Divergent cAMP Signaling Pathways Regulate Growth and Pathogenesis in the Rice Blast Fungus Magnaporthe grisea

Abstract

cAMP is involved in signaling appressorium formation in the rice blast fungus Magnaporthe grisea. However, null mutations in a protein kinase A (PKA) catalytic subunit gene, CPKA, do not block appressorium formation, and mutations in the adenylate cyclase gene have pleiotropic effects on growth, conidiation, sexual development, and appressorium formation. Thus, cAMP signaling plays roles in both growth and morphogenesis as well as in appressorium formation. To clarify cAMP signaling in M. grisea, we have identified strains in which a null mutation in the adenylate cyclase gene (MAC1) has an unstable phenotype such that the bypass suppressors of the Mac1(-) phenotype (sum) could be identified. sum mutations completely restore growth and sexual and asexual morphogenesis and lead to an ability to form appressoria under conditions inhibitory to the wild type. PKA assays and molecular cloning showed that one suppressor mutation (sum1-99) alters a conserved amino acid in cAMP binding domain A of the regulatory subunit gene of PKA (SUM1), whereas other suppressor mutations act independently of PKA activity. PKA assays demonstrated that the catalytic subunit gene, CPKA, encodes the only detectable PKA activity in M. grisea. Because CPKA is dispensable for growth, morphogenesis, and appressorium formation, divergent catalytic subunit genes must play roles in these processes. These results suggest a model in which both saprophytic and pathogenic growth of M. grisea is regulated by adenylate cyclase but different effectors of cAMP mediate downstream effects specific for either cell morphogenesis or pathogenesis.

A Mitogen-Activated Protein Kinase Cascade Regulating Infection-Related Morphogenesis in Magnaporthe grisea

“…The pmk1, mst7, and mst11 deletion mutants had intracellular cAMP levels comparable with that of the wild-type strain ( Table 3), suggesting that deletion of PMK1, MST7, or MST11 did not have significant effects on the cAMP signaling pathway. However, in transformants DA5 and DA3-4 expressing the MST7 S212D T216E allele, the intracellular cAMP level was significantly lower than that of the wild-type strain 70-15 (Table 3) but higher than that of the mac1 mutant with the adenylate cyclase gene deleted (Adachi and Hamer, 1998). Therefore, expression of the MST7 S212D T216E allele likely had a negative effect on the cAMP signaling pathway.…”

PDE1 Encodes a P-Type ATPase Involved in Appressorium-Mediated Plant Infection by the Rice Blast Fungus Magnaporthe grisea

“…Strong attachment of the germ tube to the leaf surface and alterations in cell wall conformation, mediated in part by the MPG1 hydrophobin, contribute to surface perception (Hamer et al, 1988;Talbot et al, 1996), and the outcome is rapid activation of a G-protein-adenylate cyclase-cAMP-dependent signaling pathway. This, in turn, triggers multiple mitogen-activated protein kinase cascades, resulting in differentiation of the appressorium, transport of lipid and carbohydrate reserves to the infection cell, and generation of appressorial turgor (Mitchell and Dean, 1995;Xu and Hamer, 1996;Choi and Dean, 1997;Liu and Dean, 1997;Xu et al, 1997Xu et al, , 1998Adachi and Hamer, 1998;Dixon et al, 1999;Thines et al, 2000). How appressorium development culminates in plant infection, however, is less clear because the genetic components required for penetration peg formation have not yet been identified.…”

Regulatory network of genes associated with stimuli sensing, signal transduction and physiological transformation of appressorium inMagnaporthe oryzae

“…Also, excessive accumulation of intracellular cAMP was detected in the RGS gene deletion mutant and in the point mutant of magA G187S (RGS insensitive Gαs) or magA Q208L (without GTPase activity) that generated appressorium on a hydrophilic surface (Liu et al 2007). Upon detection of extracellular stimuli like surface hydrophobicity, Gα subunit dissociates from the G-proteins complex and binds to the membrane-bound Adenylate cyclase ( MAC 1) which in turn is activated and converts ATP to cAMP (Adachi and Hamer 1998; Deka et al 2016). One of the most well-known cell-surface membrane proteins that integrate the hydrophobic signal is PTH 11, whose gene deletion mutant was non-pathogenic to the host due to a defect in appressorium differentiation (DeZwaan et al 1999).…”

“…Generation of an intracellular cAMP signal is essential for successful initiation of appressorium differentiation on an inductive surface (Adachi and Hamer 1998). The cAMP/PKA signal cascade is further mediated via its binding with the PKA regulatory subunit SUM 1, allowing the release of the PKA catalytic subunit for phosphorylating the downstream targets in the nucleus like transcriptional factors including CDTF 1, SOM 1, MSTU 1 in M. oryzae (Filippi 2004) and SFL 1 in budding yeast (Song and Carlson 1998; Galeote et al 2007).…”

“…Though lipid translocation from conidia to appressoria was unaffected, lipolysis did not take place during maturation process in the CpkA mutant (Mitchell and Dean 1995). The ∆ sum 1-99 mutant possessing a mutation in the regulatory subunit of PKA exhibited a different phenotype in lipid mobilisation and utilisation during the appressorium development, as the process took place within 12 h of induction (Adachi and Hamer 1998). …”