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

1 IntroductionThe reports on the number of microorganisms associated with seeds have increasedgradually during the latter half of the 20th century. This is obvious from the firstand the most recent editions of An Annotated List of Seed-Borne Diseases by Noble,De Tempe, and Neergaard (1958), and Richardson (1990), respectively. The organismsoccur with seed either as contaminants adhering to the seed surface, looselymixed with seed, or as an infection present inside the seed tissues. This bookdescribes penetration and location of microorganisms in seed tissues. Informationon location of infection in seeds using histological techniques alone is considered.The presence of internal infection in seed detected in transmission studies is beyondthe scope of this book.In order to appreciate penetration, course of infection, location, and the effectof infection of microorganisms in floral and seed tissues, it is necessary to have agood understanding of the structure of flowers and changes in fertile appendages —stamen and carpel — leading to the formation of seed. Neergaard (1979) has includedinformation on morphology and anatomy of seed in relation to transmission ofpathogens. This account is fragmentary and does not include information on manycritical steps in the formation of seed. Considerable new knowledge, includingultrastructure of reproductive components — embryo sac, endosperm, embryo, andtheir surrounding tissues — has been provided (Johri, 1984; Johri, Ambegaokar, andSrivastava, 1992). These data elucidate the contacts and barriers among the tissuesof developing seed. The structure of mature seed, including one-seeded dry indehiscentfruits, is also highly variable in angiosperms (Netolitzky, 1926; Corner,1976). The variations in size of the hilum, micropyle opening, nature and thicknessof the cutile, and thickness of seed coat and pericarp have shown direct correlationwith the penetration and location of fungal pathogens in certain host–parasite interfaces.Chapter 2 provides a concise up-to-date account of the structure and developmentof floral parts and the formation of seed. Chapter 3 deals with the structureof mature seed in selected families of angiosperms. Chapters 5 through 8 concernthe histopathology of seed infections by fungi, bacteria, viruses, and nematodes.Chapter 9 describes physiogenic seed disorders. Chapter 10 includes a brief accountof histopathological techniques and tips. Only a basic account is given in Chapter10; several detailed books on plant microtechnique and transmission and scanningelectron microscopy are available.1.1 THE SEEDBiologically, seed is the ripened ovule. In angiosperms, to which a majority of thecrop plants belong, the ovules are borne in the ovary, the basal part of the gynoecium1