Carbonic anhydrases are upstream regulators of CO2-controlled stomatal movements in guard cells.

The continuing rise in atmospheric CO2 causes stomatal pores in leaves to close and thus globally affects CO2 influx into plants, water use efficiency and leaf heat stress. However, the CO2-binding proteins that control this response remain unknown. Moreover, which cell type responds to CO2, mesophyll or guard cells, and whether photosynthesis mediates this response are matters of debate. We demonstrate that Arabidopsis thaliana double-mutant plants in the beta-carbonic anhydrases betaCA1 and betaCA4 show impaired CO2-regulation of stomatal movements and increased stomatal density, but retain functional abscisic-acid and blue-light responses. betaCA-mediated CO2-triggered stomatal movements are not, in first-order, linked to whole leaf photosynthesis and can function in guard cells. Furthermore, guard cell betaca-overexpressing plants exhibit instantaneous enhanced water use efficiency. Guard cell expression of mammalian alphaCAII complements the reduced sensitivity of ca1 ca4 plants, showing that carbonic anhydrase-mediated catalysis is an important mechanism for betaCA-mediated CO2-induced stomatal closure and patch clamp analyses indicate that CO2/HCO3- transfers the signal to anion channel regulation. These findings, together with ht1-2 (ref. 9) epistasis analysis demonstrate that carbonic anhydrases function early in the CO2 signalling pathway, which controls gas-exchange between plants and the atmosphere.

CO(2) signaling in guard cells: calcium sensitivity response modulation, a Ca(2+)-independent phase, and CO(2) insensitivity of the gca2 mutant.

Leaf stomata close in response to high carbon dioxide levels and open at low CO(2). CO(2) concentrations in leaves are altered by daily dark/light cycles, as well as the continuing rise in atmospheric CO(2). Relative to abscisic acid and blue light signaling, little is known about the molecular, cellular, and genetic mechanisms of CO(2) signaling in guard cells. Interestingly, we report that repetitive Ca(2+) transients were observed during the stomatal opening stimulus, low [CO(2)]. Furthermore, low/high [CO(2)] transitions modulated the cytosolic Ca(2+) transient pattern in Arabidopsis guard cells (Landsberg erecta). Inhibition of cytosolic Ca(2+) transients, achieved by loading guard cells with the calcium chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid and not adding external Ca(2+), attenuated both high CO(2)-induced stomatal closing and low CO(2)-induced stomatal opening, and also revealed a Ca(2+)-independent phase of the CO(2) response. Furthermore, the mutant, growth controlled by abscisic acid (gca2) shows impairment in [CO(2)] modulation of the cytosolic Ca(2+) transient rate and strong impairment in high CO(2)-induced stomatal closing. Our findings provide insights into guard cell CO(2) signaling mechanisms, reveal Ca(2+)-independent events, and demonstrate that calcium elevations can participate in opposed signaling events during stomatal opening and closing. A model is proposed in which CO(2) concentrations prime Ca(2+) sensors, which could mediate specificity in Ca(2+) signaling.