Mannitol transport by vacuoles of storage parenchyma of celery petioles operates by facilitated diffusion

The sugar alcohol mannitol is the main water-soluble carbohydrate of the storage parenchyma of celery petioles. Compartmentation studies had indicated that mannitol was about equally distributed between the large central vacuole and the cytosol of these cells (Keller, F. and P. Matile, New Phytol., 113, 291-299 [1989]). Evidence is now presented for a tonoplast-bound mannitol carrier that operates by facilitated diffusion, i.e. protein-mediated and passively. With isolated parenchyma vacuoles it was shown that the uptake rate of mannitol, at 1 mmol/L external concentration, was about twice that of L-glucose and glucitol (sorbitol), both established markers for simple diffusion. Mannitol uptake was a saturable process with apparent K-m and V-max values of 14 mmol/L and 0.067 nmol/mu L vacuoles.min, respectively. It was also quite specific as indicated by competition studies with glucitol, galactitol, glucose, and fructose. The functionality of the mannitol carrier was unaffected by three protein-modifying reagents, the sulfhydryl group modifiers p-chloro mercuribenzenesulfonic acid (PCMBS) and N-ethylmaleimide (NEM) and the histidine modifier diethyl pyrocarbonate (DEPC). Mannitol uptake was neither stimulated by energization with ATP or pyrophosphate nor was it impaired by dissipation of the pH gradient or the membrane potential and is, therefore, passive. The implications of this facilitated diffusion mechanism for the proposed physiological functions of mannitol in celery are discussed.