Growth Maintenance of the Maize Primary Root at Low Water Potentials Involves Increases in Cell-Wall Extension Properties, Expansin Activity, and Wall Susceptibility to Expansins

Previous work on the growth biophysics of maize (Zea mays L.) primary roots suggested that cell walls in the apical 5 mm of the elongation zone increased their yielding ability as an adaptive response to low turgor and water potential (Ψ_w). To test this hypothesis more directly, we measured the acid-induced extension of isolated walls from roots grown at high (-0.03 MPa) or low (-1.6 MPa) Ψ_w using an extensometer. Acid-induced extension was greatly increased in the apical 5 mm and was largely eliminated in the 5- to 10-mm region of roots grown at low Ψ_w. This pattern is consistent with the maintenance of elongation toward the apex and the shortening of the elongation zone in these roots. Wall proteins extracted from the elongation zone possessed expansin activity, which increased substantially in roots grown at low Ψ_w. Western blots likewise indicated higher expansin abundance in the roots at low Ψ_w. Additionally, the susceptibility of walls to expansin action was higher in the apical 5 mm of roots at low Ψ_w than in roots at high Ψ_w. The basal region of the elongation zone (5-10 mm) did not extend in response to expansins, indicating that loss of susceptibility to expansins was associated with growth cessation in this region. Our results indicate that both the increase in expansin activity and the increase in cell-wall susceptibility to expansins play a role in enhancing cell-wall yielding and, therefore, in maintaining elongation in the apical region of maize primary roots at low Ψ_w.