The Arabidopsis immune receptor FLS2 senses the bacterial flagellin epitope flg22 to activate transient elevation of cytosolic calcium ions, production of reactive oxygen species (ROS), and other signaling events to coordinate antimicrobial defenses, such as stomatal closure that limits bacterial invasion. However, how FLS2 regulates these signaling events remains largely unknown. Here we show that the receptor-like cytoplasmic kinase BIK1, a component of the FLS2 immune receptor complex, not only positively regulates flg22-triggered calcium influx but also directly phosphorylates the NADPH oxidase RbohD at specific sites in a calcium-independent manner to enhance ROS generation. Furthermore, BIK1 and RbohD form a pathway that controls stomatal movement in response to flg22, thereby restricting bacterial entry into leaf tissues. These findings highlight a direct role of the FLS2 complex in the regulation of RbohD-mediated ROS production and stomatal defense.

As a master regulator of jasmonic acid (JA)–signaled plant immune responses, the basic helix-loop-helix (bHLH) Leu zipper transcription factor MYC2 differentially regulates different subsets of JA–responsive genes through distinct mechanisms. However, how MYC2 itself is regulated at the protein level remains unknown. Here, we show that proteolysis of MYC2 plays a positive role in regulating the transcription of its target genes. We discovered a 12-amino-acid element in the transcription activation domain (TAD) of MYC2 that is required for both the proteolysis and the transcriptional activity of MYC2. Interestingly, MYC2 phosphorylation at residue Thr328, which facilitates its turnover, is also required for the MYC2 function to regulate gene transcription. Together, these results reveal that phosphorylation-coupled turnover of MYC2 stimulates its transcription activity. Our results exemplify that, as with animals, plants employ an “activation by destruction” mechanism to fine-tune their transcriptome to adapt to their ever-changing environment.

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