Involved in a reduction step in the biosynthesis of the plant steroid, brassinolide (BL); acts at the second step in brassinolide biosynthesis in the 5alpha-reduction of (24R)- 24-methylcholest-4-en-3-one, which is further modified to form campestanol. Can use progesterone, testosterone, androstenedione and campestenone as substrate. Catalyzes also the conversion of campest-4-en-3-one (campesta-4-en-3-one, 4-en-3-one) to campest-3-one (campesta-3-one, 3-one), of (22S,24R)-22-hydroxyergost-4-en-3-one (22-hydroxy-campesta-4-en-3-one, 22-OH-4-en-3-one) to (22S,24R)-22-hydroxy-5alpha-ergostan-3-one (22-hydroxy-campesta-3-one, 22-OH-3-one), and of (22R,23R)-22,23-dihydroxy-5alpha-campestan-3-one (22,23,diOH-4-en-3-one) to (22R,23R)-22,23-dihydroxycampest-4-en-3-one (6-deoxo3DT) (PubMed:22822057). Required for the brassinosteroid- (BR) dependent regulation of seed size and shape as well as embryo development (PubMed:23771896). a 3-oxo-5alpha-steroid + NADP(+) = a 3-oxo-Delta(4)-steroid + H(+) + NADPH 5alpha-campestan-3-one + NADP(+) = campest-4-en-3-one + H(+) + NADPH (22S,24R)-22-hydroxy-5alpha-ergostan-3-one + NADP(+) = (22S)-22-hydroxycampest-4-en-3-one + H(+) + NADPH 3-dehydro-6-deoxoteasterone + NADP(+) = (22R,23R)-22,23-dihydroxycampest-4-en-3-one + H(+) + NADPH Inhibited by the 4-azasteroids 4-MA. Optimum pH is 6.8. Plant hormone biosynthesis; brassinosteroid biosynthesis. Brassinolide-reversible dark- and light-grown defects; small dark-green dwarf plants with a limited leaves expension due to a reduced number of cells per leaf blade (PubMed:10394910, PubMed:11867704, PubMed:8602526, PubMed:9401120). Small and less elongated seeds due to a decreased seed cavity, reduced endosperm volume and integument cell length (PubMed:22822057). Delay embryo development, with a reduction in both the size and number of embryo cells (PubMed:22822057). Belongs to the steroid 5-alpha reductase family.