Proton electrochemical gradients and sucrose accumulation in the maize scutellum

Abstract

Estimates of proton and sucrose gradients across the plasmalemma of maize scutellum cells were made in order to test previous conclusions that sucrose uptake occurs by a proton-sucrose symport with a 1:1 stoichiometry. Membrane potential and cytoplasmic pH were estimated from the distribution between cells and bathing solution of [ 14C]tetraphenylphosphonium ions and 2,4-dinitrophenol (DNP), respectively. External pH in the range 3–8 had no effect on cytoplasmic pH which was 8.0, but external pH strongly influenced membrane potential which was 40 mV (inside negative) at pH 3 and 140 mV at pH 7–8. Additions of KCl and NaCl caused some depolarization of the membrane potential, as did the energy poison, DNP, but mannose (an energy poison in scutellum cells) had no effect. The electrochemical proton gradient (protonmotive force) varied from 340 mV at pH 3 to 163 mV at pH 7.6. During 5 hr incubations at pH 4 or 7, bathing solution sucrose [S] o appeared not to reach a steady-state with tissue sucrose [S] i . The non-steady-state sucrose gradients were too small at pH 4 and too large at pH 7 to be accounted for only by the operation of a proton-sucrose symport. Sucrose gradients also were calculated from the measured protonmotive forces on the assumption that cytosol [S], in contrast to [S] i , had reached a steady-state with [S] o at both pHs. It was concluded from these calculations that leak and/or ‘slip’ (sucrose exit via the porter without a proton) pathways for sucrose as well as a proton-sucrose cotransporter are present in the plasmalemma and that sucrose gradients exist across the tonoplast.