Genetic regulation of elevated stearic acid concentration in soybean oil

Abstract

Soybean [Glycine max (L.) Merr.] oil from commercial cultivars typically contains ca. 3% stearic acid (18∶0). However, germplasm carrying different mutations at the locus governing stearic acid (Fas) may contain 3% to about 35% 18∶0. Among these germplasm, a newly developed line, FAM94-41 (9% 18∶0), carries a serendipitous natural mutation that is temporarily designated as the recessive fas _nc allele, and the germplasm A6 (26% 18∶0) carries the recessive fas ^a allele. Mendelian genetic analysis of progeny from FAM94-41×A6 revealed that fas _nc and fas ^a are allelic to each other and represent different mutations in the same structural gene. However, the gene products (enzymes) produced by these alleles are unknown. The observation that 18∶0 concentrations among progeny from FAM94-41×A6 increased primarily at the expense of unsaturated C₁₈ FA suggests that fas alleles may reduce either 18∶0-acyl carrier protein (AcP) desaturase or 18∶1-ACP thioesterase activity. However, it also is conceivable that elevated 18∶0 concentrations may result from increased 3-keto-acyl-ACP synthetase (KAS) II activity. To test the latter possibility, a population was created that segregated for the fas _nc and the fap ₂ alleles (the latter of which is associated with reduced KAS-II activity). Mendelian genetic analysis showed that these alleles represent independent genes at different gene loci and interact in an additive genetic manner to increase the total saturate concentration in this population. Based on this finding, we speculate that fas alleles probably encode 18∶0-ACP desaturase or 18∶1-ACP thioesterase in soybeans.