<scp>MITOGEN</scp> ‐ <scp>ACTIVATED PROTEIN KINASE9</scp> fine‐tunes hypoxia signaling by phosphorylating <scp>ERF</scp> ‐ <scp>VII</scp> transcription factors in Arabidopsis

In plants, hypoxia sensing is controlled by the stabilization of group VII ethylene response factors (ERF-VIIs), which are post-translationally activated by the mitogen-activated protein kinases (MAPKs) MPK3 and MPK6. However, how plants fine-tune the MPK3/MPK6-mediated phosphorylation of ERF-VIIs to maintain cellular homeostasis remains largely unknown. Here, we show that the Arabidopsis thaliana TDY-type MAPK MPK9 is rapidly activated under hypoxia and submergence conditions. MPK9 knockout mutants show enhanced tolerance to hypoxia, whereas transgenic lines overexpressing MPK9 display decreased tolerance to hypoxia. MPK9 interacts with and phosphorylates ERF-VII RELATED TO APETALA2.12 (RAP2.12) at its Thr-86, Ser-87, Thr-88, Ser-210, Thr-280, and Thr-313 residues, which suppresses RAP2.12 accumulation in response to hypoxia. Consistent with this finding, overexpressing MPK9 significantly attenuated the hypoxia-tolerant phenotypes of RAP2.12 overexpressors. Compared with the hypoxia-triggered nucleus-localized MPK6-GFP, MPK9-GFP constitutively localized to the cytoplasm. Cytoplasmic MPK9 likely functions in hypoxia signaling by phosphorylating RAP2.12 to fine-tune its stability during hypoxia stress. Taken together with the finding that the MPK3/MPK6-ERF-VII module activates hypoxia signaling, our observations demonstrate that MPK9 provides a braking mechanism that tempers hypoxia responses, which may play a central role in protecting plants from hypoxic stress.