Abscisic acid signaling.

The plant hormone abscisic acid (ABA) regulates the development and germination of seeds, as well as the adaptation of vegetative tissues to conditions of environmental stress. During the past year, considerable insights have been gained into the molecular nature of the complex signaling network that mediates the actions of ABA. Biophysical studies indicate that at least some of the effects of ABA in stomatal guard cells involve intracellular receptors. Also, increasing evidence supports the view that guard cells contain redundant ABA transduction pathways, and that cytoplasmic Ca2+ acts as a second messenger in at least one of these pathways. Finally, mutational analysis in Arabidopsis indicates that the multiple effects of ABA at the whole plant level are mediated 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 transcriptional activator.