THE MOVEMENT OF VIRUSES WITHIN THE PLANT - A REVIEW

In order to generate a productive infection, a virus must move within its host plant. There are 2 forms of virus movement within a plant, na mely, short distance and long distance. The short-distance movement is from cell to cell. During this process, the virus moves to an adjacen t cell through the protoplasmic bridges, the plasmodesmata. Several re sults indicate that this form of spread is virus controlled. Indeed, p roteins to which a movement function has been assigned have been ident ified in a number of plant viruses, such as TMV, TRV, CPMV, AMV and Ca MV, to name a few. The long-distance movement takes place in the vascu lar tissues and it is a phenomenon essential in the establishment of a systemic infection. The mechanism of the long-distance spread is poor ly understood. it is not yet clear whether movement in the vascular sy stem requires the activity of a spread function or whether it is a pas sive process within the phloem elements. In certain combinations of vi ruses, one virus, which normally only establishes a subliminal or tiss ue-localized infection, invades other tissues along with the other vir us. The ''helper'' virus apparently provides a function, presumably in volved in spread, that the ''helped'' virus lacks in that plant. This review has sought to deal with the different aspects of virus spread w ithin a plant and also with the mechanisms of plant resistance to viru s movement.