lncreased Activity of Xyloglucan Endotransglycosylase and Its Possible Regulation by Abscisic Acid

Previous work suggested that an increase in cell wall-loosening contributes to the maintenance of maize (Zea mays 1.) primary root elongation at low water potentials (h). It was also shown that root elongation at low +w requires increased levels of abscisic acid (ABA). In this study we investigated the effects of low +w and ABA status on xyloglucan endotransglycosylase (XET) adivity in the root elongation zone. XET is believed to contribute to wall-loosening by reversibly cleaving xyloglucan molecules that tether cellulose microfibrils. l h e adivity of XET per unit fresh weight in the apical 10 mm (encompassing the elongation zone) was constant at high $, but increased by more than 2-fold ata +w of -1.6 MPa. Treatment with fluridone to decrease ABA accumulation greatly delayed the increase in activity at low GW. This effect was largely overcome when interna1 ABA levels were restored by exogenous application. Spatial distribution studies showed that XET activity was increased in the apical 6 mm at low +w whether expressed per unit fresh weight, total soluble protein, or cell wall dry weight, corresponding to the region of continued elongation. Treatment with fluridone progressively inhibited the increase in activity with distance from the apex, correlating with the pattern of inhibition of elongation. Added ABA partly restored activity at all positions. l h e increase in XET activity at low +w was due to maintenance of the rate of deposition of adivity despite decreased deposition of wall material. The loss of activity associated with decreased ABA was due to inhibition of the deposition of activity. l h e results demonstrate that increased XET activity i s associated with maintenance of root elongation at low qW and that this response requires increased ABA.