ANALYSIS OF POTATO-VIRUS-X COAT PROTEIN GENES IN RELATION TO RESISTANCE CONFERRED BY THE GENES NX, NB AND RX1 OF POTATO

The coat protein gene nucleotide sequences from eight previously uncha racterized strains of potato virus X (PVX) were determined. Comparison of the deduced amino acid sequences showed that two classes of PVX co at protein, designated types X and B, could be distinguished based on protein length and overall amino acid identities. In all there were 14 amino acid positions where all of the type X proteins differed from a ll of the type B proteins. The PVX coat protein is the principal viral determinant of the outcome of interactions between the virus and pota toes carrying either the Nx or Rx1 resistance genes. The different str ains of PVX were tested for their ability to overcome resistance confe rred by three potato resistance genes: Nx, Nb and Rx1. All of the stra ins that were avirulent on potato cultivars carrying the Nx resistance gene were found to have type X coat proteins whereas strains capable of overcoming the Nx resistance had type B coat proteins.