The Calcineurin B-Like Calcium Sensors CBL1 and CBL9 Together with Their Interacting Protein Kinase CIPK26 Regulate the Arabidopsis NADPH Oxidase RBOHF

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Stimulus-specific accumulation of second messengers like reactive oxygen species (ROS) and Ca2+ are central to many signaling and regulation processes in plants. However, mechanisms that govern the reciprocal interrelation of Ca2+ and ROS signaling are only beginning to emerge. NADPH oxidases of the respiratory burst oxidase homolog (RBOH) family are critical components contributing to the generation of ROS while Calcineurin B-like (CBL) Ca2+ sensor proteins together with their interacting kinases (CIPKs) have been shown to function in many Ca2+- signaling processes. In this study, we identify direct functional interactions between both signaling systems. We report that the CBL-interacting protein kinase CIPK26 specifically interacts with the N-terminal domain of RBOHF in yeast two-hybrid analyses and with the full-length RBOHF protein in plant cells. In addition, CIPK26 phosphorylates RBOHF in vitro and co-expression of either CBL1 or CBL9 with CIPK26 strongly enhances ROS production by RBOHF in HEK293T cells. Together, these findings identify a direct interconnection between CBL–CIPK-mediated Ca2+ signaling and ROS signaling in plants and provide evidence for a synergistic activation of the NADPH oxidase RBOHF by direct Ca2+-binding to its EF-hands and Ca2+-induced phosphorylation by CBL1/9–CIPK26 complexes.

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