Abscisic Acid (ABA)

Abscisic acid, the plant's principal stress hormone, was isolated independently by two research groups in the early 1960s before they were shown to be studying the same compound. Addicott et al. isolated it in 1963 from young cotton bolls and called it Abscisin II, while Eagles and Wareing (1963-64) isolated a dormancy-inducing substance from Betula leaves and named it dormin; Cornsforth et al. established in 1965 that dormin and abscisin were chemically identical, and the compound was renamed Abscisic Acid. ABA accumulates abundantly inside the chloroplasts of green cells, and is synthesised via the mevalonic acid pathway or from xanthophylls, giving it a carotenoid-related structure. It moves through the plant by diffusion as well as through both phloem and xylem, and its activity is classically assayed using rice coleoptiles, where ABA inhibits the straight growth normally induced by IAA. Physiologically, ABA is a powerful growth inhibitor (it can cause up to 50% growth inhibition in oat coleoptile), reduces transpiration by closing stomata (by inhibiting K+ uptake and promoting malic acid leakage in guard cells), breaks down chlorophyll, protein and nucleic acids to cause leaf yellowing, inhibits cell division and elongation, induces bud and seed dormancy, promotes abscission of leaves, flowers and fruits, has anti-auxin and anti-gibberellin activity, and plays a central role in helping plants cope with water stress and drought by triggering loss of turgor and stomatal closure. It also promotes senescence by reducing photosynthesis and altering protein and nucleic acid synthesis. Agriculturally, ABA induces male flower form …