| Summary: | Thioredoxins (Trxs) are small, ubiquitous enzymes that catalyze disulphide–dithiol interchange in target enzymes. The large set of chloroplast Trxs, including <i>f</i>, <i>m</i>, <i>x</i> and <i>y</i> subtypes, use reducing equivalents fueled by photoreduced ferredoxin (Fdx) for fine-tuning photosynthetic performance and metabolism through the control of the activity of redox-sensitive proteins. Although biochemical analyses suggested functional diversity of chloroplast Trxs, genetic studies have established that deficiency in a particular Trx subtype has subtle phenotypic effects, leading to the proposal that the Trx isoforms are functionally redundant. In addition, chloroplasts contain an NADPH-dependent Trx reductase with a joint Trx domain, termed NTRC. Interestingly, <i>Arabidopsis</i> mutants combining the deficiencies of <i>x-</i> or <i>f-</i>type Trxs and NTRC display very severe growth inhibition phenotypes, which are partially rescued by decreased levels of 2-Cys peroxiredoxins (Prxs). These findings indicate that the reducing capacity of Trxs <i>f</i> and <i>x</i> is modulated by the redox balance of 2-Cys Prxs, which is controlled by NTRC. In this study, we explored whether NTRC acts as a master regulator of the pool of chloroplast Trxs by analyzing its functional relationship with Trxs <i>y</i>. While Trx <i>y</i> interacts with 2-Cys Prxs in vitro and in planta, the analysis of <i>Arabidopsis</i> mutants devoid of NTRC and Trxs <i>y</i> suggests that Trxs <i>y</i> have only a minor effect, if any, on the redox state of 2-Cys Prxs.
|
|---|