Esophageal bulb function of Xphinema index and
associated root ce11responses,assessedby video-enhanced
contrast light microscopy
Urs
WYSS*,
Walter M.
and David L . TRUDGILL**
ROBERTSON**
* Institut fiir Phytopathologie, Univers& Kiel, Hermann-Rodewald-Str. 9, 2300 Kiel, W! Germany and
** Scottish Crop Research Institute, Invergowrie, Dundee, DD2 5DA, U.K.
SUMMARY
Xiphinema index fed on. a c.olumn of progessively deeper cells in root-tips of Ficus carica seedlings and in each the pattern
of behaviour and responses was generally similar. Following penenation of a root ce11wall, secretions (saliva) from the dorsal
œsophageal gland were injected by the nematode. Siiultaneously globules of fluid, thought to arise from the two subventral glands,
passed backwards through the œsophago-intestinal valve. There then followed several periods of œsophageal bulb pumping during
which the contents of the perforated uninucleate ce11were removed within about two minutes. Each period of pumping was
separated by a short pause at the beginning of which further secretions from the dorsal gland were even more forcefully and rapidly
injected. The pattem of feeding was similar on uninucleate and modified binucleate cells, except that it took longer on the latter.
When saliva was injected, the cytoplasm of the root ce11in the vicinity of the protruded stylet-t.ip losts its structure and viscosity
within a few seconds. With each subsequent injection of saliva this change spread to the rest of the cytoplasm and the nucleus,
facilitating the ingestion of their liquefied contents. As feeding continued the nucleolus progressively shrank until only a condensed
residue remained. The amyloplast membranes were dissolved, releasing their enclosed starch grains but the plasmalemma and
nuclear envelope apparently remained intact. Occasionally, feeding nematodes became inactive for c. 15 mn immediately after their
stylet-tip had penetrated the next deeper cell. A plug was then observed forming around the stylet-tip, but otherwise no cellular
changes were recorded. During the extended periods of ingestion that followed an inactive phase, the plug interfered with the
passage of saliva for some time SOthat in these cases the disintegration of the ce11contents was retarded.
RÉSUMÉ
Fonctionnement du bulbe cesophagien chez Xiphinema index, et modifications des cellules radiculaires par le nématode,
réve?éspar la microscopie en contraste de phase couplée à la vidéo à haute résolution
Xiphinema index se nourrit à partir d’une colonne de cellules s’accroissant progressivement en profondeur, et pour chacune de
ces cellules le comportement du nématode et la réponse de l’hôte sont similaires. Après avoir percé la paroi cellulaire, le nématode
injecte les sécrétions (salive) de la glande œsophagienne dorsale. Simultanément, des globules de fluide, supposés parvenir des deux
glandes subventrales, passent vers l’arrière, à travers la valve œsophago-intestinale. Suivent plusieurs périodes de pompage du bulbe
cesophagien au cours desquelles le contenu de la cellule uninucléée perforée est vidé, en deux minutes environ. Les périodes de
pompage sont séparées par une courte pause au début de laquelle de nouvelles sécrétions de la glande œsophagienne dorsale sont
injectées plus rapidement et avec plus de force. Ce schéma de prise de nourriture est semblable pour les cellules uninucléées et
pour les cellules modifiées, binucléées, toutefois le processus est plus long pour ces dernières. Lorsque la salive a été injectée, le
cytoplasme de la cellule radiculaire située au voisinage de l’extrémité du stylet perd sa structure et sa viscosité en quelques secondes.
Au cours des injections suivantes, ces modifications s’étendent au reste du cytoplasme et au noyau, facilitant ainsi l’ingestion du
contenu cellulaire. La prise de nourriture continuant, le nucléole diminue progressivement et il n’en demeure qu’un résidu
condensé. Les membranes des amyloplastes sont dissoutes, libérant les granules d’amidon, mais les plasmalemmes et l’enveloppe
nucléaire restent apparemment intacts. 11arrive que le nématode reste inactif pendant environ quinze minutes immédiatement après
que son stylet ait pénétré dans la cellule voisine, située plus en profondeur. On observe alors la formation d’un bouchon autour
de l’extrémité du stylet, mais aucune autre modification de la cellule n’intervient. Pendant les périodes d’ingestion qui suivent une
telle période de repos, ce bouchon gêne le passage de la salive pendant quelque temps, ce qui retarde la désintégration du contenu
cellulaire.
Xiphinenza
index, the vector of grapevine fanleaf
virus, is an economically important pest and hence one
of the most intensively studied ectoparasitic nematodes.
Revue Nématol. II (2) : 253-261 (1988)
It feeds readily on the roots of host plants growing in
agar culture,
and several studies have been made of its
relationship with its specifïc hosts. Feedmg is largely
253
U. Wyss, W M. Robertson & D. L. Tmdgill
restricted to the root-tips which, when attacked, cesse
growing and develop into terminal galls (wyss, 1981).
Sections through these galls reveal areas of collapsed
necrotic cells surrounded by metabolically active multinucleate ce& (Wyss ah Weischer, 1976; Wyss, 1978;
Rumpenhorst & Weischer, 1978; Wyss, Lehmann 8r
Jank-Ladwig, 1980; Bleve-Zacheo & Zacheo, 1983).
However, little is known about how the necrotic cells
have been killed or what induces the adjacent cells to
become multinucleate.
Attempts to determine qualitative changes in free
amino acids and proteins in aseptic Ficus carica fed
upon by X. index (Poehling, Wyss & Neuhoff, 1980;
Poehling & Wyss, 1980) gave only a few clues about the
possible means of induction of the galls. However, it
seems likely that any chemicals involved in gall formation are synthesized in the cesophageal bulb of the
nematode. The œsophageal bulb is a complex organ
used for ingestion and also contains three gland cells.
There are two subventral glands which lie midway along
the bulb. Each has a short duct which enters the pump
chamber towards the rear of the bulb (Robertson &
Wyss, 1979; 1983). The third gland ce11is the dorsal,
which is much larger than the subventrals and has a
complex duct system. Two ducts extend dorsally along
almost the entire length of the bulb and four ventral
branches extend almost half-way along the bulb. These
six ducts join together at the anterior end of the bulb and
enter the food canal through a valve just anterior to the
pump chamber. Fluids have been seen moving forward
through the duct system of the dorsal gland a few
seconds after the stylet-tip had perforated the wall of a
root ce11and also during the short pauses of ingestion
pumping (Wyss, 1977; Wyss & Inst. ‘Wiss. Film, 1977).
However, although observations were made at high
magnification with interference contrast light microscopy, it was not possible to confirm that these fluids
were injected into the root cell. Also, the immediate
response of the protoplast of the attacked cell, which was
usuahy several cells deep, remained obscure. Videoenhanced contrast light microscopy (Wyss & Zunke,
1986a; 1986b; Wyss, 1987) enabled these limitations to
be overcome and this paper reports more detailed studies on the functioning of the three gland cells during
feeding and on the response of the attacked root cell.
Materials
and methsds
Aseptic fig (Ficus carica) seedlings and X. index were
maintained in agar culture as previously described
(Wyss, 1978). The observation chambers were the same
as those described by Wyss and Zunke (1985) except
that fine sand particles were omitted. Numerous periods
of feeding by X. index (mainly adult females) were
recorded using a high resolution video system (YVyss &
Zunke, 1986a) and later analysed. In some feeds the
254
functioning of the dorsal gland and two subventral gland
cells were studied whereas in others the main objective
was to study the associated root ce11responses.
esults
FUNCTIONINGOFTHE
CESOPHAGEALBULB
Female X. index usually inserted their stylet two to
three cells deep before commencing feeding. A few
seconds after stylet protraction had ceased, the bulb was
observed to suddenly stretch and the duct system of the
dorsal gland dilated, especially at the anterior end.
Dilation of the ducts was accompanied by the rapid
disappearance (they appeared to burst) of many of the
secretory granules along the entire duct system. This was
immediately followed by the ducts collapsing as their
contents were passed forward. The whole process of
duct dilation and collapse took, on average, 9 seconds
(n = 30). After a further c. 8 seconds the bulb had
shortened and ingestion pumping commenced (Fig. 1).
Observations on the posterior end of the bulb showed
that between the stretching and shortening of the bulb
(i.e. before ingestion-pumping commenced) the pump
plates were slightly opened and that there was a steady
flow of fluids through the œsophageal-intestinal valve
(Fig. 3 F). When ingestion-pumping
started, fluids
withdrawn from the root ce11 could be seen passing
through the œsophageal-intestinal valve. Feeding on
uninucleate cells within newly attacked root-tips typically comprised 2-4 periods of ingestion interspersed
with short pauses (Fig. 1). During the pauses fluids from
within the dorsal gland were again passed forward.
These very rapid movements of dorsal gland ce11fluids
(hereafter termed saliva) were initiated by a sudden
contraction of the dilator muscles (Fig. 3 D) that opened
the valve where the main collecting duct enters the food
canal in front of the pump chamber. During these
movements of saliva no fluids were seen passing backwards through the œsophageal-intestinal valve. Depletion of the dorsal gland ce11ducts usually took c. 1
second and 3-4 second later ingestion pumping was
resumed (Fig. 1). During ingestion pumping the ducts
in the dorsal gland ce11progressively dilated (Figs 3 A,
B). However, during salivations between periods of
ingestion, a breakdown of secretory granules around the
ducts of the dorsal gland ce11was never observed, neither
during duct dilation nor during depletion (Fig. 3 C).
Figure 1 represents typical events observed in the
anterior feeding apparatus as a female X. index fed
progressively deeper on four uninucleate meristematic
cells within a newly attacked root-tip. On slightly swollen
root-tips containing binucleate &lls, feeding was identical to that on newly attacked root-tips, with the exception that the time spent on one ce11was now longer (up
to 8 min> with an average of 12 ingestion periods.
Revue iVémnto1. 11 (2) : 253-261 (1988)
ClZsophageal bulb function
In addition to tbis general pattern of feeding, most
nematodes showed occasional periods of œsophageal
bulb inactivity when feeding within the root-tip. The
inactive phases, which lasted on average 15 minutes
(n = 18), apparently
occurred at random but always
immediately after penetration of tbe wall of a living cell.
Tbe ducts of the dorsal gland ce11stayed slightly dilated
ofXiphinema
throughout
tbe periods of inactivity and, apart from a
very occasional twitcbing of muscles near t.he ce11 nucleus and a slight movement of secretory granules, the
bulb appeared to be completely inactive. The ducts and
the nuclei (Fig. 3 E) of the subventral glands were then,
as always, surrounded by many secretory granules and
most were seen to move. Occasionally,
some of the
Dd
C
Dd
r
130
190
160
index
220
I
250
350
380
480
510 sec
Dd
260
290
C
sr
C
420
390
450
Fig. 1. Characteristic events in the cesophageal bulb of a X. indexfemale feeding on a root-tip (with uninucleate meristematic cells)
of a Ficus carica seedling. r : stylet protraction ceased, œsophageal bulb no longer pulled forward, followed by bu!b stretching, dorsal
gland duct dilation and subsequent depletion; Dd : dorsal gland ducts depleted; p (dark area) : œsophageal bulb pumping for
ingestion, accompanied by a dllation of the duct system of the dorsal gland; C : sudden contraction of dilator muscles at anterior
end of bulb and fluids of dorsal gland passing into food canal in front of pump chamber; f (shaded arca) : bulb being pulled forward
as stylet is being pushed deeper; sr : stylet being retracted. Vertical arrows represent approximate time when wall of next root ce11
just perforatcd by stylet.
0
2.s
130
160
190
4.s
260
t
290
3.s
390
320
t
I
220
ltg
l
250
2.s
1;s
1
350
380
2.s
I
l.s
I
450
3.s
q*
510
Fig. 2. Characterlstic events in uninucleate mot-tip ce& of a Ficus carica seedling fed upon by a X index female. O-770 sec. : events
in a succession of seven cells fed upon. A, B, C : third, fourth and flfth ce11shown in Fig. 3 G-J. D : seventh ce11(root cap cell),
shown in Fig. 5 : L-M. Vertical arrows represent stylet-tip inserted in successive living mot cells; S : injections of fluids from dorsal
œsophageal gland cell; p (dark area) : ce11contents being withdrawn by pumping action of œsophageal bulb; f (shaded area) :
stylet-tip being pushed through empty cell; sr : stylet being retracted from root-tip.
Revue Néntatol. Il (2) : 253-261 (1988)
255
U. Wyss, KM.
Robertson & D. L. Trudgill
Fig. 3. Females of X. index feedmg from cells within root-tips of Ficus catica seedlings. Al1 bars = 5 prn. A-C : anterior end of
œsophageal bulb. A : bulb pulsating during food ingestion, showing nucleus (nd) of dorsal gland ce11and one of four dilated ventral
branches (vd) of its duct system; main duct (md) of dorsal gland cell slightly dilated. g, some of secretory granules surrounding
duct system; B : 13 sec. after A, main duct (md) and ventral branch now more dilated, just before duct depletion; C : 0.8 sec. later,
ducts depleted; D : anterior end of œsophageal bulb during depletion of dorsal gland duct system. Arrow points to region of bulb
pulled in by contraction of dilator muscles tbat open orifice of food canal (fc) into which main duct (md) enters. nd, nucleus of
dorsal gland; pc, pump chamber; E : one of two subventral gland nuclei (nv), densely surrounded by secretory granules (g); F :
posterior end of œsophageal bulb, showing passage of secretory fluids (arrows) from two subventral glands into intestine (i). pc,
pump chamber slightly opened; G-H : feeding from root-tip ce11(ce11C in Fig. 2), seven layers beneath root surface. G : stylet-tip
(St) just inserted into this cell, marked by four arrows. Nucleus (nu)‘iritact like that (nu 3) of an unaffected cell. nu 1; nu 2, degradeclnuclei of cells A and B (Fig. 2), previously fed on; H : 58 sec. after G, contents of ce11partially ingested, nucleolus (n) now more
refractive, here just before second injection of saliva; 1: 1 sec. after H, showing amount of saliva injected (compare location of arrows
with H); J : 32 sec. after 1, after a total of three injections of saliva. Stylet-tip now just being pushed deeper.
256
Revue Nématol. Il (2) : 253-261 (1988)
CEsophageal bulb function of Xiphinema index
granules around the ducts were observed breaking
down, but breakdown was rather less rapid than that
previously described for the dorsal gland cell. Following
periods of inactiviq the duration of ingestion prior to
penetrating the next deeper ce11was considerably prolonged (8-39 mn). However, pumping was still typically
intermittent, i.e. periods of ingestion alternated with
short pauses during which the ducts of the dorsal gland
ce11were depleted.
ROOT CELLRESPONSESTOFEEDING
On Young, healthy root-tips (with uninucleate meristematic cells) X index fed progressively deeper on a
column of cells (Fig. 2). Food was usually not withdrawn
from root cap and epidermal cells. However, in the
adjacent deeper cells penetration was followed by ingestion.
The stylet-tip of a female X. index which had just been
inserted into a meristematic cell, seven ce11layers below
the root surface, is shown in Fig. 3 G. This ce11was the
fifth in the series that were fed upon (cell C, Fig. 2). No
changes were observed within perforated cells in the fiist
12-19 seconds (n = 25). However, coincidental with the
breakdown of secretory granules and depletion of the
ducts in the dorsal gland cell, fluids were seen entering
the cell through the stylet-tip (1 .S in Fig. 2) and, from
these observations, it is certain that saliva from the dorsal
gland ce11is injected into the root. The injection of this
saliva caused immediate changes in the cytoplasm
around the stylet-tip with a loss of structure and an
apparent decrease in viscosity. The injection of saliva
was followed after about 10 seconds by withdrawal of
cytoplasm (Fig. 2). During ingestion, the wall of the root
ce11vibrated and the nucleus oscillated at the same rate
as the pulsations of the œsophageal bulb. After about
15-40 seconds saliva was again injected (2.S in Fig. 2),
but now with much greater volume and force than in the
first injection (Fig. 3 H-I). As illustrated in Fig. 2 the
contents of uninucleate meristematic cells were usually
consumed within 2 minutes by 2-4 periods of ingestion
and a total of 3-5 injections of saliva from the dorsal
gland ce11(n = 25).
A full sequence of ce11penetration, and responses
following salivation and ingestion is shown in Fig. 4 A-I.
Perforation of the wall of this ce11(Fig. 4 A) was achieved
by a few stylet thrusts and the stylet-tip came to rest
2-4 um within the cytoplasm (Fig. 4 B). Following the
injection of saliva, liquefaction of the cytoplasm spread
rapidly (Fig. 4 C), even while the bulb was pumping
(Fig. 4 D). The viscosity of the nucleoplasm was also
reduced. The nucleolus was drawn towards the stylet-tip
and started to oscillate at the same rate as the pulsations
of the œsophageal bulb. The second injection of saliva
was SO large that the nucleolus was visibly repelled
(compare Figs 4 D and 4 E). At the end of the third and
final period of food withdrawal the remaining cytoplasm
was totally liquefied, and the size of the nucleolus, still
Revue Nématol. Il (2) : 253-261 (1988)
retained within the nuclear envelope, had been decreased considerably (Fig. 4 F). The last (fourth) injection of saliva (Fig. 4 G) was followed by a further
disintegration of the nucleolus wich was reduced to a
dense tore (Fig. 4 H). When the stylet-tip was pushed
deeper through the dead ce11by a series of short jabs at
a rate of 2-3 second, it appeared that the nuclear
envelope had not been dissolved (Fig. 4 1).
The cellular responses described above were typical
of those observed in a11uninucleate root-tip cells even
when, occasionally, epidermal cells were attacked.
Feeding on an epidermal ce11is illustrated in Fig. 5 A-D.
The ce11was killed within 72 seconds after stylet-tip
insertion (Fig. 5 A). The cytoplasm disintegrated rapidly
after the first injection of saliva (Fig. 5 B). The plasmalemma appeared not to be destroyed during feeding,
as it was pulled away intact from the ce11wall by the
suction created during food withdrawal (Fig. 5 C). Also,
occasionally the plasmalemma was observed cloaking
the tip of the protruding stylet during penetration to the
next deeper cell.
A few feeds by juveniles were recorded and, except
that their stylets were shorter and could not penetrate SO
deeply, their effects (Fig. 5 E and F) and behaviour were
similar in all respects to those of adult females. Not yet
swollen root-tips of fig were sufficiently small for the
fully protracted stylet of a female to reach the outside
of the root on the opposite side. Females usually fed
from these cells, which contained amyloplasts (e.g. ce11
D in Fig. 2, illustrated by Figs 5 L-M). The first injection of saliva produced a rapid disintegration of the
amyloplast membrane surrounding groups of starch
grains. The released grains (Fig. 5 M) were not ingested.
Root-tips attacked by two to three females soon formed
galls which contained columns of collapsed, necrotic
cells surrounded by expanding meristematic cells
(Fig. 5 G), When the stylet-tip of a feeding nematode
encountered a necrotic ce11it had more diffïculty in
penetrating the wall because of the lack of turgor
pressure. Feeding nematodes appeared to recognise
when they had reached a dead ce11and immediately
penetrated more deeply until a living ce11was contacted.
Root-tips which had been fed upon for one day were
visibly swollen and contained binucleate cells (Fig. 5 H)
which could be recognised up to five ce11layers below
the root surface with the video system used. Feeding
from binucleate cells (Fig. 5 1) was similar to that on
uninucleate meristematic ce&, except that it took longer
(average 5 mn) and there were more periods of ingestion
(average twelve per cell, n = 12). As described previously, degraded nucleoli were left behind (Fig. 5 J).
Two days after the initial nematode attack the modified
cells contained four nuclei (Fig. 5 K). Feeding from the
larger and denser multinucleate cells was, however, more
difficult to observe but took even longer than in binucleate cells.
During the periods when nematodes became inactive
257
U. Wyss, W M. Robertson & D. L. Trudgill
Fig. 4. Female of X. index, feeding from a root-tip ce11of Ficus catica seedling. Bar = 5 prn. A : stylet-tip (St) just before ce11wall
perforation. nu, nucleus; n, nucleolus; B : stylet-tip inserted and at rest, 12 sec. before tïrst injection of saliva from dorsal œsophageal
gland cell; C : 11 sec. after first injection of saliva, cytoplasm around stylet-tip liquefied, marked by arrows; D : nucleus and
nucleolus drawn towards stylet-tip during food withdrawal, here 1 sec. before second injection of saliva; E : saliva just injected,
nucleolus pressed back (compare length of arrow with that of D); F : just before fourth and last injection of saliva. Nucleus degraded,
cytoplasm of whole ce11liquefied; G : saliva just injected; H : ce11partially emptied, nucleolus shrunk to a dense tore, stylet-tip
now being pushed deeper; 1 : stylet-tip approaching opposite wall of fed-on cell, nuclear envelope (ne) of degraded nucleus (arrows)
apparently still intact.
258
Revue Nématol. 11 (2) : 253-261 (1988)
CEsophageal bulb function of Xiphinema index
immediately after cell penetration, secretions emanating
from the stylet-tip were observed forming a plug-like
structure at the end of the stylet-tip (Fig. 5 N-O). A clear
zone developed in the cytoplasm surrounding the growing plug, but otherwise no ce11changes were observed.
Once ingestion had commenced, the usual forceful
injections of saliva were attenuated by the presence of
the plug, which was never seen to become dislodged
from the stylet-tip. Degradation of the cytoplasm and
nucleus within the ce11containing the plug was slow, in
this particular case, taking about five times as long as
during normal feeding. In other cases it was even more
prolonged. When the nematode injected saliva, fluids
were seen to escape through the slit in the stylet into the
previous adjacent ce11where, if it had not been previously fed upon, they caused a gradua1 degradation of
the cell’s protoplast (Fig. 5 0).
Discussion
Our observations show that when X. index feeds on
root-tip cells the activities within the œsophageal bulb
generally follow a fiied pattern which is similar for
uninucleate and bi- to multinucleate cells except that
more salivation and ingestion phases occur for the latter,
SOthat feeding then lasts longer.
The dorsal gland ce11appears to have at least two and
possibly three ways of functioning viz. 1) Following
penetration of a root-cell, the dorsal gland ducts fil1 with
fluid con-ring, at least in part, from the breakdown of
secretory granules lining the gland ducts. 2) In subsequent salivations into the ce11between periods of ingestion duct contents are again injected into the root cell.
However, duct dilation and depletion is then not accompanied by granule breakdown, indicating that the dorsal
gland ce11is capable of producing at least two types of
secretion. There are many nerves running through the
œsophageal bulb which probably control its complex
activities. 3) During the inactive periods slight movements of the muscles in the region of the dorsal gland
nucleus were observed indicating that the dorsal gland
ce11may be slowly secreting the material which forms the
plug at the stylet-tip in the root cell. These inactive
periods were not confined to particular cells or to certain
depths within the root and did not involve the dilation
and depletion of the dorsal gland duct system. Otherwise
they resemble the salivation phase reported for Longidorus species (Towle & Doncaster, 1978; Robertson,
Trudgill & Griffths, 1984). Similar inactive periods have
been reported for X. diversicaudutum (Trudgill, 1976).
The subventral gland cells are presumed to act in
unison and appear to have one and possibly two or three
functions viz. 1) They supply secretions to the intestine
immediately prior to the first period of ingestion pumping. 2) Breakdown of granules during the inactive
period indicates that subventral gland secretions may
pass forwards during plug formation and, if they are
produced, may act in combination with dorsal gland ce11
Revue Nématol. Il (2) : 253-261 (1988)
secretions. 3) Secretions may be added to the food taken
from the plant during ingestion pumping. Our observations therefore demonstrate the complex way in which
the œsophageal bulb of X. index functions and complement earlier ultrastructural studies (Robertson & Wyss,
1979; Robertson, in press).
Changes in the root cells during feeding also have a
regular pattern. The first injection of saliva (involving
breakdown of the dorsal gland granules) which follows
penetration of the ce11wall causes liquefaction of the
cytoplasm and nucleoplasm, thereby enabling them to
be readily ingested. Subsequent injections of saliva
accelerate the liquefaction process. This could be caused
by pH, ionic or enzymic effects. The release of secretory
proteins is strongly suggested by the observed bursting
(exocytosis) of secretory granules derived from the
rough endoplasmic reticulum via the Golgi complex (cf.
Robertson & Wyss, 1979). Neither the plasmalemma nor
the nuclear membranes are destroyed, but the nucleoplasm is extracted leaving only a residue of the nucleolus
condensed within the membrane. In contrast, amyloplast membranes are readily broken down to release
starch grains which are not ingested. Feeding on bi-to
multinucleate cells follows a similar pattern but takes
longer because the cells are larger and metabolically
highly active with an increased cytoplasmic density.
The most obvious response to feeding is the formation of a gall containing multinucleate cells. It is not
certain which of the individual effects of feeding is
responsible for these changes and it may be that several
are involved. Production of the plug material during the
inactive periods had little immediate effect either in the
ce11penetrated or in surrounding cells. However, the
inactive periods, and the more prolonged periods of
ingestion which follow may play a crucial role in gall
formation; little damage takes place during the inactive
periods and there is ample time before ingestion begins
for a gall-initiating
substance to be transported to
neighbouring cells. Similarly, the withdrawal of large
volumes of solutes may initiate some of the changes
invoked in the surrounding cells.
Our observations on the functioning of the œsophageal bulb during feeding also pose a number of questions regarding the role of its secretions during transmission of grapevine fanleaf virus by X index. The force
with which saliva is expelled during ingestion pauses is
considerable and may be sufficient to dislodge virus
particles from their site of retention on the food canal
wall without any specific release mechanism being required. However, potentially three different types of
salivation behaviour have been identified, any one of
which could involve a specific release mechanism.
Release of virus during the inactive phase is an attractive
proposition as the ce11is then not rapidly destroyed as
it is during normal feeding. The possibility that the
subventral gland secretions may be involved in plug
formation also mises the possibility that particles adsorb259
U. Wyss, W M. Robertson & D. L. Trudgill
Fig. 5. X index feeding from root-tip cells of Ficus carica seedlings. Al1 bars = 5 ~III. A-D : female feeding from an epidermal
cell. A : stylet-tip (St) just inserted. n, nucleolus; B : during food withdrawal, 23 sec. after first injection of saliva from dorsal
œsophageal gland cell. Most cytoplasm liquefied; C : during food withdrawal, 19 sec. after second injection of saliva. Plasmalemma
(arrows) pulled away from ce11wall by suction created; D : 36 sec. later, stylet-tip now inserted in next deeper cell. Nucleolus (n)
of fed on ce11shnmk to a dense tore. E-F : L 3-juvenile feeding from subepidermal ce11in region of root elongation; E : stylet-tip
(st) just inserted. nu, nucleus; F : 2 min. later, arrow points to remnants of nucleus;.G : collapsed necrotic ce11(nc) between
260
Revue Nématol. Il (2) : 253-261 (19881
CEsophageal bulb jùnction of Kiphinema index
ed onto the walls of the pump chamber (Taylor &
Robertson, 1970) could be passed forward into the root
and are not lost for the purpose of transmission as had
been previously supposed. However, it could equally be
argued that the formation of the plug material would
prevent the passage of the virus particles into the plant
cell. Detailed experiments are now required to resolve
these questions and to identify the mechanisms of gall
formation.
REFERENCES
BLEVE-ZACHEO, T. & ZACHEO, G. (1983). Early stage of disease
in fig. roots induced by Xiphinema index. Nematol. medit.,
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Acceptépour publication le 6 août 1987.
expanding meristematic cells; H : necrotic meristematic cells (nc) surrounded by binucleate cells, one day after initial nematode
attack, here three cell layers below surface of swollen root-tip; 1 : female feeding from binucleate cell, here 3 min after stylet-tip
(St) insertion. Slightly degraded nucleoli (dn) after 7 injections of saliva. c, cytoplasm; n, nucleolus of neighbouring cell; J :
comparison between two binucleate cells. a, has been fed upon for 8 min, now with shmnken nucleoli (dn); b, neighbouring cell,
with dense cytoplasm (c) and normal nucleoli (n); K : cells (asterisk) with four nuclei, two days after initial nematode attack, here
four ce11layers below surface of swollen root-tip. L-M : female feeding from root cap ce11(ce11D in Fig. 2) with amyloplasts (a);
L : 1 sec. after first injection of saliva from dorsal œsophageal gland cell. st, inserted stylet-tip; M : 19 sec. later, during food
withdrawal. Amyloplasts disintegrated (da), starch grains no longer retained within amyloplast membrane. N-O : female feeding
from root-tip cell; N : towards end of a 20 min ” inactivity ” phase. Secretions, emanated through stylet-tip aperture, hardened
into plug-like material (p). Nucleolus (n) of penetrated ce11and nucleolus (n 1) of previous adjacent ce11still Ünaffected; 0 : 8 min
+ after food withdrawal had started and stylet-tip now just retracted. Note degradation of nucleoli of both cells.
Revue Nématol. 11 (2) : 253-261 (1988)
261