| Summary: | The capacity of pea root plastids for galactolipid synthesis was investigated utilizing radiolabelled acetate and UDP-galactose. Galactolipid biosynthesis was completely dependent on an exogenous supply of UDP-galactose. UDP-galactose stimulated both total lipid biosynthesis from acetate and the proportion of radioactivity accumulated in monogalactosyldiacylglycerol (MGDG). The proportion of MGDG synthesized was saturated at 30$ mu$M UDP-galactose and represented approximately 30% of the total lipid radioactivity after a one hour incubation. However, total lipid biosynthesis continued to increase with concentrations of UDP-galactose up to 75$ mu$M while the proportion of radioactivity in MGDG remained at 30%. MGDG biosynthesis was always accompanied by a corresponding decrease in the amount of diacylglycerol (DAG) accumulated. Digalactosyldiacylglycerol (DGDG) synthesis was not routinely observed in these experiments. These results suggest that the in vitro pathway for MGDG synthesis in the root plastids of pea (an 18:3 plant) is similar to 16:3 plants (FFA's$ to$PA$ to$DAG$ to$MGDG). The endogenous lipids, consistent with the thought of pea as an 18:3 plant, contained 80% C$ sb{18}$ in the fatty acids of MGDG, DGDG, TG and PC. However, in labelled acetate experiments palmitate was the predominately labelled fatty acid in all lipids except PC (where 80% was 18:1). The precursors PA and DAG had ratios of 16:0, 18:0, and 18:1 similar to that of MGDG. 70-80% of the label was associated with the sn-2 position of glycerolipids. The cofactors required for fatty acid synthesis were generally not as required for galactolipid synthesis. The results suggest that galactolipid synthesis relies primarily on endogenous DAG and only partly involves de novo fatty acid synthesis. (Abstract shortened by UMI.)
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