Egg production was maintained for several days or even weeks as long as thefemales continued feeding on these galls. Depending on the age of the gall, modifiedcells within it were now greatly enlarged. Expansion of the multinucleate cells wasso pronounced that early necrotic cells were heavily crushed, nearly disappearingbetween the expanding cells. Fig. 7 shows the probable fate of early necrotic cells,now degraded at the plane of section to 'intercellular spaces' between 2 expandingmodified cells. The spaces are filled with dense multilamellar structures which, asdescribed before, are typical constituents of necrotic cells. Modified cells in 10- to12-day-old galls could reach dimensions 10-15 times the size of early binucleateinterphase cells. In some of them more than 16 nuclei, clustered together, werecounted.

Many nuclei within modified cells had a highly invaginated, nearly amoeboidprofile (Fig. 6). Their nucleoli were hypertrophied and most had large vacuoles.Mitochondria and plastids had increased in number in modified cells and theirshapes became strikingly heteromorphic. Massive aggregations of these organelles,mainly near cell walls, were common (Figs. 7, 11). A large proportion of the denselystaining plastids contained starch grains (Figs. 8, n ) and most of them possessed aninterplastidial membrane system (Fig. 8). Apart from these features, high metabolicactivity in transformed cells was further expressed by the dense cytoplasm and theabundance of rough endoplasmic reticulum commonly arranged in parallel strands(Figs. 11, 12). Occasionally the strands were arranged in concentric whorls aroundvacuoles (Fig. 9). The high cytoplasmic density of modified cells in root-tip gallssupporting nematode reproduction is clearly illustrated in Fig. 12. The cytoplasm ofthe cell with 3 nuclei in the plane of section is so dense that, at this level, the totalarea of the numerous vacuoles scattered within the cytoplasm would hardly exceedthat of the smallest nucleus.

Enlarged, previously active, but now necrotic cells were found next to metabolicallyhighly active cells (Fig. 11). Again the necrotic response did not spread into thesecells. Occasionally, however, ingrowths were noted on the walls of modified cellsadjacent to necrotic cells (Figs. 10, 11). These wall protuberances were only found inmodified cells near the periphery of necrotic root-tip galls. Some protuberances, withanastomosing branches extending relatively deeply into the cells (Fig. 10), were welldeveloped. As in typical wall ingrowths of 'transfer cells' the plasmalemma surround-ing the protuberances was not ruptured.

In older modified cells stubs attached to the cell walls were commonly noted(Figs. 13-17), and, less frequently, wall fragments embedded in cytoplasm (Fig. 13)were also recorded. An interesting feature within some older cells was the formationof new cell plates, surrounded by typical phragmoplasts with newly formed nuclei attheir poles (Figs. 15, 17) and associated with dictyosomes (Fig. 16). The detection ofnew cell plates in older cells suggests that thin-walled long stubs (Figs. 14, 15) orirregular thin walls fusing with thicker walls (Fig. 15, arrow) might have been formedrecently by the fusion of the cell-plate vesicles. Fig. 16 shows a cell plate apparentlygrowing towards an old wall stub.

U. Wyss, H. Lehmann and R. Jank-Ladwig


DISCUSSION

The cellular modifications induced by the ectoparasitic and migratory dorylaimidnematode Xiphinema index resemble in many respects those induced by the specializedendoparasitic and sedentary tylenchid nematodes of the genera Globodera, Heterodera,Meloidogyne (Heteroderidae) and Nacobbus, Rotylenchulus (Nacobbidae) so farexamined. At the ultrastructural level the feeding sites of the genera mentioned andof X. index show characteristic features of metabolically active cells. This is expressedespecially in the enlarged nuclei with usually irregular, lobed outlines and withhypertrophied nucleoli, in increased cytoplasmic density with the cytoplasm containingnumerous small vacuoles, in the proliferation of rough endoplasmic reticulum and inincreased numbers of mitochondria and plastids. Apart from considerable changes insize and form, the plastids of X. »n&.v--transformed cells, but not of control root cells,contained thylakoids. This is probably not so surprising (though inexplicable), as theFicus carica seedlings were exposed to light, bearing in mind that chloroplasts withfew lamellae developed in Meloidogyne incognita-infected tomato root galls, but onlywhen these were exposed to light (Orion, Gommers & van Bezooijen, 1973). Syncytiaof Heterodera schachtii-'mfected roots turn green when these are exposed to light(J. Miiller, personal communication).

Well developed wall ingrowths occur in the syncytia and giant cells induced byHeteroderidae but not in the syncytia induced by Nacobbidae (Jones & Payne, 1977).Wall ingrowths with anastomosing branches were occasionally also observed inX. in^fet-transformed cells, at the periphery of the galled root-tips and only onlimited areas in the immediate vicinity to empty necrotic cells. Wall ingrowths arethought to develop as a response to the flow of solutes from conducting elements ofthe xylem and phloem across the plasmalemma of adjacent cells, and it is suggestedthat their function is to increase the surface area of the plasmalemma for the uptakeof solutes (cf., for example, Jones & Dropkin, 1975). Cells with such characteristicsare common in plants and are termed transfer cells (Gunning & Pate, 1969). Thehypothesis that syncytia or giant cells, induced by sedentary Heteroderidae are

Figs. 7-10. Feeding of Xiphinema index on root-tips of Fiais carica seedlings. LSthrough 6- to 12-day-old root-tip galls.

Fig. 7. The intercellular space (tip) between greatly enlarged modified cells isfilled with lamellar structures which are most probably degraded remnants of crushednecrotic cells. The lamellar structures (within black rectangle) are shown at highermagnification in the inset. Note the aggregation, of mitochondria and plastids, thelatter with interplastidial membranes, x 8000; inset, x 40000.

Fig. 8. Single plastid with 2 starch grains (st) and showing the interplastidialmembrane system, t, thylakoids x 41000.

Fig. 9. Modified cells are rich in endoplasmic reticulum (er). Here it is arranged inconcentric whorls around a vacuole. x 9000.

Fig. 10. Wall ingrowths (1) invested by an intact plasmalemma (/>) are occasionallyformed adjacent to necrotic cells. In this case the intercellular space between 3modified cells is a necrotic cell (nc), x 7000.

14 CEL 41

2O2 U. Wyss, H. Lehmann and R. Jank-Ladwig


multinucleate transfer cells and that the wall ingrowths are formed as a consequenceof continuous nematode demands for nutrients (Jones & Northcote, 1972) is nowwidely accepted. Fingerlike wall ingrowths were also observed in root-tip cells ofcelery, parasitized by the ectoparasitic and migratory dorylaimid nematode Longidorusapulus (Bleve-Zacheo, Zacheo, Lamberti & Arrigoni, 1977) which belongs to thesame family as X index. The authors suggest that in this particular case, in whichthe feeding site of the parasite is not fixed (and where thus the nutrient demand is notcontinuous), the ingrowths may have a double function: to stop the spread of thenecrotic area adjacent to the cells and to improve the cell-to-cell transport when themeristematic tissue of the root-tip is inactivated. In this connexion it is worthwhilementioning the unusual finding by Gunning, Pate & Green (1970), who observedtufts of wall ingrowths of yet unknown function around intercellular spaces in leaftraces of the goosegrass (Gallium aparine).

Modified root-tip cells induced by X. index on its host F. carica also show similaritiesin their ultrastructure to cells of the nutritive tissue in galls induced by zoocecidiasuch as by Cecidomyidae (Rohfritsch, 1971), Chermesidae (Rohfritsch, 1977) andCynipidae (Rohfritsch, 1974). The nutritive cells of these zoocecidia are characterizedby a large lobed nucleus, nuclear hypertrophy, abundant cytoplasm, reduction andfragmentation of the vacuome, abundance of ribosomes, plastids and mitochondria.X. index-modified cells show even greater similarities to nutritive cells in gallsinduced by certain acarocecidia which disturb cytokinesis so that bi- or multinucleatecells with wall fragments attached to the mother-cell wall are formed (Westphal, 1977).

Multinucleate cells induced by X. index are apparently formed by repeated mitoseswithout cytokinesis (Wyss, 1978; Rumpenhorst & Weischer, 1978), and they thusresemble in this respect giant cells induced by Meloidogyne spp. (Huang & Maggenti,1969; Jones & Payne, 19780). According to Bird (1979), the term coenocyte shouldbe used for multinucleate cells formed by repeated mitoses without cell division,whereas any enlarged uninucleate cell may be a giant cell. The multinucleate stateof X. zncfoc-induced coenocytes arises by synchronous mitoses (Rumpenhorst &Weischer, 1978; Wyss, unpublished) which are typical of most naturally and arti-ficially created multinucleate cells (Fowke, Bech-Hansen, Gamborg & Constabel,

Figs. 11-13. Feeding of Xiphinema index on. root-tips of Ficus carica seedlings. LSthrough 6- to 12-day-old root-tip galls.

Fig. 11. Enlarged modified cells adjacent and near a necrotic cell (nc). The largecell at the centre contains dense cytoplasm and endoplasmic reticulum (er) arrangedin parallel strands. Wall ingrowths (i) grow into the cell adjacent to the necrotic cell.Note the aggregation of plastids (p) with starch grains in a neighbouring cell, x 3200.

Fig. 12. A necrotic cell (nc) surrounded by modified cells. The cell with 3 nucleiat the plane of section is filled with extremely dense cytoplasm (compare the smallvacuoles (uj) with those (t>j) of a neighbouring cell), x 2000.

Fig. 13. Wall stubs (foi) are quite commonly found in older modified cells.Occasionally wall fragments, not attached to cell walls, are also found, x 2300.

14-2


In the early stage of X. index-coenocyte formation cell wall stubs, attached to themother cell wall, were seen to protrude into binucleate cells at some distance fromparasitized necrotic cells. These stubs resulted most probably from incomplete fusionof cell plate vesicles. Similar protruding wall fragments are formed when roots aretreated with caffeine (Roper & Roper, 1977; Jones & Payne, 19786) which does notaffect mitosis. From the description by Jones & Payne (19786) o-i % caffeine appliedto growing roots of Impatiens balsamina inhibited root growth and caused root-tipsto swell. This effect is very similar to the early stage of root response to X. indexattack (Wyss, 1978). Cell wall fragments observed in older X. in^ex-coenocytes wereattributed to cell wall dissolution (Wyss, 1978; Lehmann & Wyss, 1978). We nowhesitate to maintain this statement, as holes in a continuous wall were very rare andas such striking symptoms of cell wall lysis as shown in affected cells by Rotylenchuhisreniformis (Rebois, Madden & Eldridge, 1975) and Longidorus apulus (Bleve-Zacheoet al. 1977) were never observed. Thin and rather irregular cell walls in older coeno-cytes are undoubtedly formed by renewed cytokinesis. To our knowledge thisfeature, involving typical phragmoplast and cell plate formation, is unique innematode-transformed plant cells. It possibly represents the initial stage of renewedmeristematic activities at the onset of new lateral root formation. As described earlier(Wyss, 1978) lateral roots were often seen to emerge from X. index-mduceA root-tipgalls.

Modified feeding sites are also induced by invertebrate - (e.g. Poinar & Hess, 1974)and vertebrate-parasitic nematodes (Wright, 1974; Lee & Wright, 1978). Themigratory trichurid nematode Capillaria hepatica induces for instance in the liver ofits host mouse a syncytial feeding site, consisting of multinucleate food cells whosecontents are ingested and which ultimately degenerate (Wright, 1974). It is suggestedthat the parasite progressively induces new feeding sites on its migration through theliver. A similar host-parasite relationship was shown for Trichuris muris in thecaecum and colon of its mouse host (Lee & Wright, 1978). In this respect X. indexshows in its behaviour and in conjunction with associated responses of the hosttissue, closer affinities to the mammal parasites mentioned than to sedentary plantparasitic nematodes which, as long as they live, do not destroy the cellular alterationsthey induce and maintain.

Figs. 14-17. Feeding of Xiphinema index on root-tips of Ficus carica seedlings. LSthrough 6- to 12-day-old root-tip galls.

Fig. 14. Modified cells with several wall stubs (tos). x 1600.Fig. 15. Two older modified cells with new cell plates {cp), each surrounded by a

phragmoplast. Nuclei (n) are found at both poles of the phragmoplasts. Note the twothin and irregular walls, one fusing with a thick wall with a stub (tos) at its end(arrowhead), x 3000.

Fig. 16. A new cell plate (cp), surrounded by dictyosomes (d), is orientated towardsthe stub (tus) of an older cell wall, x 13000.

Fig. 17. New cell plate (cp) surrounded by a phragmoplast between 2 newly formednuclei (w). The figure shows the same cell plate as the vertical plate in Fig. 15, but ata slightly different plane and at a higher magnification, x 6000.

206 U. Wyss, H. Lehtnann and R. Jank-Ladtmg

The cells killed by X. index feeding show similarities to the degenerated cells thatresult as a hypersensitive response to Meloidogyne incognita infection in resistanttomato roots (Paulson & Webster, 1972), especially with respect to the rather rapidincrease in electron-density of the affected cytoplasm. In addition the stimulus thatcauses the disorganization of the parasitized cells does not spread to adjacent cells.The trigger, however, which initiates the mitotic and cytokinetic aberrations obviouslypasses into neighbouring cells. This implies a symplastic movement of the trigger,i.e. via plasmodesmata, which in turn implies a relatively low molecular weight. Theexpansion of the neighbouring cells is later facilitated by the necrotic, easily crushable,cells that surround them.

Little is yet known about the chemical nature of the trigger that induces modifiedcells in specific plant-nematode interactions. In X. index the trigger can only bereleased into perforated cells via the parasite's odontostyle, having been produced insecretory gland cells that open into the food canal. In a recent study (Robertson &Wyss, 1979) it was shown that X. index possesses in its basal oesophageal bulb alarge and very active gland cell whose duct system was seen to deplete after the lastperforation thrust of the stylet and during ingestion pauses. There are severalindications that only secretions from this cell will be injected into perforated plantcells. Microanalytical studies on biochemical events at the site of the host-parasiteinteraction: F. carica-X. index are in progress (Poehling, Wyss & Neuhoff, 1979)and it is hoped that they will finally provide an answer.

This study, financed by the Deutsche Forschungsgemeinschaft, was made in the Arbeits-gemeinschaft fur Elektronenmikroskopie der Tierarztlichen Hochschule Hannover. Theauthors thank Ulrike Ahrendt for technical assistance and Dr M. G. K. Jones, Department ofBiochemistry, University of Cambridge, England, for helpful comments.

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WYSS, U. (19776). Feeding mechanisms and feeding behaviour of Xiphinema index. Med.Fac. Landbouvnv. Rijksuniv. Gent 42, 1513-1519.

WYSS, U. (1978). Root and cell responses to feeding by Xiphinema index. Nematologica 24,159-166.

WYSS, U. & INST. VVISS. FILM (1977). Xiphinema index (Nematoda)-Saugen an Wurzeln vonSSmlingen (Feige). Film E 2375 des IWF, Gdttingen. Publ. Wiss. Film. (Sekt. BioL), Ser.10, Nr 61, E 2375, 20 pp.

(Received 21 May 1979)

ultrastructure of modified root-tip cells in ficus … · ultrastructure of modified root-tip cells...

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