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.