Abscisic acid (ABA) is found in all higher plants and mosses. ABA regulatesdormancy and is central to plant responses to stress. The nature of the moleculemeans that it can exist in several forms. First, the carboxyl group at the end of the side chain may be cis or trans (Fig. 4), while the C at position one of the ring is asymmetric and gives either (+) or (-) (S or R) enantiomers. The active formof ABA is (+) cis ABA; (+)-2-trans-ABA also exists in plants and is active in some long-term ABA responses; it may be converted to the (+) cis form in tissues.
ABA has a variety of effects related (i) to seed dormancy and (ii) to stress responses. ABA levels rise initially during embryo development within theseed and then decline. ABA regulates the expression of genes for proteins in
the embryo that prepare it for the final stages of seed development in which the seed desiccates and becomes dormant; it also activates genes for seed storage proteins (Topic H4). ABA also keeps some seeds dormant until the environment becomes suitable for growth. Controlling dormancy is very important in temperate climates since precocious germination may lead to the death of the seedling. ABA also accumulates in the dormant buds of woody species, although control of dormancy here is likely to be the result of the action of several hormones.
ABA also regulates several plant stress responses. Rising ABA levels in water stress initially cause stomatal closure (Topic I2) and subsequently increases the ability of root tissue to carry water; it also promotes root growth and inhibits shoot growth.
ABA biosynthesis begins in chloroplasts and amyloplasts. Synthesis, like that of gibberellins and cytokinins, involves the isoprene subunit in isopentenyl pyrophosphate, which is used to produce an oxygenated carotenoid compound,
zeaxanthin. Zeaxanthin is modified in a multi-stage process to neoxanthin, which is cleaved to the C15 compound xanthoxin; xanthoxin is then modified in two stages to produce ABA. ABA is degraded, either by oxidation or by conjugation, to form ABA-glucosyl ester.
ABA is synthesized in roots and shoots, and at much higher levels in tissues undergoing stress. Water-stressed roots, for instance, produce up to 1000 times more ABA that is transported through the xylem to the shoot. ABA is also transported from shoot to root in the phloem.