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BIOENERGETICS AND PHOTOSYNTHESIS

A Mutant of Arabidopsis Lacking the Triose-Phosphate/Phosphate Translocator Reveals Metabolic Regulation of Starch Breakdown in the Light^1,[w]

Department of Plant Sciences, University of Oxford, Oxford OX1 3RB, United Kingdom (R.G.W., D.G.I., N.J.K.); and Robert Hill Institute, Department of Molecular Biology and Biotechnology, University of Sheffield, Sheffield S10 2TN, United Kingdom (P.H.)

The chloroplast envelope triose-phosphate/phosphate translocator (TPT) is responsible for carbohydrate export during photosynthesis. Using measurements of carbohydrates, partitioning of assimilated ¹⁴CO₂, photosynthetic gas exchange, and chlorophyll fluorescence, we show that a mutant of Arabidopsis lacking the TPT increases synthesis of starch compared to the wild type, thereby compensating for a deficiency in its ability to export triose-phosphate from the chloroplast. However, during growth under high light, the capacity for starch synthesis becomes limiting so that the chloroplastic phosphate pool is depleted, resulting in a restriction on electron transport, a reduction in the rate of photosynthesis, and slowed plant growth. Under the same conditions but not under low light, we observe release of ¹⁴C label from starch, indicating that its synthesis and degradation occur simultaneously in the light. The induction of starch turnover in the mutant specifically under high light conditions leads us to conclude that it is regulated by one or more metabolic signals, which arise as a result of phosphate limitation of photosynthesis.