Guard cell sensory systems: recent insights on stomatal responses to light, abscisic acid, and CO2

• Environmental regulation of stomatal apertures invokes complex signaling networks. • Blue light-induced stomatal opening involves starch and triacylglycerol degradation. • Anion channel phosphorylation by kinases is a key guard cell regulatory mechanism. • SNRK2-dependent anion channel activation evolved in a moss/tracheophyte ancestor. • Regulatory components in guard cell CO 2 response are being elucidated. By controlling the opening and closure of the stomatal pores through which gas exchange occurs, guard cells regulate two of the most important plant physiological processes: photosynthesis and transpiration. Accordingly, guard cells have evolved exquisite sensory systems. Here we summarize recent literature on guard cell sensing of light, drought ( via the phytohormone abscisic acid (ABA)), and CO 2 . New advances in our understanding of how guard cells satisfy the energetic and osmotic requirements of stomatal opening and utilize phosphorylation to regulate the anion channels and aquaporins involved in ABA-stimulated stomatal closure are highlighted. Omics and modeling approaches are providing new information that will ultimately allow an integrated understanding of guard cell physiology.