ABSCISIC-ACID SIGNALING

The plant hormone abscisic acid (ABA) regulates the development and ge rmination of seeds, as well as the adaptation of vegetative tissues to conditions of environmental stress. During the past year, considerabl e insights have been gained into the molecular nature of the complex s ignaling network that mediates the actions of ABA. Biophysical studies indicate that at least some of the effects of ABA in stomatal guard c ells involve intracellular receptors. Also, increasing evidence suppor ts the view that guard cells contain redundant ABA transduction pathwa ys, and that cytoplasmic Ca2+ acts as a second messenger in at least o ne of these pathways. Finally, mutational analysis in Arabidopsis indi cates that the multiple effects of ABA at the whole plant level are me diated by overlapping branches of a highly ramified signaling network. Two Arabidopsis loci that determine ABA sensitivity have already been cloned and found to encode a protein phosphatase and a transcriptiona l activator.