Required to form the C-22 double bond in the sterol side chain. Possesses in vitro C-22 desaturase activity toward beta-sitosterol to produce stigmasterol. No activity with 24-epi-campesterol. 5-dehydroepisterol + H(+) + NADPH + O2 = ergosta-5,7,22,24(28)-tetraen-3beta-ol + 2 H2O + NADP(+) kcat is 10 min(-1) for beta-sisterol. Steroid biosynthesis; sterol biosynthesis. Belongs to the cytochrome P450 family. Was called CYP710A7 in EMBL:BAE93156. According to the cytochrome P450 homepage, CYP710A7 is a rice cytochrome.