ANALYSIS OF THE RESISTANCE-BREAKING DETERMINANTS OF POTATO-VIRUS-X (PVX) STRAIN HB ON DIFFERENT POTATO GENOTYPES EXPRESSING EXTREME RESISTANCE TO PVX

Different potato genotypes expressing extreme resistance to potato vir us X were inoculated with potato virus X strains cp, CP4, and HB and w ith three PVX mutants constructed in vitro. Using these viral strains and mutants, the involvement of the coat protein, specifically the sin gle amino acid residue at position 121 within this protein, in the res istance-breaking capacity of strain HB was confirmed for all resistanc e genes tested. We concluded that the extreme resistance expressed in Solanum x chaucha, S. x curtilobum, S. x juzepczukii, and S. vernei is conferred by Rx genes (Rx(cha), Rx(cur), Rx(juz), and Rx(vrn), respec tively) that employ the same mechanism of interaction with the PVX coa t protein as Rx, Rx(acl), and RX(adg). Although S. sucrense clone OCH 11926.4 was resistant to both common PVX strains and PVX(HB), it was s usceptible to the mutant isolates. We hypothesized that at least two v iral determinants, one of which is not located in the coat protein cod ing region, interact with the S. sucrense resistance gene.