Guard cell signal transduction mechanisms controlling stomatal movements by light, abscisic acid, CO2, VPD and heat.

Guard cell pairs form dynamic stomatal gas exchange pores in aerial tissues of plants. Guard cells regulate CO2 influx and water loss through stomata by integrating multiple environmental and internal signals. Guard cells serve as highly specialized sensory and signal processing cells, thereby orchestrating stomatal movements in response to changing conditions. This review describes recent advances and early discoveries in elucidation of guard cell signaling networks. Present day understanding of the molecular and cellular mechanisms mediating stomatal opening by blue light, red light, heat stress and photosynthesis-mediated low CO2 in intercellular leaf spaces are reviewed. Furthermore, up-to-date knowledge of the molecular mechanisms and pathways that trigger stomatal closing by abscisic acid (ABA), elevated CO2 and low humidity/high vapor pressure difference is synthesized. These pathways promote stomatal closure and water conservation. Critical functions of other tissues are described, including mesophyll cells as producers of messengers for red light-induced stomatal opening and vascular tissues in drought-induced ABA synthesis and transport. Guard cell ion channels and pumps drive the ion fluxes required for the cellular osmotic motor that opens and closes stomatal pores. Roles of and recent advances toward understanding the regulatory mechanisms of these membrane transporters as downstream targets of guard cell signaling cascades are described. Synthesis of these signaling cascades into signaling networks is reviewed and open questions for future research are highlighted throughout. Understanding these integrated mechanisms is critical for future improvement of plant water use efficiency and environmental stress resilience of crops.