Blue light and CO2 signals converge to regulate light-induced stomatal opening.

Stomata regulate gas exchange between plants and atmosphere by integrating opening and closing signals. Stomata open in response to low CO2 concentrations to maximize photosynthesis in the light; however, the mechanisms that coordinate photosynthesis and stomatal conductance have yet to be identified. Here we identify and characterize CBC1/2 (CONVERGENCE OF BLUE LIGHT (BL) AND CO2 1/2), two kinases that link BL, a major component of photosynthetically active radiation (PAR), and the signals from low concentrations of CO2 in guard cells. CBC1/CBC2 redundantly stimulate stomatal opening by inhibition of S-type anion channels in response to both BL and low concentrations of CO2. CBC1/CBC2 function in the signaling pathways of phototropins and HT1 (HIGH LEAF TEMPERATURE 1). CBC1/CBC2 interact with and are phosphorylated by HT1. We propose that CBCs regulate stomatal aperture by integrating signals from BL and CO2 and act as the convergence site for signals from BL and low CO2.