Non-heme iron oxygenase involved in sterols biosynthesis by catalyzing the removal of the first methyl group at the C-4 position (PubMed:14653780). 4,4-dimethyl-9-beta,19-cyclopropylsterols such as 24-methylenecycloartanol are the preferred substrates (PubMed:14653780). Acts as a rate-limiting enzyme in the sterol pathway via interaction with ACBP1; sterols serve as lipid modulators for gene expression of homeodomain-leucine zipper IV transcription factors (PubMed:28500265). Together with SMO1-2, involved in the maintenance of sterol composition to balance auxin and cytokinin activities during embryogenesis (PubMed:31341004). 4,4-dimethyl-5alpha-cholest-7-en-3beta-ol + 6 Fe(II)-[cytochrome b5] + 5 H(+) + 3 O2 = 4alpha-carboxy-4beta-methyl-5alpha-cholest-7-ene-3beta-ol + 6 Fe(III)-[cytochrome b5] + 4 H2O 24-methylenecycloartanol + 6 Fe(II)-[cytochrome b5] + 5 H(+) + 3 O2 = 4alpha-carboxy-4beta,14alpha-dimethyl-9beta,19-cyclo-5alpha-ergost-24(24(1))-en-3beta-ol + 6 Fe(III)-[cytochrome b5] + 4 H2O Interacts with ACBP1. Expressed in rosettes, stems, roots, floral buds, flowers and siliques. Strongly expressed in developing ovules (PubMed:28500265). During embryogenesis, expressed from the globular stage to the mature stage in both the embryo and endosperm (PubMed:31341004). From the torpedo to the mature embryo stages, strongly expressed in the provascular cells of the developing hypocotyl and in the shoot apical meristem (SAM) (PubMed:31341004). In leaves, strongly expressed in vascular tissues and stomata (PubMed:31341004). In roots, accumulates in the stele and at lateral root formation sites (PubMed:31341004). In flowers, observed in anthers and pistils (PubMed:31341004). In siliques, high levels in seedpods (PubMed:31341004). The histidine box domains may contain the active site and/or be involved in metal ion binding. Proembryo abortion in the double mutant lacking both SMO1-1 and ACBP1 associated with altered fatty acids (FAs) and sterols profiles (PubMed:28500265). The double mutant smo1-1 smo1-3 shows no obvious phenotype (PubMed:31341004). The double mutant smo1-1 smo1-2, which accumulates dramatically 4,4-dimethylsterols, is embryo lethal, with embryo exhibiting severe defects, including no cotyledon or shoot apical meristem formation, abnormal division of suspensor cells, and twin embryos, and is associated with altered auxin and cytokinin homeostasis (PubMed:31341004). Requires a membrane-bound cytochrome b5 as an obligatory electron carrier from NAD(P)H to SMO. Belongs to the sterol desaturase family.