Fig 1.
The Pmk1 and Mps1 pathways and their relationship with cAMP signaling in Magnaporthe oryzae.
A. Distinct and overlapping functions of the cAMP-PKA pathway and Pmk1 and Mps1 MAPK cascades during plant infection. B. Physical and chemical signals known to trigger appressorium formation include surface hydrophobicity and hardness, cutin monomers, plant surface waxes, and primary alcohols. Msb2, Sho1, Pth11, and Cbp1 are involved in recognizing extracellular or surface signals to activate the downstream cAMP-PKA pathway and Mst11-Mst7-Pmk1 MAPK cascade. Both the trimeric G-proteins and Ras monomeric G-proteins are functionally related to these two pathways that regulate appressorium formation, penetration, and invasive growth. Although its upstream sensors have not been characterized, the Bck1-Mkk2-Mps1 cascade likely functions downstream from PKC and is important for sporulation, appressorium penetration, and pathogenesis via downstream Mig1, Swi6, and possibly other transcription factors. The adapter protein Mst50 is involved in both Pmk1 and Mps1 pathways. Mip11 is a RACK protein that interacts with both Mst50 and Mck1. Pmk1 positively regulates Mst12, Mcm1, Sfl1, and likely other transcription factors during different infection processes. Together with the Cyc8-Tup1 corepressor complex, Sfl1 also functions as a transcriptional repressor for hyphal growth. cAMP, cyclic adenosine monophosphate; MAPK, MAP kinase; PKA, protein kinase A; PKC, protein kinase C; RACK, receptor for activated C kinase.
Table 1.
MAP kinases characterized in plant pathogenic fungi.
Fig 2.
Functional diversity of the HOG pathway in yeast and plant-pathogenic fungi.
Schematic model of the HOG pathway that is mainly involved in hyperosmoregulation in S. cerevisiae. Although lacking redundant MEK kinases, filamentous ascomycetes generally have orthologs of all these conserved components of the Hog1 pathway, including MEKK, MEK, and upstream phosphorelay and sensor proteins. Besides its conserved role in osmoregulation, this pathway has species-specific functions in pathogenesis, vegetative growth, fungicide sensitivity, sexual and asexual development, and responses to oxidative, cell wall, and other stresses in different plant pathogenic fungi. Filled and empty circles indicate that the Hog1/OS-2 kinase is important or dispensable, respectively, for specific functions characterized in different fungi. HOG, high-osmolarity glycerol; MEK, MAPK kinase; MEKK, MEK kinase.