The BIG protein distinguishes the process of CO₂-induced stomatal closure from the inhibition of stomatal opening by CO₂

We conducted an infrared thermal imaging-based genetic screen to identify Arabidopsis mutants displaying aberrant stomatal behavior in response to elevated concentrations of CO₂. This approach resulted in the isolation of a novel allele of the Arabidopsis BIG locus (At3g02260) that we have called CO₂ insensitive 1 (cis1). BIG mutants are compromised in elevated CO₂-induced stomatal closure and bicarbonate activation of S-type anion channel currents. In contrast with the wild-type, they fail to exhibit reductions in stomatal density and index when grown in elevated CO₂. However, like the wild-type, BIG mutants display inhibition of stomatal opening when exposed to elevated CO₂. BIG mutants also display wild-type stomatal aperture responses to the closure-inducing stimulus abscisic acid (ABA). Our results indicate that BIG is a signaling component involved in the elevated CO₂-mediated control of stomatal development. In the control of stomatal aperture by CO₂, BIG is only required in elevated CO₂-induced closure and not in the inhibition of stomatal opening by this environmental signal. These data show that, at the molecular level, the CO₂-mediated inhibition of opening and promotion of stomatal closure signaling pathways are separable and BIG represents a distinguishing element in these two CO₂-mediated responses.