Thioredoxins f (Trx f) are chloroplastic proteins that have been shown to be essential for the redox-based regulation of several steps of carbon assimilation, such as the Calvin-Benson-Bassham cycle. However, the effective impact of Trx f activity on photosynthetic performance and carbon primary metabolism, including under varying CO2 mole fraction, is not well documented. In this study, we provide a physiological and metabolomic characterization of leaves in transplastomic Trx f over-expressing tobacco (Nicotiana tabacum L., cv. Petit Havana SR1) grown under either ambient or elevated CO2 (400 or 800μmolmol-1). Trx f overexpression strongly increased starch synthesis under both ambient and elevated CO2 but was not accompanied by a stimulation of net photosynthetic CO2 fixation. Rather, Trx f-overexpressing plants had a lower photorespiration rate due to an increase in internal (mesophyll) conductance for CO2 (with the consequent increase in CO2 mole fraction at the carboxylation site, cc), and a higher decrease (compared with the wild-type) in total photosynthetic electron flux upon acclimation to elevated CO2. There were also changes in a number of metabolites, such as enrichment in sugar phosphates and free phosphate, and depletion in alanine, threonine and free sugars. Our results suggest that over-expressing Trx f has an influence on chloroplastic metabolism by simultaneously stimulating the carboxylation-to-oxygenation ratio and starch synthesis, with side effects on amino acid metabolism. The potential mechanisms involved are discussed.

This work was funded by the Spanish National Research and Development Programme (AGL2010-15107, AGL2011-30386-C02-2 and Ramón y Cajal program).

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