ABSTRACT
Calcium-dependent protein kinases (CDPKs/CPKs) are key regulators of plant stress signaling that translate calcium signals into cellular responses by phosphorylating diverse substrate proteins. However, the molecular mechanism by which plant cells relay calcium signals in response to hypoxia remains elusive. Here, we show that one member of the CDPK family in Arabidopsis thaliana, CPK12, is rapidly activated during hypoxia through calcium-dependent phosphorylation of its Ser-186 residue. Phosphorylated CPK12 shuttles from the cytoplasm to the nucleus, where it interacts with and phosphorylates the group VII ethylene-responsive transcription factors (ERF-VII) that are core regulators of plant hypoxia sensing, to enhance their stabilities. Consistently, CPK12 knockdown lines show attenuated tolerance of hypoxia, whereas transgenic plants overexpressing CPK12 display improved hypoxia tolerance. Nonethelss, loss of function of five ERF-VII proteins in an erf-vii pentuple mutant could partially suppress the enhanced hypoxia-tolerance phenotype of CPK12-overexpressing lines. Moreover, we also discovered that phosphatidic acid and 14-3-3κ protein serve as positive and negative modulators of the CPK12 cytoplasm-to-nucleus translocation, respectively. Taken together, these findings uncover a CPK12-ERF-VII regulatory module that is key to transducing calcium signals from the cytoplasm into the nucleus to potentiate hypoxia sensing in plants.
