Silencing Leaf Sorbitol Synthesis Alters Long-Distance Partitioning and Apple Fruit Quality

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 103; no. 49; pp. 18842 - 18847
• Main Authors: Teo, Gianni, Suzuki, Yasuo, Uratsu, Sandie L., Lampinen, Bruce, Ormonde, Nichole, Hu, William K., DeJong, Ted M., Dandekar, Abhaya M.
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 05.12.2006; National Acad Sciences
• Series: Colloquium Paper
• Subjects: Apples; Biological Sciences; Dehydrogenases; Enzymes; Fruit - enzymology; Fruit - genetics; Fruit - metabolism; Fruit trees; Fruits; Gene Silencing; Kinases; Leaves; Malus - enzymology; Malus - genetics; Malus - metabolism; Malus domestica; Metabolism; Phloem; Photosynthesis; Plant Leaves - enzymology; Plant Leaves - genetics; Plant Leaves - metabolism; Plants; Plants, Genetically Modified; Sorbitol - antagonists & inhibitors; Sorbitol - metabolism; Starches; Sugar Alcohol Dehydrogenases - genetics; Sugars; Transgenic plants
• DOI: 10.1073/pnas.0605873103
• ISSN: 0027-8424

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-6phosphate 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 ≈90% and in phloem exudates by ≈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.