Abstract
In the rice blast fungus Magnaporthe oryzae, the cAMP-PKA pathway regulates surface recognition, appressorium turgor generation, and invasive growth. However, deletion of CPKA failed to block appressorium formation and responses to exogenous cAMP. In this study, we generated and characterized the cpk2 and cpkA cpk2 mutants and spontaneous suppressors of cpkA cpk2 in M. oryzae. Our results demonstrate that CPKA and CPK2 have specific and overlapping functions, and PKA activity is essential for appressorium formation and plant infection. Unlike the single mutants, the cpkA cpk2 mutant was significantly reduced in growth and rarely produced conidia. It failed to form appressoria although the intracellular cAMP level and phosphorylation of Pmk1 MAP kinase were increased. The double mutant also was defective in plant penetration and Mps1 activation. Interestingly, it often produced fast-growing spontaneous suppressors that formed appressoria but were still non-pathogenic. Two suppressor strains of cpkA cpk2 had deletion and insertion mutations in the MoSFL1 transcription factor gene. Deletion of MoSFL1 or its C-terminal 93-aa (MoSFL1ΔCT) was confirmed to suppress the defects of cpkA cpk2 in hyphal growth but not appressorium formation or pathogenesis. We also isolated 30 spontaneous suppressors of the cpkA cpk2 mutant in Fusarium graminearum and identified mutations in 29 of them in FgSFL1. Affinity purification and co-IP assays showed that this C-terminal region of MoSfl1 was essential for its interaction with the conserved Cyc8-Tup1 transcriptional co-repressor, which was reduced by cAMP treatment. Furthermore, the S211D mutation at the conserved PKA-phosphorylation site in MoSFL1 partially suppressed the defects of cpkA cpk2. Overall, our results indicate that PKA activity is essential for appressorium formation and proper activation of Pmk1 or Mps1 in M. oryzae, and phosphorylation of MoSfl1 by PKA relieves its interaction with the Cyc8-Tup1 co-repressor and suppression of genes important for hyphal growth.
Highlights
Magnaporthe oryzae is the causal agent of rice blast, which is one of the most important rice diseases worldwide
The terminal region of MoSfl1 was found to be essential for its interaction with the MoCyc8 co-repressor, which may be negatively regulated by PKA
Our results indicate that PKA activity is essential for appressorium formation in M. oryzae, and phosphorylation of MoSfl1 by PKA likely relieves its interaction with the Cyc8-Tup1 corepressor and suppression of genes important for hyphal growth and appressorium development
Summary
Magnaporthe oryzae is the causal agent of rice blast, which is one of the most important rice diseases worldwide. In the past two decades, M. oryzae has been developed as a model organism to study fungal-plant interactions because of its economic importance and the experimental tractability [1,2,3]. The narrow penetration peg differentiates into bulbous invasive hyphae [5] that grow biotrophically inside penetrated plant cells [6]. Various apoplastic and cytoplasmic effectors are known to play critical roles in suppressing plant defense responses during different stages of invasive growth [7]. Lesions are formed and the pathogen produces conidiophores and conidia on diseased plant tissues under favorable conditions
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