Unraveling the role of transient starch in the response of Arabidopsis to elevated CO2 under long-day conditions

Previous studies on Arabidopsis under long-term exposure to elevated CO2 have been conducted using starch synthesis and breakdown mutants cultured under short day conditions. These studies showed that starch synthesis can ameliorate the photosynthetic reduction caused by soluble sugar-mediated feedback regulation. In this work we characterized the effect of long-term exposure to elevated CO2 (800 ppm) on growth, photosynthesis and content of primary photosynthates in long-day grown wild type plants as well as the near starch less (aps1) and the starch-excess (gwd) mutants. Notably, elevated CO2 promoted growth of both wild type and aps1 plants but had no effect on gwd plants. Growth promotion by elevated CO2 was accompanied by an increased net photosynthesis in WT and aps1 plants. However, the plants with the highest starch content (wild type at elevated CO2, gwd at ambient CO2, and gwd at elevated CO2) were the ones that suffered decreased in in vivo maximum carboxylation rate of Rubisco, and therefore, photosynthetic down-regulation. Further, the photosynthetic rates of wild type at elevated CO2 and gwd at elevated CO2 were acclimated to elevated CO2. Notably, elevated CO2 promoted the accumulation of stress-responsive and senescence-associated amino acid markers in gwd plants. The results presented in this work provide evidence that under long-day conditions, temporary storage of overflow photosynthate as starch negatively affect Rubisco performance, These data are consistent with earlier hypothesis that photosynthetic acclimation can be caused by accelerated senescence and hindrance of CO2 diffusion to the stroma due to accumulation of large starch granules.