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

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Previous work showed that primary root elongation in maize (Zeamays L.) seedlings at low water potentials was severely inhibited when accumulation of abscisic acid (ABA) was decreased using either fluridone or the vp5 mutant to inhibit carotenoid (and ABA) biosynthesis. The objective of this study was to confirm that the inhibition of root elongation resulting from these treatments was indeed attributable to the decrease in ABA levels. Seedlings were transplanted after germination to vermiculite at water potentials of −1.6 or − 0.3 MPa. ABA was mixed at various concentrations with the vermiculite to test whether the effects of fluridone and the vp5 mutation on root elongation could be overcome. In both treatments, restoration of ABA levels in the root apical 10 mm, which encompassed the elongation zone, resulted in recovery of root elongation rate at both −1.6 MPa and −0.3 MPa. Analysis of the spatial distribution of elongation rate showed that the recovery of overall root elongation resulted from restoration of the profile of elongation, and did not involve over-promotion of local elongation rate at any position. The recovery of root elongation was shown to be independent of effects of ABA status on shoot growth. When ABA was applied at high water potential, such that levels of ABA in the root tip reached those associated with maintenance of elongation at low water potentials, root elongation was inhibited. Thus, the response of root elongation to bulk tissue ABA content varied with the tissue water status. The results confirm that accumulation of ABA is required for the maintenance of maize primary root elongation at low water potentials.

Conditions of soil drying and plant growth that lead to non-hydraulic inhibition of leaf elongation and stomatal conductance in maize (Zea mays L.) were investigated using plants grown with their root systems divided between two containers. The soil in one container was allowed to dry while the other container was kept well-watered. Soil drying resulted in a maximum 35% inhibition of leaf elongation rate which occurred during the light hours, with no measurable decline in leaf water potential (ψw). Leaf area was 15% less than in control plants after 18 d of soil drying. The inhibition of elongation was observed only when the soil ψw declined to below that of the leaves and, thus, the drying soil no longer contributed to transpiration. However, midday root ψw in the dry container (-0.29 MPa) remained much higher than that of the surrounding soil (-1.0 MPa) after 15 d of drying, indicating that the roots in drying soil were rehydrated in the dark.To prove that the inhibition of leaf elongation was not caused by undetectable changes in leaf water status as a result of loss of half the watergathering capacity, one-half of the root system of control plants was excised. This treatment had no effect on leaf elongation or stomatal conductance. The inhibition of leaf elongation was also not explained by reductions in nutrient supply.Soil drying had no effect on stomatal conductance despite variations in the rate or extent of soild drying, light, humidity or nutrition. The results indicate that non-hydraulic inhibition of leaf elongation may act to conserve water as the soil dries before the occurrence of shoot water deficits.

Development of aerenchyma (soft cortical tissue with large intercellular air spaces) in flooded plants results from cell-wall hydrolysis and eventual cell lysis and i s promoted by endogenous ethylene. Despite its adaptive significance, the molecular mechanisms behind aerenchyma development remain unknown. We recently isolated a flooding-induced maize (Zea mays 1.) gene (wus/7005[gful; abbreviated as 7005) encoding a homolog of xyloglucan endo-transglycosylase (XET), a putative cell-wall-loosening enzyme active during germination, expansion, and fruit softening. XET and related enzymes may also be involved in cell-wall metabolism during flooding-induced aerenchyma development. Under flooding, 1005 mRNA accumulated in root and mesocotyl locations that subsequently exhibited aerenchyma development and reached maximum levels within 12 h of treatment. Aerenchyma development was observed in the same locations by 48 h of treatment. Treatment with the ethylene synthesis inhibitor (aminooxy)acetic acid (AOA), which prevented cortical air space formation under flooding, almost completely inhibited 1005 mRNA accumulation in both organs. AOA treatment had little effect on the accumulation of mRNA encoded by adhl, indicating that it did not cause general suppression of flooding-responsive genes. Additionally, ethylene treatment under aerobic conditions resulted in aerenchyma development as well as induction of 7005 in both organs. These results indicate that 7005 is responsive to ethylene. Treatment with anoxia, which suppresses ethylene accumulation and aerenchyma development, also resulted in 7005 induction. However, in contrast to flooding, AOA treatment under anoxia did not affect 1005 mRNA accumulation, indicating that 7005 is induced via different mechanisms under flooding (hypoxia) and anoxia.Plants respond to flooding by undergoing changes at the molecular, biochemical, and cell structural levels. To date, the majority of molecular-leve1 research on flooding responses has focused on the enzymology of energy production, namely the induction of genes encoding enzymes of Glc-P metabolism ( e g ADH, enolase, and glyceraldehyde-3-P dehydrogenase). These enzymes allow limited energy production in the face of limited oxygen supply and thus aid in survival during short-term flooding (reviewed by Sachs et al., 1996).