Role of Abscisic Acid in the Regulation of Root Growth at Low Water Potentials

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9306935 Sharp Studies in the Principal Investigator's laboratory have shown that accumulation of the hormone abscisic acid (ABA) is required for the maintenance of primary root elongation in maize under water-limited conditions. Recent evidence indicates that one role of ABA in this regard is the regulation of proline accumulation for osmotic adjustment. The proposed research aims to determine the cellular mechanisms underlying this relationship. Building upon recent data and a review of the published work in plant as well as animal and bacterial systems, a hypothesis is developed that ABA accumulation in the root growth zone at low water potentials regulates membrane ion transport and intracellular ion levels, which in turn induces mechanisms for growth maintenance including the accumulation of proline. While the role of ABA in regulating ion transport in stomatal guard cells is well-established, such information is scant with regard to the action of ABA in growth regulation. The primary objective is to test this hypothesis using electrophysiological approaches, emphasizing changes in membrane potential and ion concentrations and fluxes in the root growth zone caused by low water potentials and manipulation of endogenous ABA levels. Emphasis will be placed on measurements of K+ and H+. As in previous work, endogenous ABA will be manipulated within physiologically meaningful levels in roots growing at low water potentials using both the inhibitor fluridone and the vp5 mutant of maize to inhibit carotenoid (and ABA) biosynthesis. In all experiments a high degree of spatial resolution will be emphasized to address differential responsiveness to ABA within the root growth zone. Conventional and ion- selective microelectrodes will be used to measure changes in membrane potential, cytoplasmic ion levels and ion transport between roots grown at high and low water potential, and in roots at low water potentials in which endogenous ABA levels are altered. A sol ution culture system has been developed for this work, and preliminary experiments have shown changes in membrane potentials in the root growth zone in response to altered water potentials and to exogenous ABA. The electrophysiological measurements of ion levels will be compared with values obtained by x-ray microanalysis (EDX). Appropriate collaborations have been established to facilitate the electrophysiological and EDX measurements. Parallel to the studies of ionic regulation, the biochemistry of proline accumulation in the root growth zone at low water potentials and as affected by ABA will be examined. The contributions of increased synthesis, decreased catabolism, increased uptake, and decreased incorporation into protein will be assessed. The proposed research is a step toward the long-range goal of identifying specific plant processes or traits which could be used in crop germplasm improvement programs. The studies should provide much needed information in the fields of ABA physiology and root growth regulation under water- limited conditions, which have been characterized by slow progress. ***