Dear Editor,During CO2 assimilation,the reductive pentose phosphate cycle generates triose phosphates (triose-P) at the expense of ATP and NADPH generated by photosynthetic light reactions.Triose-P is retained within chloroplasts to either regenerate the CO2 acceptor ribulose-1,5-bisphosphate or to feed into transitory starch synthesis.Alteatively,triose-P can be exported into the cytosol to produce sucrose.To maintain optimum rates of photosynthesis,the rate of triose-P export from the chloroplast stroma to the cytosol is necessarily adjusted to the demands for photoassimilate export.The triose-P/phosphate (Pi) transiocator (TPT) mediates the export of stromal triose-P into the cytosol in a counter exchange for cytosolic Pi.The rate of triose-P export is primarily regulated by the availability of Pi liberated by cytosolic sucrose synthesis.When sucrose synthesis slows down during the day,the restriction in Pi availability reduces TPT transport activity and redirects assimilated carbon into transitory starch synthesis in the chloroplast (B(o)ke and Sonnewald,2011).