Type A and type B monogalactosyldiacylglycerol synthases are spatially and functionally separated in the plastids of higher plants

Mono- and digalactosyldiacylglycerol (MGDG and DGDG, respectively) constitute the bulk of membrane lipids in plant chloroplasts. The final step in MGDG biosynthesis occurs in the plastid envelope and is catalyzed by MGDG synthase. In Arabidopsis, the three MGDG synthases are classified into type A (atMGD1) and type B MGD isoforms (atMGD2 and atMGD3). atMGD1 is an inner envelope membrane-associated protein of chloroplasts and is responsible for the bulk of galactolipid biosynthesis in green tissues. MGD1 function is indispensable for thylakoid membrane biogenesis and embryogenesis. By contrast, type B atMGD2 and atMGD3 are localized in the outer envelopes and have no important role in chloroplast biogenesis or plant development under nutrient-sufficient conditions. These type B MGD genes are, however, strongly induced by phosphate (Pi) starvation and are essential for alternative galactolipid biosynthesis during Pi starvation. MGD1 gene expression is up-regulated by light and cytokinins. By contrast, Pi starvation-dependent expression of atMGD2/3 is suppressed by cytokinins but induced through auxin signaling pathways. These growth factors may control the functional sharing of the inner envelope pathway by atMGD1 and the outer envelope pathway by atMGD2/3 according to the growth environment.