Sodium transport measured in plasma membrane vesicles isolated from wheat genotypes with differing K+/Na+ discrimination traits

Abstract

ABSTRACTRight‐side‐out plasma membrane vesicles were isolated from wheat roots using an aqueous polymer two‐phase system. The purity and orientation of the vesicles were confirmed by marker enzyme analysis. Membrane potential (Ψ)‐dependent 22Na+ influx and sodium/proton (Na+/ H+) antiport‐mediated efflux across the plasma membrane were studied using these vesicles. Membrane potentials were imposed on the vesicles using either K+ gradients in the presence of valinomycin or H+ gradients. The ΔΨ was quantified by the uptake of the lipophilic cation tetraphenylphosphonium. Uptake of Na+ into the vesicles was stimulated by a negative ΔΨ and had a Km for extrav‐esicular Na+ of 34.8 ± 5.9 mol m3. The ΔΨ‐dependent uptake of Na+ was similar in vesicles from roots of hexaploid (cv. Troy) and tetraploid (cv. Langdon) wheat differing in a K+/Na+ discrimination trait, and was also unaffected by growth in 50 mol m−3 NaCl. Inhibition of ΔΨ‐dependent Na+ uptake by Ca2+ was greater in the hexaploid than in the tetraploid.Sodium/proton antiport was measured as Na+‐dependent, amiloride‐inhibited pH gradient formation in the vesicles. Acidification of the vesicle interior was measured by the uptake of 14C‐methylamine. The Na+/H+ antiport had a Km, for intravesicular Na+ of between 13 and 19 mol m−3. In the hexaploid, Na+/H+ antiport activity was greater when roots were grown in the presence of 50 mol m−3NaCl, and was also greater than the activity in salt‐grown tetraploid wheat roots. Antiport activity was not increased in a Langdon 4D chromosome substitution line which carries a trait for K+/Na+ discrimination. It is concluded that neither of the transport processes measured is responsible for the Na+/K+ discrimination trait located on the 4D chromosome of wheat.