Plastid anionic lipids are required for membrane development and protochlorophyllide synthesis in etioplasts

Abstract

Dark-germinated angiosperms develop the chloroplast precursors called etioplasts in cotyledon cells. Etioplasts develop lattice membrane structures called prolamellar bodies (PLBs), where the chlorophyll intermediate protochlorophyllide (Pchlide) forms a ternary complex with NADPH and light-dependent NADPH-Pchlide oxidoreductase (LPOR). The lipid bilayers of etioplast membranes are mainly composed of galactolipids, which play important roles in membrane-associated processes in etioplasts. Although etioplast membranes also contain two anionic lipids, phosphatidylglycerol (PG) and sulfoquinovosyldiacylglycerol (SQDG), the roles of these anionic lipids are unknown. To reveal the importance of PG and SQDG for the development of etioplasts, we characterized etiolated Arabidopsis mutants deficient in the biosynthesis of PG and SQDG. A partial deficiency in PG biosynthesis loosened the lattice structure of PLBs and impaired the insertion of Mg2+ into protoporphyrin IX, leading to a significant decrease in Pchlide content. Although a complete lack of SQDG biosynthesis did not notably affect both PLB formation and Pchlide biosynthesis, the lack of SQDG in addition to the partial deficiency of PG caused strong impairments of these processes. The results suggested that PG is required for PLB formation and Pchlide biosynthesis, whereas SQDG plays an auxiliary role in these processes. Notably, the PG deficiency and the lack of SQDG oppositely affected the dynamics of LPOR complexes after photoconversion, suggesting different involvements of PG and SQDG in the organization of LPOR complexes. Our data demonstrate pleiotropic roles of anionic lipids in etioplast development.