CO2 Sensing and CO2 Regulation of Stomatal Conductance: Advances and Open Questions

Guard cells form epidermal stomatal gas-exchange valves in plants and regulate the aperture of stomatal pores in response to changes in the carbon dioxide (CO2) concentration ([CO2]) in leaves. Moreover, the development of stomata is repressed by elevated CO2 in diverse plant species. Evidence suggests that plants can sense [CO2] changes via guard cells and via mesophyll tissues in mediating stomatal movements. We review new discoveries and open questions on mechanisms mediating CO2-regulated stomatal movements and CO2 modulation of stomatal development, which together function in the CO2 regulation of stomatal conductance and gas exchange in plants. Research in this area is timely in light of the necessity of selecting and developing crop cultivars that perform better in a shifting climate. Research on how plant guard cells, which form epidermal stomatal gas-exchange valves, regulate the aperture of stomatal pores in response to changes in [CO2] is of current interest given the necessity of selecting and developing crop cultivars that perform better in a shifting global climate. Understanding of the underlying CO2 response mechanisms is also needed for modeling efforts to better understand plant responses to rising atmospheric CO2 levels. Recent discoveries in guard cell CO2 and secondary messenger signaling, the contributions of the subcellular localization of the CO2-binding carbonic anhydrases, the interplay with the stress hormone ABA, and the role of photosynthesis in stomatal responses to the CO2 stimulus indicate new models and raise new open questions in CO2 signal transduction. Elucidation of the molecular mechanisms controlling stomatal development and the identification of initial mechanisms mediating elevated CO2 repression of stomatal development points to a signaling model and to new avenues for further research on this pathway.