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

Location of Fungal Hyphae in Seeds 105sorghum embryo (Figure 5.3C, D) is incited by C. lunata (Rastogi, Singh, and Singh,1990) and in that of wheat by B. sorokiniana (Yadav, 1984). In rubber (Heveabrasiliansis) seeds infected with B. theobromae, some cells in mesophyll of cotyledonsbecame hypertrophied and thick-walled (Varma, Singh, and Singh, 1990). Theembryo in heavily infected seeds shows conspicuous axial elongation, comprisingnarrow elongated cells in sorghum and wheat infected by C. lunata (Rastogi, Singh,and Singh, 1990) and D. tetramera (Yadav, 1984), respectively.Common histological manifestations at the subcellular level in heavily affectedcells of cotyledons in soybean by F. oxysporum and R. bataticola (Mathur, 1992;Sharma, 1992) are the loosening of cell wall fibrils, the broadening of plasmodesmata,the depletion of plasma membrane, the degradation of cytoplasm and cellorganelles, and the deformation of protein bodies, lipid bodies, and nuclear membrane.The soybean cotyledons infected with R. bataticola also revealed the appearanceof striated excretory bodies not found in uninfected cotyledons (Figure 5.3E,F). Thus, the changes at subcellular level show symptoms of disturbed metabolicactivity, including triggering of some new pathways.5.4. MIXED INFECTIONSSeeds usually do not have unifungal infection, but information on histopathologyof seeds infected with more than one fungus is limited (Wilson, Noble, and Gray,1945; Kunwar, Singh, and Sinclair, 1985; Sharma, 1992). Wilson, Noble, and Gray(1945) distinguished the hyphae of Gloetinia granigena (blind seed fungus) fromthat of the endophyte in rye grains on the basis of hyphal morphology. Gloetiniagranigena occurred in different parts of the seed and its expanse depended on thestage at which infection took place. But the mycelium of the endophyte was usuallyconfined to a layer immediately outside the aleurone layer (probably the seed coat).In germinating caryopsis, the endophyte spread to the plumule, while G. granigenawas usually restricted to the endosperm and scutellum.Kunwar, Singh, and Sinclair (1985) found Colletotrichum truncatum and Phomopsissp. or Cercospora sojina in soybean seeds. Based on hyphal color (unstained),width, presence or absence of oil globules, and reaction to stains (safranin-lightgreen and trypan blue), the mycelium of these fungi could be distinguished fromone another (Table 5.1). These pathogens compete with one another for colonizationof seed tissues. The hyphae of Phomopsis sp., an aggressive pathogen, are restrictedto seed coat layers when C. truncatum is present. Similarly, C. sojina hyphae aremore numerous in the seed coat than C. truncatum hyphae in seeds having mixedinfection of these pathogens. An additive effect is seen on deterioration of tissuesof seed coat when infected with C. truncatum and Phomopsis sp.Seeds of soybean can be infected with F. oxysporum and Papulaspora coprophila,a saprophyte. Hyphae of F. oxysporum are hyline, branched, septate, 2 to3.5 mm broad and stained green or light red, whereas those of P. coprophila arebrown to dark brown, much branched, shortly septate, 4 to 10.5 mm broad and darkred brown in safranin-fast green stained preparations. With trypan blue the hyphaeof F. oxysporum are stained blue, but those of P. coprophila retained the naturalbrown color. Both the fungi invade all the components, but F. oxysporum is more