Gj. Allen et al., SODIUM-TRANSPORT MEASURED IN PLASMA-MEMBRANE VESICLES ISOLATED FROM WHEAT GENOTYPES WITH DIFFERING K+ NA+ DISCRIMINATION TRAITS/, Plant, cell and environment, 18(2), 1995, pp. 105-115
Right-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 po
tential (Delta Psi)-dependent Na-22(+) influx and sodium/proton (Na+/H
+) antiport-mediated efflux across the plasma membrane were studied us
ing these vesicles, Membrane potentials were imposed on the vesicles u
sing either K+ gradients in the presence of valinomycin or H+ gradient
s, The Delta Psi was quantified by the uptake of the lipophilic cation
tetraphenylphosphonium. Uptake of Na+ into the vesicles was stimulate
d by a negative Delta Psi and had a K-m for extravesicular Na+ of 34.8
+/-5.9 mol m(-3). The Delta Psi-dependent uptake of Na+ was similar in
vesicles from roots of hexaploid (cv, Troy) and tetraploid (cv, Langd
on) wheat differing in a K+/Na+ discrimination trait, and was also una
ffected by growth in 50 mol m(-3) NaCl. Inhibition of Delta Psi-depend
ent Na+ uptake by Ca2+ was greater in the hexaploid than in the tetrap
loid. 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 C-14-methylamine. The N
a+/H+ antiport had a K-m for intravesicular Na+ of between 13 and 19 m
ol m(-3). In the hexaploid, Na+/H+ antiport activity was greater when
roots were grown in the presence of 50 mol m(-3) NaCl, and was also gr
eater than the activity in salt-grown tetraploid wheat roots, Antiport
activity was not increased in a Langdon 4D chromosome substitution li
ne which carries a trait for K+/Na+ discrimination, It is concluded th
at neither of the transport processes measured is responsible for the
Na+/K+ discrimination trait located on the 4D chromosome of wheat.