Plant and algal galactolipids: Their function, biosynthesis and evolution

Two galactolipids, namely monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG), are the most abundant lipids in plastid membranes of plant vegetative tissues. MGDG synthesis is carried out by type A and B MGDG synthases, which are widely conserved in seed plants. Type A MGDG synthase produces the bulk of MGDG for thylakoid membranes of chloroplasts in seed plants. Type B MGDG synthase primarily supplies MGDG as a substrate for DGDG synthesis —especially in response to abiotic stress. Under phosphate-starved conditions, extraplastidial membranes lack sufficient phospholipids. To compensate for this deficiency, the DGDG produced by the type B MGDG synthase pathway is transported out of chloroplasts and becomes incorporated in extraplastidial membranes. In addition to MGDG synthase and DGDG synthase, the third enzyme of galactolipid synthesis, namely galactolipid:galactolipid galactosyltransferase, is involved in freezing tolerance. The various galactolipid synthases seem to be regulated in response to diverse abiotic stresses, but many aspects remain unclear. Considering that galactolipid synthesis is conserved among land plants and algae, the findings obtained from algal galactolipid research will further our understanding of galactolipid function. Here, we review the history and progress of galactolipid research in seed plants as well as the few, but very recent, advances achieved with algae.