Chapter 10Ultrastructure of Initial Responses of Susceptible
and Resistant Soybean Roots to Infection by the Soybean Cyst Nematode,
Cross section of susceptible 'Lee' soybean root with Heterodera
at 18 hours after inoculation. Invaginated cell
wall thickening (ICWT
) corresponds to stylet penetration
site (see fig. 217)
. But ICWT is
not a usual formation. Small vacuoles (Va
) near feeding
site and adjacent cell indicate initial stages of host response.
Two cells anteriad to nematode are part of a syncytium, but cell
into which stylet has penetrated is called the initial syncytial
, cell wall; CVa
, feeding plug; PC
, parietal cytoplasm.
Longitudinal section through stylet (St) of nematode
shown in fig. 216 at a different sectioning level. Stylet cone
is surrounded by feeding plug (FP) at cell surface region.
Deposition around cone terminus appears as invaginated cell wall
(ICW). Arrow, plasmalemma. Bar=1.0 µm.
Section through enlarged nucleus of cell adjacent to initial syncytial
cell of fig. 216. Cell wall (CW) discontinuity contributes
to syncytium formation and development. Nu, nucleolus.
Figure 219. Stylet
(St) terminus of Heterodera glycines J2 inserted
into initial syncytial cell (ISC) of susceptible soybean
cultivar 'Lee' at 18 hours after inoculation. Note very dense
cytoplasm of ISC and of adjacent syncytial cells. ISC may have
enlarged vacuole above or below this site, as in other adjacent
cells. Cytoplasm is characterized by abundance of ribosomes and
clusters of rough endoplasmic reticulum. Cell adjacent to ISC
has enlarged nucleus (N) and nucleolus (Nu).
X, xylem. Bar=1.0 µm.
Section through initial syncytial cell (ISC
) of syncytium
induced in 'Lee' soybean roots at 2 days after inoculation with
. Tangential section shows stylet
) tip surrounded by plasmalemma (arrows
within initial syncytial cell. Secretions or feeding tubes (FT
have perimeters of smooth endoplasmic reticulum. Central region
of secretions appear electron-translucent whereas perimeters are
, cell wall opening; FP
feeding plug; Mc
, mitochondria; Pd
, vacuole. Bar=1.0 µm.
Cross section of anterior region of J2 located at feeding site
of 'Lee' soybean at 2 days after inoculation. Note presence of
shaft (Sh) of extended stylet, full complement of anterior
sensilla (Se), cephalic framework (CF), and
termini of muscles (mu) and attachment sites (hd)
as nematode is appressed to feeding site. Bar=1.0 µm.
Section of 'Lee' soybean roots at 3 days after inoculation with
, showing cell wall openings (CWO
and masses of membrane proliferations (MP
), one of which
has attachment to cell wall and associated plasmalemma. CWT
thickened cell wall. Bar=1.0 µm.
Section through adjacent cells of syncytium where nucleoplasm
(Np) of a nucleus (N) extends across two cells.
Cell at right contains two hypertrophied nucleoli (Nu).
Section of 'Lee' soybean root at 3 days after inoculation
with Heterodera glycines. Polyhedral-to-spherical vesicles
(Ve) accumulate at termini of separated walls within
syncytium. Mc, mitochondria. Bar=1.0 µm.
Syncytium formed in 'Lee' soybean root at 3 days after inoculation
with Heterodera glycines
, showing combination of susceptible
host reactions. Cell wall dissolution is extensive. Cell adjacent
to initial syncytial cell (ISC
) has large vacuole (Va
Group of cells toward lower left of micrograph indicates presence
of hyperplastic parenchyma (PaC
wall material; GA
, Golgi; Mt
, nematode. Bar=1.0 µm.
Figure 226. Submedian
longitudinal section showing stylet (St) of Heterodera
glycines J2 in wall of syncytium (Syn) induced in
susceptible cultivar 'Lee', at 3 days after inoculation. Stylet
is surrounded by characteristic feeding plug (FP) showing
continuity with electron-opaque material emerging from amphidial
canals (AC). Feeding plug site is apparently reused for
stylet reentry after withdrawal during feeding and molt, as demonstrated
with H. schachtii in radish roots (Wyss 1992). ISC,
initial syncytial cell; NP, nerve process; Pm,
protractor muscle; Sm, somatic muscle. Bar=1.0 µm.
Syncytium in 'Lee' soybean root near protoxylem (Px)
at 4 days after inoculation with Heterodera glycines.
Nematode feeding site is at lower part of figure. Cellular changes
within syncytium consist of increases in cytoplasmic density with
accumulations of endoplasmic reticulum (ER), mitochondria
(Mc), lipid droplets (LD) or possible secretion
granules, and plastids (Pd). Small- to moderate-sized
vacuoles (Va) are distributed throughout syncytium. Callose
depositions (CaD) occur on walls of adjacent cells that
form the syncytium and intercellular spaces. Hypertrophied cell
with nucleus (N) having convoluted membrane lies adjacent
to protoxylem vessel and group of parenchyma cells (PaC).
Nu, nucleolus. Bar=1.0 µm.
Initial syncytial cell (ISC) and adjacent cell
of resistant 'Bedford' soybean root at 18 hours after inoculation
with Heterodera glycines. Electron-opaque region near
nematode (Nm) is part of feeding plug (FP).
Appressed against a mass of apparently deteriorated nuclear material
(N) is smaller electron-opaque region consisting of nematode
secretions (NS) surrounded by endoplasmic reticulum (ER).
This endoplasmic reticulum tends to concentrate the secretory
mass by forming a mass of folded, smooth endoplasmic reticulum,
which in turn integrates with surrounding rough endoplasmic reticulum
(RER) that is widespread in ISC. Another distinctive
feature of host cytoplasm is presence of numerous mitochondria
(Mc). Bar=1.0 µm.
Figure 229. Cross
section of resistant soybean cultivar 'Bedford' at 18 hours after
inoculation with Heterodera glycines, showing cell-wall
dissolution and continuity of cytoplasm among adjacent pericyclic
cells (PcC). Syncytium contains numerous vacuoles (Va)
of variable size. Syncytial cytoplasm is more electron-opaque
than that in adjacent nonsyncytial cells. Rough endoplasmic reticulum
(RER) and its large cisternae (Cs) interspersed
with numerous mitochondria (Mc) are common features of
syncytial cytoplasm in resistant cultivar. CWO, cell
wall opening; ISC, initial syncytial cell; NS,
nematode secretions; Px, protoxylem. Bar=1.0 µm.
Section of 'Bedford' soybean root of fig. 229 but at a different
location, showing stylet entry into initial syncytial cell. Cisternae
) and rough endoplasmic reticulum (RER
extensive throughout feeding site of nematode. Although cell to
right of initial syncytial cell has enlarged nucleus (not shown),
the primary host response is in initial syncytial cell and cells
to left of nematode as shown in fig. 229. Pd
, stylet. Bar=1.0 µm.
Cross and tangential views of nematode secretions (NS)
or feeding tubes from serial section below site of stylet in fig.
230. Bar=1.0 µm.
Figure 232. Section
through initial syncytial cell (ISC) of resistant 'Pickett
71' soybean root at 2 days after inoculation with Heterodera
glycines. Electron-opaque regions dispersed among rough endoplasmic
reticulum (RER) indicate site of nematode secretions
(NS). ISC is almost integrated with adjacent
cell (Can), with only wall fragment (CWF) separating
the two components of the syncytium. CW, cell wall. Bar=1.0
section through lip region and stylet of Heterodera glycines
at feeding site of initial syncytial cell (ISC
) of resistant
line PI 88788, at 3 days after inoculation. An electron-opaque
mass forms a feeding plug (FP
) as is found at feeding
site in susceptible host. Plasmalemma (arrow
to surround major portion of stylet tip, except for small area
of low contrast of membrane (M
) at upper right of stylet.
Cluster of cells near necrotic (NC) or deteriorating
(DC) cells, distal to nematode feeding site, are hyperplastic
(HpC). Cells between deteriorating cell and syncytium
appear to be hyperactive (PC) as indicated by dense cytoplasm
with Golgi and extensive cell-wall depositions (CWD).
PC, parenchyma cell; Px, protoxylem; Syn,
syncytium. Bar=1.0 µm.
Figure 235. Higher
magnification of hyperactive cell of fig. 234, showing dense accumulations
of smooth (ER) and rough endoplasmic reticulum (RER)
and numerous sites of Golgi activity (GA). Cell walls
are thickened with electron-opaque, strandlike, particulate deposits
(EDD) shown in cross and tangential sections of an apparent
hyperactive cell (PC) and adjacent cells. CW,
cell wall; DC, deteriorating cell; HpC, hyperplastic
cell; Mc, mitochondrion; N, nucleus; NC,
necrotic cell. Bar=1.0 µm
Figure 236. Section
of 'Pickett 71' soybean root at 4 days after inoculation with
Heterodera glycines. Almost-empty central region of once-established
syncytium shows a resistant reaction to H. glycines infection.
Central region of syncytium is almost devoid of organelles, except
for residues of nucleoplasm (Np) at wall opening in syncytium
(Syn). EOD, electron-opaque particulate deposits;
Pd, plastid. Bar=1.0 µm.
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