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

We conducted an infrared thermal imaging-based genetic screen to identify Arabidopsis mutants displaying aberrant stomatal behavior in response to elevated concentrations of CO . This approach resulted in the isolation of a novel allele of the Arabidopsis BIG locus (At3g02260) that we have called CO insensitive 1 (cis1). BIG mutants are compromised in elevated CO -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 CO . However, like the wild-type, BIG mutants display inhibition of stomatal opening when exposed to elevated CO . 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 CO -mediated control of stomatal development. In the control of stomatal aperture by CO , BIG is only required in elevated CO -induced closure and not in the inhibition of stomatal opening by this environmental signal. These data show that, at the molecular level, the CO -mediated inhibition of opening and promotion of stomatal closure signaling pathways are separable and BIG represents a distinguishing element in these two CO -mediated responses.