Regulation of Root Growth at Low Water Potentials

Abstract

In many species, roots continue to grow at low water potentials that cause complete inhibition of shoot growth. Recent studies show that the differential sensitivity of root and shoot growth to low water potentials is regulated, at least in part, by the hormone abscisic acid (ABA). When ABA accumulation in maize seedlings at low water potential is inhibited using either fluridone or the vp5 mutant to block carotenoid (and ABA) biosynthesis, primary root elongation is severely inhibited, while shoot growth is promoted as compared with untreated or wild-type seedlings growing at the same water potential. Fluridone-induced inhibition of root elongation at low water potential may be reversed by applying exogenous ABA, confirming the requirement for increased endogenous levels of ABA for the maintenance of root growth under dry conditions. In fluridone-treated seedlings, ABA levels at low water potential decreased to a similar extent throughout the root growth zone, while inhibition of elongation increased with distance from the apex. These results suggest a developmental gradient in tissue responsiveness to endogenous ABA. Current studies suggest that ABA may have a number of roles in maintaining root cell elongation at low water potentials. Firstly, proline is a major contributor to osmotic adjustment in the growth zone of maize primary roots, and its accumulation is partially prevented by treatment with fluridone. This effect can be reversed by adding exogenous ABA. Secondly, activity of the enzyme xyloglucan endotransglycosylase (XET), which is believed to be important for cell wall yielding, is markedly increased in the root growth zone at low water potential, and this response is also inhibited by fluridone, suggesting regulation by ABA.