MYB44 regulates PTI by promoting the expression of EIN2 and MPK3/6 in Arabidopsis

Plant signaling pathway regulating pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) essentially involves mitogen-activated protein kinase (MPK) cascades that comprise sequential activation of several protein kinases and the ensuing phosphorylation of MPKs, which activate transcription factors (TFs) to promote the downstream defense responses. To identify plant TFs that regulate MPKs, we investigated TF-defective mutants of Arabidopsis thaliana and identified MYB44 as an essential constituent of the PTI pathway. MYB44 confers resistance against the bacterial pathogen Pseudomonas syringae by cooperation with MPK3 and MPK6. Under PAMP treatment, MYB44 binds to the promoters of both MPK3 and MPK6 to activate their expression, leading to the phosphorylation of MPK3 and MPK6 proteins. In turn, phosphorylated MPK3 and MPK6 phosphorylate MYB44 in a functionally redundant manner, thus enabling MYB44 to activate MPK3 and MPK6 expression and to further activate the downstream defense responses. Activation of defense responses has also been attributed to the activation of transcription of the EIN2 gene by MYB44, which has been previously shown to affect PAMP recognition and PTI development. In essence, AtMYB44 functions as an integral component of the PTI pathway by connecting transcriptional and posttranscriptional regulation of the MPK3/6 cascade.