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Silencing leaf sorbitol synthesis alters long-distance partitioning and apple fruit quality

Teo, G., Suzuki, Y., Uratsu, S.L., Lampinen, B., Ormonde, N., Hu, W.K., DeJong, T.M., Dandekar, A.M.
Proceedings of the National Academy of Sciences of the United States of America 2006 v.103 no.49 pp. 18842-18847
Malus domestica, plant physiology, sorbitol, biosynthesis, leaves, transgenic plants, gene silencing, apples, fruit quality, carbohydrate composition, fructose, glucose, starch, malic acid, gene expression, messenger RNA, carbohydrate metabolism
Sorbitol and sucrose are major products of photosynthesis distributed in apple trees (Malus domestica Borkh. cv. "Greensleeves") that affect quality in fruit. Transgenic apple plants were silenced or up-regulated for sorbitol-6-phosphate dehydrogenase by using the CaMV35S promoter to define the role of sorbitol distribution in fruit development. Transgenic plants with suppressed sorbitol-6-phosphate dehydrogenase compensated by accumulating sucrose and starch in leaves, and morning and midday net carbon assimilation rates were significantly lower. The sorbitol to sucrose ratio in leaves was reduced by approximately equal to 90% and in phloem exudates by approximately equal to 75%. The fruit accumulated more glucose and less fructose, starch, and malic acid, with no overall differences in weight and firmness. Sorbitol dehydrogenase activity was reduced in silenced fruit, but activities of neutral invertase, vacuolar invertase, cell wall-bound invertase, fructose kinase, and hexokinase were unaffected. Analyses of transcript levels and activity of enzymes involved in carbohydrate metabolism throughout fruit development revealed significant differences in pathways related to sorbitol transport and breakdown. Together, these results suggest that sorbitol distribution plays a key role in fruit carbon metabolism and affects quality attributes such as sugar-acid balance and starch accumulation.