Trx-z is a chloroplastic thioredoxin, exhibiting a usual WCGPC active site, but whose biochemical properties are unknown. We demonstrate here that Trx-z supports the activity of several plastidial antioxidant enzymes, such as thiol-peroxidases and methionine sulfoxide reductases, using electrons provided by ferredoxin-thioredoxin reductase. Its disulfide reductase activity requires the presence of both active site cysteines forming a catalytic disulfide bridge with a midpoint redox potential of -251 mV at pH7. These in vitro biochemical data suggest that, besides its decisive role in the regulation of plastidial transcription, Trx-z might also be involved in stress response.