Exhibits zinc-dependent disulfide isomerase activity. Required for seedling and chloroplast development under heat stress, probably by maintaining plastid-encoded RNA polymerase (PEP)-dependent transcription. Catalyzes the rearrangement of -S-S- bonds in proteins. Interacts with HSP21; the formed complex associates with the plastid-encoded RNA polymerase (PEP) complex not only during transcription initiation, but also during elongation and termination, and with a stronger efficiency in illuminated chloroplasts. Binds to promoter regions of PEP-dependent genes, especially after a heat stress (PubMed:23922206). Interacts with FLN2 (PubMed:24019900). Ivory phenotype under heat stress.