Abscisic acid (ABA) is a classical plant hormone that regulates various aspects of plant growth and development. This chapter reviews current understanding of ABA synthesis, metabolism, transport, and signal transduction, emphasizing knowledge from Arabidopsis studies. Genetic, molecular, and biochemical studies have identified nearly all enzymes involved in ABA metabolism, almost 200 loci regulating ABA response, and thousands of genes regulated by ABA in various contexts. Some regulators are involved in cross-talk with other developmental, environmental, or hormonal signals. ABA signaling mechanisms vary among tissues or developmental stages, discussed in the context of ABA effects on seed maturation, germination, seedling growth, vegetative stress responses, stomatal regulation, pathogen response, flowering, and senescence. ABA is a 15-carbon weak acid first identified in the 1960s as a growth inhibitor. It regulates many aspects of plant growth and development, including embryo maturation, seed dormancy, germination, cell division and elongation, floral induction, and responses to environmental stresses. ABA is produced by plants and some phytopathogenic fungi, bacteria, and metazoans. Although some signaling aspects are conserved across kingdoms, there are at least two biosynthetic pathways: fungi produce ABA directly from farnesyl pyrophosphate, whereas plants synthesize it indirectly from carotenoids. ABA is mostly uncharged in the apoplastic compartment of plants and can easily enter cells across the plasma membrane. Control of ABA distribution among plant cell compartments was thought to primarily follow the "anion trap" concept. However, multiple plasma membrane-localized transporters have been identified, including two ATP-binding cassette (ABC) transporters for ABA. Genetic analyses demonstrated their importance for ABA responses including stomatal regulation, gene regulation, germination inhibition, and stress tolerance. ABA is ubiquitous in plants and is also produced by some phytopathogenic fungi, bacteria, and metazoans. ABA is a sesquiterpenoid with one asymmetric, optically active carbon atom at C-1'. The naturally occurring form is S-(+)-ABA; the side chain of ABA is 2-cis,-4-trans. Trans, trans-ABA is biologically inactive, but R-(-)-ABA is active in some assays. Structure-function studies identified the 2-cis, 4-trans side chain configuration, the stereochemistry at C-1', the presence of the 7' methyl group, and the C4' ketone as important for many ABA-like activities. ABA biosynthesis begins in plastids with the MEP pathway. The next major phase of ABA biosynthesis is production of carotenoids. Sequential condensation reactions catalyzed by geranyl geranyl diphosphate synthase add one isoprene unit at a time to successively generate C10, C1Abscisic acid (ABA) is a classical plant hormone that regulates various aspects of plant growth and development. This chapter reviews current understanding of ABA synthesis, metabolism, transport, and signal transduction, emphasizing knowledge from Arabidopsis studies. Genetic, molecular, and biochemical studies have identified nearly all enzymes involved in ABA metabolism, almost 200 loci regulating ABA response, and thousands of genes regulated by ABA in various contexts. Some regulators are involved in cross-talk with other developmental, environmental, or hormonal signals. ABA signaling mechanisms vary among tissues or developmental stages, discussed in the context of ABA effects on seed maturation, germination, seedling growth, vegetative stress responses, stomatal regulation, pathogen response, flowering, and senescence. ABA is a 15-carbon weak acid first identified in the 1960s as a growth inhibitor. It regulates many aspects of plant growth and development, including embryo maturation, seed dormancy, germination, cell division and elongation, floral induction, and responses to environmental stresses. ABA is produced by plants and some phytopathogenic fungi, bacteria, and metazoans. Although some signaling aspects are conserved across kingdoms, there are at least two biosynthetic pathways: fungi produce ABA directly from farnesyl pyrophosphate, whereas plants synthesize it indirectly from carotenoids. ABA is mostly uncharged in the apoplastic compartment of plants and can easily enter cells across the plasma membrane. Control of ABA distribution among plant cell compartments was thought to primarily follow the "anion trap" concept. However, multiple plasma membrane-localized transporters have been identified, including two ATP-binding cassette (ABC) transporters for ABA. Genetic analyses demonstrated their importance for ABA responses including stomatal regulation, gene regulation, germination inhibition, and stress tolerance. ABA is ubiquitous in plants and is also produced by some phytopathogenic fungi, bacteria, and metazoans. ABA is a sesquiterpenoid with one asymmetric, optically active carbon atom at C-1'. The naturally occurring form is S-(+)-ABA; the side chain of ABA is 2-cis,-4-trans. Trans, trans-ABA is biologically inactive, but R-(-)-ABA is active in some assays. Structure-function studies identified the 2-cis, 4-trans side chain configuration, the stereochemistry at C-1', the presence of the 7' methyl group, and the C4' ketone as important for many ABA-like activities. ABA biosynthesis begins in plastids with the MEP pathway. The next major phase of ABA biosynthesis is production of carotenoids. Sequential condensation reactions catalyzed by geranyl geranyl diphosphate synthase add one isoprene unit at a time to successively generate C10, C1