MKD1, a novel MAPKKK, is required for disease resistance against Fusarium Species.

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MAPKKK delat-1 (MKD1), a novel Raf-like MAPKKK, was isolated as a subunit of a complex including the Arabidopsis transcription factor AtNFXL1. An MKD1-dependent cascade positively regulates disease resistance against PstDC3000 and the trichothecene mycotoxin-producing fungal pathogen Fusarium sporotrichioides. MKD1 directly interacted with MKK1 and MKK5 in vivo, and phosphorylated MKK1 and MKK5 in vitro. Correspondingly, mkk1 mutant and MKK5RNAi transgenic plants showed enhanced susceptibility to F. sporotrichioides. MKD1 was required for full activation of two MAPKs (MPK3 and MPK6) by the T-2 toxin. Finally, quantitative phosphoproteomics revealed that an MKD1-dependent cascade controlled phosphorylation of a disease resistance protein, SUMOs, and a mycotoxin-detoxifying enzyme. Our findings reveal that the MKD1-MKK1/MKK5-MPK3/MPK6-dependent signalling cascade plays an important role in plant immune responses against both bacterial and fungal phytopathogens.