Alteration by thioredoxin f over-expression of primary carbon metabolism and its response to elevated CO₂ in tobacco (Nicotiana tabacum L.)

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, c_c), 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.