Changes in the 14C-labelling of molecular species of 3-monogalactosyl-1,2-diacylglycerol in leaves of Vicia faba treated with compound San 9785.

The effects of the substituted pyridazinone herbicide. San 9785, on the biosynthesis of monogalactosyldiacylglycerol (MGDG) molecular species and on the diacylglycerol precursors were studied. Kinetic experiments with [14C]glucose-infiltrated Vicia faba leaf tissue showed that San 9785 inhibited desaturation of MGDG linoleic acid (C18:2) to form linolenic acid (C18:3) and increased the degree of unsaturation of the diacylglycerol molecular species used in MGDG biosynthesis. These results confirmed that the diacylglycerol precursor of MGDG contains highly unsaturated fatty acids, particularly C18:2 and C18:3. The results also indicated that the MGDG 3/3 molecular species (a molecular species of MGDG containing two C18:3 molecules) is derived from MGDG 2/3 (i.e. containing one C18:2 and one C18:3 fatty acid moiety) and, likely, MGDG 2/2 (i.e. containing two C18:2 fatty acid moieties), via sequential fatty acid desaturation in situ. A model for MGDG 3/3 biosynthesis in 'C18:3-plants', incorporating several sites for fatty acid desaturation (in the phospholipid and diacylglycerol precursors, which are not inhibited by San 9785, and in MGDG, which is inhibited by San 9785) is discussed.