Catalyzes the NAD-dependent reduction of a carbon-carbon double bond in an enoyl moiety that is covalently linked to an acyl carrier protein (ACP). Catalyzes the last reduction step in the de novo synthesis cycle of fatty acids. Involved in the elongation cycle of fatty acids which are used in lipid metabolism. Required for normal plant growth (By similarity). a 2,3-saturated acyl-[ACP] + NAD(+) = a (2E)-enoyl-[ACP] + H(+) + NADH Lipid metabolism; fatty acid biosynthesis. Homotetramer. Belongs to the short-chain dehydrogenases/reductases (SDR) family. FabI subfamily.