Non-heme iron oxygenase involved in sterols biosynthesis by catalyzing the removal of the first methyl group at the C-4 position (By similarity). 4,4-dimethyl-9-beta,19-cyclopropylsterols such as 24-methylenecycloartanol are the preferred substrates (By similarity). 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:29288621). Together with SMO1-1, 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 embryo sacs, pollen and trichomes (PubMed:29288621). Observed in leaves, roots, siliques and flowers (PubMed:31341004). Highly expressed in pollen grains at all floral stages and in pollen tubes at anthesis (PubMed:29288621). Observed in transmitting tracts along pistils, nectaries and ovules, and in the entire pistils at anthesis (PubMed:29288621). In anthers, present in pollen and tapetal cells (PubMed:29288621). Accumulates in imbibed pollen grains and in germinating pollen tubes (PubMed:29288621). Detected in embryo sacs at stages 13-14 (PubMed:29288621). During embryogenesis, expressed from the globular stage to the mature stage in the embryo but not in the endosperm, especially in the embryonic root meristem (PubMed:31341004). Accumulates also in trichomes on rosettes and stems, and the vasculature in roots (PubMed:29288621). In developing seeds, expressed in the basal suspensor cells and chalazal seed coat at the heart stage of embryogenesis, and in the radicle and cotyledons of mature green embryos (PubMed:29288621). In leaves, strongly expressed in vascular tissues (PubMed:31341004). In roots, detected in root tips, mostly in the root stem cell niche and columella cells (PubMed:31341004). In flowers, accumulates in styles, petals and anther filaments (PubMed:31341004). In siliques, present in funiculi (PubMed:31341004). The histidine box domains may contain the active site and/or be involved in metal ion binding. Double mutants lacking SMO1-2 and ACBP1 are impaired in seed development, embryo sac development, male and female gamete transmission, and pollen function, as well as altered fatty acids (FAs) and sterols profiles (PubMed:29288621). 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 an 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.