Inhibition of Sorbitol Synthesis In Leaves Altered the Primary Metabolism In Transgenic ‘Greensleeves' Apple Fruit

Antisense suppression of aldose-6-phosphate reductase decreased the concentration of sorbitol but increased the concentration of sucrose in leaves of the transgenic ‘Greensleeves’ apple trees. Since most of the carbon used by fruit is supplied from leaves, we investigated the metabolism of sugars, organic acids, and amino acids in the fruit of the transgenic trees. The concentration of sorbitol was lower whereas that of sucrose, glucose, galactose, and raffinose was higher in the transgenic fruit than in the untransformed control, and these differences became larger as fruit developed. The concentration of fructose and starch was similar in the transgenic and non-transgenic fruits. Sorbitol dehydrogenase activity was lower; the activity of acid invertase was slightly higher, whereas the activity of neutral invertase and sucrose synthase remained the same in the transgenic fruit. The concentration of organic acids such as malic acid, fumaric acid, 2-oxoglutaric acid, and oxaloacetic acid was lower but citric acid concentration was higher in the transgenic fruit. Respiration rate and the activity of citrate synthase, cis-conitase and NADP-malic enzyme were higher in the transgenic fruit. The concentration of amino acids such as aspartic acid, alanine, serine, proline, threonine, and methionine was higher in the transgenic fruit. The activity of NADH-dependent glutamate synthase, glutamate dehydrogenase, and asparaginase was higher in the transgenic fruit at the early developmental stages. These results indicate that inhibition of sorbitol synthesis in leaves altered not only carbohydrate metabolism but also the metabolism of organic acids and amino acids in fruit.