THE CELLULAR PATHWAY OF POSTPHLOEM SUGAR-TRANSPORT IN DEVELOPING TOMATO FRUIT

The cellular pathway of postphloem sugar transport was elucidated in t he outer pericarp of tomato (Lycopersicon esculentum Mill cv. Floradad e) fruit at 13-14 and 23-25 days after anthesis (DAA). These developme ntal stages are characterized by phloem-imported sugars being accumula ted as starch and hexose, respectively. The symplasmic tracer, 5(6)-ca rboxyfluorescein, loaded into the storage parenchyma cells of pericarp discs, moved readily in the younger fruit but was immobile in fruit a t 23-25 DAA. Symplasmic mobility of [C-14]glucose was found to be iden tical to 5(6)-carboxyfluorescein. For the older fruit, the pericarp ap oplasm was shown to be freely permeable to the apoplasmic tracer, tris odium 3-hydroxy-5,8,1 0-pyrenetrisulfonate. Indeed, the transport capa city of the pericarp apoplasm was such that the steady-state rate of i n-vitro glucose uptake by pericarp discs accounted fully for the estim ated rate of in-vivo glucose accumulation. For fruit at 23-25 DAA, the inhibitory effects of the sulfhydryl group modifier, p-chloromercurib enzenesulfonic acid (PCMBS), on [C-14]glucose and [C-14]fructose uptak e by the pericarp discs depended on the osmolality of the external sol ution. The inhibition was most pronounced for pericarp discs enriched in storage parenchyma. Consistent with the PCMBS study, strong fluores cent signals were exhibited by the storage parenchyma cells of pericar p discs exposed to the membrane-impermeable thiol-binding fluorochrome , monobromotrimethylammoniobimane. The fluorescent weak acid, sulphorh odamine G, was accumulated preferentially by the storage parenchyma ce lls. Accumulation of sulphorhodamine G was halted by the ATPase inhibi tor erythrosin B, suggesting the presence of a plasma-membrane-bound H +-ATPase. A linkage between the putative H+-ATPase activity and hexose transport was demonstrated by an erythrosin-B inhibition of [C-14]glu cose and [C-14]fructose uptake. In contrast, comparable evidence for a n energy-coupled hexose porter could not be found in the pericarp of y ounger fruit at 13-14 DAA. Overall, the data are interpreted to indica te that: (i) The postphloem cellular pathway in the outer fruit perica rp shifts from the symplasm during starch accumulation (13-14 DAA) to the apoplasm for rapid hexose accumulation (23-25 DAA). (ii) An energy -coupled plasma-membrane hexose carrier is expressed specifically in s torage parenchyma cells at the latter stage of fruit development.