Plastids are a diverse family of plant organelles and chloroplasts are the most typical form of plastids that develop the thylakoid membrane inside to perform oxygenic photosynthesis. In angiosperms germinated in the light, proplastids of cotyledon cells differentiate directly to chloroplasts. However, in the dark, proplastids differentiate to etioplasts as precursors of chloroplasts. Etioplasts have lattice membrane structures named prolamellar bodies (PLBs), where protochlorophyllide, a chlorophyll intermediate, accumulates with light-dependent protochlorophyllide reductase (LPOR). With light exposure, etioplasts rapidly differentiate to chloroplasts to establish photoautotrophic growth. The differentiation from etioplasts to chloroplasts involves the dynamic transformation of PLBs to thylakoids, which is accompanied by protochlorophyllide-to-chlorophyll conversion, LPOR degradation, and accumulation of photosynthetic complexes. In contrast to the drastic changes in pigment and protein compositions, the composition of glycerolipids is almost unchanged during PLB-to-thylakoid transformation, suggesting that the lipid molecules in PLBs are directly used for the development of the thylakoid membrane.
The lipid bilayer matrix of PLB and thylakoid membrane mainly consists of four unique lipid classes—monogalactosyldiacylglycerol, digalactosyldiacylglycerol, sulfoquinovosyldiacylglycerol, and phosphatidylglycerol. Our research group revealed that each lipid has specific roles in membrane-associated processes during the development of etioplasts and the differentiation from etioplasts to chloroplasts (1-4). Particularly, phosphatidylglycerol plays an essential role in chlorophyll metabolism and membrane organization in these processes.