Yl. Ruan et al., PROTOPLAST HEXOSE CARRIER ACTIVITY IS A DETERMINATE OF GENOTYPIC DIFFERENCE IN HEXOSE STORAGE IN TOMATO FRUIT, Plant, cell and environment, 20(3), 1997, pp. 341-349
Post-phloem sugar transport in developing tomato (Lycopersicon esculen
tum Mill. cv. Flora-Dade) fruit follows an apoplastic route during the
rapid phase of sugar accumulation. The pathway is characterized by su
gar retrieval by the storage parenchyma cells from the fruit apoplast.
Two tomato genotypes differing in fruit hexose content were compared
in terms of the transport and transfer processes controlling fruit sug
ar levels. The genotypic difference in fruit sugar content was indepen
dent of photoassimilate export from source leaves. Discs of pericarp t
issue were cultured in a medium based on analyses of the fruit apoplas
tic sap. The cultured discs maintained a composition, a relative growt
h rate and a respiration rate similar to those of the pericarp tissue
of intact fruit. Estimates of hexose fluxes into metabolic and storage
pools suggested that membrane transport regulated the genotypic diffe
rence in hexose accumulation. Short-term [C-14]hexose uptake experimen
ts demonstrated a genotypic difference in V-max for glucose, fructose
and 3-O-methylglucose, and this difference was abolished in the presen
ce of the inhibitor p-chloromercuribenzenesulphonic acid (PCMBS). The
results support the hypothesis that the activity of energized hexose c
arriers on the plasma membranes of storage parenchyma cells is a signi
ficant determinate of the genotypic difference in hexose accumulation.