title: Proton Fluxes Associated with Sugar Uptake in Vicia faba Leaf Tissues.
abstract: Vicia 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 millimolar sucrose causes a transient (20 to 40 minutes) alkalinization (0.05 to 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 (Delrot, Despeghel, Bonnemain 1980 Planta 149: 144-148). Addition of 20 millimolar glucose, fructose, or 3-O-methylglucose induces weaker alkalinization than sucrose. Sequential additions of sugars show that: (a) sucrose- and hexose-induced proton fluxes are nearly saturated at 20 millimolar sugar (b) there is no competition between sucrose and hexoses for inducing proton influxes whereas (c) 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 [(14)C]sucrose into the veins within 2 minutes of uptake, whereas [(14)C]glucose and 3-O-methyl [(14)C]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 millimolar sucrose (about 1.9 sucrose per proton taken up) but increases up to 6 sucrose per proton, between 5 and 15 millimolar sucrose. This confirms our previous study indicating that above 5 millimolar 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, thus suggesting that sucrose and K(+) uptake can compete in dissipating the protonmotive force.
authors: Station Biologique de Beau-Site, 25 Faubourg St-Cyprien, 86000 Poitiers, France.
authors: S
authors: S
authors: Delrot