title: Quantitative competition of calcium with sodium or magnesium for sorption sites on plasma membrane vesicles of melon (Cucumis melo L.) root cells.
abstract: The presence of Ca2+ ions in solution is vital for root growth. The plasma membrane is one of the first sites where competition between Ca2+ and other ions occurs. We studied the competition between Ca2+ and Na+ or Mg2+ for sorption sites on the plasma membrane of melon root cells. Sorption of 45Ca2+ to right-side-out PM vesicles of melon (Cucumis melo L.) roots (prepared by aqueous two-phase partitioning) was studied at various Ca2+ concentrations, in the presence of increasing concentrations of Na+ or Mg2+ chlorides. Experimentally determined amounts of Ca2+ sorbed to the plasma membrane vesicles agreed fairly well with those calculated from a competitive sorption model. The best fit of the model to the experimental data was obtained for an average surface area of 370 A2 per charge, and binding coefficients for Na+, Mg2+ and Ca2+ of 0.8, 9 and 50 M-1, respectively. Our results suggest that nonphospholipid components in the plasma membrane contribute significantly to Ca2+ binding. The high affinity of Ca2+ binding to the plasma membrane found in this study might explain the specific role of Ca2+ in relieving salt stress in plant roots.
authors.affiliation: Department of Field and Vegetable Crops, Faculty of Agriculture, Hebrew University of Jerusalem, Rehovot, Israel.
authors.forename: U
authors.initials: U
authors.lastname: Yermiyahu
authors.forename: S
authors.initials: S
authors.lastname: Nir
authors.forename: G
authors.initials: G
authors.lastname: Ben-Hayyim
authors.forename: U
authors.initials: U
authors.lastname: Kafkafi
mesh_terms: D001665:Binding Sites; D001667:Binding, Competitive; D002118:Calcium / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D008274:Magnesium / Q000378:metabolism*; D008954:Models, Biological; D010944:Plants / Q000033:anatomy & histology / Q000378:metabolism* / Q000648:ultrastructure; D012964:Sodium / Q000378:metabolism*
chemical_list: D012964:Sodium; D008274:Magnesium; D002118:Calcium