Involved in a reduction step in the biosynthesis of the plant steroid, brassinolide (BL) (PubMed:17565582). Promotes cotton fibers (seed trichomes) initiation and elongation (PubMed:17565582). a 3-oxo-5alpha-steroid + NADP(+) = a 3-oxo-Delta(4)-steroid + H(+) + NADPH Plant hormone biosynthesis; brassinosteroid biosynthesis. Accumulates in fibers (seed trichomes) during both their initiation and elongation phases. Also present in roots, hypocotyls, leaves, flowers and ovules, and barely in cotyledons. Impaired fiber initiation and elongation. Belongs to the steroid 5-alpha reductase family.