Involved in nuclear gene expression via the retrograde plastid-to-nucleus signaling (PubMed:17395793, PubMed:19140931, PubMed:20605896, PubMed:21673514). Plastid signals that depend on GUN1 can affect photomorphogenesis developments, such as cotyledon opening and expansion, anthocyanin biosynthesis, and hypocotyl elongation (PubMed:19140931, PubMed:21673514). Defective in retrograde plastid-to-nucleus signaling. Belongs to the PPR family. P subfamily.