The ratio of campesterol to sitosterol that modulates growth in Arabidopsis is controlled by STEROL METHYLTRANSFERASE 2;1

The Arabidopsis genome contains three distinct genes encoding sterol-C24-me thyltransferases (SMTs) involved in sterol biosynthesis. The expression of one of them, STEROL METHYLTRANSFERASE 2;1, was modulated in 35S::SMT2;1 Ara bidopsis in order to study its physiological function. Plants overexpressin g the transgene accumulate sitosterol, a 24-ethylsterol which is thought to be the typical plant membrane reinforcer, at the expense of campesterol. T hese plants displayed a reduced stature and growth that could be restored b y brassinosteroid treatment. Plants showing co-suppression of SMT2;1 were c haracterized by a predominant 24-methylsterol biosynthetic pathway leading to a high campesterol content and a depletion in sitosterol. Pleiotropic ef fects on development such as reduced growth, increased branching, and low f ertility of high-campesterol plants were not modified by exogenous brassino steroids, indicating specific sterol requirements to promote normal develop ment. Thus SMT2;? has a crucial role in balancing the ratio of campesterol to sitosterol in order to fit both growth requirements and membrane integri ty.