Drought is an environmental stress that causes severe crop loss. Drought stress can induce abscisic acid (ABA) accumulation and cytoplasmic calcium oscillation. Calcium-dependent protein kinases (CPKs) constitute a group of Ser/Thr protein kinases decoding calcium signals. However, the function and molecular mechanisms of most CPKs in oilseed rape (Brassica napus) remain unknown. Here, we report the functional characterization of BnaCPK5 in drought stress tolerance. BnaCPK5 belongs to Group I of the CPK family and was localized at the plasma membrane and nuclei. Overexpression of BnaCPK5 enhanced drought stress tolerance compared with the control. A screening of interacting proteins identified that BnaCPK5 interacted strongly with two ABA-Responsive Element Binding Factors (ABF/AREBs), BnaABF3 and BnaABF4. BnaCPK5 was shown to phosphorylate both BnaABF3 and BnaABF4 in a kinase assay. Further, it was found that the phosphorylation of BnaABF3 and BnaABF4 by BnaCPK5 increased their transcriptional activities against the famous drought stress marker gene, Responsive to Dehydration (RD) 29B and protein stability. Taken together, these data demonstrate that BnaCPK5 acts as a positive regulator of drought tolerance by, at least in part, phosphorylating two core ABA-signaling components to modulate Late-Embryogenesis Abundant (LEA)-like RD29B expression.
Keywords: ABA-Responsive Element Binding Factors; Brassica napus; Calcium-dependent protein kinase; Drought; Phosphorylation.
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