VARIATIONS IN THE VPG PROTEIN ALLOW A POTYVIRUS TO OVERCOME VA GENE RESISTANCE IN TOBACCO

The va gene is used in commercial Burley tobacco cultivars including c v TN 86 to confer resistance to tobacco vein mottling virus (TVMV) and , to some extent, other potyviruses. A naturally occurring strain of T VMV (TVMV-S), which overcomes this resistance, was isolated from TN 86 plants. To investigate the mechanism by which TVMV-S overcomes va gen e resistance, a cDNA clone encompassing the complete genome of TVMV-S was produced. Using chimeric transcripts combining regions of TVMV-S a nd regions of the wild-type strain (TVMV-WT) to which TN 86 is resista nt, it was demonstrated that a domain within the VPS protein is respon sible for overcoming va resistance in TN 86, DNA sequence analysis rev ealed six amino acid differences between the two strains of TVMV withi n this domain. Inclusion of sequences for four of the TVMV-S VPS amino acids was sufficient to confer the resistance-overcoming phenotype to all corresponding transcripts. Coinoculation experiments suggested th at the resistance of TN 86 to TVMV-WT was not due to the induction of a general host defense response. The results are compatible with the h ypothesis that VPS must assume an appropriate configuration in order t o interact with appropriate host components and facilitate systemic vi rus movement. (C) 1997 Academic Press.