Early ABA signaling events in guard cells

The plant hormone abscisic acid (ABA) regulates a wide variety of plant physiological and developmental processes, particularly responses to environmental stress, such as drought. In response to water deficiency, plants redistribute foliar ABA and/or upregulate ABA synthesis in roots, leading to roughly a 30-fold increase in ABA concentration in the apoplast of stomatal guard cells. The elevated ABA triggers a chain of events in guard cells, causing stomatal closure and thus preventing water loss. Although the molecular nature of ABA receptor(s) remains unknown, considerable progress in the identification and characterization of its downstream signaling elements has been made by using combined physiological, biochemical, biophysical, molecular, and genetic approaches. The measurable events associated with ABA-induced stomatal closure in guard cells include, sequentially, the production of reactive oxygen species (ROS), increases in cytosolic free Ca2+ levels ([Ca2+](i)), activation of anion channels, membrane potential depolarization, cytosolic alkalinization, inhibition of K+ influx channels, and promotion of K+ efflux channels. This review provides an overview of the cellular and molecular mechanisms underlying these ABA-evoked signaling events, with particular emphasis on how ABA triggers an "electronic circuitry" involving these ionic components.