CBL-interacting protein kinase 25 contributes to root meristem development

Abstract

Coordination of auxin and cytokinin activities determines root meristem size during post-embryonic development. Calcineurin B-like proteins (CBLs) and their interacting protein kinases (CIPKs) constitute signaling modules that relay calcium signals. Here we report that CIPK25 is involved in regulating the root meristem size. Arabidopsis plants lacking CIPK25 expression displayed a short root phenotype and a slower root growth rate with less meristem cells. This phenotype was rescued by restoration of CIPK25 expression. CIPK25 interacted with CBL4 and -5, and displayed a strong gene expression in the flower and root except in the cell proliferation domain in the root apical meristem. Its expression in root was positively and negatively regulated by auxin and cytokinin, respectively. cipk25 T-DNA insertion line was compromised in auxin transport and auxin-responsive promoter activity. cipk25 mutant line showed altered expression of auxin efflux carriers and an Aux/IAA family gene SHY2. Decreased PIN1 and PIN2 expression in cipk25 mutant line was completely restored when combined with SHY2 loss-of-function mutation resulting in recovery of root growth. SHY2 and PIN1 expression was partially regulated by cytokinin even in absence of CIPK25, suggesting existence of CIPK25-independent cytokinin signaling pathway(s) as well. Our results suggested a role of CIPK25 in root meristem development.