Enzymatic studies of high stearic acid sunflower seed mutants

Three high stearic acid sunflower (Helianthus annuus L.) mutants, CAS-3, CAS-4 and CAS-8, accumulating 28, 15 and 14 % of stearic acid in the seed lipids have been biochemically characterised. In vivo conversion rate of palmitic acid into stearic acid is not altered in the mutants but the conversion rate of stearic acid into oleic acid shows a reduction that correlated with the total stearic acid content of seed lipid mutants. Two enzymatic activities are found to be involved in the mutant phenotype, the acyl-ACP thioesterase (EC 3.1.2.14) and the stearoyl-ACP desaturase (EC 1.12.99.6). Our data suggest that the high stearic phenotype is due to the combined effect of a reduced stearoyl-ACP desaturase activity and an acyl-ACP thioesterase with higher activity on stearoyl-ACP. The same thioesterase activity increment, found on stearoyl-ACP, was also found on palmitoyl-ACP, suggesting that the affected thioesterase activity could be a FatB type.