The plant hormone abscisic acid (ABA) serves as a physiological monitor to assess the water status of plants and, under drought conditions, induces stomatal pore closure by activating specific ion channels, such as a slow-anion channel (SLAC1) that, in turn, mediate ion efflux from the guard cells. Earlier genetic analyses uncovered a protein kinase (OST1) and several 2C-type phosphatases, as respective positive and negative regulators of ABA-induced stomatal closure. Here we show that the OST1 kinase interacts with the SLAC1 anion channel, leading to its activation via phosphorylation. PP2CA, one of the PP2C phosphatase family members acts in an opposing manner and inhibits the activity of SLAC1 by two mechanisms: (1) direct interaction with SLAC1 itself, and (2) physical interaction with OSTI leading to inhibition of the kinase independently of phosphatase activity. The results suggest that ABA signaling is mediated by a physical interaction chain consisting of several components, including a PP2C member, SnRK2-type kinase (OST1), and an ion channel, SLAC1, to regulate stomatal movements. The findings are in keeping with a paradigm in which a protein kinase-phosphatase pair interacts physically with a target protein to couple a signal with a specific response.

### Calcium Signaling and Calcium Sensors: A General Paradigm Many extracellular signals, including light, biotic, and abiotic stress factors, elicit changes in cellular Ca2+ concentration in plants ([Trewavas and Knight, 1994][1]; [Bush, 1995][2]; [Braam et al., 1997][3]; [McAinsh et al., 1997][4

Summary: Calcium signalling involves sensor proteins that decode temporal and spatial changes in cellular Ca²⁺ concentration. Calcineurin B‐like proteins (CBLs) represent a unique family of plant calcium sensors that relay signals by interacting with a family of protein kinases, designated as CBL‐interacting protein kinases (CIPKs). In a reverse genetic screen for altered drought tolerance, we identified a loss‐of‐function allele of CIPK23 as exhibiting a drought‐tolerant phenotype. In the cipk23 mutant, reduced transpirational water loss from leaves coincides with enhanced ABA sensitivity of guard cells during opening as well as closing reactions, without noticeable alterations in ABA content in the plant. We identified the calcium sensors CBL1 and CBL9 as CIPK23‐interacting proteins that targeted CIPK23 to the plasma membrane in vivo. Expression analysis of the CIPK23, CBL1 and CBL9 genes suggested that they may function together in diverse tissues, including guard cells and root hairs. In addition, expression of the CIPK23 gene was induced by low‐potassium conditions, implicating a function of this gene product in potassium nutrition. Indeed, cipk23 mutants displayed severe growth impairment on media with low concentrations of potassium. This phenotype correlates with a reduced efficiency of K⁺ uptake into the roots. In support of the conclusion that CBL1 and CBL9 interact with and synergistically serve as upstream regulators of CIPK23, the cbl1 cbl9 double mutant, but not the cbl1 or cbl9 single mutants, exhibit altered phenotypes for stomatal responses and low‐potassium sensitivity. Together with the recent identification of the potassium channel AKT1 as a target of CIPK23, these results imply that plasma membrane‐localized CBL1– and CBL9–CIPK23 complexes simultaneously regulate K⁺ transport processes in roots and in stomatal guard cells.