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

Seed Infection by Viruses 2073. The movement of virus through plasmodesmata may take place on accountof the capacity of the virus to alter the structure of plasmodesmata or maybe facilitated by the spread and movement proteins coded by the virus(Meshi et al., 1987; Wolf et al., 1989).4. Long-distance movement of virus occurs through conducting tissues, particularlythe phloem. The existence of functional plasmodesmata betweenmesophyll cells and phloem has been observed, providing a symplasticpathway for the movement of viruses. The open ends of the vascularelements in the leaf mesophyll may also provide sites for the phloemloading with virus. The virus moves rapidly over long distances as evidentfrom the classic experiments of Samuel (1934) on tobacco mosaic virus(TMV) in tomato. Samuel inoculated one terminal leaflet and then followedthe spread of the virus in the infected plant with the help ofinfectivity tests and found a systematic spread of the pathogen downwardas well as upward. Schippers (1963) examined the spread of BCMV inbean plants. He inoculated the middle leaflet of the first compound leafon the main stem and thereafter determined its presence in flower budsand ovaries at different nodes (Figure 7.1). Five days after inoculation,the virus was detected for the first time in buds and ovaries from nodesfour and five and from axillary shoots at nodes one and three. After 6days of inoculation, the virus also could be detected in floral buds andovaries at nodes zero and two (Figure 7.1). This showed upward as wellas downward transport of virus in the plant.5. An important question is the form in which the virus moves from cell tocell systematically in the plant. Three options are possible: (1) as virusparticle; (2) as virus nucleic acid; and (3) as virus-specific nucleoprotein.It is also possible that the virus may move in more than one form. TheTMV infection spreads from cell to cell in the absence of protein coat intomato (Siegel, Zaitlin, and Sehgal, 1962; Dawson, Bubrick, andGranthan, 1988) but for long-distance transport, virus particles areprobably essential (Saito, Yamanaka, and Okada, 1990). BSMV causesinfection in the absence of the protein coat and may become systemic(Maule, 1991).7.3.2 OVULE AND SEEDNeergaard (1979) and Mathews (1991) believe that the viruses may not enter ovuleson account of the lack of plasmodesmatal connections with the surrounding tissues.Neergaard (1979) further concluded that (1) the virus may not be capable of establishinga compatible relationship with the gametes or the embryo sac of the host;(2) the virus is lethal to the gametes, thus causing sterility and preventing productionof infected seed;and (3) the virus is not capable of infecting male and female gametesor the young embryo, either due to lack of virulence to these stages of the host ordue to their resistance. Such conclusions seem to have been made mainly on thebasis of detrimental effects of virus infection on the floral, fruit, and seed parts.Caldwell (1962) has observed that tomato aspermy virus is not seed-transmitted in