The chloroplast NADPH thioredoxin reductase C, NTRC, controls non-photochemical quenching of light energy and photosynthetic electron transport in Arabidopsis

High irradiances may lead to photooxidative stress in plants, and non-photochemical quenching (NPQ) contributes to protection against excess excitation. One of the NPQ mechanisms, qE, involves thermal dissipation of the light energy captured. Importantly, plants need to tune down qE under light-limiting conditions for efficient utilization of the available quanta. Considering the possible redox control of responses to excess light implying enzymes, such as thioredoxins, we have studied the role of the NADPH thioredoxin reductase C (NTRC). Whereas Arabidopsis thaliana plants lacking NTRC tolerate high light intensities, these plants display drastically elevated qE, have larger trans-thylakoid pH and have 10-fold higher zeaxanthin levels under low and medium light intensities, leading to extremely low linear electron transport rates. To test the impact of the high qE on plant growth, we generated an ntrc-psbs double-knockout mutant, which is devoid of qE. This double mutant grows faster than the ntrc mutant and has a higher chlorophyll content. The photosystem II activity is partially restored in the ntrc-psbs mutant, and linear electron transport rates under low and medium light intensities are twice as high as compared with plants lacking ntrc alone. These data uncover a new role for NTRC in the control of photosynthetic yield. Excess excitation and the means by which plants respond to defend themselves against light-induced damage have been extensively studied, and the importance of qE is well documented. Nevertheless, the success of a plant also depends on the capacity to restrict the induction of acclimation mechanisms, such as qE, in order to avoid wasting the energy absorbed. Given the possible redox control and implication of thioredoxins in protection mechanisms against excess light, we have investigated the role of the chloroplast NADPH-dependent thioredoxin reductase NTRC. This enzyme has been previously implied in peroxide detoxification and abiotic stress tolerance. Surprisingly, NTRC proved to be essential, not for the induction of protection mechanisms under strong irradiance, but for the maintenance of optimal photosynthetic efficiency under normal-light conditions and down-regulation of qE through control of the trans-thylakoid pH gradient.