The catalytic subunit of magnesium-protoporphyrin IX monomethyl ester cyclase forms a chloroplast complex to regulate chlorophyll biosynthesis in rice.

KEY MESSAGE: YGL8 has the dual functions in Chl biosynthesis: one as a catalytic subunit of MgPME cyclase, the other as a core component of FLU-YGL8-LCAA-POR complex in Chl biosynthesis. Magnesium-protoporphyrin IX monomethyl ester (MgPME) cyclase is an essential enzyme involved in chlorophyll (Chl) biosynthesis. However, its roles in regulating Chl biosynthesis are not fully explored. In this study, we isolated a rice mutant yellow-green leaf 8 (ygl8) that exhibited chlorosis phenotype with abnormal chloroplast development in young leaves. As the development of leaves, the chlorotic plants turned green accompanied by restorations in Chl content and chloroplast ultrastructure. Map-based cloning revealed that the ygl8 gene encodes a catalytic subunit of MgPME cyclase. The ygl8 mutation caused a conserved amino acid substitution (Asn182Ser), which was related to the alterations of Chl precursor content. YGL8 was constitutively expressed in various tissues, with more abundance in young leaves and panicles. Furthermore, we showed that expression levels of some nuclear genes associated with Chl biosynthesis were affected in both the ygl8 mutant and YGL8 RNA interference lines. By transient expression in rice protoplasts, we found that N-terminal 40 amino acid residues were enough to localize the YGL8 protein to chloroplast. In vivo experiments demonstrated a physical interaction between YGL8 and a rice chloroplast protein, low chlorophyll accumulation A (OsLCAA). Moreover, bimolecular fluorescence complementation assays revealed that YGL8 also interacted with the other two rice chloroplast proteins, viz. fluorescent (OsFLU1) and NADPH:protochlorophyllide oxidoreductase (OsPORB). These results provide new insights into the roles of YGL8, not only as a subunit with catalytic activity, but as a core component of FLU-YGL8-LCAA-POR complex required for Chl biosynthesis.