Abscisic acid as a natural growth regulator

Abstract

Abscisic acid (ABA) is accepted as one of the five major classes of natural plant growth regulators. In many tests ABA inhibits growth and metabolism, and enhances degradative changes, as in ripening and senescence. Its sites of biosynthesis appear not to be strictly localized and it is transported within the plant both within the phloem and by cell-to-cell transport. The best authenticated example of its function as a growth regulator is in the geotropic responses of roots in which growth curvature is apparently brought about by the asymmetric distribution of ABA produced in the root cap. The evidence for a rôle of ABA in the dormancy of seeds and buds, and in the ripening and abscission of fruits, is suggestive but at present incomplete. Experiments involving both exogenous and endogenous ABA suggest that ABA may play a significant role in tuberization in potato and other species. ABA inhibits hormone-induced nucleic acid and protein synthesis and there is evidence for its action both at the transcription and at the translation levels. On the other hand, ABA induces very rapid inhibition of growth which appears to be mediated via its effects on cell membrane properties. Its effects on potassium uptake may be mediated through inhibition of proton excretion by the cell. ABA counteracts the effects of other growth promoting hormones in various tests but the nature of this interaction is unknown. It seems likely that ABA plays a general role in the regulation of growth and certain aspects of metabolism, through its interaction with other growth substances.