ultrastructure of modified root-tip cells in ficus carica, induced by the ...
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J. Cell Sci. 4i, 193-208 (1980)<br />
Pr<strong>in</strong>ted <strong>in</strong> Great Brita<strong>in</strong> © Company <strong>of</strong> Biologists Limited 1980<br />
SUMMARY<br />
ULTRASTRUCTURE OF MODIFIED ROOT-TIP<br />
CELLS IN FICUS CARICA, INDUCED BY THE<br />
ECTOPARASITIC NEMATODE<br />
XIPHINEMA INDEX<br />
U. WYSS,' H. LEHMANNf AND R. JANK-LADWIGf<br />
• Institut fUr Pflanzenkrankhciten und Pflanzenschutz der Universitat Hannover,<br />
Herrenhauser Str. 2, .D-3000 Hannover 21, FRG, and<br />
f Botanisches Institut der Tieraratlichen Hochschule Hannover,<br />
BOnteweg 17 d, D-3000 Hannover 71, FRG<br />
The migratory ectoparasitic <strong>root</strong> nematode Xiph<strong>in</strong>ema <strong>in</strong>dex, added to Ficus <strong>carica</strong> seedl<strong>in</strong>gs<br />
<strong>in</strong> sterile agar culture, fed exclusively on <strong>the</strong> <strong>tip</strong>s <strong>of</strong> <strong>the</strong> <strong>root</strong>s. As a response <strong>the</strong> <strong>tip</strong>s started to<br />
swell and became transformed <strong>in</strong>to term<strong>in</strong>al galls as long as feed<strong>in</strong>g was cont<strong>in</strong>ued.<br />
When <strong>the</strong> cytology <strong>of</strong> swollen <strong>root</strong>-<strong>tip</strong>s was exam<strong>in</strong>ed 24 h after <strong>the</strong> first nematode attack,<br />
necrotic <strong>cells</strong>, scattered s<strong>in</strong>gly or <strong>in</strong> small groups with<strong>in</strong> <strong>the</strong> <strong>root</strong> apex, were found <strong>in</strong> ultrath<strong>in</strong><br />
sections. These <strong>cells</strong>, whose protoplasts showed features <strong>of</strong> a hypersensitive reaction, were most<br />
probably those fed upon <strong>by</strong> <strong>the</strong> nematodes. Each necrotic cell was surrounded <strong>by</strong> several<br />
enlarged, mostly b<strong>in</strong>ucleate <strong>cells</strong> with dense cytoplasm. One day later <strong>the</strong> b<strong>in</strong>culeate <strong>cells</strong><br />
were mult<strong>in</strong>ucleate, conta<strong>in</strong><strong>in</strong>g 4 or even 8 nuclei. The clear-cut demarcation between necrotic<br />
and <strong>modified</strong> <strong>cells</strong> <strong>in</strong>dicated that only <strong>the</strong> stimulus for <strong>the</strong> <strong>in</strong>duction <strong>of</strong> <strong>modified</strong> <strong>cells</strong> but not<br />
<strong>the</strong> stimulus for cell necrosis passed <strong>in</strong>to neighbour<strong>in</strong>g <strong>cells</strong>.<br />
Root-<strong>tip</strong> galls that provided <strong>the</strong> appropriate food for egg production <strong>in</strong> nematodes conta<strong>in</strong>ed<br />
greatly enlarged mult<strong>in</strong>ucleate <strong>cells</strong> between necrotic <strong>cells</strong>. The <strong>modified</strong> <strong>cells</strong> showed features<br />
<strong>of</strong> high metabolic activities, expressed <strong>in</strong> nuclear and nucleolar hypertrophy, <strong>in</strong>vag<strong>in</strong>ation <strong>of</strong><br />
<strong>the</strong> nuclear envelope, <strong>in</strong>creased cytoplasmic density, abundance <strong>of</strong> mitochondria, plastids and<br />
rough endoplasmic reticulum. Wall <strong>in</strong>growths, typical <strong>of</strong> transfer <strong>cells</strong>, were rare and if present<br />
occurred only adjacent to necrotic <strong>cells</strong>. In older <strong>modified</strong> <strong>cells</strong> new cell plates, surrounded<br />
<strong>by</strong> phragmoplasts, were formed.<br />
INTRODUCTION<br />
Xiph<strong>in</strong>ema <strong>in</strong>dex Thorne & Allen, 1950, with a world-wide distribution <strong>in</strong> soils<br />
where v<strong>in</strong>es are grown, was <strong>the</strong> first nematode species proved to be <strong>the</strong> vector <strong>of</strong> a<br />
plant pathogenic virus (Hewitt, Raski & Goheen, 1958), and various aspects <strong>of</strong> its<br />
biology, ecology and pathogenicity have s<strong>in</strong>ce been studied <strong>in</strong> more detail. The host<br />
range appears to be restricted to a few plants amongst which v<strong>in</strong>e (Vitis v<strong>in</strong>ifera L.)<br />
and fig (Ficus <strong>carica</strong> L.) support a rapid population <strong>in</strong>crease under controlled conditions<br />
<strong>in</strong> glasshouses with temperatures above 20 °C. Attacked <strong>root</strong>-<strong>tip</strong>s <strong>of</strong> both<br />
plants become transformed <strong>in</strong>to term<strong>in</strong>al gals. Sections through <strong>the</strong>se galls reveal<br />
<strong>modified</strong> enlarged mult<strong>in</strong>ucleate <strong>cells</strong> (Radewald & Raski, 1962; Weischer & Wyss,<br />
1976; Wyss, 1978; Lehmann & Wyss, 1978; Rumpenhorst & Weischer, 1978) which
194 U. Wyss, H. Lehmann and R. Jank-Ladwig<br />
are thought to play an important role <strong>in</strong> <strong>the</strong> successful development <strong>of</strong> <strong>the</strong> parasite<br />
(Weischer & Wyss, 1976; Wyss, 1978).<br />
X. <strong>in</strong>dex is a migratory ectoparasitic nematode which frequently changes its<br />
feed<strong>in</strong>g sites. The feed<strong>in</strong>g behaviour on v<strong>in</strong>e and fig <strong>root</strong>s has been studied (Fisher<br />
& Raski, 1967; Weischer & Wyss, 1976; Wyss, 1977 a, b) and recorded <strong>in</strong> a research<br />
film (Wyss & Inst. wiss. Film, 1977). Roots <strong>of</strong> fig seedl<strong>in</strong>gs grown <strong>in</strong> agar are usually<br />
first attacked <strong>in</strong> <strong>the</strong> region <strong>of</strong> cell elongation or <strong>in</strong> <strong>the</strong> transition zone between <strong>the</strong><br />
<strong>root</strong> apex and cell elongation. The nematode <strong>in</strong>serts its odontostyle 3-4 <strong>cells</strong> deep<br />
before it starts feed<strong>in</strong>g on a column <strong>of</strong> <strong>cells</strong>, with <strong>in</strong>gestion periods rarely exceed<strong>in</strong>g<br />
10 m<strong>in</strong> per <strong>in</strong>dividual cell dur<strong>in</strong>g <strong>the</strong> <strong>in</strong>itial attacks on <strong>root</strong>-<strong>tip</strong>s. Root growth is soon<br />
retarded if feed<strong>in</strong>g <strong>by</strong> a s<strong>in</strong>gle female or late larval stage (L4) is cont<strong>in</strong>ued for several<br />
hours at different sites along <strong>the</strong> <strong>root</strong>-<strong>tip</strong>. The <strong>tip</strong> starts to swell and gradually<br />
becomes transformed <strong>in</strong>to a term<strong>in</strong>al gall as long as <strong>the</strong> nematode ma<strong>in</strong>ta<strong>in</strong>s its<br />
attack for several days. Galled <strong>root</strong>-<strong>tip</strong>s rema<strong>in</strong> strongly attractive to feed<strong>in</strong>g nematodes<br />
and obviously provide <strong>the</strong> appropriate food source for egg production <strong>in</strong><br />
females. Meristematic activities with<strong>in</strong> <strong>the</strong> <strong>root</strong>-<strong>tip</strong> swell<strong>in</strong>gs and galls are usually<br />
only temporarily arrested (Wyss, 1978).<br />
The ma<strong>in</strong> purpose <strong>of</strong> this study was to obta<strong>in</strong> a deeper <strong>in</strong>sight <strong>in</strong>to <strong>the</strong> development<br />
and ultrastnicture <strong>of</strong> <strong>modified</strong> <strong>root</strong> <strong>cells</strong> <strong>in</strong>duced <strong>by</strong> a migratory ectoparasitic nematode<br />
and to compare <strong>the</strong>se <strong>cells</strong> with <strong>the</strong> well-known cellular adaptations <strong>in</strong>duced <strong>by</strong><br />
sedentary endoparasitic nematodes.<br />
MATERIALS AND METHODS<br />
Dried Ficus <strong>carica</strong> fruits (Izmir figs) were soaked overnight <strong>in</strong> distilled water. On <strong>the</strong><br />
follow<strong>in</strong>g morn<strong>in</strong>g <strong>the</strong> seeds were removed, washed and surface-sterilized for 20 m<strong>in</strong> <strong>in</strong> a<br />
filtered 4% Ca(OCl)i. 4HtO solution. The seeds were <strong>the</strong>n washed for 1 h <strong>in</strong> sterile water<br />
and transferred on to o-8 % distilled water agar <strong>in</strong> plastic Petri dishes for germ<strong>in</strong>ation under<br />
artificial light or <strong>in</strong> daylight at about 25 °C. When <strong>the</strong> seedl<strong>in</strong>gs were 2-3 weeks old <strong>the</strong>y were<br />
<strong>in</strong>serted s<strong>in</strong>gly <strong>in</strong>to a layer 2 mm thick <strong>of</strong> o-6 % distilled water agar <strong>in</strong> plastic Petri dishes and<br />
a few drops <strong>of</strong> Hoagland's solution No. 1 were added. The plates were sealed with ParafUm<br />
and stored at 25 °C and about 3000 lux (16 h exposure/day) until <strong>the</strong>y were <strong>in</strong>oculated with<br />
sterile nematodes.<br />
Stock cultures <strong>of</strong> nematodes were kept <strong>in</strong> growth chambers (25 °C) on fig plants, cultured<br />
<strong>in</strong> sand with little organic matter, <strong>in</strong> which high population densities <strong>of</strong> up to 800 nematodes/<br />
100 ml soil developed. After extraction, batches <strong>of</strong> about 100 females and late larval stages<br />
(L4) were transferred <strong>in</strong>to a 0-03 % NaNs solution <strong>in</strong> sterile sta<strong>in</strong><strong>in</strong>g blocks where <strong>the</strong> nematodes<br />
were soon immobilized. After 1 h <strong>the</strong> sterilant was replaced twice <strong>in</strong>. quick succession <strong>by</strong><br />
sterile distilled water and <strong>the</strong> nematodes were transferred on to agar <strong>in</strong> Petri dishes. One day<br />
later <strong>the</strong> most active females and late larval stages were removed with a microneedle and placed<br />
s<strong>in</strong>gly or <strong>in</strong> batches <strong>of</strong> 10 on to fig seedl<strong>in</strong>gs with <strong>root</strong>s that had just started to grow well.<br />
The cultures were <strong>the</strong>n kept at 25 °C, but now <strong>the</strong> light <strong>in</strong>tensity was reduced to 500-700 lux<br />
(16 h exposure/day). Nematode feed<strong>in</strong>g was exam<strong>in</strong>ed daily and attacked <strong>root</strong>-<strong>tip</strong>s <strong>of</strong> different<br />
developmental stages from <strong>the</strong> first nematode attack were cut and processed for electronmicroscopic<br />
studies.<br />
Excised <strong>root</strong>-<strong>tip</strong>s were fixed for 2 h <strong>in</strong> 3% glutaraldehyde <strong>in</strong> 0-05 M sodium cacodylate<br />
buffer (pH 6-8), washed <strong>in</strong> 12 changes <strong>of</strong> buffer and postfixed for 2 h <strong>in</strong> 2 % osmium tetroxide,<br />
all at room temperature. After a few wash<strong>in</strong>gs <strong>in</strong> buffer, <strong>the</strong> postfixed <strong>root</strong>-<strong>tip</strong>s were dehydrated<br />
<strong>in</strong> an acetone series, followed <strong>by</strong> propylene oxide, and embedded <strong>in</strong> Durcupan (Fluka).<br />
Ultrath<strong>in</strong> sections were cut on a LKB Ultratome I with glass knives and mounted on Formvar-
, > , • • . - .<br />
Modified <strong>root</strong>-<strong>tip</strong> <strong>cells</strong> <strong>in</strong> Ficus<br />
1/V.S<br />
••'1 i / m<br />
Fig. i. LS through a parasitized swollen <strong>root</strong>-<strong>tip</strong>, i day after <strong>the</strong> first nematode<br />
attack. Necrotic <strong>cells</strong> (TIC) with<strong>in</strong> <strong>the</strong> transition zone between <strong>root</strong> apex and cell<br />
elongation are surrounded <strong>by</strong> <strong>modified</strong>, mostly b<strong>in</strong>ucleate <strong>cells</strong> with lobed nuclei<br />
(arrows). The <strong>modified</strong> <strong>cells</strong> bulge <strong>in</strong>to <strong>the</strong> necrotic <strong>cells</strong> whose contents have most<br />
probably been removed <strong>by</strong> nematode feed<strong>in</strong>g. Nucleoli (mi) are occasionally left <strong>in</strong><br />
<strong>the</strong> necrotic <strong>cells</strong>. B<strong>in</strong>ucleate <strong>cells</strong> with wall stubs (tos) attached to <strong>the</strong> mo<strong>the</strong>r cell wall<br />
occur at some distance from <strong>the</strong> necrotic <strong>cells</strong>, x 1700.<br />
WS<br />
1
196 U. Wyss, H. Lehmann and R. Jank-Ladwig<br />
coated copper grids. The sections were sta<strong>in</strong>ed for 10 m<strong>in</strong> with 5 % aqueous uranyl acetate<br />
and <strong>the</strong>n for 15 m<strong>in</strong> with lead citrate. The ultrath<strong>in</strong> sections were exam<strong>in</strong>ed <strong>in</strong> <strong>the</strong> EM 10A<br />
(Zeiss) or Elmiskop IA (Siemens).<br />
RESULTS<br />
Modified <strong>cells</strong> <strong>in</strong> swollen <strong>root</strong>-<strong>tip</strong>s, 1-3 days after <strong>the</strong> first nematode attack<br />
Sections through <strong>the</strong> apical region <strong>of</strong> unattacked, well grow<strong>in</strong>g <strong>root</strong>-<strong>tip</strong>s <strong>of</strong> Ficus<br />
<strong>carica</strong> seedl<strong>in</strong>gs displayed characteristic features <strong>of</strong> meristematic <strong>cells</strong>. All <strong>cells</strong> were<br />
un<strong>in</strong>ucleate and conta<strong>in</strong>ed relatively large, more or less spherical nuclei embedded<br />
<strong>in</strong> dense cytoplasm. However, when parasitized swollen <strong>root</strong>-<strong>tip</strong>s were exam<strong>in</strong>ed<br />
12-14 h after <strong>the</strong> first attack <strong>by</strong> a s<strong>in</strong>gle nematode or several <strong>in</strong>dividuals, dist<strong>in</strong>ct<br />
cellular alterations were observed <strong>in</strong> <strong>the</strong> transition zone between <strong>the</strong> <strong>root</strong> apex and<br />
cell elongation. A general view <strong>of</strong> <strong>the</strong>se changes is given <strong>in</strong> Fig. 1. A few necrotic<br />
<strong>cells</strong>, ei<strong>the</strong>r devoid <strong>of</strong> cytoplasm or still partially filled with degraded contents, were<br />
surrounded <strong>by</strong> <strong>cells</strong> undergo<strong>in</strong>g modification. These <strong>cells</strong> were now 2-3 times as<br />
large as correspond<strong>in</strong>g <strong>cells</strong> <strong>in</strong> unattacked well grow<strong>in</strong>g <strong>root</strong>-<strong>tip</strong>s, and <strong>the</strong>y were<br />
usually b<strong>in</strong>ucleate. Their nuclei were now considerably lobed and slightly enlarged<br />
and were embedded <strong>in</strong> dense cytoplasm. There was always a clear-cut demarcation<br />
between <strong>the</strong> necrotic and <strong>modified</strong> <strong>cells</strong> (Figs. 1, 2), a necrotic response was never<br />
seen to spread <strong>in</strong>to <strong>cells</strong> border<strong>in</strong>g <strong>the</strong> site <strong>of</strong> necrosis. The necrotic <strong>cells</strong> resulted<br />
most probably from direct <strong>in</strong>jury <strong>by</strong> <strong>the</strong> nematode's odontostyle and from <strong>the</strong><br />
subsequent <strong>in</strong>jection <strong>of</strong> saliva and removal <strong>of</strong> cell contents dur<strong>in</strong>g <strong>in</strong>gestion. In an<br />
earlier light-microscopic study on X. <strong>in</strong>dex-feed<strong>in</strong>g sites (Wyss, 1978) necrotic <strong>cells</strong><br />
were occasionally found whose walls were perforated <strong>by</strong> holes <strong>of</strong> approximately <strong>the</strong><br />
same diameter as that <strong>of</strong> <strong>the</strong> nematode's odontostyle. Fig. 2 shows characteristic<br />
features <strong>of</strong> necrotic <strong>cells</strong>, still partially filled with degraded contents. Disorganized<br />
nuclei, nucleoli and cytoplasm were more electron-dense <strong>in</strong> <strong>the</strong>se than <strong>in</strong> adjacent<br />
<strong>modified</strong> <strong>cells</strong>. The numerous vacuoles appeared to have fused, and myel<strong>in</strong>-like<br />
multilamellar dense structures were common (Fig. 2, <strong>in</strong>set). Organelles such as<br />
mitochondria and plastids were no longer recognizable.<br />
In spite <strong>of</strong> numerous exam<strong>in</strong>ations, no stages <strong>of</strong> cell-plate development could be<br />
detected between daughter nuclei <strong>of</strong> b<strong>in</strong>ucleate <strong>cells</strong> adjacent or close to <strong>the</strong> necrotic<br />
Figs. 2-4. Feed<strong>in</strong>g <strong>of</strong> Xiph<strong>in</strong>ema <strong>in</strong>dex on <strong>root</strong>-<strong>tip</strong>s <strong>of</strong> Ficus <strong>carica</strong> seedl<strong>in</strong>gs. LS<br />
through parasitized swollen <strong>root</strong>-<strong>tip</strong>s.<br />
Fig. 2. Necrotic <strong>cells</strong> (nc) surrounded <strong>by</strong> <strong>modified</strong> <strong>cells</strong>, 1 day after <strong>the</strong> first nematode<br />
attack. Note <strong>the</strong> clear-cut demarcation between <strong>the</strong> 2 cell types. The degraded cytoplasmic<br />
contents and nuclei (n) <strong>of</strong> <strong>the</strong> necrotic <strong>cells</strong> are more electron-dense than<br />
those <strong>of</strong> <strong>the</strong> <strong>modified</strong> <strong>cells</strong>. Vacuoles <strong>in</strong> <strong>the</strong> necrotic <strong>cells</strong> have fused, and multilamellar<br />
dense structures are common. These structures (with<strong>in</strong> white outl<strong>in</strong>e) are<br />
shown at higher magnification <strong>in</strong> <strong>the</strong> <strong>in</strong>set, x 4800; <strong>in</strong>set, x 17000.<br />
Fig. 3. Two b<strong>in</strong>ucleate <strong>cells</strong>, a few cell layers distant to <strong>the</strong> necrotic <strong>cells</strong>, 2 days<br />
after <strong>the</strong> first nematode attack. Wall stubs (ws) are attached to <strong>the</strong> mo<strong>the</strong>r cell wall <strong>in</strong><br />
<strong>the</strong> region <strong>of</strong> <strong>the</strong> equatorial plane, x 3900.<br />
Fig. 4. Magnification <strong>of</strong> a wall stub shown <strong>in</strong> Fig. 4. Plasmodesmata (pd) are<br />
present, and <strong>the</strong> end <strong>of</strong> <strong>the</strong> stub is bent and slightly swollen, x 11000.
•i*<br />
Modified <strong>root</strong>-<strong>tip</strong> <strong>cells</strong> <strong>in</strong> Ficus
198 U. Wyss, H. Lehmann and R. Jank-Ladwig<br />
Figs. 5, 6. Feed<strong>in</strong>g <strong>of</strong> Xiph<strong>in</strong>ema <strong>in</strong>dex on <strong>root</strong>-<strong>tip</strong>s <strong>of</strong> Ficus <strong>carica</strong> seedl<strong>in</strong>gs. LS<br />
through parasitized swollen <strong>root</strong>-<strong>tip</strong>s.<br />
Fig. 5. Necrotic <strong>cells</strong> (nc) with rema<strong>in</strong><strong>in</strong>g nucleoli (nu) surrounded <strong>by</strong> enlarged<br />
<strong>modified</strong> <strong>cells</strong> with up to 4 nuclei at <strong>the</strong> plane <strong>of</strong> section, 2 days after <strong>the</strong> first nematode<br />
attack. Numerous small vacuoles are dispersed <strong>in</strong> <strong>the</strong> cytoplasm <strong>of</strong> <strong>the</strong> <strong>modified</strong> <strong>cells</strong>,<br />
x 2000.<br />
Fig. 6. Illustration <strong>of</strong> 2 enlarged nuclei with <strong>in</strong>vag<strong>in</strong>ated envelopes, 3 days after <strong>the</strong><br />
first nematode attack, x 6000.<br />
<strong>cells</strong>. Wall stubs were, however, seen at some distance from necrotic <strong>cells</strong> (Fig. 1).<br />
These wall stubs were always attached to <strong>the</strong> mo<strong>the</strong>r cell wall, approximately <strong>in</strong> <strong>the</strong><br />
region <strong>of</strong> <strong>the</strong> equatorial plane. Fig. 3 shows 2 b<strong>in</strong>ucleate <strong>cells</strong> with such wall stubs at<br />
a higher magnification. In a number <strong>of</strong> <strong>cells</strong>, stubs opposite to each o<strong>the</strong>r were present<br />
on <strong>the</strong> mo<strong>the</strong>r cell walls. Plasmodesmata were fully developed <strong>in</strong> <strong>the</strong> stubs (Fig. 4).<br />
Sections through 2- to 3-day-old <strong>root</strong>-<strong>tip</strong> swell<strong>in</strong>gs revealed <strong>modified</strong> <strong>cells</strong> that had<br />
<strong>in</strong>creased <strong>in</strong> size and which were now mult<strong>in</strong>ucleate, possess<strong>in</strong>g 4-8 enlarged nuclei<br />
(Fig. 5). Aga<strong>in</strong>, no cell wall stubs were present <strong>in</strong> <strong>the</strong> <strong>modified</strong> <strong>cells</strong> adjacent to<br />
necrotic <strong>cells</strong>. The necrotic <strong>cells</strong>, devoid <strong>of</strong> cytoplasm, frequently conta<strong>in</strong>ed several<br />
well preserved but ra<strong>the</strong>r compact nucleoli which <strong>in</strong>dicated that <strong>the</strong>se <strong>cells</strong> were<br />
mult<strong>in</strong>ucleate before <strong>the</strong>y were disorganized <strong>by</strong> nematode attack.<br />
Modified <strong>cells</strong> <strong>in</strong> <strong>root</strong>-<strong>tip</strong> galls, 6-12 days after <strong>the</strong> first nematode attack<br />
When s<strong>in</strong>gle or several nematodes cont<strong>in</strong>ued feed<strong>in</strong>g on <strong>the</strong> same <strong>root</strong>-<strong>tip</strong>, <strong>the</strong><br />
term<strong>in</strong>al swell<strong>in</strong>g became gradually transformed <strong>in</strong>to a gall which <strong>in</strong> many cases<br />
rema<strong>in</strong>ed strongly attractive to <strong>the</strong> feed<strong>in</strong>g nematodes. Five to six days after <strong>the</strong> first<br />
attack some galls were fed upon <strong>by</strong> females which <strong>in</strong> <strong>the</strong> meantime started to produce
Modified <strong>root</strong>-<strong>tip</strong> <strong>cells</strong> <strong>in</strong> Ficus 199<br />
eggs. Egg production was ma<strong>in</strong>ta<strong>in</strong>ed for several days or even weeks as long as <strong>the</strong><br />
females cont<strong>in</strong>ued feed<strong>in</strong>g on <strong>the</strong>se galls. Depend<strong>in</strong>g on <strong>the</strong> age <strong>of</strong> <strong>the</strong> gall, <strong>modified</strong><br />
<strong>cells</strong> with<strong>in</strong> it were now greatly enlarged. Expansion <strong>of</strong> <strong>the</strong> mult<strong>in</strong>ucleate <strong>cells</strong> was<br />
so pronounced that early necrotic <strong>cells</strong> were heavily crushed, nearly disappear<strong>in</strong>g<br />
between <strong>the</strong> expand<strong>in</strong>g <strong>cells</strong>. Fig. 7 shows <strong>the</strong> probable fate <strong>of</strong> early necrotic <strong>cells</strong>,<br />
now degraded at <strong>the</strong> plane <strong>of</strong> section to '<strong>in</strong>tercellular spaces' between 2 expand<strong>in</strong>g<br />
<strong>modified</strong> <strong>cells</strong>. The spaces are filled with dense multilamellar structures which, as<br />
described before, are typical constituents <strong>of</strong> necrotic <strong>cells</strong>. Modified <strong>cells</strong> <strong>in</strong> 10- to<br />
12-day-old galls could reach dimensions 10-15 times <strong>the</strong> size <strong>of</strong> early b<strong>in</strong>ucleate<br />
<strong>in</strong>terphase <strong>cells</strong>. In some <strong>of</strong> <strong>the</strong>m more than 16 nuclei, clustered toge<strong>the</strong>r, were<br />
counted.<br />
Many nuclei with<strong>in</strong> <strong>modified</strong> <strong>cells</strong> had a highly <strong>in</strong>vag<strong>in</strong>ated, nearly amoeboid<br />
pr<strong>of</strong>ile (Fig. 6). Their nucleoli were hypertrophied and most had large vacuoles.<br />
Mitochondria and plastids had <strong>in</strong>creased <strong>in</strong> number <strong>in</strong> <strong>modified</strong> <strong>cells</strong> and <strong>the</strong>ir<br />
shapes became strik<strong>in</strong>gly heteromorphic. Massive aggregations <strong>of</strong> <strong>the</strong>se organelles,<br />
ma<strong>in</strong>ly near cell walls, were common (Figs. 7, 11). A large proportion <strong>of</strong> <strong>the</strong> densely<br />
sta<strong>in</strong><strong>in</strong>g plastids conta<strong>in</strong>ed starch gra<strong>in</strong>s (Figs. 8, n) and most <strong>of</strong> <strong>the</strong>m possessed an<br />
<strong>in</strong>terplastidial membrane system (Fig. 8). Apart from <strong>the</strong>se features, high metabolic<br />
activity <strong>in</strong> transformed <strong>cells</strong> was fur<strong>the</strong>r expressed <strong>by</strong> <strong>the</strong> dense cytoplasm and <strong>the</strong><br />
abundance <strong>of</strong> rough endoplasmic reticulum commonly arranged <strong>in</strong> parallel strands<br />
(Figs. 11, 12). Occasionally <strong>the</strong> strands were arranged <strong>in</strong> concentric whorls around<br />
vacuoles (Fig. 9). The high cytoplasmic density <strong>of</strong> <strong>modified</strong> <strong>cells</strong> <strong>in</strong> <strong>root</strong>-<strong>tip</strong> galls<br />
support<strong>in</strong>g nematode reproduction is clearly illustrated <strong>in</strong> Fig. 12. The cytoplasm <strong>of</strong><br />
<strong>the</strong> cell with 3 nuclei <strong>in</strong> <strong>the</strong> plane <strong>of</strong> section is so dense that, at this level, <strong>the</strong> total<br />
area <strong>of</strong> <strong>the</strong> numerous vacuoles scattered with<strong>in</strong> <strong>the</strong> cytoplasm would hardly exceed<br />
that <strong>of</strong> <strong>the</strong> smallest nucleus.<br />
Enlarged, previously active, but now necrotic <strong>cells</strong> were found next to metabolically<br />
highly active <strong>cells</strong> (Fig. 11). Aga<strong>in</strong> <strong>the</strong> necrotic response did not spread <strong>in</strong>to <strong>the</strong>se<br />
<strong>cells</strong>. Occasionally, however, <strong>in</strong>growths were noted on <strong>the</strong> walls <strong>of</strong> <strong>modified</strong> <strong>cells</strong><br />
adjacent to necrotic <strong>cells</strong> (Figs. 10, 11). These wall protuberances were only found <strong>in</strong><br />
<strong>modified</strong> <strong>cells</strong> near <strong>the</strong> periphery <strong>of</strong> necrotic <strong>root</strong>-<strong>tip</strong> galls. Some protuberances, with<br />
anastomos<strong>in</strong>g branches extend<strong>in</strong>g relatively deeply <strong>in</strong>to <strong>the</strong> <strong>cells</strong> (Fig. 10), were well<br />
developed. As <strong>in</strong> typical wall <strong>in</strong>growths <strong>of</strong> 'transfer <strong>cells</strong>' <strong>the</strong> plasmalemma surround<strong>in</strong>g<br />
<strong>the</strong> protuberances was not ruptured.<br />
In older <strong>modified</strong> <strong>cells</strong> stubs attached to <strong>the</strong> cell walls were commonly noted<br />
(Figs. 13-17), and, less frequently, wall fragments embedded <strong>in</strong> cytoplasm (Fig. 13)<br />
were also recorded. An <strong>in</strong>terest<strong>in</strong>g feature with<strong>in</strong> some older <strong>cells</strong> was <strong>the</strong> formation<br />
<strong>of</strong> new cell plates, surrounded <strong>by</strong> typical phragmoplasts with newly formed nuclei at<br />
<strong>the</strong>ir poles (Figs. 15, 17) and associated with dictyosomes (Fig. 16). The detection <strong>of</strong><br />
new cell plates <strong>in</strong> older <strong>cells</strong> suggests that th<strong>in</strong>-walled long stubs (Figs. 14, 15) or<br />
irregular th<strong>in</strong> walls fus<strong>in</strong>g with thicker walls (Fig. 15, arrow) might have been formed<br />
recently <strong>by</strong> <strong>the</strong> fusion <strong>of</strong> <strong>the</strong> cell-plate vesicles. Fig. 16 shows a cell plate apparently<br />
grow<strong>in</strong>g towards an old wall stub.
U. Wyss, H. Lehmann and R. Jank-Ladwig
DISCUSSION<br />
Modified <strong>root</strong>-<strong>tip</strong> <strong>cells</strong> <strong>in</strong> Ficus 201<br />
The cellular modifications <strong>in</strong>duced <strong>by</strong> <strong>the</strong> ectoparasitic and migratory dorylaimid<br />
nematode Xiph<strong>in</strong>ema <strong>in</strong>dex resemble <strong>in</strong> many respects those <strong>in</strong>duced <strong>by</strong> <strong>the</strong> specialized<br />
endoparasitic and sedentary tylenchid nematodes <strong>of</strong> <strong>the</strong> genera Globodera, Heterodera,<br />
Meloidogyne (Heteroderidae) and Nacobbus, Rotylenchulus (Nacobbidae) so far<br />
exam<strong>in</strong>ed. At <strong>the</strong> ultrastructural level <strong>the</strong> feed<strong>in</strong>g sites <strong>of</strong> <strong>the</strong> genera mentioned and<br />
<strong>of</strong> X. <strong>in</strong>dex show characteristic features <strong>of</strong> metabolically active <strong>cells</strong>. This is expressed<br />
especially <strong>in</strong> <strong>the</strong> enlarged nuclei with usually irregular, lobed outl<strong>in</strong>es and with<br />
hypertrophied nucleoli, <strong>in</strong> <strong>in</strong>creased cytoplasmic density with <strong>the</strong> cytoplasm conta<strong>in</strong><strong>in</strong>g<br />
numerous small vacuoles, <strong>in</strong> <strong>the</strong> proliferation <strong>of</strong> rough endoplasmic reticulum and <strong>in</strong><br />
<strong>in</strong>creased numbers <strong>of</strong> mitochondria and plastids. Apart from considerable changes <strong>in</strong><br />
size and form, <strong>the</strong> plastids <strong>of</strong> X. »n&.v--transformed <strong>cells</strong>, but not <strong>of</strong> control <strong>root</strong> <strong>cells</strong>,<br />
conta<strong>in</strong>ed thylakoids. This is probably not so surpris<strong>in</strong>g (though <strong>in</strong>explicable), as <strong>the</strong><br />
Ficus <strong>carica</strong> seedl<strong>in</strong>gs were exposed to light, bear<strong>in</strong>g <strong>in</strong> m<strong>in</strong>d that chloroplasts with<br />
few lamellae developed <strong>in</strong> Meloidogyne <strong>in</strong>cognita-<strong>in</strong>fected tomato <strong>root</strong> galls, but only<br />
when <strong>the</strong>se were exposed to light (Orion, Gommers & van Bezooijen, 1973). Syncytia<br />
<strong>of</strong> Heterodera schachtii-'mfected <strong>root</strong>s turn green when <strong>the</strong>se are exposed to light<br />
(J. Miiller, personal communication).<br />
Well developed wall <strong>in</strong>growths occur <strong>in</strong> <strong>the</strong> syncytia and giant <strong>cells</strong> <strong>in</strong>duced <strong>by</strong><br />
Heteroderidae but not <strong>in</strong> <strong>the</strong> syncytia <strong>in</strong>duced <strong>by</strong> Nacobbidae (Jones & Payne, 1977).<br />
Wall <strong>in</strong>growths with anastomos<strong>in</strong>g branches were occasionally also observed <strong>in</strong><br />
X. <strong>in</strong>^fet-transformed <strong>cells</strong>, at <strong>the</strong> periphery <strong>of</strong> <strong>the</strong> galled <strong>root</strong>-<strong>tip</strong>s and only on<br />
limited areas <strong>in</strong> <strong>the</strong> immediate vic<strong>in</strong>ity to empty necrotic <strong>cells</strong>. Wall <strong>in</strong>growths are<br />
thought to develop as a response to <strong>the</strong> flow <strong>of</strong> solutes from conduct<strong>in</strong>g elements <strong>of</strong><br />
<strong>the</strong> xylem and phloem across <strong>the</strong> plasmalemma <strong>of</strong> adjacent <strong>cells</strong>, and it is suggested<br />
that <strong>the</strong>ir function is to <strong>in</strong>crease <strong>the</strong> surface area <strong>of</strong> <strong>the</strong> plasmalemma for <strong>the</strong> uptake<br />
<strong>of</strong> solutes (cf., for example, Jones & Dropk<strong>in</strong>, 1975). Cells with such characteristics<br />
are common <strong>in</strong> plants and are termed transfer <strong>cells</strong> (Gunn<strong>in</strong>g & Pate, 1969). The<br />
hypo<strong>the</strong>sis that syncytia or giant <strong>cells</strong>, <strong>in</strong>duced <strong>by</strong> sedentary Heteroderidae are<br />
Figs. 7-10. Feed<strong>in</strong>g <strong>of</strong> Xiph<strong>in</strong>ema <strong>in</strong>dex on <strong>root</strong>-<strong>tip</strong>s <strong>of</strong> Fiais <strong>carica</strong> seedl<strong>in</strong>gs. LS<br />
through 6- to 12-day-old <strong>root</strong>-<strong>tip</strong> galls.<br />
Fig. 7. The <strong>in</strong>tercellular space (<strong>tip</strong>) between greatly enlarged <strong>modified</strong> <strong>cells</strong> is<br />
filled with lamellar structures which are most probably degraded remnants <strong>of</strong> crushed<br />
necrotic <strong>cells</strong>. The lamellar structures (with<strong>in</strong> black rectangle) are shown at higher<br />
magnification <strong>in</strong> <strong>the</strong> <strong>in</strong>set. Note <strong>the</strong> aggregation, <strong>of</strong> mitochondria and plastids, <strong>the</strong><br />
latter with <strong>in</strong>terplastidial membranes, x 8000; <strong>in</strong>set, x 40000.<br />
Fig. 8. S<strong>in</strong>gle plastid with 2 starch gra<strong>in</strong>s (st) and show<strong>in</strong>g <strong>the</strong> <strong>in</strong>terplastidial<br />
membrane system, t, thylakoids x 41000.<br />
Fig. 9. Modified <strong>cells</strong> are rich <strong>in</strong> endoplasmic reticulum (er). Here it is arranged <strong>in</strong><br />
concentric whorls around a vacuole. x 9000.<br />
Fig. 10. Wall <strong>in</strong>growths (1) <strong>in</strong>vested <strong>by</strong> an <strong>in</strong>tact plasmalemma (/>) are occasionally<br />
formed adjacent to necrotic <strong>cells</strong>. In this case <strong>the</strong> <strong>in</strong>tercellular space between 3<br />
<strong>modified</strong> <strong>cells</strong> is a necrotic cell (nc), x 7000.<br />
14 CEL 41
2O2 U. Wyss, H. Lehmann and R. Jank-Ladwig
Modified <strong>root</strong>-<strong>tip</strong> <strong>cells</strong> <strong>in</strong> Ficus 203<br />
mult<strong>in</strong>ucleate transfer <strong>cells</strong> and that <strong>the</strong> wall <strong>in</strong>growths are formed as a consequence<br />
<strong>of</strong> cont<strong>in</strong>uous nematode demands for nutrients (Jones & Northcote, 1972) is now<br />
widely accepted. F<strong>in</strong>gerlike wall <strong>in</strong>growths were also observed <strong>in</strong> <strong>root</strong>-<strong>tip</strong> <strong>cells</strong> <strong>of</strong><br />
celery, parasitized <strong>by</strong> <strong>the</strong> ectoparasitic and migratory dorylaimid nematode Longidorus<br />
apulus (Bleve-Zacheo, Zacheo, Lamberti & Arrigoni, 1977) which belongs to <strong>the</strong><br />
same family as X <strong>in</strong>dex. The authors suggest that <strong>in</strong> this particular case, <strong>in</strong> which<br />
<strong>the</strong> feed<strong>in</strong>g site <strong>of</strong> <strong>the</strong> parasite is not fixed (and where thus <strong>the</strong> nutrient demand is not<br />
cont<strong>in</strong>uous), <strong>the</strong> <strong>in</strong>growths may have a double function: to stop <strong>the</strong> spread <strong>of</strong> <strong>the</strong><br />
necrotic area adjacent to <strong>the</strong> <strong>cells</strong> and to improve <strong>the</strong> cell-to-cell transport when <strong>the</strong><br />
meristematic tissue <strong>of</strong> <strong>the</strong> <strong>root</strong>-<strong>tip</strong> is <strong>in</strong>activated. In this connexion it is worthwhile<br />
mention<strong>in</strong>g <strong>the</strong> unusual f<strong>in</strong>d<strong>in</strong>g <strong>by</strong> Gunn<strong>in</strong>g, Pate & Green (1970), who observed<br />
tufts <strong>of</strong> wall <strong>in</strong>growths <strong>of</strong> yet unknown function around <strong>in</strong>tercellular spaces <strong>in</strong> leaf<br />
traces <strong>of</strong> <strong>the</strong> goosegrass (Gallium apar<strong>in</strong>e).<br />
Modified <strong>root</strong>-<strong>tip</strong> <strong>cells</strong> <strong>in</strong>duced <strong>by</strong> X. <strong>in</strong>dex on its host F. <strong>carica</strong> also show similarities<br />
<strong>in</strong> <strong>the</strong>ir <strong>ultrastructure</strong> to <strong>cells</strong> <strong>of</strong> <strong>the</strong> nutritive tissue <strong>in</strong> galls <strong>in</strong>duced <strong>by</strong> zoocecidia<br />
such as <strong>by</strong> Cecidomyidae (Rohfritsch, 1971), Chermesidae (Rohfritsch, 1977) and<br />
Cynipidae (Rohfritsch, 1974). The nutritive <strong>cells</strong> <strong>of</strong> <strong>the</strong>se zoocecidia are characterized<br />
<strong>by</strong> a large lobed nucleus, nuclear hypertrophy, abundant cytoplasm, reduction and<br />
fragmentation <strong>of</strong> <strong>the</strong> vacuome, abundance <strong>of</strong> ribosomes, plastids and mitochondria.<br />
X. <strong>in</strong>dex-<strong>modified</strong> <strong>cells</strong> show even greater similarities to nutritive <strong>cells</strong> <strong>in</strong> galls<br />
<strong>in</strong>duced <strong>by</strong> certa<strong>in</strong> acarocecidia which disturb cytok<strong>in</strong>esis so that bi- or mult<strong>in</strong>ucleate<br />
<strong>cells</strong> with wall fragments attached to <strong>the</strong> mo<strong>the</strong>r-cell wall are formed (Westphal, 1977).<br />
Mult<strong>in</strong>ucleate <strong>cells</strong> <strong>in</strong>duced <strong>by</strong> X. <strong>in</strong>dex are apparently formed <strong>by</strong> repeated mitoses<br />
without cytok<strong>in</strong>esis (Wyss, 1978; Rumpenhorst & Weischer, 1978), and <strong>the</strong>y thus<br />
resemble <strong>in</strong> this respect giant <strong>cells</strong> <strong>in</strong>duced <strong>by</strong> Meloidogyne spp. (Huang & Maggenti,<br />
1969; Jones & Payne, 19780). Accord<strong>in</strong>g to Bird (1979), <strong>the</strong> term coenocyte should<br />
be used for mult<strong>in</strong>ucleate <strong>cells</strong> formed <strong>by</strong> repeated mitoses without cell division,<br />
whereas any enlarged un<strong>in</strong>ucleate cell may be a giant cell. The mult<strong>in</strong>ucleate state<br />
<strong>of</strong> X. zncfoc-<strong>in</strong>duced coenocytes arises <strong>by</strong> synchronous mitoses (Rumpenhorst &<br />
Weischer, 1978; Wyss, unpublished) which are typical <strong>of</strong> most naturally and artificially<br />
created mult<strong>in</strong>ucleate <strong>cells</strong> (Fowke, Bech-Hansen, Gamborg & Constabel,<br />
Figs. 11-13. Feed<strong>in</strong>g <strong>of</strong> Xiph<strong>in</strong>ema <strong>in</strong>dex on. <strong>root</strong>-<strong>tip</strong>s <strong>of</strong> Ficus <strong>carica</strong> seedl<strong>in</strong>gs. LS<br />
through 6- to 12-day-old <strong>root</strong>-<strong>tip</strong> galls.<br />
Fig. 11. Enlarged <strong>modified</strong> <strong>cells</strong> adjacent and near a necrotic cell (nc). The large<br />
cell at <strong>the</strong> centre conta<strong>in</strong>s dense cytoplasm and endoplasmic reticulum (er) arranged<br />
<strong>in</strong> parallel strands. Wall <strong>in</strong>growths (i) grow <strong>in</strong>to <strong>the</strong> cell adjacent to <strong>the</strong> necrotic cell.<br />
Note <strong>the</strong> aggregation <strong>of</strong> plastids (p) with starch gra<strong>in</strong>s <strong>in</strong> a neighbour<strong>in</strong>g cell, x 3200.<br />
Fig. 12. A necrotic cell (nc) surrounded <strong>by</strong> <strong>modified</strong> <strong>cells</strong>. The cell with 3 nuclei<br />
at <strong>the</strong> plane <strong>of</strong> section is filled with extremely dense cytoplasm (compare <strong>the</strong> small<br />
vacuoles (uj) with those (t>j) <strong>of</strong> a neighbour<strong>in</strong>g cell), x 2000.<br />
Fig. 13. Wall stubs (foi) are quite commonly found <strong>in</strong> older <strong>modified</strong> <strong>cells</strong>.<br />
Occasionally wall fragments, not attached to cell walls, are also found, x 2300.<br />
14-2
204 U. Wyss, H. Lehmann and R. Jank-Ladwig
Modified <strong>root</strong>-<strong>tip</strong> <strong>cells</strong> <strong>in</strong> Ficus 205<br />
In <strong>the</strong> early stage <strong>of</strong> X. <strong>in</strong>dex-coenocyte formation cell wall stubs, attached to <strong>the</strong><br />
mo<strong>the</strong>r cell wall, were seen to protrude <strong>in</strong>to b<strong>in</strong>ucleate <strong>cells</strong> at some distance from<br />
parasitized necrotic <strong>cells</strong>. These stubs resulted most probably from <strong>in</strong>complete fusion<br />
<strong>of</strong> cell plate vesicles. Similar protrud<strong>in</strong>g wall fragments are formed when <strong>root</strong>s are<br />
treated with caffe<strong>in</strong>e (Roper & Roper, 1977; Jones & Payne, 19786) which does not<br />
affect mitosis. From <strong>the</strong> description <strong>by</strong> Jones & Payne (19786) o-i % caffe<strong>in</strong>e applied<br />
to grow<strong>in</strong>g <strong>root</strong>s <strong>of</strong> Impatiens balsam<strong>in</strong>a <strong>in</strong>hibited <strong>root</strong> growth and caused <strong>root</strong>-<strong>tip</strong>s<br />
to swell. This effect is very similar to <strong>the</strong> early stage <strong>of</strong> <strong>root</strong> response to X. <strong>in</strong>dex<br />
attack (Wyss, 1978). Cell wall fragments observed <strong>in</strong> older X. <strong>in</strong>^ex-coenocytes were<br />
attributed to cell wall dissolution (Wyss, 1978; Lehmann & Wyss, 1978). We now<br />
hesitate to ma<strong>in</strong>ta<strong>in</strong> this statement, as holes <strong>in</strong> a cont<strong>in</strong>uous wall were very rare and<br />
as such strik<strong>in</strong>g symptoms <strong>of</strong> cell wall lysis as shown <strong>in</strong> affected <strong>cells</strong> <strong>by</strong> Rotylenchuhis<br />
reniformis (Rebois, Madden & Eldridge, 1975) and Longidorus apulus (Bleve-Zacheo<br />
et al. 1977) were never observed. Th<strong>in</strong> and ra<strong>the</strong>r irregular cell walls <strong>in</strong> older coenocytes<br />
are undoubtedly formed <strong>by</strong> renewed cytok<strong>in</strong>esis. To our knowledge this<br />
feature, <strong>in</strong>volv<strong>in</strong>g typical phragmoplast and cell plate formation, is unique <strong>in</strong><br />
nematode-transformed plant <strong>cells</strong>. It possibly represents <strong>the</strong> <strong>in</strong>itial stage <strong>of</strong> renewed<br />
meristematic activities at <strong>the</strong> onset <strong>of</strong> new lateral <strong>root</strong> formation. As described earlier<br />
(Wyss, 1978) lateral <strong>root</strong>s were <strong>of</strong>ten seen to emerge from X. <strong>in</strong>dex-mduceA <strong>root</strong>-<strong>tip</strong><br />
galls.<br />
Modified feed<strong>in</strong>g sites are also <strong>in</strong>duced <strong>by</strong> <strong>in</strong>vertebrate - (e.g. Po<strong>in</strong>ar & Hess, 1974)<br />
and vertebrate-parasitic nematodes (Wright, 1974; Lee & Wright, 1978). The<br />
migratory trichurid nematode Capillaria hepatica <strong>in</strong>duces for <strong>in</strong>stance <strong>in</strong> <strong>the</strong> liver <strong>of</strong><br />
its host mouse a syncytial feed<strong>in</strong>g site, consist<strong>in</strong>g <strong>of</strong> mult<strong>in</strong>ucleate food <strong>cells</strong> whose<br />
contents are <strong>in</strong>gested and which ultimately degenerate (Wright, 1974). It is suggested<br />
that <strong>the</strong> parasite progressively <strong>in</strong>duces new feed<strong>in</strong>g sites on its migration through <strong>the</strong><br />
liver. A similar host-parasite relationship was shown for Trichuris muris <strong>in</strong> <strong>the</strong><br />
caecum and colon <strong>of</strong> its mouse host (Lee & Wright, 1978). In this respect X. <strong>in</strong>dex<br />
shows <strong>in</strong> its behaviour and <strong>in</strong> conjunction with associated responses <strong>of</strong> <strong>the</strong> host<br />
tissue, closer aff<strong>in</strong>ities to <strong>the</strong> mammal parasites mentioned than to sedentary plant<br />
parasitic nematodes which, as long as <strong>the</strong>y live, do not destroy <strong>the</strong> cellular alterations<br />
<strong>the</strong>y <strong>in</strong>duce and ma<strong>in</strong>ta<strong>in</strong>.<br />
Figs. 14-17. Feed<strong>in</strong>g <strong>of</strong> Xiph<strong>in</strong>ema <strong>in</strong>dex on <strong>root</strong>-<strong>tip</strong>s <strong>of</strong> Ficus <strong>carica</strong> seedl<strong>in</strong>gs. LS<br />
through 6- to 12-day-old <strong>root</strong>-<strong>tip</strong> galls.<br />
Fig. 14. Modified <strong>cells</strong> with several wall stubs (tos). x 1600.<br />
Fig. 15. Two older <strong>modified</strong> <strong>cells</strong> with new cell plates {cp), each surrounded <strong>by</strong> a<br />
phragmoplast. Nuclei (n) are found at both poles <strong>of</strong> <strong>the</strong> phragmoplasts. Note <strong>the</strong> two<br />
th<strong>in</strong> and irregular walls, one fus<strong>in</strong>g with a thick wall with a stub (tos) at its end<br />
(arrowhead), x 3000.<br />
Fig. 16. A new cell plate (cp), surrounded <strong>by</strong> dictyosomes (d), is orientated towards<br />
<strong>the</strong> stub (tus) <strong>of</strong> an older cell wall, x 13000.<br />
Fig. 17. New cell plate (cp) surrounded <strong>by</strong> a phragmoplast between 2 newly formed<br />
nuclei (w). The figure shows <strong>the</strong> same cell plate as <strong>the</strong> vertical plate <strong>in</strong> Fig. 15, but at<br />
a slightly different plane and at a higher magnification, x 6000.
206 U. Wyss, H. Lehtnann and R. Jank-Ladtmg<br />
The <strong>cells</strong> killed <strong>by</strong> X. <strong>in</strong>dex feed<strong>in</strong>g show similarities to <strong>the</strong> degenerated <strong>cells</strong> that<br />
result as a hypersensitive response to Meloidogyne <strong>in</strong>cognita <strong>in</strong>fection <strong>in</strong> resistant<br />
tomato <strong>root</strong>s (Paulson & Webster, 1972), especially with respect to <strong>the</strong> ra<strong>the</strong>r rapid<br />
<strong>in</strong>crease <strong>in</strong> electron-density <strong>of</strong> <strong>the</strong> affected cytoplasm. In addition <strong>the</strong> stimulus that<br />
causes <strong>the</strong> disorganization <strong>of</strong> <strong>the</strong> parasitized <strong>cells</strong> does not spread to adjacent <strong>cells</strong>.<br />
The trigger, however, which <strong>in</strong>itiates <strong>the</strong> mitotic and cytok<strong>in</strong>etic aberrations obviously<br />
passes <strong>in</strong>to neighbour<strong>in</strong>g <strong>cells</strong>. This implies a symplastic movement <strong>of</strong> <strong>the</strong> trigger,<br />
i.e. via plasmodesmata, which <strong>in</strong> turn implies a relatively low molecular weight. The<br />
expansion <strong>of</strong> <strong>the</strong> neighbour<strong>in</strong>g <strong>cells</strong> is later facilitated <strong>by</strong> <strong>the</strong> necrotic, easily crushable,<br />
<strong>cells</strong> that surround <strong>the</strong>m.<br />
Little is yet known about <strong>the</strong> chemical nature <strong>of</strong> <strong>the</strong> trigger that <strong>in</strong>duces <strong>modified</strong><br />
<strong>cells</strong> <strong>in</strong> specific plant-nematode <strong>in</strong>teractions. In X. <strong>in</strong>dex <strong>the</strong> trigger can only be<br />
released <strong>in</strong>to perforated <strong>cells</strong> via <strong>the</strong> parasite's odontostyle, hav<strong>in</strong>g been produced <strong>in</strong><br />
secretory gland <strong>cells</strong> that open <strong>in</strong>to <strong>the</strong> food canal. In a recent study (Robertson &<br />
Wyss, 1979) it was shown that X. <strong>in</strong>dex possesses <strong>in</strong> its basal oesophageal bulb a<br />
large and very active gland cell whose duct system was seen to deplete after <strong>the</strong> last<br />
perforation thrust <strong>of</strong> <strong>the</strong> stylet and dur<strong>in</strong>g <strong>in</strong>gestion pauses. There are several<br />
<strong>in</strong>dications that only secretions from this cell will be <strong>in</strong>jected <strong>in</strong>to perforated plant<br />
<strong>cells</strong>. Microanalytical studies on biochemical events at <strong>the</strong> site <strong>of</strong> <strong>the</strong> host-parasite<br />
<strong>in</strong>teraction: F. <strong>carica</strong>-X. <strong>in</strong>dex are <strong>in</strong> progress (Poehl<strong>in</strong>g, Wyss & Neuh<strong>of</strong>f, 1979)<br />
and it is hoped that <strong>the</strong>y will f<strong>in</strong>ally provide an answer.<br />
This study, f<strong>in</strong>anced <strong>by</strong> <strong>the</strong> Deutsche Forschungsgeme<strong>in</strong>schaft, was made <strong>in</strong> <strong>the</strong> Arbeitsgeme<strong>in</strong>schaft<br />
fur Elektronenmikroskopie der Tierarztlichen Hochschule Hannover. The<br />
authors thank Ulrike Ahrendt for technical assistance and Dr M. G. K. Jones, Department <strong>of</strong><br />
Biochemistry, University <strong>of</strong> Cambridge, England, for helpful comments.<br />
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(Received 21 May 1979)