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.