Influence of amino acid at position 132 in VPg on replication and systemic infection of Barley yellow mosaic virus.

A substitution of Lys with Asn or His at the amino-acid position 132 in VPg (VPg132) correlates with overcoming rym4-gene resistance by European strain 2 of Barley yellow mosaic virus (BaYMV-2). From the full-length cDNA clones for a Japanese BaYMV isolate JK05 (BaYMV-JK05) we generated virus mutants with Tyr, Lys, Asn, and Ala substituted for wild-type His at the VPg132. Only Tyr and Asn mutants replicated efficiently in protoplasts from barley varieties that are susceptible to wild-type virus. The Tyr mutant also infected susceptible barley plants systemically with the emergence of virus populations with partial or complete reversion to His, whereas the Asn mutant did not cause systemic infection. Thus, the VPg132 amino acid is essential for both efficient replication and systemic infection. Neither wild-type virus nor any of the mutants replicated in protoplasts from a rym4 barley genotype. Therefore, substitution of the VPg132 amino acid alone cannot enable breaking rym4-mediated resistance in the BaYMV-JK05 background.