Amino Acid Changes in Pepper mild mottle virus Coat Protein That Affect L 3 Gene-mediated Resistance in Pepper

Several field isolates of Pepper mild mottle virus (PMMoV) were obtained from Capsicum annuum (L 3 / + ) that exhibited systemic mosaic or systemic necrosis. The deduced amino acid sequences of the coat protein (CP) of these isolates differed from those of previously reported PMMoV strains that are able to overcome L 3 gene-mediated resistance. CP mutants having these amino acid changes were constructed using cDNA clones pTPWl and pTPC4350. These clones produce in vitro transcripts TPW1, which induces a resistance response resulting in local infection, and TPC4350, which causes no response on inoculated leaves resulting in systemic infection in the Capsicum plants carrying the L 3 gene (L 3 /L 3 ). These CP mutants were tested for infection and symptomatology in Capsicum spp. plants (L 3 /L 3 and L 3 / + ). An A/S (from serine to alanine) change at position 81 in the CP of TPW1 conferred a resistance-breaking activity in the L 3 /L 3 plants, whereas an S/A change at position 7 in the CP of either TPW1 or TC4350 conferred resistance-inducing activities. These results suggest that amino acid changes of A/S at position 81 and S/A at position 7 in CP had inverse effects on the L 3 gene-mediated resistance. These and other amino acid changes in PMMoV CP are also discussed in relation to the dosage effects of the L 3 gene on virus resistance.