Contributes to fatty acids elongation and stockage in developing seeds. Active on both saturated and mono-unsaturated acyl-CoAs of 16 and 18 carbons. Required for the elongation of C18 to C20 and of C20 to C22 fatty acids. Mediates also the synthesis of VLCFAs from 20 to 26 carbons in length (e.g. C20:1, C20, C22:1, C22, C24:1, C24, C26) (PubMed:15277688, PubMed:16765910, PubMed:7734965, PubMed:9263455, Ref.4, Ref.5, PubMed:12135493, PubMed:23145136). Has no activity with polyunsaturated C18:2 and C18:3 or with acyl-CoAs having 22 carbons or longer chain length (PubMed:12135493). a very-long-chain acyl-CoA + H(+) + malonyl-CoA = a very-long-chain 3-oxoacyl-CoA + CO2 + CoA Inhibited by K3 herbicides such as allidochlor, cafenstrole and flufenacet (PubMed:15277688). Strongly inhibited by metazachlor and only slightly by mefluidide (PubMed:22284369). Optimum pH is 6.6-7.5. Lipid metabolism; fatty acid biosynthesis. Expressed specifically in seeds, especially in embryos (PubMed:11575726, PubMed:7734965). Expressed in siliques (PubMed:18465198). Accumulates in upper part of roots, hypocotyls and cotyledons of developing embryo. Levels increase in early stages (torpedo, 4 days after flowering) and decrease during late stages of embryo development. Repressed by herbicides such as flufenacet and benfuresate. Strong fatty acid chain length ratio (CLR) reduction. CLR.2, the major quantitative trait loci (QTL) involved in seed lipid metabolism is associated with the single point mutation L407V. Belongs to the thiolase-like superfamily. Chalcone/stilbene synthases family.