Proton Fluxes Associated with Sugar Uptake in Vicia faba Leaf Tissues

Abstract

V. faba leaf fragments bring the pH of their incubation medium to about 4.7, whatever the initial pH value. At this pH, addition of 20 mM sucrose causes a transient (20-40 min) alkalinization (0.05-0.10 pH unit) of the medium. The alkalinization is not observed in the presence of p-chloromercuribenzenesulfonic acid which blocks the sucrose carrier involved in phloem loading without affecting the ATPase. Addition of 20 mM glucose, fructose, or 3-O-methylglucose induces weaker alkalinization than sucrose. Sequential additions of sugars show that sucrose- and hexose-induced proton fluxes are nearly saturated at 20 mM sugar; there is no competition between sucrose and hexoses for inducing proton influxes whereas glucose and 3-O-methylglucose are competing for a common system. Autoradiographs performed under the conditions used for the observation of proton fluxes show a slight accumulation of [14C]sucrose into the veins within 2 min of uptake, whereas [14C]glucose and 3-O-methyl [14C]glucose are localized in the mesophyll. These data support the proton-sucrose cotransport hypothesis of phloem loading and show that mesophyll cells are able to take up hexoses by symport with protons. The apparent sucrose/proton stoichiometry is constant below 5 mM sucrose (about 1.9 sucrose per proton taken up) but increases up to 6 sucrose per proton, between 5 and 15 mM sucrose. This confirms a previous study indicating that above 5 mM sucrose, a system which exhibits little pH dependence is involved in the uptake. Simultaneous measurements of H+ and K+ fluxes indicate that sucrose uptake is accompanied by a reduction of K+ uptake rate, suggesting that sucrose and K+ uptake can compete in dissipating the protonmotive force.