Phosphorylation of the CAMTA3 transcription factor triggers its destabilization and nuclear export.
The calmodulin-binding transcription activator 3 (CAMTA3) is a repressor of immunity-related genes but an activator of cold-induced genes in plants. Post-transcriptional or -translational mechanisms have been proposed to control CAMTA3's role in the crosstalk between immune and chilling responses. Here, we show that treatment with the bacterial flg22 elicitor, but not cold stress, induces a phospho-mobility shift of CAMTA3 proteins. Correspondingly, CAMTA3 is directly phosphorylated by two flg22-responsive mitogen-activated protein kinases (MAPKs), MPK3 and MPK6, which triggers CAMTA3 nuclear export and destabilization. SR1IP1, a substrate E3 ubiquitin ligase adaptor required for pathogen-induced CAMTA3 degradation, is shown here to be likely plasma-membrane-localized and therefore cannot physically interact with the nuclear CAMTA3. Despite the flg22-inducible re-localization of CAMTA3 to the cytoplasm, we failed to detect CAMTA3-SR1IP1 complexes. Hence, the role of SR1IP1 for CAMTA3 degradation needs to be re-evaluated. Surprisingly, flg22 elicitation can still induce nuclear export and phospho-mobility shift of a phospho-null CAMTA3 that cannot be phosphorylated by MAPKs, suggesting the participation of additional flg22-responsive kinase(s). A constitutively-active calcium-dependent protein kinase, CPK5, can stimulate a phospho-mobility shift in CAMTA3 similar to that induced by flg22. Although CPK5 can interact with CAMTA3, it did not directly phosphorylate CAMTA3, suggesting the requirement of a still unidentified downstream kinase or additional components. Overall, at least two flg22-responsive kinase pathways target CAMTA3 to induce degradation that presumably serves to remove CAMTA3 from target promoters and de-repress expression of defence genes.
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