OsFBN7-OsKAS I module promotes formation of plastoglobules clusters in rice chloroplasts

Plastoglobules (PGs) contiguous with the outer leaflets of thylakoid membranes regulate lipid metabolism, plastid developmental transitions, and responses to environmental stimuli. However, the function of OsFBN7, a PG-core fibrillin gene in rice, has not been elucidated. Using molecular genetics and physiobiochemical approaches, we observed that OsFBN7 overexpression promoted PG clustering in rice chloroplasts. OsFBN7 interacted with two KAS I enzymes, namely OsKAS Ia and OsKAS Ib, in rice chloroplasts. Lipidomic analysis of chloroplast subcompartments, including PGs in the OsFBN7 overexpression lines, confirmed that levels of diacylglycerol (DAG), a chloroplast lipid precursor and monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG), the main chloroplast membrane lipids, were increased in PGs and chloroplasts. Furthermore, OsFBN7 enhanced the abundances of OsKAS Ia/Ib in planta and their stability under oxidative and heat stresses. In addition, RNA sequencing and real-time quantitative reverse-transcription polymerase chain reaction (qRT-PCR) analyses showed that the expression of the DAG synthetase gene PAP1 and MGDG synthase gene MDG2 was upregulated by OsFBN7. In conclusion, this study proposes a new model in which OsFBN7 binds to OsKAS Ia/Ib in chloroplast and enhances their abundance and stability, thereby regulating the chloroplast and PG membrane lipids involved in the formation of PG clusters.