Mediates the synthesis of VLCFAs from 22 to 26 carbons in length (e.g. C22, C24, C26) (PubMed:15277688). Functionally redundant with KCS2 in the two-carbon elongation of C22 fatty acids that is required for cuticular wax and root suberin biosynthesis (PubMed:19619160). a very-long-chain acyl-CoA + H(+) + malonyl-CoA = a very-long-chain 3-oxoacyl-CoA + CO2 + CoA Inhibited by K3 herbicides such as alachlor, allidochlor, anilofos, cafenstrole, fentrazamide and flufenacet (PubMed:15277688). Strongly inhibited by metazachlor and only slightly by mefluidide (PubMed:22284369). Lipid metabolism; fatty acid biosynthesis. Expressed in aerial organs (PubMed:15914083). Expressed in leaves, flowers, siliques and stems (PubMed:18465198). Expressed in roots, young seedlings, leaves, flowers and siliques (PubMed:19619160). Repressed by herbicides such as flufenacet and benfuresate (PubMed:12916765). Down-regulated by darkness and low temperature, and up-regulated by drought and osmotic stress (PubMed:18465198). Up-regulated twofold by drought, but no effect of other stress treatments (PubMed:19619160). No visible phenotype, but reduced root growth. Kcs2 and kcs20 double mutants have a glossy green appearance due to a significant reduction of the amount of epicuticular wax crystals on the stems and siliques, a significant reduction of C22 and C24 VLCFA derivatives in aliphatic suberin and a roots growth retardation and abnormal lamellation of the suberin layer in the endodermis. Belongs to the thiolase-like superfamily. Chalcone/stilbene synthases family.