New Approaches to the Biology of Stomatal Guard Cells

CO(2) acts as an environmental signal that regulates stomatal movements. High CO(2) concentrations reduce stomatal aperture, whereas low concentrations trigger stomatal opening. In contrast to our advanced understanding of light and drought stress responses in guard cells, the molecular mechanisms underlying stomatal CO(2) sensing and signaling are largely unknown. Leaf temperature provides a convenient indicator of transpiration, and can be used to detect mutants with altered stomatal control. To identify genes that function in CO(2) responses in guard cells, CO(2)-insensitive mutants were isolated through high-throughput leaf thermal imaging. The isolated mutants are categorized into three groups according to their phenotypes: (i) impaired in stomatal opening under low CO(2) concentrations; (ii) impaired in stomatal closing under high CO(2) concentrations; and (iii) impaired in stomatal development. Characterization of these mutants has begun to yield insights into the mechanisms of stomatal CO(2) responses. In this review, we summarize the current status of the field and discuss future prospects.