Root Growth Maintenance at Low Water Potentials (Increased 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 L.) primary root elongation at low water potentials (psi]w). It was also shown that root elongation at low psi]w requires increased levels of abscisic acid (ABA). In this study we investigated the effects of low psi]w and ABA status on xyloglucan endotransglycosylase (XET) activity in the root elongation zone. XET is believed to contribute to wall-loosening by reversibly cleaving xyloglucan molecules that tether cellulose microfibrils. The activity of XET per unit fresh weight in the apical 10 mm (encompassing the elongation zone) was constant at high psi]w but increased by more than 2-fold at a psi]w of -1.6 MPa. Treatment with fluridone to decrease ABA accumulation greatly delayed the increase in activity at low psi]w. This effect was largely overcome when internal ABA levels were restored by exogenous application. Spatial distribution studies showed that XET activity was increased in the apical 6 mm at low psi]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. The increase in XET activity at low psi]w was due to maintenance of the rate of deposition of activity despite decreased deposition of wall material. The loss of activity associated with decreased ABA was due to inhibition of the deposition of activity. The results demonstrate that increased XET activity is associated with maintenance of root elongation at low psi]w and that this response requires increased ABA.