Sugar metabolism and accumulation in the fruit of transgenic apple trees with decreased sorbitol synthesis

Both sorbitol and sucrose are synthesized in source leaves and transported to fruit for supporting fruit growth in tree fruit species of the Rosaceae family.In apple(Malus domestica),antisense suppression of aldose-6-phosphate reductase,the key enzyme for sorbitol synthesis,significantly decreased the sorbitol concentration but increased the sucrose concentration in leaves,leading to a lower sorbitol but a higher sucrose supply to fruit in these plants.In response to this altered carbon supply,the transgenic fruit had lower concentration of sorbitol and much higher concentration of glucose but similar levels of fructose,sucrose,and starch throughout fruit development relative to the untransformed control.Activities of sorbitol dehydrogenase,fructokinase,and sucrose phosphate synthase were lower,whereas activities of neutral invertase,sucrose synthase,and hexokinase were higher in the transgenic fruit during fruit development.Transcript levels of MdSOT1,MdSDHs,MdFK2,and MdSPS3/6 were downregulated,whereas transcript levels of MdSUC1/4,MdSUSY1-3,MdNIV1/3,MdHKs,and MdTMT1 were upregulated in the transgenic fruit.These findings suggest that the Sucrose cycle and the sugar transport system are very effective in maintaining the level of fructose and provide insights into the roles of sorbitol and sucrose in regulating sugar metabolism and accumulation in sorbitol-synthesizing species.

Increased activity of MdFRK2, a high-affinity fructokinase, leads to upregulation of sorbitol metabolism and downregulation of sucrose metabolism in apple leaves

To investigate the functions of fructokinase(FRK)in apple(Malus domestica)carbohydrate metabolism,we cloned the coding sequences of MdFRK1 and MdFRK2 from the‘Royal Gala’apple.The results showed that MdFRK2 expression was extremely high in shoot tips and young fruit.Analyses of heterologously expressed proteins revealed that MdFRK2 had a higher affinity for fructose than did MdFRK1,with Km values of 0.1 and 0.62 mM for MdFRK2 and MdFRK1,respectively.The two proteins,however,exhibited similar Vmax values when their activities were significantly inhibited by high concentrations of fructose.MdFRK2 ectopic expression was associated with a general decrease in fructose concentration in transgenic lines.In leaves,increased FRK activity similarly resulted in reduced concentrations of glucose and sucrose but no alterations in sorbitol concentration.When compared with those in the untransformed control,genes involved in sorbitol synthesis(A6PR)and the degradation pathway(SDH1/2)were significantly upregulated in transgenic lines,whereas those involved in sucrose synthesis(SPS1)and other degradation processes(SUSY4,NINV1/2,and HxK2)were downregulated.The activity of enzymes participating in carbohydrate metabolism was proportional to the level of gene expression.However,the growth performance and photosynthetic efficiency did not differ between the transgenic and wild-type plants.These results provide new genetic evidence to support the view that FRK plays roles in regulating sugar and sorbitol metabolism in Rosaceae plants.

Effects of location within the tree canopy on carbohydrates,organic acids,amino acids and phenolic compounds in the fruit peel and flesh from three apple(Malus×domestica)cultivars

Fruits from three cultivars of apple(Malus × domestica Borkh.)—‘McIntosh’,‘Gala’and‘Mutsu’—were harvested from the exterior and interior of the tree canopy.Peel and flesh tissues were sampled separately to determine how the position of the fruit on the tree might affect the levels of the primary and secondary metabolites in the fruit.Fruit from the outer-canopy had a higher fresh weight and a higher soluble solids content compared with inner-canopy fruit.Both the flesh and peel of the outer-canopy fruit had higher concentrations of soluble sugars and sugar alcohols,but lower starch concentrations than the inner-canopy fruit.Canopy position did not significantly affect malic acid concentrations,except in the peel of‘McIntosh’and the flesh of‘Mutsu’.Although levels of ascorbic and succinic acids were higher in the peel of the outer-canopy fruit,the responses of other organic acids to canopy position depended on tissue type and cultivar.Except for histidine,lysine,threonine and glycine,most amino acids accumulated at higher concentrations in the inner-canopy fruit.By contrast,levels of phenolic compounds from both the peel and flesh were significantly higher in the outer-canopy fruit.The significant effects of location within the canopy on both primary metabolites and secondary metabolites demonstrate the importance of light exposure on apple fruit quality.

Effect of Metaxenia on Sugars and Organic Acids and Taste Quality of Bagged Fuji Apple Fruits

In order to provide reference for the selection of appropriate pollination varieties facilitating the improvement of the flavor quality of bagged Fuji apple, the metaxenia effects of 5 pollination varieties on sugars, organic acids and flavor quality of bagged Fuji fruit were studied. The results showed that the 5 pollination varieties had significant metaxenia effects on sugars, organic acids and flavor quality of bagged Fuji fruit. For the sugar components, the fruits pollinated with Golden Delicious and Jinxiuhaitang showed higher glucose contents, and the fruit pollinated with Gala had the highest sucrose content; the fruits pollinated by Gala, Golden Delicious and Ruby exhibited higher malic acids, and the oxalic acid content and the succinic acid content were higher in the fruits pollinated with Jinxiuhaitang and Ruby ; and the fruit pollinated with Gala had the highest total sugar content, and the total acid content was higher in the fruits pollinated by Jinxiuhaitang and Ruby. The sugar-acid ratio was higher in the fruits pollinated with Gala and Golden Delicious, and the lowest in the fruit pollinated with Ruby. The senso- ry evaluation of fruits showed that the fruit pollinated with Ruby tasted sour-sweet, while the fruits pollinated with other 4 pollination varieties were sour and sweet moderately.