Arabidopsis thalianaCML25 mediates the Ca2+ regulation of K+ transmembrane trafficking during pollen germination and tube elongation

The concentration alteration of cytosolic-free calcium ([Ca2+](cyt)) is a well-known secondary messenger in plants and plays important roles during pollen grain germination and tube elongation. Here we demonstrate that CML25, a member of calmodulin-like proteins, has Ca2+-binding activity and plays a role in pollen grain germination, tube elongation and seed setting. CML25 transcript was abundant in mature pollen grains and pollen tubes, and its product CML25 protein was primarily directed to the cytoplasm. Two independent CML25 loss-of-function T-DNA insertion mutants suffered a major reduction in both the rate of pollen germination and the elongation of the pollen tube. Also, pollen grains of cml25 mutants were less sensitive to the external K+ and Ca2+ concentration than wild-type pollen. The disruption of CML25 increased the [Ca2+](cyt) in both the pollen grain and the pollen tube, which in turn impaired the Ca2+-dependent inhibition of whole-cell inward K+ currents in protoplasts prepared from these materials (pollen grain and pollen tube). Complementation of cml25-1 mutant resulted in the recovery of wild-type phenotype. Our findings indicate that CML25 is an important transducer in the Ca2+-mediated regulation of K+ influx during pollen germination and tube elongation. Ca2+ is a ubiquitous secondary messenger and plays an essential role in regulation of pollen grain germination and pollen tube elongation, but the fundamental regulatory network downstream still remains largely unknown. In this study, the Arabidopsis CML25 (Calmodulin-Like protein 25), a member of predicated Ca2+ sensor family, was found to be involved in cytosolic Ca2+ regulation of K+ influx of pollen grains and pollen tubes, and subsequently to regulate pollen grain germination, pollen tube growth and seed setting rate positively. The physiological roles and the molecular mechanisms of CML25 in pollen grains and pollen tubes uncovers a new signal pathway downstream of Ca2+ signal.