PT  - JOURNAL ARTICLE
AU  - Lauri Nikkanen
AU  - Jouni Toivola
AU  - Andrea Trotta
AU  - Manuel Guinea Diaz
AU  - Mikko Tikkanen
AU  - Eva-Mari Aro
AU  - Eevi Rintamäki
TI  - Regulation of cyclic electron flow by chloroplast NADPH-dependent thioredoxin system
AID  - 10.1101/261560
DP  - 2018 Jan 01
TA  - bioRxiv
PG  - 261560
4099  - http://biorxiv.org/content/early/2018/04/19/261560.short
4100  - http://biorxiv.org/content/early/2018/04/19/261560.full
AB  - Linear electron transport in the thylakoid membrane drives both photosynthetic NADPH and ATP production, while cyclic electron flow (CEF) around photosystem I only promotes the translocation of protons from stroma to thylakoid lumen. The chloroplast NADH-dehydrogenase-like complex (NDH) participates in one CEF route transferring electrons from ferredoxin back to the plastoquinone pool with concomitant proton pumping to the lumen. CEF has been proposed to balance the ratio of ATP/NADPH production and to control the redox poise particularly in fluctuating light conditions, but the mechanisms regulating the NDH complex remain unknown. We have investigated potential regulation of the CEF pathways by the chloroplast NADPH-thioredoxin reductase (NTRC) in vivo by using an Arabidopsis knockout line of NTRC as well as lines overexpressing NTRC. Here we present biochemical and biophysical evidence showing that NTRC activates the NDH-dependent CEF and regulates the generation of proton motive force, thylakoid conductivity to protons and redox balance between the thylakoid electron transfer chain and the stroma during changes in light conditions. Further, protein–protein interaction assays suggest a putative thioredoxin-target site in close proximity to the ferredoxin binding domain of NDH, thus providing a plausible mechanism for regulation of the NDH ferredoxin:plastoquinone oxidoreductase activity by NTRC.One sentence summary Chloroplast thioredoxins regulate photosynthetic cyclic electron flow that balances the activities of light and carbon fixation reactions and improves plant fitness under fluctuating light conditions.