Required for the transfer of reducing equivalents from stroma to thylakoid lumen. Involved in the biogenesis of the plastid cytochrome b6f complex by probably transferring reducing equivalents from stromal m-type thioredoxin (Trx-m) to the lumenal thioredoxin HCF164. A number of isoforms are produced. According to EST sequences. High chlorophyll fluorescence and deficiency in the accumulation of the cytochrome b6f complex subunits. Dwarf due to very slow growth and pale green leaves. Belongs to the DsbD family.