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

Abstract

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 Ϸ90% and in phloem exudates by Ϸ75%. 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.gene silencing ͉ sugar-acid balance ͉ translocation ͉ starch accumulation A pples are in the family Rosaceae, which includes temperate species with fleshy fruit. The family produces sorbitol, sucrose, and starch as primary products of photosynthesis ( Fig. 1). Sorbitol is synthesized via reduction of glucose-6-phosphate to sorbitol-6-phosphate by aldose-6-phosphate reductase (EC 1.1.1.200), also called sorbitol-6-phosphate dehydrogenase (S6PDH). Sorbitol is the main sugar present in apple leaves and is transported in phloem with sucrose ( Fig. 1) (1). Sugars are distributed through a network of sieve elements to sink tissues such as developing fruit, seed, and leaves in a complex process regulated by photosynthetic rate, phloem loading, long-distance translocation and unloading, postphloem transport, and metabolism within sink tissues (2, 3). Transporters of sugar alcohols like sorbitol and other structural components involved in phloem unloading of sorbitol into the apoplast of apple fruit have been identified (4-6).In sink tissues such as fruit, sucrose is metabolized by invertases and sucrose synthase, whereas sorbitol is converted to fructose by sorbitol dehydrogenase (SDH, EC 1.1.1.14) (7,8). Expression of SDH in developing apple fruit is highest 2-3 weeks after bloom, a period sensitive to carbon availability (9). Additionally, sorbitol and other sugars may regulate expression of SDH mRNA and protein in pear fruit slices (10).In this article, we describe how sorbitol distribution affects fruit quality using transgenic apple plants with altered levels of S6PDH. Understanding the relationship between sorbitol accumulation and fruit quality is important not only for a...

Metabolism and Accumulation of Sugars Translocated to Fruit and Their Regulation

“…Transgenic apples containing the introduced antisense S6PDH gene decreased S6PDH activity, but did not influence SPS activity, but increased fructose 1,6-bisphosphatase activity (Cheng et al, 2005). Similar transgenic apples reduced SDH activity in fruit, but did not influence the activities of AIV, fructokinase or hexokinase (Teo et al, 2006). Such transgenic apples reduced sorbitol synthesis and SDH activity in the shoot tip, but did not influence AIV activity, and increased SuSy activity (Zhou et al, 2006).…”

Physiological Roles of Polyols in Horticultural Crops

“…This increase in Brix value and decrease in acidity favor high fruit quality. Kanamaru et al (2004) and Teo et al (2006) examined apple vegetative growth and found some inhibition after suppression of S6PDH expression and promotion after overexpression of S6PDH, suggesting that sorbitol is a favorable photosynthate for apple vegetative growth.…”

“…Cheng et al (2005) further showed that starch is also increased in transgenic apple plants with suppressed expression of S6PDH. Fruit from apples transformed with S6PDH cDNA was analyzed by Teo et al (2006), who found that although fruit weight and firmness did not change, there was an increase in glucose and a decrease in malate content after suppression of S6PDH expression. This increase in Brix value and decrease in acidity favor high fruit quality.…”

Gene silencing: concepts, applications, and perspectives in woody plants

“…According to these reports, it is possible to produce transgenic hypoallergenic apples using RNAi. Research on leaf sorbitol silencing suggests that sorbitol distribution affects fruit quality such as starch accumulation and sugar-acid balance (Teo et al, 2006). In Citrus, the down regulation of putative thermostable pectin methylesterase genes is projected as a possible solution for the undesirable separation of juice into clear serum and particulate phase (Guo et al, 2005).…”

“…(Jouanin et al, 2000;Kumar & Fladung, 2001;Hawkins et al, 2003), Solanum dulcamara L. (Curtis et al, 2000), Malus domestica Borkh. (Ko et al, 1998;Dandekar et al, 2004;Viss et al, 2003;Broothaerts et al, 2004;Gilissen et al, 2005;Teo et al, 2006), Pinus radiata D. Don. (Moller et al, 2005;Tang et al, 2005a;2005b;Wagner et al, 2005;Tang et al, 2006), Juglans regia L. (Escobar et al, 2002), Coffea canephora (Ogita et al, 2003), Eucalyptus sp.…”