Gallium ferredoxin as a tool to study the effects of ferredoxin binding to photosystem I without ferredoxin reduction.

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.

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.