Galactolipids rule in seed plants.

Chloroplast membranes contain high levels of the galactolipids monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG). The isolation of the genes involved in the biosynthesis of MGDG and DGDG, and the identification of galactolipid-deficient Arabidopsis mutants has greatly facilitated the analysis of galactolipid biosynthesis and function. Galactolipids are found in X-ray structures of photosynthetic complexes, suggesting a direct role in photosynthesis. Furthermore, galactolipids can substitute for phospholipids, as suggested by increases in the galactolipid:phospholipid ratio after phosphate deprivation. The ratio of MGDG to DGDG is also crucial for the physical phase of thylakoid membranes and might be regulated.