The r�le of abscisic acid in root growth and gravireaction: A critical review

Abstract

A brief account is given of the discovery of abscisic acid (ABA) in roots and root caps of higher plants as well as the techniques by which ABA may be demonstrated in these tissues . The remainder of the review is concerned with examining the role of ABA in the regulation of root growth . In this regard, it is well established that when ABA is supplied to roots their elongation is usually inhibited, although at low external concentrations a stimulation of growth may also be found . Fewer observations have been directed at exploring the connection between root growth and the level of naturally occurring, endogenous ABA . Nevertheless, the evidence here also suggests that ABA is an inhibitory regulator of root growth . Moreover, ABA appears to be involved in the differential growth that arises in response to a gravitational stimulus . Recent reports that deny a role for ABA in root gravitropism are considered inconclusive. The response of roots to osmotic stress and the changes in ABA levels which ensue, are summarised ; so are the interrelations between ABA and other hormones, particularly auxin (e .g . indoleacetic acid) ; both are considered in the context of the root growth and development . Quantitative changes in auxin and ABA levels may together provide the root with a flexible means of regulating its growth .

Suppression of gravitropic response of primary roots by submergence

“…Roots of maize seedlings that had been germinated under Ca 2+ solution also did not show curvature under conditions of submergence (data not shown). Leakage of growth substances from roots under water may cause a suppression of curvature (Pilet and Barlow 1987). In the present experiments, neither auxins, antiauxins, nor abseisic acid induced a curvature under water (data not shown).…”

“…Thus, the generality of suppression of gravitropic curvature by submergence has not always been confirmed. On the other hand, it has been proposed that anaerobiosis (Wilkins and Shaw 1967;Nantawisarakul and Newman 1992), removal of mucilage (Moore and Evans 1986), or leakage of growth substances, such as indole-3-acetic acid and abscisic acid (Pilet and Barlow 1987) may be associated with the modifications of gravitropism under conditions of submergence. In rice coleoptiles, submergence suppresses development of starchfilled amyloplasts, which appears to cause the delayed onset of gravitropic curvature in air (Kutschera et al 1990).…”

Abscisic Acid Elicits the Water-Stress Response in Root Hairs of Arabidopsis thaliana

“…ABA is believed to play a critical role in these responses. Exogenously applied ABA at low concentrations tends to elicit an increase in the root to shoot ratio by preferentially inhibiting shoot growth (2,11,13,14), whereas at high concentrations, it generally inhibits root growth (12,13); these differences depend on the time and method of ABA application. It is well known that drought stress elicits the accumulation of ABA in roots, particularly at the tips (12,14).…”

Development and Growth of Crop Root Systems

“…It is well known that such growth substances as ABA and indoleacetic acid (IAA) can regulate root elongation (Saugy et aI., 1989). These hormones are also involved in the root growth response to gravity (Pilet and Barlow, 1987), along with other factors such as calcium (Leopold and Wettlaufer, 1988).…”