5'-PROXIMAL POTYVIRAL SEQUENCES MEDIATE POTATO-VIRUS-X POTYVIRAL SYNERGISTIC DISEASE IN TRANSGENIC TOBACCO

The interaction of potato virus X (PVX) and potato virus Y (PW) in tob acco causes a synergistic disease characterized by a dramatic increase in symptom severity, a change in the regulation of PVX RNA replicatio n, and an increase in accumulation of PVX. In this study we demonstrat e that PVX also interacts synergistically with three other members of the potyvirus group of plant viruses, tobacco vein mottling virus (TVM V), tobacco etch virus (TEV), and pepper mottle virus. These synergism s resemble the classic PVX/PVY synergism with respect to both the incr ease in host response and the change in PVX replication. To determine if the induction of PVX/potyviral synergism requires potyviral genome replication per se or if the response is mediated by expression of one or more potyviral genes, we used tobacco plants stably transformed wi th various subsets of the TVMV genome. PVX infections of transgenic pl ants expressing the 5'-proximal region of the TVMV genome, including t he protease-l, helper component protease, and protein-3 genes, result in symptoms resembling those of PVX/potyviral synergism. A similar syn ergistic-like response occurs when transgenic tobacco plants expressin g the analogous but smaller region from the 5'-proximal region of the TEV genome were infected with PVX. Replication of PVX RNA is altered i n transgenic plants expressing 5'-proximal sequences of either TVMV or TEV, and in a manner similar to that observed in double infections. T hese results indicate that replication of the potyviral genome is not required for PVX/potyviral synergism and that the response is mediated by expression of potyviral sequences which have been localized to the 5'-proximal third of the genomic RNAs of both TVMV and TEV. (C) 1995 Academic Press, Inc.