CHARACTERISTICS OF SUCROSE TRANSPORT AND SUCROSE-INDUCED H+ TRANSPORT ON THE TONOPLAST OF RED BEET (BETA-VOLGARIS L) STORAGE TISSUE

Sucrose-induced changes of the energization state of the red beet root (Beta vulgaris L. ssp. conditiva) vacuolar membrane were observed with the fluorescent dyes 6-chloro-9-{[4-(diethylamino)-1-methylbutyl]-amino}-2-methoxyacridine dihydrochloride, as a pH monitor, and 9-amino-6-chloro-2-methoxyacridine (ACMA). Consequently, transient acidification of the surrounding suspension medium could be measured with a pH electrode. This signal was specific for Suc and was not seen for sorbitol, mannitol, or maltose. Sucrose-induced medium acidification was sensitive to the same inhibitors that were efficient in inhibiting sucrose transport, including the monoclonal antibodies TNP56-12 and C-50-5-3. It was seen with vacuoles and vesicles energized with MgATP before sucrose was added but also with vacuoles not artificially energized previously. Using bafilomycin A, for the inhibition of the vacuolar ATPase of vacuoles previously energized by MgATP, apparent K-m values for H+ export from the vacuoles to the medium could be calculated taking into account the passive proton leak. Apparent K, values for H+ export determined from data obtained with pH measurements in the medium and with ACMA corresponded to those obtained previously for sucrose uptake. Comparing sucrose uptake rates with corresponding H+ export rates at the respective sucrose concentrations and at K-m, a stoichiometry of approximately one proton per transported sucrose was estimated.