Sensory circuitry controls cytosolic calcium-mediated phytochrome B phototransduction.

Although Ca has long been recognized as an obligatory intermediate in visual transduction, its role in plant phototransduction remains elusive. Here, we report a Ca signaling that controls photoreceptor phyB nuclear translocation in etiolated seedlings during dark-to-light transition. Red light stimulates acute cytosolic Ca increases via phyB, which are sensed by Ca-binding protein kinases, CPK6 and CPK12 (CPK6/12). Upon Ca activation, CPK6/12 in turn directly interact with and phosphorylate photo-activated phyB at Ser80/Ser106 to initiate phyB nuclear import. Non-phosphorylatable mutation, phyB, abolishes nuclear translocation and fails to complement phyB mutant, which is fully restored by combining phyB with a nuclear localization signal. We further show that CPK6/12 function specifically in the early phyB-mediated cotyledon expansion, while Ser80/Ser106 phosphorylation generally governs phyB nuclear translocation. Our results uncover a biochemical regulatory loop centered in phyB phototransduction and provide a paradigm for linking ubiquitous Ca increases to specific responses in sensory stimulus processing.