CHANGES OF SOME PROPERTIES OF THE PLASMA MEMBRANE-ENRICHED FRACTION OF BARLEY ROOTS RELATED TO ALUMINUM STRESS - MEMBRANE-ASSOCIATED ATPASE, ALUMINUM AND CALCIUM

A microsomal membrane fraction was prepared from barley roots which were treated with and without 1 mm AlCl3. Plasma membrane was dominant in the fraction based on the inhibitor experiment of ATPase activity. ATPase activity of the microsomal membrane fraction decreased after treatment of roots with aluminum. The microsomal membrane fraction was fractionated on a continuous density gradient of sucrose, and the amount of Ca2+ and aluminum associated with the membrane was investigated. The ATPase activity of roots treated with 1 mol m-3 AlCl3 for 5 d decreased to 55.3% of that of the control sample. The amount of Ca2+ associated with the membrane of the Al-treated sample was approximately half as large as that of the control, while substantial binding of Al to the membrane was observed in the sample prepared from the aluminum-treated roots. The K(m) values for ATP of the fractionated ATPase was 0.45 mol m-3 in both the aluminum-treated and control sample while V(max) was smaller in the Al-treated sample. Furthermore, the optimum pH was different between the ATPase prepared from the control and that from the aluminum-treated roots. Activation of ATPase with asolectin, a soybean phospholipid, was revealed in the sample prepared from the Al-treated roots but not in the control one. These results suggest that the structure of the plasma membrane in barley roots was impaired by aluminum stress.