EFFECT OF ANTISENSE REPRESSION OF THE CHLOROPLAST TRIOSE-PHOSPHATE TRANSLOCATOR ON PHOTOSYNTHETIC METABOLISM IN TRANSGENIC POTATO PLANTS

The introduction of an antisense DNA into transgenic potato (Salanum t uberosum L.) plants decreased the expression of the chloroplast triose -phosphate translocator and lowered its activity by 20-30%. With plant s propagated from tubers, the effect of the transformation on photosyn thetic metabolism was analysed by measuring photosynthesis, the format ion of leaf starch, and the total and subcellular metabolite contents in leaves. Although the transformants, in contrast to those propagated from cell cultures, did not differ from the wild-type plants in respe ct to rates of photosynthesis, plant appearance, growth and tuber prod uction, their photosynthetic metabolism was found to be severely affec ted. The results show that the decrease in activity of the triose-phos phate translocator in the transformants caused a fourfold increase in the level of 3-phosphoglycerate and a corresponding decrease in inorga nic phosphate in the stromal compartment, resulting in a large increas e in the synthesis of starch. Whereas during a 12-h day period wild-ty pe plants deposited 43% of their CO2 assimilate into starch, this valu e rose to 61-89% in the transformants. In contrast to the wild-type pl ants, where the rate of assimilate export from the leaves during the n ight period was about 75% of that during the day, the export rate from leaves of transformants appeared to be much higher during the night t han during the day. As the mobilisation of starch occurs in part hydro lytically, resulting in the formation of glucose, the triose-phosphate translocator loses its exclusive function in the export of carbohydra tes from the chloroplasts when the photoassimilates are temporarily de posited as starch. It appears that by directing the CO2 assimilates ma inly into starch, the transformants compensate for the deficiency in t riose-phosphate translocator activity in such a way that the productiv ity of the plants is not affected by the transformation.