Two types of MGDG synthase genes, found widely in both 16 : 3 and 18 : 3 plants, differentially mediate galactolipid syntheses in photosynthetic and nonphotosynthetic tissues in Arabidopsis thaliana

In Arabidopsis, monogalactosyldiacylglycerol (MGDG) is synthesized by a mul tigenic family of MGDG synthases consisting of two types, of enzymes differ ing in their N-terminal portion: type A (atMGD1) and type B (atMGD2 and atM GD3). The present paper compares type B isoforms with the enzymes of type A that are known to sit in the inner membrane of plastid envelope. The occur rence of types A and B in 16:3 and 18:3 plants shows that both types are no t specialized isoforms for the prokaryotic and eukaryotic glycerolipid bios ynthetic pathways. Type A atMGD1 gene is abundantly expressed in green tiss ues and along plant development and encodes the most active enzyme. Its mat ure polypeptide is immunodetected in the envelope of chloroplasts from Arab idopsis leaves after cleavage of its transit peptide. atMGD1 is therefore l ikely devoted to the massive production of MGDG required to expand the inne r envelope membrane and build up the thylakoids network. Transient expressi on of green fluorescent protein fusions in Arabidopsis leaves and in vitro import experiments show that type B precursors are targeted to plastids, ow ing to a different mechanism. Noncanonical addressing peptides, whose proce ssing could not be assessed, are involved in the targeting of type B precur sors, possibly to the outer envelope membrane where they might contribute t o membrane expansion. Expression of type B enzymes was higher in nongreen t issues, i.e., in inflorescence (atMGD2) and roots (atMGD3), where they conc eivably influence the eukaryotic structure prominence in MGDG. In addition, their expression of type B enzymes is enhanced under phosphate deprivation .