Thiol-disulfide oxidoreductase probably involved in the redox regulation of chloroplastic enzymes. Required for chloroplast biogenesis and differentiation. Functions as an electron donor for plastidial 2-Cys peroxiredoxins and participates in hydrogen peroxide scavenging system in chloroplasts. Possesses reducing activity towards insulin disulfide bonds. Expressed in leaves and at lower levels in flowers. By de-etiolation. Semi-dwarf plants with pale-green leaves, abnormal chloroplasts, reduced carotenoid and chlorophyll content and increased hydrogen peroxide levels. Belongs to the thioredoxin family. Plant M-type subfamily.