Cadmium induces acidosis in maize root cells

Abstract: Cadmium (Cd) stress increases cell metabolic demand for sulfur, reducing equivalents, and carbon skeletons, to sustain phytochelatin biosynthesis for Cd detoxification. In this condition the induction of potentially acidifying anaplerotic metabolism in root tissues may be expected. For these reasons the effects of Cd accumulation on HCO3- anaplerotic metabolism, glycolysis, and cell pH control mechanisms were investigated in maize (Zea mays) roots. The study compared root apical segments, excised from plants grown for 24 h in a nutrient solution supplemented, or not, with 10 mu M CdCl2, using physiological, biochemical and P-31-nuclear magnetic resonance (NMR) approaches. Cadmium exposure resulted in a significant decrease in both cytosolic and vacuolar pH of root cells and in a concomitant increase in the carbon fluxes through anaplerotic metabolism leading to malate biosynthesis, as suggested by changes in dark CO2 fixation, metabolite levels and enzyme activities along glycolysis, and mitochondrial alternative respiration capacity. This scenario was accompanied by a decrease in the net H+ efflux from the roots, probably related to changes in plasma membrane permeability. It is concluded that anaplerotic metabolism triggered by Cd detoxification processes might lead to an imbalance in H+ production and consumption, and then to cell acidosis.