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

Seed Infection by Viruses 205Unlike conventional viruses, not all cryptoviruses are transmitted through vectors,grafting, or mechanical processes. They seem unable to move from cell to cell.Propagation and distribution of cryptic viruses occur only through cell multiplication(Boccardo et al., 1987; Mink, 1993). Pollination experiments among carrier andnoncarrier parents have shown that cryptic viruses are transmitted through pollen orovule or both. When both parents are carriers, all progeny carry the virus. Whenboth parents are free of the virus, the progeny remains free.7.2 CELLULAR CONTACTS, ISOLATION, ANDTRANSPORT SYSTEMS IN OVULE AND SEEDThe recent information, based on ultrastructure, has yielded valuable details aboutcell contacts and barriers in the ovule and developing seed. A brief summary is givenbecause this situation has been largely misunderstood, resulting in several erroneousconclusions. Neergaard (1979) has written that “The virus may not invade the ovuleor embryo. The ovule is isolated by lack of plasmodesmatal connections with thesurrounding tissues….” Mathews (1991) considers that a few viruses that are confinedto vascular elements may be unable to enter the ovule, which has no vascularconnection with the parent.The ovule, progenitor of seed, is connected through the funiculus to the ovarywall at the placenta. Each ovule receives a vascular supply originating from theventral carpellary trace which, in turn, is in continuation of the pedicel and stemvascular supply. Initially, the ovular supply is of procambial cells, but the xylemand phloem elements differentiate during later stages. The cells of the integument,nucellus, and chalaza in the ovule have plasmodesmatal connections. However, thecuticles of integument and that of the nucellus act as barriers between them. Thenucellus is the site of differentiation, and development of the female gametophyteand cell contacts and separation in these tissues are as follows:1. Megasporocyte, functional megaspore, two- and four-nucleate embryosacs have plasmodesmatal connections with the surrounding nucellar cells.2. By the time the mature embryo sac is formed, the plasmodesmatal connectionswith the surrounding cells are severed.3. In mature female gametophyte, the synergids and egg have no cell wallin the chalazal one third part, which is delimited by plasmalemma. Thecentral cell is also without the cell wall adjacent to the egg apparatus.4. After fertilization, the zygote develops the cell wall all around.5. The proembryo has plasmodesmatal connections among its cells, but it isisolated (no plasmodesmata) from the surrounding endosperm.6. Wall ingrowths occur in the basal cells of the proembryo, particularlythose of suspensor, and are prominent at the globular or precordate stageof the embryo.7. The embryo proper in early stages is covered by the cuticle, which isabsent over the suspensor cells.