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 z one increased their yielding ability as an adaptive response to low tu rgor and water potential (psi(w)). To test this hypothesis more direct ly, we measured the acid-induced extension of isolated walls from root s grown at high (-0.03 MPa) or low (-1.6 MPa) psi(w) using an extensom eter. 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 psi(w). This pattern is consistent with the maintenance of elonga tion toward the apex and the shortening of the elongation zone in thes e roots. Wall proteins extracted from the elongation zone possessed ex pansin activity, which increased substantially in roots grown at low p si(w). Western blots likewise indicated higher expansin abundance in t he roots at low psi(w). Additionally, the susceptibility of walls to e xpansin action was higher in the apical 5 mm of roots at low psi(w) th an in roots at high psi(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 pla y a role in enhancing cell-wall yielding and, therefore, in maintainin g elongation in the apical region of maize primary roots at low psi(w) .