The SMO1 Family of Sterol 4α-Methyl Oxidases Is Essential for Auxin- and Cytokinin-Regulated Embryogenesis

Abstract

In the plant sterol biosynthetic pathway, sterol 4α-methyl oxidase1 (SMO1) and SMO2 enzymes are involved in the removal of the first and second methyl groups at the C-4 position, respectively. SMO2s have been found to be essential for embryonic and postembryonic development, but the roles of SMO1s remain unclear. Here, we found that the three Arabidopsis (<i>Arabidopsis thaliana</i>) <i>SMO1</i> genes displayed different expression patterns. Single <i>smo1</i> mutants and <i>smo1-1 smo1-3</i> double mutants showed no obvious phenotype, but the <i>smo1-1 smo1-2</i> double mutant was embryo lethal. The <i>smo1-1 smo1-2</i> embryos exhibited severe defects, including no cotyledon or shoot apical meristem formation, abnormal division of suspensor cells, and twin embryos. These defects were associated with enhanced and ectopic expression of auxin biosynthesis and response reporters. Consistently, the expression pattern and polar localization of PIN FORMED1, PIN FORMED7, and AUXIN RESISTANT1 auxin transporters were dramatically altered in <i>smo1-1 smo1-2</i> embryos. Moreover, cytokinin biosynthesis and response were reduced in <i>smo1-1 smo1-2</i> embryos. Tissue culture experiments further demonstrated that homeostasis between auxin and cytokinin was altered in <i>smo1-1 smo1-2</i> heterozygous mutants. This disturbed balance of auxin and cytokinin in <i>smo1-1 smo1-2</i> embryos was accompanied by unrestricted expression of the quiescent center marker <i>WUSCHEL-RELATED HOMEOBOX5</i> Accordingly, exogenous application of either auxin biosynthesis inhibitor or cytokinin partially rescued the embryo lethality of <i>smo1-1 smo1-2</i> Sterol analyses revealed that 4,4-dimethylsterols dramatically accumulated in <i>smo1-1 smo1-2</i> heterozygous mutants. Together, these data demonstrate that SMO1s function through maintaining correct sterol composition to balance auxin and cytokinin activities during embryogenesis.