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United States Department of Agriculture

Agricultural Research Service

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Chapter 10—Ultrastructure of Initial Responses of Susceptible and Resistant Soybean Roots to Infection by the Soybean Cyst Nematode, Heterodera glycines

Figure Captions

Figure 216. Cross section of susceptible 'Lee' soybean root with Heterodera glycines 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 (ISC). CW, cell wall; CVa, central vacuole; FP, feeding plug; PC, parietal cytoplasm. Bar=1.0 µm.


Figure 217. 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.


Figure 218. 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. Bar=1.0 µm.

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.

Figure 220. Section through initial syncytial cell (ISC) of syncytium induced in 'Lee' soybean roots at 2 days after inoculation with Heterodera glycines. Tangential section shows stylet (St) 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 electron-opaque. CWO, cell wall opening; FP, feeding plug; Mc, mitochondria; Pd, plastid; Va, vacuole. Bar=1.0 µm.


Figure 221. 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.


Figure 222. Section of 'Lee' soybean roots at 3 days after inoculation with Heterodera glycines, 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.


Figure 223. 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). Bar=1.0 µm.

Figure 224.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.

Figure 225. 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). CWM, cell wall material; GA, Golgi; Mt, microtubules; Nm, 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.

Figure 227.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.

Figure 228.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.

Figure 230. Section of 'Bedford' soybean root of fig. 229 but at a different location, showing stylet entry into initial syncytial cell. Cisternae (Cs) and rough endoplasmic reticulum (RER) are 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, plastid; St, stylet. Bar=1.0 µm.


Figure 231. 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 µm.

Figure 233.Tangential 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) appears to surround major portion of stylet tip, except for small area of low contrast of membrane (M) at upper right of stylet. Bar=1.0 µm.


Figure 234. 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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Last Modified: 8/12/2016
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