Characterization of two group III potato CDPKs, StCDPK22 and StCDPK24, that contain three EF-Hand motifs in their CLDs.

Four members of the potato (Solanum tuberosum L.) calcium-dependent protein kinase (CDPK) family StCDPK22/23/24 and StCDPK27, present three functional EF-hands motifs in their calmodulin-like domain (CLD). StCDPK22/23/24 are clustered in clade III-b1 with tomato and Arabidopsis CDPKs that lack the first EF-hand motif, while StCDPK27 is clustered in clade III-b3 with CDPKs that lack EF-hand 2. Members of each clade share similar intron-exon structures and acylation profiles. 3D model predictions suggested that StCDPK22 and StCDPK24 are active kinases that undergo a conformational switch in the presence of Ca even when lacking one functional EF-hand motif; however, assays performed with recombinant proteins indicated that StCDPK24:6xHis was active in all the conditions tested, and its activity was enhanced in the presence of Ca, but StCDPK22:6xHis had scarce or null activity. Both kinases share with AtCPK8 the same autophosphorylation pattern in the autoinhibitory (AD) and C-terminal variable (CTV) domains, suggesting that it could be a characteristic of clade III-b1. RT-qPCR analysis revealed that StCDPK22 is mainly expressed in early stages of tuberization, but not limited to, while StCDPK24 expression is more ubiquitous. In silico analysis predicted several abiotic stress-responsive elements in its promoters. Accordingly, StCDPK24 expression peaked at 10 h in in vitro plants exposed to salt shock and then declined. Moreover, a significant increase was observed at 2 h in stems of salt-treated greenhouse plants, suggesting that this CDPK could participate in the early events of the signaling cascade triggered in response to salt.