Starch synthesis in tomato remains constant throughout fruit development and is dependent on sucrose supply and sucrose synthase activity |
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Authors: | N'tchobo H; Dali N; Nguyen-Quoc B; Foyer C; Yelle S |
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Institution: | Centre de Recherche en Horticulture, FSAA, Université Laval, Quebec, Canada G1K 7P4; Department of Biochemistry and Physiology, IACR-Rothamsted, Harpenden, Herts AL5 2JQ, UK; Corresponding author e-mail: nguyenb@plg.ulaal.ca |
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Abstract: | Studies designed to investigate the cellular pathway of phloem unloading
were conducted on two tomato lines with either high or low fruit invertase
activities. Experiments were based on determination of the degree to which
3H label from
3H]-(fructosyl)-sucrose was randomized between
fructose and glucose following exposure of excised fruit to a pulse of
labelled sucrose delivered through pedicels. Fruit from the low invertase
line harvested 10, 20 and 40 d after anthesis had similar sucrose uptake
kinetics to the high invertase line. A positive correlation was found
between sucrose synthase activity and sucrose uptake in both low and high
invertase lines. In contrast, no correlation was observed between acid or
neutral invertase activities and sucrose uptake. Within the putative
apoplasmic sap collected from fruit, label in
3H]-(fructosyl)-sucrose was randomized between the
free hexoses and sucrose hexose moieties. Label asymmetry was retained in
sucrose on arrival within the tissues. Randomization patterns were similar
in both the low and high acid invertase lines. These data support the view
that sucrose imported into the fruit was not exposed to extracellular
hydrolysis. This suggests that movement from the phloem is likely to occur
predominantly through a symplastic pathway. About 25% of the sucrose taken
up by the fruit was converted into starch regardless of fruit age,
suggesting that starch turnover remains constant throughout fruit
development and that starch synthesis was dependent on sucrose
supply. |
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Keywords: | |
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