Thioredoxins and thioredoxin reductase in chloroplasts: A review

The chloroplastic thioredoxins (Trxs), a family of thiol-disulphide oxidoreductases, are reduced by either ferredoxin (Fd)-dependent Trx reductase (FTR) or reduced nicotinamide adenine dinucleotide phosphate (NADPH)-dependent Trx reductase (NTR). Two Trx systems are present in chloroplasts including Trxs, Trx-like proteins, and reductase FTR and NTRC. FTR is the main reductant for Trxs in chloroplasts, while the flavoprotein NTRC integrates NTR and Trx activity, and plays multiple roles in the Calvin cycle, the oxidative pentose phosphate pathway (OPPP), anti-peroxidation, tetrapyrrole metabolism, ATP and starch synthesis, and photoperiodic regulation. In addition, not only there exists a reduction potential transfer pathway across the thylakoid membrane, but also FTR and NTRC coordinate with each other to regulate chloroplast redox homeostasis. Herein, we summarise the physiological functions of these two Trx reduction systems, discuss how both regulate redox homeostasis in plant plastids, and emphasize the significance of chloroplast thioredoxin systems in maintaining photosynthetic efficiency in plants.