Emergence of a resistance-breaking isolate of Rice yellow mottle virus during serial inoculations is due to a single substitution in the genome-linked viral protein VPg

The recessive gene rymv-1, responsible for the high resistance of Oryza sativa 'Gigante' to Rice yellow mottle virus (genus Sobemovirus), was overcome by the variant Cl4*, which emerged after serial inoculations of the non-resistance-breaking (nRB) isolate Cl4. By comparison of the full-length sequences of Cl4 and Cl4*, a non-synonymous mutation was identified at position 1729, localized in the putative VPg domain, and an assay was developed based on this single-nucleotide polymorphism. The mutation G1729T was detected as early as the first passage in resistant plants and was found in all subsequent passages. Neither reversion nor any additional mutation was observed. The substitution G1729T, introduced by mutagenesis into the VPg of an nRB infectious clone, was sufficient to induce symptoms in uninoculated leaves of O. sativa 'Gigante'. This is the first evidence that VPg is a virulence factor in plants with recessive resistance against viruses outside the family Potyviridae.