Histopathology of Seed-Borne Infections - Applied Research Center ...

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Seed Infection by Viruses 209Wilcoxson, Johnson, and Frosheiser (1975) for alfalfa mosaic virus (AMV) in alfalfa;Carroll and Mayhew (1976a,b) and Mayhew and Carroll (1974) using BSMV inbarley; and Hunter and Bowyer (1993) for LMV in lettuce.A second course of invasion of the ovule may take place through infected pollengrains (Mandahar, 1981; Mandahar and Gill, 1984; Mink, 1993). Mandahar (1981)listed 37 viruses as pollen-transmitted, but according to Mink (1993) the current listincludes only 9 viruses. Pollen grains may be contaminated by virus, or the virusmay occur inside in the cytoplasm and in sperm. BSMV particles in barley pollengrains occur in the cytoplasm as well as the nucleus of sperm cells (Carroll, 1974;Carroll and Mayhew, 1976a). Brlansky, Carroll, and Zaske (1986) have shown thatBSMV-infected barley pollen grains are viable, germinate, and on germination thevirus particles pass into the pollen tube and reach the embryo sac via the stigmaand style. The cytoplasmic contents of the pollen tube together with the sperms aredischarged in the degenerating synergid in the embryo sac.A third course for viruses to reach the ovule and embryo is through the funiculus,its vascular supply, or parenchyma cells. Once the infection has reached the ovule,it may spread through symplastic pathways in cells of the integument and thenucellus. Using pea seed-borne mosaic virus (PSbMV) in pea, Wang and Maule(1992, 1993, 1994) traced this course of virus entry. They found abundant virusparticles in and around carpellary vascular bundles of unfertilized carpels (Figure7.2A, B), but none in the ovule, including the egg cell, synergids, and antipodalcells (i.e., the embryo sac). After fertilization, virus was detected in and around thevascular supply of the ovule (Figure 7.2C to E). Subsequently, it spread in the cellsof integument.7.4 LOCALIZATION IN REPRODUCTIVE SHOOT,OVULE, AND SEEDTable 7.4 lists all those cases that convincingly show the presence of virus in seedcomponents. Histopathological determinations using ultra-thin sections and electronmicroscopy are few. The detection of seed-borne viruses in mature dry seeds isdifficult because of the very laborious procedures for preparing material for electronmicroscopy transmission and also because of the inactivation of viruses in matureseeds in many cases. The most comprehensive investigations are on BSMV infectionin barley by Carroll and co-workers and on PSbMV infection in pea (Wang andMaule, 1992, 1994; Maule and Wang, 1996).There have been no studies using histopathological techniques of viruses of thecryptovirus group, which are transmitted with high efficiency through pollen andseed (Boccardo et al., 1987; Mathews, 1991).7.4.1 BARLEY STRIPE MOSAIC VIRUS (BSMV) AND SIMILARVIRUSESBSMV is seed-borne and seed-transmitted. It has seed-transmitted as well as nonseed-passage(NSP) strains, and its response in different cultivars also varies.Figure 7.3 is a schematic representation of the infection cycle of a seed-transmitted
210 Histopathology of Seed-Borne InfectionsovcwcwovABCDfummEFFIGURE 7.2 Distribution of pea seed-borne mosaic virus (PSbMV) RNA in the ovary tissuebefore and after fertilization (dark areas represent virus RNA concentration also marked byarrow). A, B, Ls through unfertilized ovaries of cultivars Progreta and Vedette, respectively,showing viral RNA restricted mostly to the carpel tissue. C, D, Ls fertilized ovaries showingthe ingress of the virus into the ovule along the vascular tissue (arrow). E, F, magnified viewsof ovules from C, D to show the ingress of the virus. (Abbreviations: cw, carpel wall; fu,funiculus; m, micropyle; ov, ovule.) (From Wang, D. and Maule, A.J. 1994. Plant Cell 6:777–787. With permission.)strain (MI-1) in a susceptible cultivar Atlas. The infection reaches the primordia forfertile appendages — stamen and pistil — quite early. In the anther, cells of walllayers, connective, anther sac-male archesporium, and its derivatives (i.e., microsporemother cells, dyads, and microspores in tetrads), pollen grains contain virus particlesin the cytoplasm individually, in clusters, and attached to the microtubules. The virusoccurs in vegetative cells and in sperm cells (Figure 7.4A to C), and in the latter inthe cytoplasm as well as the nucleus (Carroll, 1974; Carroll and Mayhew, 1976a).The pollen grains are viable and after pollination, they germinate, and pollen tubesenter the stigma and style, ovule, and embryo sac, and discharge their contents inthe cytoplasm of the degenerating synergid (Figure 7.5A to D). BSMV particlesoccur in pollen tubes at all stages and also in the contents discharged in the embryosac (Brlansky, Carroll, and Zaske, 1986).In the ovule, the BSMV particles have been observed in the integument, nucellus,female archesporium (Figure 7.6A, B), megaspore mother cell, megaspores, two-,four-, and eight-nucleate embryo sacs, and in cells (synergids, egg, and antipodalcells) of the organized embryo sac (Figure 7.6C to E) (Carroll and Mayhew, 1976b;Mayhew and Carroll, 1974). After fertilization, clusters of BSMV particles are seen
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Histopathology ofSeed-Borne Infecti
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PrefaceThe book deals with only one
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The AuthorsDalbir Singh, Ph.D., for
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ContentsChapter 1 Introduction ....
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4.3.2 Routes for Infection from Ova
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8.4 Survival in Seed ..............
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2 Histopathology of Seed-Borne Infe
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4 Histopathology of Seed-Borne Infe
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2Reproductive Structuresand Seed Fo
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Reproductive Structures and Seed Fo
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14 Histopathology of Seed-Borne Inf
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3Structure of SeedsThe seed habit i
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Structure of Seeds 49(1971) of usin
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Structure of Seeds 51is found in as
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Structure of Seeds 53dry seed of Ly
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Structure of Seeds 553.3.2 SEED COA
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Structure of Seeds 57Endosperm: One
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Structure of Seeds 59scendembepsmuA
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Structure of Seeds 61Endosperm: Sca
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Structure of Seeds 63Chalaza simple
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Structure of Seeds 65stystypmerA Bc
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Structure of Seeds 67epsent endCADB
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Structure of Seeds 69perhscendembpe
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Structure of Seeds 71eposgepi′epo
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Structure of Seeds 73(Zea and Sorgh
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Structure of Seeds 75Baker, D.M. an
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Structure of Seeds 77Maheshwari Dev
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Structure of Seeds 79Vries, M.A. de
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100 Histopathology of Seed-Borne In
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TABLE 5.6Location of Mycelium of St
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