DIFFERENTIAL ABSCISIC-ACID REGULATION OF GUARD-CELL SLOW ANION CHANNELS IN ARABIDOPSIS WILD-TYPE AND ABI1 AND ABI2 MUTANTS

Abscisic acid (ABA) regulates vital physiological responses, and a num ber of events in the ABA signaling cascade remain to be identified. To allow quantitative analysis of genetic signaling mutants, patch-clamp experiments were developed and performed with the previously inaccess ible Arabidopsis guard cells from the wild type and ABA-insensitive (a bi) mutants. Slow anion channels have been proposed to play a rate-lim iting role in ABA-induced stomatal closing. We now directly demonstrat e that ABA strongly activates slow anion channels in wild-type guard c ells. Furthermore, ABA-induced anion channel activation and stomatal c losing were suppressed by protein phosphatase inhibitors. In abi1-1 an d abi2-1 mutant guard cells, ABA activation of slow anion channels and ABA-induced stomatal closing were abolished. These impairments in ABA signaling were partially rescued by kinase inhibitors in abi1 but not in abi2 guard cells. These data provide cell biological evidence that the abi2 locus disrupts early ABA signaling, that abi1 and abi2 affec t ABA signaling at different steps in the cascade, and that protein ki nases act as negative regulators of ABA signaling in Arabidopsis. New models for ABA signaling pathways and roles for abi1, abi2, and protei n kinases and phosphatases are discussed.