Catalyzes the condensation reaction of fatty acid synthesis by the addition to an acyl acceptor of two carbons from malonyl-ACP. KAS III catalyzes the first condensation reaction which initiates fatty acid synthesis and may therefore play a role in governing the total rate of fatty acid production. Possesses both acetoacetyl-ACP synthase and acetyl transacylase activities (By similarity). acetyl-CoA + H(+) + malonyl-[ACP] = 3-oxobutanoyl-[ACP] + CO2 + CoA No inhibition by the antibiotic cerulenin. Lipid metabolism; fatty acid biosynthesis. Expressed in both leaf and root. Belongs to the thiolase-like superfamily. FabH family.