Functional characterization of the strawberry calcium-dependent protein kinase FvCPK28

Calcium-dependent protein kinases (CDPKs) translate cellular calcium signals into phosphorylation events that regulate plant development and immunity. In Arabidopsis thaliana, CPK28 restrains pattern-triggered immunity by limiting the accumulation of the receptor-like cytoplasmic kinase BOTRYTIS-INDUCED KINASE 1 (BIK1), and contributes to roles in development such as stem elongation. CPK28 orthologs have been described in several crops, but not in Rosaceae, a family that includes economically important fruits such as strawberry, apple, peach, and raspberry. Phylogenetic analysis of CDPKs and CDPK-related kinases (CRKs) in four Rosaceae species and Arabidopsis identified CPK28 orthologs forming a clade with Arabidopsis CPK28. Using the woodland strawberry (Fragaria vesca) ortholog, FvCPK28, as a case study, we show that FvCPK28 is a calcium-dependent kinase that phosphorylates both Arabidopsis AtBIK1 and a strawberry FvBIK1 homolog in vitro and associates with both proteins in planta. When expressed in the Arabidopsis cpk28-1 mutant, FvCPK28 restored stem elongation and immune responses to wild-type levels. These results show that the strawberry ortholog retains the biochemical and biological properties of CPK28. This work extends the functional study of CPK28 to the Rosaceae and identifies FvCPK28 as a candidate for modulating the balance between growth and defense in strawberry.