Biochemische Untersuchungen zur Protonenabscheidung von Proteoidwurzeln der Weißlupine

Abstract

White lupin is able to develop proteoid roots in the course of a morphological and physiological adaptation to phosphorus deficiency in soil. These proteoid roots may release large amounts of organic acid, acid phosphatase, and phenolic substances to mobilize the insoluble phosphorus in the rhizosphere. An agar technique with bromocresol purple as pH indicator has shown a strong acidification of the rhizosphere around proteoid roots of white lupin. Due to high cytosolic pH the organic acids may only be released as anions and therefore a second transport systems for the efflux of H⁺ has to be postulated. Proton release out of plant cells results from the activity of plasma membrane H⁺ ATPase. This enzyme acts as a primary transporter by pumping protons out of the cell, thereby creating pH and electric potential differences across the plasmalemma. The proton-motive force created by H⁺ ATPase may be involved in the efflux of the organic anions. Therefore, the adaptation of plasma membrane H⁺ ATPase in proteoid roots to P deficiency may be a prerequisite for the synthesis and release of organic acids. In our study, white lupin (Lupinus albus L.) was grown under P deficiency conditions in a nutrient solution for four weeks. Plasma membrane of active proteoid roots was isolated by means of two-phase partitioning in aqueous dextran T-500 and PEG-3350.

In comparison to the laterals of P sufficient plants, the plasma membrane H⁺ ATPase of proteoid roots induced by P deficiency showed an increase in catalytic activity, a more acidic pH optimum, an increase in V _max’ unchanged K _m’ lower activation energy and higher H⁺ pumping activity.