Predicted kinase-3a motif of a resistance gene analogue as a unique markerfor virus resistance

A DNA fragment (ADG2, 310 bp) 77% homologous to the gene N (resistance to t obacco mosaic virus in Nicotiana glutinosa) and 53% homologous to RPP5 (res istance to Peronospora parasitica in Arabidopsis thaliana) was amplified by PCR from the diploid potato genotype 2x(v-2)7 that carries the gene Ry(adg ) located on chromosome XI and conferring extreme resistance to potato viru s Y(PVY). Sequence comparison revealed that ADG2 spans a region correspondi ng to the predicted kinase-2 and kinase-3a motifs in N and RPP5. One of the 12 nucleotide differences detected between ADG2 and a homologous fragment from a PVY-susceptible potato genotype was located within the predicted kin ase-3a motif. This single nucleotide substitution of G --> C, resulting in an amino-acid substitution Ser --> Thr, abolished the BbvI recognition site of ADG2, which was shown to distinguish all tested potato genotypes carryi ng Ry(adg) from those lacking: this gene, irrespective of the genetic backg round and ploidy level. This PCR-based resistance marker, developed using a resistance gene analogue as a target, is the first example of a PCR-based marker that is generally applicable for selection of an economically import ant trait in potato.