Thiol-disulfide oxidoreductase that participates in various redox reactions in the chloroplast. Mediates the reduction of PSI-N in the thylakoid lumen. May interact and probably reduce other target proteins of the thylakoid membrane, such as FTSH2, FTSH8, LHCB5, atpA, atpB, atpE, petA and petC. Involved in the biogenesis of the plastid cytochrome b6f complex. Reducing equivalents are provided by stromal M-type thioredoxins and probably transduced through the thylakoid membrane by CCDA. Possesses low insulin disulfide bonds reducing activity. Interacts in vitro with LTO1. High chlorophyll fluorescence and deficiency in the accumulation of the cytochrome b6f complex subunits. Belongs to the thioredoxin family. The active site contains a CEVC motif wich differs from the conserved CGPC motif.