Genetic control of fatty acid biosynthesis in rapeseed (Brassica napus L.)

Abstract

Isolation of rapeseed plants containing no erucic acid in their seed oil gave simultaneous selection for low eicosenoic acid. Genetic analy-sis of populations segregating for erucic acid content indicated that the synthesis of these fatty acids was controlled, in the developing embryo, by two genes which displayed no domi-nance and acted in an additive manner. As the genetic capacity for erucic acid synthesis de-creased, there was an increase in percentage of oleic acid with no corresponding decrease in total oil content. The percentage of eicosenoic acid re-mained relatively constant with decreasing erucic acid except in the zero erucic acid genotype where only 1% eicosenoic was detected instead of approx 12%. These observations were in-terpreted to mean that eicosenoic and erucic acids were formed by a genetically controlled carbon chain lengthening system operating by the addi-tion of acetate molecules to the carboxyl end of oleic acid. This hypothesis was supported by data obtained from the injection of radioactive sodium acetate into immature rapeseed pods. In the monoene fraction of the oil, eicosenoate had three times the specific activity of oleate and erucate had twice the activity of eicosenoate. On oxidation, the monocarboxylic fragments of these three acids had low and similar specific activities while in the dicarboxylic acids, where chain elongation has presumably taken place, activity increased in the ratio of 1:5:19.