Thioredoxins are involved in the activation of the PMK1 MAP kinase pathway during appressorium penetration and invasive growth in Magnaporthe oryzae

In Magnaporthe oryzae, the Mst11-Mst7-Pmk1 MAP kinase pathway is essential for appressorium formation and invasive growth. To determine their roles in Pmk1 activation and plant infection, we characterized the two thioredoxin genes, TRX1 and TRX2, in M. oryzae. Whereas the Delta trx1 mutants had no detectable phenotypes, deletion of TRX2 caused pleiotropic defects in growth, conidiation, light sensing, responses to stresses and plant infection progresses. The Delta trx1 Delta trx2 double mutant had more severe defects than the Delta trx2 mutant and was non-pathogenic in infection assays. The Delta trx2 and Delta trx1 Delta trx2 mutant rarely formed appressoria on hyphal tips and were defective in invasive growth after penetration. Pmk1 phosphorylation was barely detectable in the Delta trx2 and Delta trx1 Delta trx2 mutants. Deletion of TRX2 affected proper folding or intra-/inter-molecular interaction of Mst7 and expression of the dominant active MST7 allele partially rescued the defects of the Delta trx1 Delta trx2 mutant. Furthermore, Cys305 is important for Mst7 function and Trx2 directly interacts with Mst7 in co-IP assays. Our data indicated that thioredoxins play important roles in intra-cellular ROS signalling and pathogenesis in M. oryzae. As the predominant thioredoxin gene, TRX2 may regulate the activation of Pmk1 MAPK via its effects on Mst7.