Group IIc WRKY transcription factors regulate cotton resistance to Fusarium oxysporum by promoting GhMKK2-mediated flavonoid biosynthesis

WRKY transcription factors (TFs) are crucial regulators in response to pathogen infection. However, the regulatory mechanisms of WRKY TFs in response to Fusarium oxysporum f. sp. vasinfectum (Fov), the most devastating pathogen of cotton, remain unclear. Here, transcriptome sequencing indicated that the group ΙΙc WRKY TF subfamily was the most important TF subfamily in response to Fov. Gain-of-function and loss-of-function analyses showed that group ΙΙc WRKY TFs positively regulated cotton resistance to Fov. A series of ChIP-seq, yeast one-hybrid assay and EMSA experiments indicated that group ΙΙc WRKY TFs directly bound to the promoter of GhMKK2 and regulated its expression. Importantly, a novel MAPK cascade composed of GhMKK2, GhNTF6 and GhMYC2 was identified. The functional analysis indicated that group ΙΙc WRKY TFs induced the GhMKK2-GhNTF6 pathway to increase resistance to Fov by upregulating the GhMYC2-mediated expression of several flavonoid biosynthesis-related genes, which led to flavonoids accumulation. In conclusion, our study demonstrated a novel disease defense mechanism by which the WRKY-MAPK pathway promotes flavonoid biosynthesis to defend against pathogen infection. This pathway improves our understanding of the interaction mode between WRKY TFs and MAPK cascades in plant immunity and the vital role of plant flavonoids in pathogen defense.