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

206 Histopathology of Seed-Borne Infections8. At the cordate stage of embryo, the suspensor usually shows weakeningand obliteration, and simultaneously the cuticle around the embryo properdisappears.9. The developing endosperm also develops wall ingrowths along the embryosac boundary as seen in endosperm of Vicia faba (Johansson and Walles,1993).10. In ovule and developing seed, the sieve elements of ovular supply havenumerous plasmodesmatal connections with companion cells, and alsosome with the parenchyma cells. The companion cells in turn are connectedthrough plasmodesmata to the parenchyma cells (Thorne, 1980,1981; Offler and Patrick, 1984; Offler, Nerlich, and Patrich, 1989; Singh,1998).The above information shows that there are two kinds of transport systemsoperating in ovule and seed: (1) cells with plasmodesmatal connections have symplastictransport, and (2) those with wall ingrowths (transfer cell structure) carry onapoplastic transport. Johansson and Walles (1993) have concluded that cells withwall ingrowths are common at sites at the junction of different generations (oldmaternal and the new sporophyte) in ovules.7.3 VIRUS MOVEMENT7.3.1 INFECTED PLANTThe information on the movement of viruses in vegetative parts, particularly leafand stem, of the infected plant has been summarized in several excellent reviewarticles (Broadbent, 1976; Maule, 1991; Mink, 1993; Lucas and Gilbertson, 1994;Johansen, Edwards, and Hampton, 1994). A brief summary of various aspects relatedto virus movement in these parts is given with the view that similar systems probablyoperate in the ovule and seed.1. The conventional viruses that are seed-borne and seed-transmitted, afterentering the leaf epidermis or mesophyll cells through wounds or byvectors, multiply and spread to neighboring cells through cell-to-cellmovement. When this infection comes in contact with the vascular tissues,it enters the phloem sieve elements. In the sieve tubes, the virus movesover long distances. Long-distance spread through the phloem is alsoimportant for phloem-limited viruses.2. Virus particles have been seen in plasmodesmatal channels and sieve pores(Esau, Cronshaw, and Hoefert, 1967; Kitajima and Lauritus, 1969). Plasmodesmatavary considerably in diameter, 20 to 200 mm, in youngtobacco leaves. The properties of plasmodesmata may be altered as aresult of virus infection. The altered plasmodesmata have consistentlylarger openings of uniform diameter throughout their length (Kitajimaand Lauritus, 1969).