The Calcineurin B-Like Ca2+ Sensors CBL1 and CBL9 Function in Pollen Germination and Pollen Tube Growth in Arabidopsis

ABSTRACTCa2+ has been established as an important second messenger regulating pollen germination and tube growth. However, to date, only a few signaling components have been identified to decode and relay Ca2+ signals in growing pollen tubes. Here, we report a function for the calcineurin B-like (CBL) Ca2+ sensor proteins CBL1 and CBL9 from Arabidopsis in pollen germination and tube growth. Both proteins are expressed in mature pollen and pollen tubes and impair pollen tube growth and morphology if transiently overexpressed in tobacco pollen. The induction of these phenotypes requires efficient plasma membrane targeting of CBL1 and is independent of Ca2+ binding to the fourth EF-hand of CBL1. Overexpression of CBL1 or its closest homolog CBL9 in Arabidopsis renders pollen germination and tube growth hypersensitive towards high external K+ concentrations while disruption of CBL1 and CBL9 reduces pollen tube growth under low K+ conditions. Together, our data identify a crucial function for CBL1 and CBL9 in pollen germination and tube growth and suggest a model in which both proteins act at the plasma membrane through regulation of K+ homeostasis.