Abstract:

Sterols are essential molecules for multiple biological processes, including embryogenesis, cell elongation, and endocytosis. The plant sterol biosynthetic pathway is unique in the involvement of two distinct sterol 4α-methyl oxidase (smo) families,smo1andsmo2, which contain three and two isoforms, respectively, and are involved in sequential removal of the two methyl groups at C-4. In this study, we characterized thebiological functions of members of thesmo2gene family.smo2-1was strongly expressed in most tissues during Arabidopsis (Arabidopsis thaliana) development, whereassmo2-2showed a more specific expression pattern. Although singlesmo2mutantsdisplayed no obvious phenotype, thesmo2-1 smo2-2double mutant was embryonic lethal, and thesmo2-1 smo2-2/+mutant was dwarf, whereas thesmo2-1/+ smo2-2mutant exhibited a moderate phenotype. The phenotypes of thesmo2mutants resembled those of auxin-defective mutants. Indeed, the expression ofDR5rev:GFP, an auxin-responsive reporter, was reduced and abnormal insmo2-1 smo2-2embryos. Furthermore, the expression and subcellular localization of the PIN1 auxin efflux facilitator also were altered. Consistent with these observations, either the exogenous application of auxin or endogenous auxin overproduction (YUCCA9overexpression) partially rescued thesmo2-1 smo2-2embryonic lethality. Surprisingly, the dwarf phenotype ofsmo2-1 smo2-2/+was completely rescued byYUCCA9overexpression. Gas chromatography-mass spectrometry analysis revealed a substantial accumulation of 4α-methylsterols, substrates ofsmo2, insmo2heterozygous double mutants. Together, our data suggest thatsmo2s are important for correct sterol composition and function partially through effects on auxin accumulation, auxin response, and PIN1 expression to regulate Arabidopsis embryogenesis and postembryonic development.