Molecular Analysis of Potato Virus X Isolates in Relation to the Potato Hypersensitivity Gene Nx. Simon Santa Cruz. The Sainsbury Laboratory, Norwich Research Park, Colney, Norwich NR4 7UH, U.K.. David C. Baulcombe. The Sainsbury Laboratory, Norwich Research Park, Colney, Norwich NR4 7UH, U.K.. MPMI 6:707-714. Accepted 9 July 1993. Copyright 1993 The American Phytopathological Society.

The coat protein gene of potato virus X is known to affect the outcome of interactions between different strains of the virus and potato plants carrying the Nx resistance gene. To analyze the role of the coat protein in interactions with Nx hosts, we used the potato virus X strain PVX_DX, which induces a hypersensitive response on potato cultivars carrying the Nx resistance gene and the strain PVX_DX4, which was originally derived from PVX_DX and which overcomes Nx-mediated resistance. Sequencing of cloned coat protein genes representing the strains PVX_DX and PVX_DX4 showed that they differed at a single nucleotide. This change results in the substitution of glutamine-78 in the PVX_DX coat protein for proline in PVX_DX4. We constructed hybrid viral genomes by replacing the coat protein gene of a full-length clone of isolate PVX_UK3 with the corresponding sequence from either PVX_DX or PVX_DX4. Progeny virus, derived from in vitro transcripts of these hybrid clones, showed that the single nucleotide difference between the coat protein genes of isolates PVX_DX and PVX_DX4 was sufficient to alter the outcome of the interaction between the virus and potato plants carrying the resistance gene Nx. Additional coat protein mutants generated in planta from transcript-derived inocula induced an intermediate host response on Nx potato cultivars which is influenced by the presence of a second, PVX-specific, resistance gene in the host plant genome.