Catalyzes the C22-alpha-hydroxylation step in brassinosteroid biosynthesis, which is the rate-limiting step in this biosynthetic pathway (PubMed:16369540). Catalyzes the conversion of campesterol (CR) to (22S)-22-hydroxycampesterol (22-OHCR, 22-hydroxyCR) and of campestanol (CN) to 6-deoxycathasterone (6-deoxoCT) (PubMed:16369540). Required for auxin responses involved in the regulation of epidermal cells length of the lamina joint (PubMed:19605414). campesterol + O2 + reduced [NADPH--hemoprotein reductase] = (22S)-22-hydroxycampesterol + H(+) + H2O + oxidized [NADPH--hemoprotein reductase] campestanol + O2 + reduced [NADPH--hemoprotein reductase] = 6-deoxycathasterone + H(+) + H2O + oxidized [NADPH--hemoprotein reductase] Plant hormone biosynthesis; brassinosteroid biosynthesis. Highly expressed in roots and leaf blades. Expressed in shoot apex, stems, leaf sheaths, inflorescences and flowers. Down-regulated by brassinolide (BL). Slight dwarf phenotype and erect leaves (PubMed:16369540). Impaired response to auxin leading to reduced epidermal cells length of the lamina joint (PubMed:19605414). Null mutant plants for CYP90B1/DWARF4 produce increased biomass and grain yield at high density planting. Belongs to the cytochrome P450 family. Truncated N-terminus.