Figure 11.

Model of the functions of SMO2 proteins in Arabidopsis embryonic and postembryonic development. The SMO2-1 and SMO2-2 enzymes catalyze the conversion of 4α-methylsterols (mainly 24-ethylidene lophenol and 24-ethyl lophenol) to 4α-caboxysterols, which are further converted into membrane sterols. Normal membrane sterol composition is required for correct PIN1 auxin efflux facilitator positioning and PIN1-dependent auxin gradient formation, which are essential for embryonic and postembryonic development. It is also possible that 4α-caboxysterols are needed for PIN1 localization and embryonic development and that 24-ethylidene lophenol and 24-ethyl lophenol are deleterious to Arabidopsis embryos.