Cu2+ Reduction by Tomato Root Plasma Membrane Vesicles.

Reduction of Cu2+ by plasma membrane vesicles isolated from tomato (Lycopersicon esculentum Mill.) roots was investigated. Plants were grown in hydroponic culture with complete nutrition for 4 weeks or were deprived of Fe for the last 7 d. Plasma membrane vesicles were prepared by aqueous two-phase partitioning. Reduction of Cu, Fe, and ferricyanide by plasma membrane vesicles was measured. An increase in the activity of all three pyridine-nucleotide-dependent activities was noted in plasma membrane preparations from Fe-deficient, compared to Fe-sufficient, plants. Solubilization and chromatographic separation of two plasma membrane electron transport systems indicated that the Fe-chelate reductase was probably responsible for reduction of Cu. Assays used a variety of Cu chelates, and for each the Cu activity in the assay was determined by the program Geochem PC. The rate of reduction of Cu correlated with the level of Cu activity, and results support the idea that free Cu2+ and not Cu chelates may serve as the true substrate for reduction. Reduction was observed only in assays in which Cu activity was equivalent to Cu-enriched or Cu-toxic soils. These results suggest that reduction of Cu by tomato root may have little or no physiological relevance under conditions experienced by the root in the soil.