Molecular dissection of the potato virus Y VPg virulence factor reveals complex adaptations to the pvr2 resistance allelic series in pepper.

The virulence properties of potato virus Y (PVY) towards an allelic series at the pvr2 locus in pepper genotypes are related to variations in the genome-linked viral protein (VPg). Eleven amino acid substitutions in the central part of the VPg were identified in strains differing by their virulence properties and were introduced, either singly or in combination, in an infectious PVY clone to get an in-depth genetic analysis of the virulence determinant. The virulence spectrum of these mutants was evaluated by inoculation of four pepper genotypes carrying different alleles at the pvr2 locus. The mutations introduced had complex effects on virulence, including antagonistic epistasis and trade-offs for virulence towards different pvr2 alleles. In addition, several mutants showed new virulence properties that were unknown in the natural environment. Such complex effects of mutations on plant virus virulence are unprecedented. They provide a better understanding of the variable levels of durability of the resistance conferred by the different pvr2 alleles, and have important consequences for a durable management of the resistances.

Different mutations in the genome-linked protein VPg of potato virus Y confer virulence on the pvr2(3) resistance in pepper.

Five different amino acid substitutions in the VPg of Potato virus Y were shown to be independently responsible for virulence toward pvr2(3) resistance gene of pepper. A consequence of these multiple mutations toward virulence involving single nucleotide substitutions is a particularly high frequency of resistance breaking (37% of inoculated plants from the first inoculation) and suggests a potentially low durability of pvr2(3) resistance. These five mutants were observed with significantly different frequencies, one of them being overrepresented. Genetic drift alone could not explain the observed distribution of virulent mutants. More plausible scenarios were obtained by taking into account either the relative substitution rates, the relative fitness of the mutants in pvr2(3) pepper plants, or both.

Mutations in potato virus Y genome-linked protein determine virulence toward recessive resistances in Capsicum annuum and Lycopersicon hirsutum.

The recessive resistance genes pot-1 and pvr2 in Lycopersicon hirsutum and Capsicum annuum, respectively, control Potato virus Y (PVY) accumulation in the inoculated leaves. Infectious cDNA molecules from two PVY isolates differing in their virulence toward these resistances were obtained using two different strategies. Chimeras constructed with these cDNA clones showed that a single nucleotide change corresponding to an amino acid substitution (Arg119His) in the central part of the viral protein genome-linked (VPg) was involved in virulence toward the pot-1 resistance. On the other hand, 15 nucleotide changes corresponding to five putative amino acid differences in the same region of the VPg affected virulence toward the pvr2(1) and pvr2(2) resistances. Substitution models identified six and five codons within the central and C terminal parts of the VPg for PVY and for the related potyvirus Potato virus A, respectively, which undergo positive selection. This suggests that the role of the VPg-encoding region is determined by the protein and not by the viral RNA apart from its protein-encoding capacity.