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PLANT PHYSIOLOGY , Vol 105, Issue 3 981-987, Copyright © 1994 by American Society of Plant Biologists
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ENVIRONMENTAL AND STRESS PHYSIOLOGY |
Proline Accumulation in Maize (Zea mays L.) Primary Roots at Low Water Potentials (I. Requirement for Increased Levels of Abscisic Acid)
E. S. Ober and R. E. Sharp
Department of Agronomy, University of Missouri-Columbia, Columbia, Missouri 65211
Previous work showed that the concentration of proline (Pro) increases
greatly in the primary root tip of maize (Zea mays L.) at low water
potentials ([psi]w). It was also shown that the maintenance of root
elongation at low [psi]w depends on increased levels of abscisic acid
(ABA). In this study we have assessed whether ABA is required for the
increase in Pro concentration. Seedlings were grown in vermiculite of
various [psi]w, and endogenous ABA levels were decreased using either
fluridone (FLU) or the vp5 mutant to inhibit carotenoid (and ABA)
synthesis. In both treatments, Pro concentrations at low [psi]w were
substantially decreased throughout the apical centimeter, which encompassed
the elongation zone. Pro concentrations in FLU-treated roots were restored
by addition of 7 [mu]M ABA to the vermiculite, which raised the internal
ABA content to the level in untreated roots at the same [psi]w. Pro and
water content profiles were combined with published growth-velocity
distributions to calculate the distribution of net Pro and water deposition
rates using the continuity equation. At a [psi]w of -1.6 MPa, the rate of
Pro deposition in the root tip was decreased by 75% in FLU-treated compared
to untreated roots. FLU treatment increased root diameter and, therefore,
water content per unit length, but water deposition rates decreased due to
the dominant influence of reduced longitudinal expansion. Thus, the
decrease in Pro concentration was attributable entirely to the decrease in
Pro deposition. The results demonstrate that increased ABA is required for
high rates of Pro deposition and, thereby, high Pro concentrations in the
growing region of maize primary roots at low [psi]w.
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