Catalyzes the last of the four reactions of the long-chain fatty acids elongation cycle. This endoplasmic reticulum-bound enzymatic process, allows the addition of 2 carbons to the chain of long- and very long-chain fatty acids/VLCFAs per cycle. This enzyme reduces the trans-2,3-enoyl-CoA fatty acid intermediate to an acyl-CoA that can be further elongated by entering a new cycle of elongation. Thereby, it participates in the production of VLCFAs of different chain lengths that are involved in multiple biological processes as precursors of membrane lipids and lipid mediators. Required for the elongation of fatty acids precursors of sphingolipids, storage lipids and cuticular waxes. a very-long-chain 2,3-saturated fatty acyl-CoA + NADP(+) = a very-long-chain (2E)-enoyl-CoA + H(+) + NADPH Lipid metabolism; fatty acid biosynthesis. Part of the fatty acid elongase complex which contains a beta-ketoacyl-CoA synthase (KCS), a beta-ketoacyl-CoA reductase (KCR), a beta-hydroxyacyl-CoA dehydratase (HCD) and an enoyl-CoA reductase (ECR). Expressed in roots, leaves, stems, flowers and siliques. Detected in mature seeds. Abnormal organ morphology and stem glossiness. Belongs to the steroid 5-alpha reductase family.