PKA activity is essential for relieving the suppression of hyphal growth and appressorium formation by MoSfl1 in <i>Magnaporthe oryzae</i>

<div><p>In the rice blast fungus <i>Magnaporthe oryzae</i>, the cAMP-PKA pathway regulates surface recognition, appressorium turgor generation, and invasive growth. However, deletion of <i>CPKA</i> failed to block appressorium formation and responses to exogenous cAMP. In this study, we generated and characterized the <i>cpk2</i> and <i>cpkA cpk2</i> mutants and spontaneous suppressors of <i>cpkA cpk2</i> in <i>M</i>. <i>oryzae</i>. Our results demonstrate that <i>CPKA</i> and <i>CPK2</i> have specific and overlapping functions, and PKA activity is essential for appressorium formation and plant infection. Unlike the single mutants, the <i>cpkA cpk2</i> 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 <i>cpkA cpk2</i> had deletion and insertion mutations in the <i>MoSFL1</i> transcription factor gene. Deletion of <i>MoSFL1</i> or its C-terminal 93-aa (<i>MoSFL1</i>^ΔCT) was confirmed to suppress the defects of <i>cpkA cpk2</i> in hyphal growth but not appressorium formation or pathogenesis. We also isolated 30 spontaneous suppressors of the <i>cpkA cpk2</i> mutant in <i>Fusarium graminearum</i> and identified mutations in 29 of them in <i>FgSFL1</i>. 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 <i>MoSFL1</i> partially suppressed the defects of <i>cpkA cpk2</i>. Overall, our results indicate that PKA activity is essential for appressorium formation and proper activation of Pmk1 or Mps1 in <i>M</i>. <i>oryzae</i>, and phosphorylation of MoSfl1 by PKA relieves its interaction with the Cyc8-Tup1 co-repressor and suppression of genes important for hyphal growth.</p>