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

Seed Infection by Bacteria 183FIGURE 6.6 (Color figure follows p. 146.) Pigeon pea (Cajanus cajan) seeds healthy (upperrow) and infected with Xanthomonas axonopodis pv. cajani. Seeds moderately shriveleddiscolored(middle row) and heavily shriveled-discolored (lower row) with water-soakedtranslucent areas. (From Sharma, M. et al. 2001. J. Mycol. Plant Pathol. 31: 216–219.)The embryo remained free of infection. Moderately shriveled and discolored seedscarried infection in all the layers of seed coat and superficial layers of the cotyledons(Figure 6.7A, D). Bacteria occurred inter- and intracellularly and were present inthe stellate parenchyma and persistent part of funiculus (Figure 6.7C). Affectedcotyledonary cells also showed necrosis. Heavily infected seeds had infection in allparts including parts of the embryo (Figure 6.7B). Bacterial masses occurred in theseed coat (Figure 6.7E), stellate parenchyma, and remnants of the funiculus. Necrosisand formation of the lytic cavities, occupied by bacterial cells, were seen. Spacesin seed, between the seed coat and embryo, between the cotyledons, and around theshoot-root axis also contained bacteria. Bacteria also occurred intracellularly incotyledonary cells (Figure 6.7F) and caused depletion of cell contents includingstarch grains, which were small and only of the simple type (simple and compoundstarch grains in unaffected cotyledons).X. axonopodis pv. malvacearum, a causal organism of bacterial blight, angularleaf spot, or black arm of cotton, occurs externally and internally in seed. Internalinfection was detected in the micropylar as well as chalazal halves of the seed coats,and rarely in the embryo (Tennyson, 1936; Brinkerhoff and Hunter 1963). Brinkerhoffand Hunter (1963) tried to determine the course of entry of the bacterium intothe seed. They soaked ginned fuzzy seeds of several varieties of cotton in aqueousdye (light green) and aqueous suspension of the bacterium. The dye readily enteredthrough the blunt chalazal end, and only after the embryo swelled, entered through