Required to form the C-22 double bond in the sterol side chain. Possesses in vitro C-22 desaturase activity toward beta-sitosterol and produces stigmasterol. 5-dehydroepisterol + H(+) + NADPH + O2 = ergosta-5,7,22,24(28)-tetraen-3beta-ol + 2 H2O + NADP(+) Expressed in stems. Detected in primary root caps and immature petals. Belongs to the cytochrome P450 family.