Figure 12.

A working model for SMO1s in embryonic development. SMO1 enzymes catalyze the conversion of 4,4-dimethylsterols into 4α-carboxy-4β-methylsterols, which are further converted into end-product 4-demethylsterols, including β-sitosterol, campesterol, stigmasterol, and cholesterol. Correct composition of these sterols is required for polar localization of PIN proteins. SMO1-related SBIs may inhibit auxin biosynthesis, affect the polar localization of AUX1 and PIN proteins, and affect cytokinin activity, thereby maintaining an appropriate ratio between auxin and cytokinin activities, which is required for embryo development. R, Side chain of the sterol.