ASSOCIATION OF DISTINCT DOUBLE-STRANDED RNAS WITH ENHANCED OR DIMINISHED VIRULENCE IN RHIZOCTONIA-SOLANI INFECTING POTATO

A virulent field isolate of Rhizoctonia solani AG 3, Rhs 1AP, has give n rise to three sectors in a period of several years. Subculturing of these sectors resulted in three hypovirulent isolates--Rhs 1A1, Rhs 1A 2, and Rhs 1A3. We reported previously that five genetically different double-stranded (ds) RNAs occur in these four isolates, with each iso late containing a unique combination of two or more dsRNAs. We report here that all five dsRNA elements occur in the cytoplasm, and none in the nucleus. The mitochondria contains low to moderate concentrations of the four larger dsRNAs. The four isolates were paired in selected c ombinations in an attempt to transmit specific dsRNAs to cultures lack ing these dsRNAs. This approach generated groups of near-isogenic line s possessing the same dsRNA elements. As many as six cultures possessi ng the same dsRNA genotype were found to have the same degree of patho genicity. Consistently, acquisition of a 6.4-kb dsRNA brought about in creased virulence, whereas the presence or acquisition of a 3.6-kb dsR NA resulted in diminished virulence in the recipient culture. All cult ures had the same mitochondrial DNA haplotype as that of the original field isolate, Rhs 1AP. This, in conjunction with the fact that all cu ltures in this study had a single nuclear DNA origin (Rhs 1AP), indica tes a strong correlation between certain phenotypic characters or chan ges thereof and dsRNA profiles or acquisition of particular dsRNAs, re spectively.