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

7Seed Infection by VirusesViruses are submicroscopic entities composed of a nucleic acid, ribonucleic acid(RNA), or deoxyribonucleic acid (DNA), surrounded by a protective protein orlipoprotein coat. They are acellular, but each virus has a characteristic shape, whichmay be a rigid rod, flexuous thread, spherical (polyhedral), or bacilliform (bulletshaped).One fourth of all known viruses cause diseases in plants. Plant virusesnumber about 2000, and new viruses are described regularly (Agrios, 1988). Approximately18 to 20% of the described plant viruses are seed-transmitted (Mathews,1991; Johansen, Edwards, and Hampton, 1994). Stace-Smith and Hamilton (1988)believe that one third of plant viruses will eventually prove to be seed-borne.Mathews (1991), Mink (1993), and Agarwal and Sinclair (1997) consider thatmost seed-transmitted viruses are carried within the embryo. The seed-borne andseed-transmitted viruses are listed in several publications (Bennett, 1969; Phatak,1974; Bos, 1977, Mink, 1993; Shukla, Ward, and Bunk, 1994; Agarwal and Sinclair,1997). Table 7.1 provides a list of seed-borne and seed-transmitted viruses, excludingcryptic viruses, using the nomemclature of the updated International Committee onTaxonomy of Viruses (ICTV) lists of 1995 and 1999. The cryptic viruses are listedseparately in Table 7.2. The occurrence of viruses in seed is shown in seed transmission,infectivity tests, and serological tests using whole seed or seed components.Histopathological investigations using TEM are few, but in recent years enzymelinkedimmunosorbent assay (ELISA), immunosorbent electron microscopy (ISEM),and molecular techniques have been used more frequently. Viroids, composed ofnaked single-stranded, low-molecular-weight, and circular RNA, were initiallytreated under viruses. They are also seed-borne and seed-transmitted. Table 7.3 liststhe most common seed-borne viroids. Histopathological information on seed infectionsby viroids is lacking at present.7.1 INFECTION AND MULTIPLICATIONViruses are unique as they multiply intracellularly. They enter cells through woundsmade mechanically, by vectors (insects, aphids, thrips, whiteflies, leafhoppers, beetles,mites, nematodes, and fungi), systemically through seeds, and by infected pollengrains. Ectodesmata have also been implicated as possible routes of infection forplant viruses (Brants, 1964; Thomas and Fulton, 1968). Outside the cell, viruses donot divide and are not known to produce any specialized reproductive structures.The viral parasitism is at the genetic level, using the internal cellular machineryduring its replication. They multiply by inducing the host cell to form more virusparticles, and in doing so, they disturb the normal cellular processes, upset the cellmetabolism, and prove injurious to functions and life of the host cell. Since viruses199