Sensory circuitry controls cytosolic calcium-mediated phytochrome B phototransduction

Although Ca2+ has long been recognized as an obligatory intermediate in visual transduction, its role in plant phototransduction remains elusive. Here, we report a Ca2+ signaling that controls photoreceptor phyB nu-clear translocation in etiolated seedlings during dark-to-light transition. Red light stimulates acute cytosolic Ca2+ increases via phyB, which are sensed by Ca2+-binding protein kinases, CPK6 and CPK12 (CPK6/12). Upon Ca2+ activation, CPK6/12 in turn directly interact with and phosphorylate photo-activated phyB at Ser80/Ser106 to initiate phyB nuclear import. Non-phosphorylatable mutation, phyBS80A/S106A, abolishes nu-clear translocation and fails to complement phyB mutant, which is fully restored by combining phyBS80A/S106A with a nuclear localization signal. We further show that CPK6/12 function specifically in the early phyB-medi-ated cotyledon expansion, while Ser80/Ser106 phosphorylation generally governs phyB nuclear transloca-tion. Our results uncover a biochemical regulatory loop centered in phyB phototransduction and provide a paradigm for linking ubiquitous Ca2+ increases to specific responses in sensory stimulus processing.

Automated summary

A study found that Ca2+ signaling in photosynthetic plants controls the nuclear translocation of photoreceptor phyB during the transition from darkness to light. Red light stimulates cytosolic Ca2+ increases via phyB, which are sensed by Ca2+-binding protein kinases CPK6 and CPK12. Upon Ca2+ activation, CPK6/12 directly interact with and phosphorylate photo-activated phyB at Ser80/Ser106 to initiate phyB nuclear import. This research uncovers a biochemical regulatory loop in phyB phototransduction and provides a paradigm for connecting ubiquitous Ca2+ increases to specific responses in sensory stimulus processing.