Thiol-disulfide oxidoreductase catalyzing disulfide bond formation of chloroplast proteins and involved in redox regulation and photosynthetic electron transport. Required for the assembly of photosystem II (PSII) through the formation of disulfide bond in PSBO, a subunit of the PSII oxygen-evolving complex in the thylakoid lumen. Involved in the formation of disulfide bonds in the lumenal protein FKBP13. In vitro, reduces phylloquinone (vitamin K1) and menaquinone (vitamin K2) to their respective quinol. Cannot reduce phylloquinone epoxide to phylloquinone (PubMed:20626653, PubMed:21781282, PubMed:22209765, PubMed:23689258). Plays an important role in regulating the thylakoid lumen redox (PubMed:25412899). Interacts with the PSII subunits PSBO1 and PSBO2 (PubMed:22209765). Interacts with TL17, TL20.3, HCF164, PETJ, VDE1, EDA3, FKBP13 and FKBP20-2 (PubMed:25412899). Expressed in cotyledons, rosette leaves, stems, cauline leaves and flowers. Cysteine residues from the thioredoxin-like domain participate in a series of disulfide-exchange reactions that regenerate the redox-active cysteine residues in the transmembrane domain. Severe growth defects due to a limitation in the electron flow from PSII. Belongs to the VKOR family. Truncated N-terminus.