Stomatal CO2 sensing in plants: control of gas exchange and interactions with environmental stimuli.

Stomatal pores in land plants rapidly and reversibly open and close in response to diurnal changes in leaf carbon dioxide (CO2) concentration. Studies have suggested that CO2 is sensed by guard cells with relevant amplifying contributions from mesophyll tissue. CO2 concentration changes trigger rapid signal transduction events involving protein phosphorylation in guard cells. Moreover, molecular crosstalk and physiological interactions of the stomatal CO2 response with other environmental conditions and stimuli, including light, temperature, drought, and abscisic acid, are reviewed here. Genetic studies have revealed several key genes and provided important insights into the stomatal CO2 sensors and signal transduction mechanisms. The primary CO2/HCO3- sensor in Arabidopsis guard cells was recently identified. Quantitative trait locus (QTL) analyses have shown that early guard cell CO2 signal transduction components regulate water use efficiency (WUE). In this review, we describe the molecular details of stomatal CO2 sensing by CO2/HCO3--induced interaction of two protein kinases, the HIGH LEAF TEMPERATURE 1 Raf-like kinase and the MPK4/MPK12 mitogen-activated protein kinases. The evolutionary emergence of, physiological relevance of, and potential for improvement of WUE of plants via the stomatal CO2 response and open questions in this research field are discussed.