Growth of the Maize Primary Root at Low Water Potentials

Abstract

Seedlings of maize (Zea mays L. cv WF9 x Mol7) growing at low water potentials in vermiculite contained greatly increased proline concentrations in the primary root growth zone. Proline levels were particularly high toward the apex, where elongation rates have been shown to be completely maintained over a wide range of water potentials. Proline concentration increased even in quite mild treatments and reached 120 millimolal in the apical millimeter of roots growing at a water potential of -1.6 megapascal. This accounted for almost half of the osmotic adjustment in this region. Increases in concentration of other amino acids and glycinebetaine were comparatively small. We have assessed the relative contributions of increased rates of proline deposition and decreased tissue volume expansion to the increases in proline concentration. Proline content profiles were combined with published growth velocity distributions to calculate net proline deposition rate profiles using the continuity equation. At low water potential, proline deposition per unit length increased by up to 10-fold in the apical region of the growth zone compared to roots at high water potential. This response accounted for most of the increase in proline concentration in this region. The results suggest that osmotic adjustment due to increased proline deposition plays an important role in the maintenance of root elongation at low water potentials.apex, resulting in a shorter growth zone. Root radial expansion was also inhibited. In the succeeding paper (18), hexose was shown to make the major contribution to osmotic adjustment in basal locations. This was due primarily to the growth inhibition in that region, because hexose deposition rates were calculated to decrease rather than increase (18). In contrast, hexose (and the other measured solutes, sucrose and potassium) accounted for little of the osmotic adjustment in the apical region, where elongation was fully maintained despite very low 0,. These results indicated that other solutes must be preferentially deposited in that region. Because cells close to the apex are only slightly vacuolated, our objective in this paper was to examine the contributions of proline and glycinebetaine to osmotic adjustment. These compounds have been suggested to act as cytoplasmic solutes, which are compatible at high concentrations with metabolism (25, 28). We show that proline accounts for as much as 50% ofthe osmotic adjustment in the apical region and that this response involves a dramatic increase in the rate ofproline deposition, expressed per unit root length or volume.

Osmolyte accumulation: can it really help increase crop yield under drought conditions?

“…OA occurs in roots and has an impact on root elongation (Voetberg & Sharp 1991). Data obtained on wheat (Morgan & Condon 1986;Morgan 1995) clearly show a greater depth of water extraction in high osmotic-adjusting lines in comparison with low osmotic-adjusting lines.…”

Translatable RNA Populations Associated with Maintenance of Primary Root Elongation and Inhibition of Mesocotyl Elongation by Abscisic Acid in Maize Seedlings at Low Water Potentials

“…This increase in activity was suppressed when ABA accumulation and elongation were inhibited by treatment with FLU and could be restored with exogenous ABA. It has also been demonstrated that the accumulation of ABA in the primary root at low i// w is required both for the dramatic increase in Pro concentration that occurs in the apical region (Voetberg and Sharp, 1991;Ober and Sharp, 1994) and for restriction of ethylene synthesis and/or sensitivity . Both effects probably involve control at the level of gene regulation.…”

Molecular Cloning and Evidence for Osmoregulation of the Δ1-Pyrroline-5-Carboxylate Reductase (proC) Gene in Pea (Pisum sativum L.)

“…In some tissues, proline levels may increase as much as 100-fold in response to stress. In corn roots grown at low water potentials, proline accumulation represented approximately 45% of the total osmotic adjustment (35).…”