Recovery and characterization of spontaneous, selenate-resistant mutants of Magnaporthe grisea, the rice blast pathogen

There have been numerous biochemical mutants of Magnaporthe grisea previous ly described. In this study, selenate-resistant (sel) mutants were recovere d and characterized. The mutants were recovered as spontaneous, fast-growin g sectors arising from initially restricted colonies grown in the presence of 0.1% sodium selenate. Over 94% of the sectors generated were resistant t o selenate, and approximately 70% of the selenate-resistant mutants were un able to utilize sulfate sulfur. The sulfate non-utilizing (sub) mutants cou ld be distinguished from wild type by their thin expansive growth with litt le or no aerial mycelium on a defined minimal medium with sulfate as the so le sulfur source. However, growth of the sul mutant was comparable to the w ild type when given L-methionine as a reduced sulfur source. Sul mutants we re recovered from all 36 isolates examined representing four distinct MGR58 6 DNA fingerprint groups commonly found on rice in Arkansas. The frequency of sul mutants recovered from the 36 isolates Varied on different media and among the different isolates examined. Generally, sectoring frequency and the number of sul mutants recovered was much higher on rice bran agar amend ed with selenate (0.1 to 1.4 sectors per colony) than on a defined minimal medium amended with selenate (0.0 to 0.7 sectors per colony). No sectors we re observed on potato dextrose agar amended with 0.1% sodium selenate. The mean sectoring frequency among isolates in MGR586 DNA fingerprint group B w as significantly higher than among isolates in three other fingerprint grou ps examined. Complementation tests with multiple sul mutants from a given i solate and sexual crosses among sul mutants of two fertile laboratory isola tes indicated that the sul mutation was at a single locus. The sul mutants tested were pathogenic on rice in greenhouse tests and indistinguishable in their virulence phenotype to the corresponding wild type. The sul mutation was stable and may be useful as a marker in epidemiological studies.