A Brand New START: Abscisic Acid Perception and Transduction in the Guard Cell

Understanding how plants respond to the hormone abscisic acid provides targets for designing crops resilient to drought. The combined daily consumption of fresh water ranges from 200 to 700 liters per capita per day in most developed countries, with ~70% used for agricultural needs. With the looming prospect of global water crisis, the success in deciphering the early steps in the signal transduction of the stress hormone abscisic acid (ABA) has ignited hopes that crops can be engineered with the capacity to maintain productivity while requiring less water input. ABA can accumulate 10- to 30-fold in plants under drought stress relative to unstressed conditions. The hormone triggers diverse adaptive pathways, permitting plants to withstand temporary bouts of water shortage. One experimental model to unravel the details of ABA signaling mechanisms is based on the hormone’s ability to elicit stomatal closure—a rapid cellular response used by plants to limit water loss through transpiration. Each stoma, or pore, is contoured by two specialized kidney-shaped cells called guard cells. The stomatal pores are the primary portals for photosynthetic carbon dioxide uptake and, by triggering closure of these pores, drought negatively affects photosynthesis and, consequently, biomass. The stomatal aperture widens and narrows by expansion and contraction, respectively, of these flanking guard cells caused by changes in the intracellular concentrations of ions and organic solutes. These events require coordination of ion channels, which generate a specific pattern of electrical signals that relay the ABA stimulus. A soluble ABA receptor that turns on and off the activities of a kinase and phosphatase pair is at the heart of the signaling complex. Results distilled from the latest structural studies on these ABA receptors, characterized by the so-called START domain, are beginning to tender exciting promise for rational design of agonists and antagonists for modulating stress adaptive ability in plants. With 4 figures, 2 tables, and 127 citations, this Review charts the extraordinary progress on understanding how ABA controls membrane transport mechanisms that evoke the stomatal closing pathway. More background details are available with the teaching tools at www.plantcell. org.gate1. inist.fr/site/teachingtools/teaching.xhtml. The soluble receptors of abscisic acid (ABA) have been identified in Arabidopsis thaliana. The 14 proteins in this family, bearing the double name of PYRABACTIN RESISTANCE/PYRABACTIN-LIKE (PYR/PYL) or REGULATORY COMPONENTS OF ABA RECEPTOR (RCAR) (collectively referred to as PYR/PYL/RCAR), contain between 150 and 200 amino acids with homology to the steroidogenic acute regulatory-related lipid transfer (START) protein. Structural studies of these receptors have provided rich insights into the early mechanisms of ABA signaling. The binding of ABA to PYR/PYL/RCAR triggers the pathway by inducing structural changes in the receptors that allows them to sequester members of the clade A negative regulating protein phosphatase 2Cs (PP2Cs). This liberates the class III ABA-activated Snf1-related kinases (SnRK2s) to phosphorylate various targets. In guard cells, a specific SnRK2, OPEN STOMATA 1 (OST), stimulates H2O2 production by NADPH oxidase respiratory burst oxidase protein F and inhibits potassium ion influx by the inward-rectifying channel KAT1. OST1, the kinase CPK23, the calcium-dependent kinase CPK21, and the counteracting PP2Cs modulate the slow anion channel SLAC1, a pathway that contributes to stomatal responses to diverse stimuli, including ABA and carbon dioxide. A minimal ABA response pathway that leads to activation of the SLAC1 homolog, SLAH3, and presumably stomatal closure has been reconstituted in vitro. The identification of the soluble receptors and core components of the ABA signaling pathway provides promising targets for crop design with higher resilience to water deficit while maintaining biomass.