Involved in sterol biosynthesis. Catalyzes the 14-alpha demethylation of obtusifoliol to 4 alpha-methyl-5 alpha-ergosta-8,14,24(28)-trien-3 beta-ol. a 14alpha-methyl steroid + 3 O2 + 3 reduced [NADPH--hemoprotein reductase] = a Delta(14) steroid + formate + 4 H(+) + 4 H2O + 3 oxidized [NADPH--hemoprotein reductase] Expressed in leaves, roots, stems, siliques, flowers, flower buds and seedlings. Lack of membrane integrity. Seedling lethality. Decreased expression of CYP51G1 by antisense leads to a semidwarf phenotype in the early growth stage and a longer life span. Disruption mutants accumulate obtusifoliol and 14-alpha-methyl-sterols and cannot be rescued by exogenous application of brassinosteroids. Belongs to the cytochrome P450 family.