A SINGLE NUCLEOTIDE SUBSTITUTION IN THE ALPHA-A GENE CONFERS OAT PATHOGENICITY TO BARLEY STRIPE MOSAIC-VIRUS STRAIN ND18

A 236-nucleotide region from the alpha a gene of strain CV42 (pathogen ic to oat), when substituted for the homologous region in strain ND18 (nonpathogenic to oat), was shown previously to confer a near wild-typ e oat pathogenicity to this strain (Weiland and Edwards, 1994, Virolog y 201: 116-126). The data suggested that six amino acid substitutions in the alpha a gene were responsible for the differences in oat pathog enicity, and that threonine-724, encoded by CV42, might be a critical amino acid in determining pathogenicity of barley stripe mosaic virus (BSMV) to oat. In the present work, codons specifying T-724, I-764, an d N-785 (encoded by CV42 RNA alpha) were substituted individually and in combination for those coding for P-724, T-764, and K-785 (encoded b y ND18 RNA alpha), respectively, by site-directed mutagenesis. The cor e K-733, T-734, and K-736 positions (CV42) were substituted for Q-733, S-734, and Q-736 (ND18) as a single block. The results of inoculation s with these mutants indicate that the C-2261 --> A(2261) nucleotide s ubstitution (P-724 --> T-724) by itself is sufficient to enable strain ND18 to infect oat plants, although poorly. Additional substitution o f CV42 codons into ND18 RNA alpha at the remaining five positions alte red symptom type, decreased the timing of the appearance of symptoms, and increased the percentage of plants infected per inoculation. Nonet heless, all mutants accumulated to similar levels in inoculated oat pr otoplasts after a 24-h period. Using a recombinant RNA beta from which beta-glucuronidase could be expressed, results were obtained suggesti ng that the multiplication of strain ND18 and the nonpathogenic varian ts generated in the study was restricted in the inoculated leaf. The d ata indicate a potential pathway by which pathogenicity to oat evolved in BSMV.