SENSITIVITY TO ABSCISIC-ACID OF GUARD-CELL K+ CHANNELS IS SUPPRESSED BY ABI1-1, A MUTANT ARABIDOPSIS GENE ENCODING A PUTATIVE PROTEIN PHOSPHATASE

Abscisic acid (ABA) modulates the activities of three major classes of ion channels-inward- and outward-rectifying K+ channels (I-K,I-in, and I-K,I-out, respectively) and anion channels-at the guard-cell plasma membrane to achieve a net efflux of osmotica and stomatal closure, Disruption of ABA sensitivity in wilty abiI-1 mutants of Arabidopsis and evidence that this gene encodes a protein phosphatase suggest that protein (de-)phosphorylation contributes to guard-cell transport control by ABA. To pinpoint the role of ABI1, the abiI-1 dominant mutant allele was stably transformed into Nicotiana benthamiana and its influence on I-K,I-in, I-K,I-out, and the anion channels was monitored in guard cells under voltage clamp, Compared with guard cells from wild-type and vector-transformed control plants, expression of the abiI-1 gene was associated with 2- to 6-fold reductions in I-K,I-out and an insensitivity of both I-K,I-in and I-K,I-out to 20 mu M ABA. In contrast, no differences between control and abiI-1 transgenic plants were observed in the anion current or its response to ABA, Parallel measurements of intracellular pH (pH(i)) using the fluorescent dye 2',7'-bis(2-carboxyethyl)-5-(and -6)-carboxyfluorescein (BCECF) in every case showed a 0.15- to 0.2-pH-unit alkalinization in ABA, demonstrating that the transgene was without effect on the pH(i) signal that mediates in ABA-evoked K+ channel control, In guard cells from the abiI-1 transformants, normal sensitivity of both K+ channels to and stomatal closure in ABA was recovered in the presence of 100 mu M H7 and 0.5 mu M staurosporine, both broad-range protein kinase antagonists, These results demonstrate an aberrant K+ channel behavior-including channel insensitivity to ABA-dependent alkalinization of pH(i)-as a major consequence of abiI-1 action and implicate ABI1 as part of a phosphatase/kinase pathway that modulates the sensitivity of guard-cell K+ channels to ABA-evoked signal cascades.