Increased Endogenous Abscisic Acid Maintains Primary Root Growth and Inhibits Shoot Growth of Maize

Roots of maize (Zea mays L.) seedlings continue to grow at low water potentials that cause complete inhibition of shoot growth. In this study, we have investigated the role of abscisic acid (ABA) in this differential growth sensitivity by manipulating endogenous ABA levels as an altemative to extemal applications of the hormone. An inhibitor of carotenoid biosynthesis (fluridone) and a mutant deficient in carotenoid biosynthesis (vp 5) were used to reduce the endogenous ABA content in the growing zones of the primary root and shoot at low water potentials. Experiments were performed on 30 to 60 hour old seedlings that were transplanted into vermiculite which had been preadjusted to water potentials of approximately -1.6 megapascals (roots) or -0.3 megapascals (shoots). Growth occurred in the dark at nearsaturation humidity. Results of experiments using the inhibitor and mutant approaches were very similar. Reduced ABA content by either method was associated with inhibition of root elongation and promotion of shoot elongation at low water potentials, compared to untreated and wild-type seedlings at the same water potential. Elongation rates and ABA contents at high water potential were little affected. The inhibition of shoot elongation at low water potential was completely prevented in fluridone-treated seedlings during the first five hours after transplanting. The results indicate that ABA accumulation plays direct roles in both the maintenance of primary root elongation and the inhibition of shoot elongation at low water potentials. A major reason behind the slow progress in the area of crop adaptation to drought is the insufficient basic understanding of the regulation of growth responses to water stress. When water is limited, shoot growth in many species is more inhibited than root growth (24), and in some cases, the absolute biomass of roots has been shown to increase relative to wellwatered controls (12, 23). In maize, roots continue to grow at low water potentials that cause complete inhibition of shoot growth (25, 30). The role of the hormone ABA in the differential growth responses of the primary root and shoot of maize to low water potentials is the subject of this paper. ABA accumulates to high concentrations in tissues ofplants Supported in part by U.S. Department of Agriculture Grant 87CRCR-1-2492 to R. E. S. Contribution from the Missouri Agricultural Experiment Station, Journal Series No. 10889. 2 Present address: Department of Biochemistry and Soil Science, University College of North Wales, Bangor, Gwynedd, LL57 2UW, United Kingdom. experiencing water stress and has been proposed to be involved in the regulation of root and shoot growth responses (3, 5, 6). Speculations on the involvement ofABA in growth responses to water stress have relied on the results of ABA applications to well-watered plants. Such applications have generally led to inhibition of shoot growth (1, 5, 11, 28, 29). In the case of roots, results have been variable; applications of ABA have resulted in growth inhibition (10, 18, 29), promotion (15, 19, 29), or have had little effect (5). In maize, applications of the hormone led to stimulation or inhibition of primary root growth depending on the initial root growth rate (19) and the concentration used (15, 19), with higher concentrations being associated with growth inhibition. Additionally, in a population ofmaize primary roots with varying growth rates, higher levels ofendogenousABA were associated with slower growth rates (22). Nevertheless, the lack of inhibition and, in some cases, stimulation of root growth by some concentrations of exogenous ABA has led to speculation that ABA may modulate the differential inhibition of root and shoot growth in water-stressed plants (5, 9, 29, 31). When ABA is applied to well-watered plants, however, even if it accumulates to realistic levels in the growing zone (5), its compartmentation may differ from that of endogenous ABA in water-stressed tissue (8). Also, other metabolic changes that may be induced by water stress, such as changes in the levels of other hormones and tissue sensitivity to hormones, are not addressed when well-watered tissue is used to study the involvement of ABA in growth responses to low water potentials. Thus, it is not surprising that in one example ABA applications produced different growth responses in wellwatered and water-stressed plants (29). Consequently, the role, if any, ofABA in the growth responses of roots and shoots to low water potentials remains equivocal. With this in mind, our aim was to critically evaluate the role of elevated levels of endogenous ABA in the growth responses of the primary root and shoot ofmaize to low water potentials, without exogenous applications of the hormone. Specifically, our objective was to reduce the accumulation of endogenous ABA in seedlings growing at low water potential using two approaches: a mutant deficient in carotenoid (and ABA) biosynthesis (vp 5, [7, 14]), and fluridone, an inhibitor of carotenoid (and ABA) biosynthesis (7, 13) applied to seedlings of a commercial variety. Use of the two approaches was intended to circumvent any side effects that the inhibitor may have on growth independently of ABA content, and to confirm results obtained using one approach. The results indicate