Auxin-regulated changes of fatty acid content and composition in soybean zygotic embryo cotyledons

The present studies were conducted to investigate the effect of exogenous auxins on zygotic embryo fatty acid content and lipid composition during somatic embryo induction. Immature soybean (Glycine max (L.) Merr.) cotyledons from two developmental stages were cultured on 2,4-dichlorophenoxyacetic acid (2,4-D) or α-naphthaleneacetic acid (NAA). The presence of 2,4-D or NAA in the medium altered the content and percentage of oleic (18:1), linoleic (18:2) and linolenic (18:3) acids, with less effects on stearic (18:0), palmitic (16:0) or total fatty acids in the embryonic cotyledons. Desaturation assays with radiolabelled fatty acids showed that 2,4-D increased both the ratios of 18:2 to 18:1 and 18:3 to 18:2. NAA at 10 mg/l resulted in an increase in the content and percentage of 18:3 but not that of 18:2 in the embryonic cotyledons. Analysis of free fatty acids, triglycerides, phosphatidylcholine, phosphatidylethanolamine, and glycolipids (monogalactosyldiacylglycerol and digalactosyldiacylglycerol) revealed that the increased ratio of 18:3 to 18:2 occurred in phosphatidylcholine, triglycerides and free fatty acids, but not in glycolipids. Our data suggest that 2,4-D increased the efficiency of the desaturation related processes in extra plastidial compartments.