Biochemical characterization of temperature-induced changes in lipid metabolism in a high oleic acid mutant of Brassica rapa.

We have characterized a Brassica rapa mutant (WR586) that has low levels of polyunsaturated fatty acids (D.L. Auld et al., 1992, Crop Sci. 32, 657-662). The mutant lacked oleoyl-phosphatidylcholine desaturase (ODS) activity when assayed in 6-day-old seedlings. To further characterize the mutant, the leaf fatty acid composition and major galactolipids and phospholipids were characterized in mutant (WR586) and control (cultivar "Tobin") plants grown at either 26 degrees C/26 degrees C or 10 degrees C/5 degrees C. Fatty acid profiles show significantly higher 18:1 levels in WR586 throughout 12 days of germination. The amount of saturated fatty acids decreased with a concomitant increase of 18:1. Ratios of 18:1/18:2 revealed that WR586 maintains higher mole percent of 18:1 than Tobin at all times and temperature regimes because of a lack of desaturation to 18:2. Values for monogalactosyl-diacylglycerol and digalactosyldiacylglycerol 18:1/18:2 ratios indicate a disparity in the concentration of 18:1 between WR586 and Tobin grown in either temperature during early germination. The phosphatidylcholine and phosphatidylethanolamine 18:1/18:2 ratios were higher in WR586 compared to Tobin and remained higher throughout the 12-day period. In WR586, 18:1 always accumulated to higher levels in the cooler temperature. In Tobin, 18:1 concentrations paralleled 18:2 in both temperature regimes. These results indicate that the lesion in the mutant WR586 resides at the ODS locus, 18:1 synthesis is chilling induced, and the microsomal desaturation pathway is the most prominent in early developing Brassica seedlings.