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

2 Histopathology of Seed-Borne Infections(pistil). The seed formation takes place in situ through a series of integrated sequentialsteps in the life cycle of the flowering plant (Maheshwari, 1950, 1963; Johri,1984). After pollination and fertilization, changes in different parts of the ovule, i.e.,the embryo sac (zygote and primary endosperm nucleus), nucellus, chalaza, andintegument, lead to the formation of seed. The term seed, when used sensu lato,includes one-seeded dry indehiscent fruits that are the dispersal propagative unitsin plants of several families such as Poaceae, Asteraceae, Apiaceae, and Chenopodiaceae.In the present treatment, the term seed is used in a loose sense.Seed is an autonomous living unit and links successive generations. Structurally,it consists of an embryo (new plantlet), a protective covering — seed coat, pericarp,or both — and reserve food material, which may be present in the endosperm,perisperm, or embryo. It has the capacity to withstand desiccation and retain viabilityunder unfavorable environments or until it germinates. These properties of seed makeit an important commodity for storage as well as transport to new areas or countries,for planting or for edible purposes. The structure of seed is fairly constant in aspecies, but varies in different plant taxa (Netolitzky, 1926; Corner, 1976).Seeds, if infected by microorganisms, will act as carriers. If the organisms remainviable they will result in development of disease in the new crop. Infected seeds areoften responsible for the spread of diseases to new areas. For this reason seed hasbecome an object of plant quarantine internationally. Countries also use domesticseed certification, including seed health testing, as a method of quality control ofseed.1.2 MICROORGANISMS IN SEEDFungi, bacteria, viruses, and nematodes are known to be seed-borne (Neergaard,1979; Maude, 1996; Agarwal and Sinclair, 1997). Fungi form a major group ofpathogens that are seed-borne as well as seed-transmitted. In addition to saprophytesand parasites, fungi are known to form an inherent association with seeds of somemembers of Cistaceae, Ericaceae, and Orchidaceae. In the Orchidaceae, the seedswill not usually germinate without the presence of a mycorhizal fungus (Rayner,1915). The seed coat in seeds of Helianthemum chamaecistus (Cistaceae) harbors afungus that seems essential for normal germination of seed. In the absence of thefungus, the plumule fails to emerge, and roots also are not formed at the time ofgermination (Boursnell, 1950).The list of saprophytic and parasitic fungi associated with seeds of differentplants is very large (Richardson, 1990), and they belong to all fungal classes. Thefungi that are discussed in this book and for which histopathological information isavailable belong to the division Eumycota, subdivision Mastigomycotina, classOomycetes, subdivisions Ascomycotina, Basidiomycotina, and Deuteromycotina.The members of Deuteromycotina dominate and these fungi belong to the classesHyphomycetes and Coelomycetes. The endophytic fungi are discussed separately.A large number of viruses, including cryptoviruses and viroids, are known tobe seed-borne, but the information on seeds infected by viruses is limited and mostlyinconclusive. The better-studied viruses are barley stripe mosaic virus (BSMV) andpea seed-borne mosaic virus (PSbMV) due to the studies of Carroll and co-workers