Carbon And Nitrogen Metabolism In Transgenic Apple Leaves With Decreased Sorbitol Synthesis

Abstract

Sorbitol serves as a main photosynthetic end-product and a primary translocated form of carbon in apple and many other tree fruit species of the Rosaceae family. Sorbitol synthesis shares the same hexose phosphate pool with sucrose synthesis in the cytosol. Previous work showed that the expression of aldose-6-phosphate reductase (A6PR, the key enzyme in sorbitol synthesis) in 'Greensleeves' apple was decreased via antisense inhibition, A6PR activity in mature leaves was decreased to approximately 15-30% of the untransformed control. The present worh showed that a consequence of this inhibition was that sorbitol synthesis was significantly decreased. Both glucose-6-phosphate (G6P) and fructose-6-phosphate (F6P) accumulated in the cytosol at the expense of inorganic phosphate (Pi), leading to up-regulation of starch synthesis without altering CO2 assimilation. Downstream metabolic responses to accumulation of G6P and F6P as well as the decreased pool of Pi in the cytosol were investigated in this study. It was found that transgenic plants had higher activities of several key enzymes in glycolysis, anaplerotic pathway and tricarboxylic acid cycle, higher respiration rate, and higher levels of organic acids and amino acids in mature leaves than the untransformed control, indicating that both organic acid metabolism and nitrogen metabolism were up-regulated in the transgenic plants. This up-regulation was mimicked, to a certain extent, by feeding detached leaves of the untransformed control with 10 mM mannose (a Pi sequester), suggesting that the decreased level of Pi in the cytosol due to the accumulation of hexose phosphates was also involved in the responses of organic acid metabolism and nitrogen metabolism in the transgenic plants. When grown under N deficiency, the transgenic plants had higher activities of several key enzymes and higher contents of several organic acids and amino acids in organic acid metabolism and nitrogen metabolism. This enabled the transgenic plants to synthesize more proteins (enzymes), thereby maintaining a higher photosynthesis per unit leaf area relative to the untransformed control under N deficiency. As a result of having a more active organic acid and nitrogen metabolism, the transgenic plants are more tolerant of N deficiency.