Metabolic mechanisms associated with alleles governing the 16 : 0 concentration of soybean oil

Soybean [Glycine max (L.) Merr.] oil typically contains ca. 11% palmitic ac id, but germplasm has been developed with less than 4% to about 35% 16:0. A number of recessive alleles associated with these phenotypes have been des cribed that represent different mutations at Fap loci, however, the gene pr oducts (enzymes) produced by these alleles are unknown. This work attempts to define the metabolic activities that are regulated by the fap(1) fap(2), and fap(nc) alleles in soybean. Observation of de novo synthesis and metab olic turnover of fatty acids esterified to phospholipids in cotyledons duri ng the period of peak oil accumulation revealed genotypic differences in th e supply of 16:0-CoA from plastids. These metabolic studies narrowed the id entification of fap(1) fap(2), and fap(nc) alleles to the genes that encode or regulate the 3-keto-acyl-ACP synthetase II (where ACP is acyl carrier p rotein), 16:0-ACP thioesterase, 18:0-ACP desaturase, or 18:1-ACP thioestera se enzymes. Kinetic analyses suggested that the fap(2) mutation results in a decreased 3-keto-acyl-ACP synthetase II activity. Deficiencies in 16:0-AC P thioesterase activity represented the most likely explanation of fap(1) a nd fap(nc) gene function. This hypothesis was strongly supported by Norther n blot assays that revealed a significant reduction in the accumulation of transcripts corresponding to the 16:0-ACP thioesterase in germplasm homozyg ous for the fap(nc) allele.