Responses of Transgenic Apple Trees with Decreased Sorbitol Synthesis to Nitrogen Deficiency

Sorbitol is the main photosynthetic end-product and the primary translocated form of carbohydrates in apple. When the expression of aldose-6-phosphate (A6PR), the key enzyme in sorbitol synthesis, was decreased via antisense inhibition in ‘Greensleeves’ apple, both A6PR activity and sorbitol concentration were significantly decreased. As a result, hexose phosphates accumulated in the cytosol at the expense of inorganic phosphate, leading to up-regulation of starch synthesis, organic acid metabolism and amino acid metabolism in the antisense plants. To determine the responses of the transgenic plants to N deficiency, a transgenic line (A10) and the untransformed control (CK) were supplied with 0.5, 2.5, 7.5 or 15mM nitrogen in Hoagland’s solution via fertigation for 7 weeks from mid-May. A10 leaves had significantly higher photosynthesis and chlorophyll concentrations than CK at both 0.5 and 2.5 mM N treatments. 2-oxoglutarate concentration in A10 leaves remained unchanged whereas that in CK increased with decreasing N supply. Oxaloacetate concentration increased with decreasing N supply in both A10 and CK, but it increased to a lesser extent in A10 than in CK. Both soluble proteins and total free amino acids in A10 and CK decreased with decreasing N supply, but A10 had significantly higher levels of soluble proteins and higher levels of aspartate and glutamate than CK under N deficiency. In addition, A10 leaves had much higher levels of trehalose and maltose than CK under N deficiency. These results indicate that more organic acids are used in amino acid and protein synthesis to maintain higher photosynthesis in the transgenic plants under N deficiency and trehalose signaling might be involved in the responses of transgenic plants to nitrogen deficiency.