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

Abstract

There have been numerous biochemical mutants of Magnaporthe grisea previously described. In this study, selenate-resistant (sel) mutants were recovered and characterized. The mutants were recovered as spontaneous, fast-growing sectors arising from initially restricted colonies grown in the presence of 0.1% sodium selenate. Over 94% of the sectors generated were resistant to selenate, and approximately 70% of the selenate-resistant mutants were unable to utilize sulfate sulfur. The sulfate non-utilizing (sul) mutants could be distinguished from wild type by their thin expansive growth with little or no aerial mycelium on a defined minimal medium with sulfate as the sole sulfur source. However, growth of the sul mutant was comparable to the wild type when given L-methionine as a reduced sulfur source. Sul mutants were recovered from all 36 isolates examined representing four distinct MGR586 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 amended 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 were observed on potato dextrose agar amended with 0.1% sodium selenate. The mean sectoring frequency among isolates in MGR586 DNA fingerprint group B was significantly higher than among isolates in three other fingerprint groups examined. Complementation tests with multiple sul mutants from a given isolate and sexual crosses among sul mutants of two fertile laboratory isolates 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.