Comparison of scytalone dehydratase activities between scytalone dehydratase inhibitor-sensitive and inhibitor-resistant Magnaporthe oryzae isolates

Abstract

Magnaporthe oryzae is the causal agent of blast disease of cereals such as rice. Recently, fungal isolates resistant to the melanin biosynthesis inhibitors-dehydratase (MBI-D), which inhibits scytalone dehydratase (SH), have been detected in Japan. The isolates have a single point mutation in the gene encoding SH. In fitness tests, resistant isolates showed reduced fitness, compared with that of sensitive isolates. To investigate the factors contributing to the reduced fitness of resistant isolates, we compared the enzyme activities of SHs purified from sensitive and resistant isolates. Enzyme kinetics analyses revealed that the SHs from MBI-D-sensitive and MBI-D-resistant isolates had Michaelis constant values of 53.2 ± 14.6 and 44.3 ± 9.9 μmol, respectively, maximal velocity values of 20.0 ± 2.1 and 10.2 ± 0.9 μmol min−1, respectively, and catalytic constant values of 38.2 ± 4.1 and 19.6 ± 1.7 s−1, respectively. Thus, the activity of MBI-D-sensitive-type SHs was approximately double that of MBI-D-resistant-type SHs. The sensitive-type SHs had higher scytalone dehydration activity than that of resistant-type SHs at temperatures ranging from 15 to 35 °C and at pHs ranging from 6.0 to 9.0. Thus, the sensitive-type SH showed higher scytalone dehydration activity than that of the resistant-type SH under the natural environmental conditions of M. oryzae. In infection tests, the MBI-D-sensitive isolates infected rice leaves faster than did the resistant isolates. Thus, the acquisition of resistance to MBI-D has negatively affected the pathogenicity of M. oryzae due to decreased SH activity. These results have implications for disease management programs.