Abscisic acid (ABA) is an optically active 15-C weak acid that was first identified in the early 1960s as a growth inhibitor accumulating in abscising cotton fruit (“abscisin II”) and leaves of sycamore trees photoperiodically induced to become dormant (“dormin”) (reviewed in Addicott, 1983). It has since been shown to regulate many aspects of plant growth and development including embryo maturation, seed dormancy, germination, cell division and elongation, and responses to environmental stresses such as drought, salinity, cold, pathogen attack and UV radiation (reviewed in Leung and Giraudat, 1998; Rock, 2000). However, despite the name, it does not appear to control abscission directly; the presence of ABA in abscising organs reflects its role in promoting senescence and/or stress responses, the processes preceding abscission. Although ABA has historically been thought of as a growth inhibitor, young tissues have high ABA levels, and ABA-deficient mutant plants are severely stunted (Figure 1) because their ability to reduce transpiration and establish turgor is impaired. Exogenous ABA treatment of mutants restores normal cell expansion and growth. Figure 1. Exogenous ABA suppresses growth inhibition of ABA-deficient mutants. Plants with one of three mutant alleles of aba1 were grown with (bottom) or without (top) ABA treatment (spraying twice weekly with 10 µM ABA for 8 weeks). (Photograph courtesy ... ABA is ubiquitous in lower and higher plants. It is also produced by some phytopathogenic fungi (Assante et al., 1977; Neill et al., 1982; Kitagawa et al., 1995) and has even been found in mammalian brain tissue (Le Page-Degivry et al., 1986). As a sesquiterpenoid, it was long thought to be synthesized directly from farnesyl pyrophosphate, as in fungi (reviewed in Zeevaart and Creelman, 1988). However, it is actually synthesized indirectly from carotenoids. As a weak acid (pKa=4.8), ABA is mostly uncharged when present in the relatively acidic apoplastic compartment of plants and can easily enter cells across the plasma membrane. The major control of ABA distribution among plant cell compartments follows the “anion trap” concept: the dissociated (anion) form of this weak acid accumulates in alkaline compartments (e.g. illuminated chloroplasts) and may redistribute according to the steepness of the pH gradients across membranes. In addition to partitioning according to the relative pH of compartments, specific uptake carriers contribute to maintaining a low apoplastic ABA concentration in unstressed plants. Despite the ease with which ABA can enter cells, there is evidence for extracellular as well as intracellular perception of ABA (reviewed in Leung and Giraudat, 1998; Rock, 2000). Multiple receptor types are also implicated by the variation in stereospecificity among ABA responses. Genetic studies, especially in Arabidopsis, have identified many loci involved in ABA synthesis and response and analyzed their functional roles in ABA physiology (reviewed in Leung and Giraudat, 1998; Rock, 2000). Many likely signaling intermediates correlated with ABA response (e.g. ABA-activated or -induced kinases and DNA-binding proteins that specifically bind ABA-responsive promoter elements) have also been identified by molecular and biochemical studies, but the relationships among these proteins are unclear. Cell biological studies have identified secondary messengers involved in ABA response. Ongoing studies combine these approaches in efforts to determine coherent models of ABA signaling mechanism(s).