Arabidopsis abi1-1 and abi2-1 phosphatase mutations reduce abscisic acid-induced cytoplasmic calcium rises in guard cells

Elevations in cytoplasmic calcium ([Ca²⁺](cyt)) are an important component of early abscisic acid (ABA) signal transduction. To determine whether defined mutations in ABA signal transduction affect [Ca²⁺](cyt) signaling, the Ca²⁺-sensitive fluorescent dye fura 2 was loaded into the cytoplasm of Arabidopsis guard cells. Oscillations in [Ca²⁺](cyt) could be induced when the external calcium concentration was increased, showing viable Ca²⁺ homeostasis in these dye-loaded cells. ABA-induced [Ca²⁺](cyt) elevations in wild-type stomata were either transient or sustained, with a mean increase of ~300 nM. Interestingly, ABA-induced [Ca²⁺](cyt) increases were significantly reduced but not abolished in guard cells of the ABA-insensitive protein phosphatase mutants abi1 and abi2. Plasma membrane slow anion currents were activated in wild-type, abi1, and abi2 guard cell protoplasts by increasing [Ca²⁺](cyt), demonstrating that the impairment in ABA activation of anion currents in the abi1 and abi2 mutants was bypassed by increasing [Ca²⁺](cyt). Furthermore, increases in external calcium alone (which elevate [Ca²⁺](cyt)) resulted in stomatal closing to the same extent in the abi1 and abi2 mutants as in the wild type. Conversely, stomatal opening assays indicated different interactions of abi1 and abi2, with Ca²⁺-dependent signal transduction pathways controlling stomatal closing versus stomatal opening. Together, [Ca²⁺](cyt) recordings, anion current activation, and stomatal closing assays demonstrate that the abi1 and abi2 mutations impair early ABA signaling events in guard cells upstream or close to ABA-induced [Ca²⁺](cyt) elevations. These results further demonstrate that the mutations can be bypassed during anion channel activation and stomatal closing by experimental elevation of [Ca²⁺](cyt).