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. No activity with campesterol. 5-dehydroepisterol + H(+) + NADPH + O2 = ergosta-5,7,22,24(28)-tetraen-3beta-ol + 2 H2O + NADP(+) kcat is 0.53 min(-1) for beta-sisterol. Steroid biosynthesis; sterol biosynthesis. Expressed in the vascular tissues of roots, shoots and leaves. Expressed in root tips and sepals. Very low expression in stems and siliques. By infection with Pseudomonas syringae. No visible phenotype under normal growth condition, but upon infection with P.syringae, no accumulation of stigmasterol and increased resistance to the pathogen. Plants overexpressing CYP710A1 show higher levels of stigmasterol and lower levels of beta-sitosterol than the wild-type. Belongs to the cytochrome P450 family.