Zm CPK 1, a calcium‐independent kinase member of the Z ea mays CDPK gene family, functions as a negative regulator in cold stress signalling

Calcium‐dependent protein kinases ( CDPKs ) have been shown to play important roles in plant environmental stress signal transduction. We report on the identification of Zm CPK 1 as a member of the maize ( Z ea mays ) CDPK gene family involved in the regulation of the maize cold stress response. Based upon in silico analysis of the Z .  mays cv. B 73 genome, we identified that the maize CDPK gene family consists of 39 members. Two CDPK members were selected whose gene expression was either increased ( Zm cpk 1) or decreased ( Zm cpk 25) in response to cold exposure. Biochemical analysis demonstrated that Zm CPK 1 displays calcium‐independent protein kinase activity. The C ‐terminal calcium‐binding domain of Zm CPK 1 was sufficient to mediate calcium independency of a previously calcium‐dependent enzyme in chimeric Zm CPK 25‐ CPK 1 proteins. Furthermore, co‐transfection of maize mesophyll protoplasts with active full‐length Zm CPK 1 suppressed the expression of a cold‐induced marker gene, Zm erf3 ( ZmCOI 6.21 ). In accordance, heterologous overexpression of Zm CPK 1 in A rabidopsis thaliana yielded plants with altered acclimation‐induced frost tolerance. Our results identify Zm CPK 1 as a negative regulator of cold stress signalling in maize.