Ultrastructure of Initial Responses of Susceptible and Resistant
Soybean Roots to Infection by the Soybean Cyst Nematode, Heterodera
The initiation and development of syncytial nurse cells in
host tissues are critical for the survival of infective cyst nematodes.
The host-parasite relationship of the soybean cyst nematode, Heterodera
glycines, in soybean (Glycine max [L.] Merr.) roots
has been observed through light microscopy (Ross
1958; Endo 1964, 1965; Endo and Veech 1970; Veech
and Endo 1970; Acedo et al.
1984), transmission electron microscopy (Gipson
et al. 1971, Riggs et al. 1973,
Endo 1978, Kim
et al. 1987), and scanning electron microscopy (Jones
and Dropkin 1976).
The feeding plugsformed at the feeding sites of soybean
roots infected by the soybean cyst nematode (Endo
1978) and other host-parasite interfaces (Wyss
et al. 1984)function as a seal between the stylet and
host syncytium during feeding and at molt. The fine-structural
observations of H. glycines infecting susceptible and resistant
cultivar roots provide important data on the mechanisms of resistance
and susceptibility (Gipson et
al. 1971, Riggs et al. 1973,
Kim et al. 1987). Syncytia induced
by cyst nematodes contain a greater number of organelles than
do the normal cells from which they are derived. At advanced stages
of infection, syncytium walls adjacent to xylem vessels are modified
into fingerlike ingrowths that resemble those of transfer cells
(Jones and Northcote 1972,
Jones and Gunning 1976, Stender et al. 1982, Wyss
et al. 1984).
Light microscopy observations of syncytia and giant cells emphasize
the presence of feeding tubes near the feeding site of these nurse
cells (Rumpenhorst 1984).
In vivo studies of the feeding process showed the interactions
of stylet movement and secretion, cytoplasmic streaming, and food
ingestion (Wyss and Zunke 1986).
The ultrastructure of feeding tubes shows that the hardened salivary
secretions are attached to the stylet of H. schachtii and
indicates the continuity between the secretions emanating from
the stylet and the feeding tube. Wyss and Zunke (1986) and Wyss
et al. (1984) also reported that continuous food uptake lasts
for up to 1 hour, including short pauses when the nematode salivates.
Similar feeding-tube formation and accumulation of secretion granules
in the dorsal gland ampulla occur in H. glycines during
the infection of soybean roots (Endo
Although light and electron microscopic observations have provided
new information on the interactions of the soybean cyst nematode
and the soybean, the initial stages of infection and their relation
to subsequent cytological modifications of host tissues have not
been systematically examined. This chapter describes the ultrastructure
of the initial stages of host-parasite interactions between the
soybean cyst nematode and the roots of susceptible and resistant
cultivars, with emphasis on nematode secretions (salivation) and
host responses in the initial syncytial cell and adjacent cells.
Early responses of susceptible and resistant soybean cultivars
to infection by the soybean cyst nematode, Heterodera glycines,
are expressed by the accumulation of smooth and rough endoplasmic
reticulum within a thick-walled initial syncytial cell. Nematode
secretions are surrounded by a narrow region of endoplasmic reticulum
that integrates with the rough endoplasmic reticulum of the host
cytoplasm. The initial syncytial cell and adjacent cells that
form the syncytium show slight enlargement at 18 hours after inoculation
but have definite disruptions in their connecting cell walls.
Although the endoplasmic reticulum closest to the stylet and associated
secretions is not associated with mitochondria, the surrounding
cytoplasm has rough endoplasmic reticulum interspersed among numerous
mitochondria, plastids, and other organelles.
In most resistant cultivars, the syncytial cytoplasm contains
extensively distributed wide cisternae, extending throughout the
rough endoplasmic reticulum. These cisternae are less prevalent
in syncytia induced in susceptible cultivars. Secretions varying
greatly in electron density are found in infections at 18 hours
to 2 days. The secretions nearest the nematode stylet appear uniform
in distribution, whereas others have darkened peripheral cylindrical
boundaries with clear to partially occluded centers. Syncytia
in susceptible and resistant cultivars at 18 hours to 4 days after
inoculation were hypertrophied and hyperplastic, contained expanded
regions of cell-wall dissolutions and cell-wall depositions of
callose, and sometimes included lysosomelike particles.
The advances in video-enhanced light microscopy coupled with
electron microscopy should provide ways to further understand
the process involved in host-parasite interaction of cyst nematode
infection and to explore means of nematode control by interrupting
these infection processes.
Ultrastructure of the initial responses of susceptible and
resistant soybean roots to infection by the soybean cyst nematode
is shown in figures 216218,
figure 219, figures
220221, figures 222223,
figure 224, figure
225, figure 226, figure
227, figure 228, figure
229, figures 230231,
figure 232, figures
233234, figure 235, and figure 236.
9 Reprinted in modified form
with permission of the editor-in-chief, Révue du Nématologie
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