The modulating effects of pH and benzyladenine on the Ca2+– and Mg2+‐ATPases in the plasmalemma of wheat root cells

Abstract

Plant cells frequently and rapidly have to respond to environmental changes for survival. Regulation of transport and other energy‐requiring processes in the plasmalemma of root cells is therefore one important aspect of the ecological adaptation of plants. Wheat (Triticum aestivum L. cv. Drabant) was grown hydroponically, with or without 50 nM benzyladenine in the medium, and plasma membranes from root cells of 8‐day‐old plants were prepared by aqueous polymer two‐phase partitioning. The influence of Ca2+ and Mg2+ on the plasmalemma ATPase activities was investigated. The presence of benzyladenine during growth increased the ATPase activity, that dependent upon Ca2+ more than that elicited by Mg2+. As a general characteristic, ATP was the preferred substrate, but all nucleotide tri‐ and diphosphates could be accepted with activities in plasma membranes from control plants of 7‐36% (Mg2+) and 40‐86% (Ca2+) and in plasma membranes from benzyladenine‐treated plants of 12‐47% (Mg2+) and 53‐102% (Ca2+) as compared with activities obtained with ATP. Nucleotidemonophosphates were not hydrolyzed by the preparations. In preparations from benzyladenine‐treated plants one peak of Ca2+‐ATPase at pH 5.2–5.6, with a tail from pH 6 and upwards, and one peak of Mg2+‐ATPase at pH 6.0–6.5 were observed in the presence of EDTA in the assay media. In preparations from control plants, the addition of EDTA to the assays resulted in a wide optimum between pH 6 and 7 for Mg2+‐ATPase and low Ca2+‐ATPase activity with no influence of pH in the range 4.5 to 8. Analysis of the pH dependence in the presence of both Ca2+ and Mg2+ indicates that the control plants mainly contain Mg2+‐ATPase corresponding to the proton pump. Preparations from benzyladenine‐treated wheat roots show, in addition, activation by Ca2+, which, in the slightly alkaline pH range may correspond to a Ca2+‐extruding (Ca2++ Mg2+)‐ATPase. In the acidic range, the responses are more complicated: the Mg2+‐ATPase is inhibited by vanadate, while the Ca2+‐ATPase is insensitive, and benzyladenine added during growth influences the interaction between Ca2+ and Mg2+ in a way that parallels the effect of high salt medium.