The role of Starch Metabolism and Guard Cell Photosynthesis in CO₂ Regulation of Stomatal Conductance

Carbon dioxide (CO₂) is one of the main regulators of stomatal aperture. Few Arabidopsis mutants that are insensitive to CO₂ have been previously studied and have revealed parts of the mechanisms involved in CO₂ regulation of guard cell signal transduction. However, CO₂ regulation of stomatal conductance still requires further investigation. Here, an innovative stomatal movement analysis in response to CO₂ shifts was developed in order to study the role of starch metabolism and guard cell photosynthesis in CO₂ regulation of stomatal conductance. These analyses revealed that high [CO₂] induced stomatal closure was impaired in Arabidopsis mutants that cannot produce starch in general (adg1), but not in pgi1-1 Arabidopsis mutants that accumulate starch specifically in their guard cells, implying that functional starch metabolism solely in guard cells is sufficient for CO₂ regulation of stomatal conductance. Additionally, kidney- shaped stomata of all three Arabidopsis GC-Chlase[Delta]N transgenic lines (#4, #5, and #8) that featured severe chlorophyll deficiency specifically in their guard cells, responded to high [CO₂]-induction by stomatal closure, while their chlorophyll-less thin-shaped and collapsed stomata remained constitutively closed, suggesting that wild-type chlorophyll levels in guard cells are not required for CO₂ regulation of stomatal conductanc