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 ΔpH 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.

Reduction of ferredoxin by photosystem I (PSI) involves the [4Fe-4S] clusters F and F harbored by PsaC, with F being the direct electron transfer partner of ferredoxin (Fd). Binding of the redox-inactive gallium ferredoxin to PSI was investigated by flash-absorption spectroscopy, studying both the P700 decay and the reduction of the native iron Fd in the presence of Fd. Fd binding resulted in a faster recombination between P700 and (F, F), a slower electron escape from (F, F) to exogenous acceptors, and a decreased amount of intracomplex Fd reduction, in accordance with competitive binding between Fd and Fd. [Fd] titrations of these effects revealed that the dissociation constant for the PSI:Fd complex is different whether (F, F) is oxidized or singly reduced. This difference in binding, together with the increase in the recombination rate, could both be attributed to a c. -30 mV shift of the midpoint potential of (F, F), considered as a single electron acceptor, due to Fd binding. This effect of Fd binding, which can be extrapolated to Fd because of the highly similar structure and the identical charge of the two Fds, should help irreversibility of electron transfer within the PSI:Fd complex. The effect of Fd binding on the individual midpoint potentials of F and F is also discussed with respect to the possible consequences on intra-PSI electron transfer and on the escape process.