ALUMINUM EFFECTS ON ATPASE ACTIVITY AND LIPID-COMPOSITION OF PLASMA-MEMBRANES IN SUGAR-BEET ROOTS

The effect of aluminium (Al) in vivo and in vitro on root plasma membranes has been studied in two sugar beet (Beta vulgaris L.) cultivars, Monohill (Al-sensitive) and Regina (relatively Al-tolerant). Although Al in vitro inhibited the MgATPase in an uncompetitive way for both cultivars raised in the absence of Al, the specific K+-activation of the MgATPase was only inhibited by Al in cv. Monohill. Arrhenius analysis of the MgATPase activity showed that the effect of Al in vitro depended on whether or not the plants were exposed to Al in vivo. Al treatment in vitro of the MgATPase from control plants cultivated at a low pH (5.4) caused an increase in the phase transition temperature from 17 to 22-degrees-C. Only at a higher pH range (pH 6.1) could a second transition temperature be induced (at 9-degrees-C). By addition of Al in vitro to plants cultivated with Al at pH 5.4, the slopes of the activity plots did not change. Aluminium changed the K(m) of the ATPase for MgATP in an opposite way by treatment in vivo and in vitro. Lipid analyses of the plasma membranes showed that the acyl composition differed little following Al treatment in vivo, but that the ratio of phosphatidylcholine: phosphatidylethanolamine increased. The changes correlated with the observed change in the K(m) for the MgATPase. We conclude that the main effect of Al on the MgATPase is not due to the formation of an Al-ATP complex. Instead, Al may bind to the membrane-bound enzyme(s) and/or modify the lipid environment.