Molecular basis for loss of virulence in Magnaporthe oryzae strain AM16

The rapid virulence variation of <i>Magnaporthe oryzae</i> (<i>M. oryzae</i>) to rice is a big challenge for rice blast control. Even though many studies have been done by scientists all over the world, the mechanism of virulence variation in <i>M. oryzae</i> remains elusive. AM16, an avirulent <i>M. oryzae</i> strain reported in our previous study, provides an excellent entry point to explore the mechanism of virulence variation in <i>M. oryzae</i>. In this study, we found that the <i>Pmk1</i> and <i>Mac1</i> had specific mutations in strain AM16. The AM16 strains overexpressing <i>Pmk1</i> ^Guy11 or (and) <i>Mac1</i> ^Guy11 allele from strain Guy11 displayed significantly increasing conidiation, functional appressorium formation, and restoring pathogenicity to rice. Moreover, we observed that the strains overexpressing <i>Mac1</i> ^Guy11 had stronger conidia forming capacity than that of the strains overexpressing <i>Pmk1</i> ^Guy11, while the appressorium formation rate of strains overexpressing <i>Pmk1</i> ^Guy11 was similar to that of strains overexpressing <i>Pmk1</i> ^Guy11-<i>Mac1</i> ^Guy11, much higher than that of the strains overexpressing <i>Mac1</i> ^Guy11. Taken together, our results reveal that the natural mutation of <i>Pmk1</i> and <i>Mac1</i> genes are important, but not the sole cause, for the loss of virulence in strain AM16. The functional difference between Pmk1 and Mac1 in the growth and development of <i>M. oryzae</i> was first discovered, providing new insight into the pathogenic mechanism of <i>M. oryzae</i>.