Carbohydrate transport in discs of storage parenchyma of celery petioles: 2. Uptake of mannitol.

Abstract

The sugar alcohol mannitol is, together with glucose and fructose, the main soluble carbohydrate of celery petioles. It is accumulated in the storage parenchyma of mature petioles and remobilized during senescence. The mechanisms of mannitol uptake into discs isolated from petiole parenchyma was studied. Uptake kinetics showed a biphasic response to increasing external concentrations of [14 C]D-mannitol with a predominant saturable component at low concentrations (below about 2 mM) and a linear component at higher concentrations (at least up to 16 mM). The saturable component showed an apparent Km of about 1 mM and a fairly sharp pH optimum around pH 6-5. It was stimulated by the plasmalemma H+ -ATPase stimulator FC and inhibited by the SH-reagents PCMBS and NEM and the uncoupler DNP. CCCP was also inhibitory, but its effect could be reversed by DTT, suggesting a dual role of CCCP as SH-reagent and uncoupler in this tissue. Competition studies with 14-different polyols and sugars showed that uptake was very specific. The saturable uptake component was clearly turgor-dependent. Lowering of cell turgor by treatment of increasing concentrations of the relatively nonpermeant glycinebetaine (0 to 0.8 M) resulted in a linear increase of Vmax and an unaltered Km . It is concluded that celery petiole parenchyma contains at least a mannitol carrier at the plasmalemma which operates in an active proton-cotransport manner. It is speculated that a superimposed mannitol carrier is present at the tonoplast and might operate by facilitated diffusion.