Inhibition of Cycloartenol Synthase (CAS) Function in Tobacco BY-2 Cells

Tobacco BY-2 cell suspensions are our preferred model for studying isoprenoid biosynthesis pathways, due to their easy genetic transformation and the efficient absorption of metabolic precursors, intermediates, and/or inhibitors. Using this model system, we have analyzed the effects of chemical and genetic blockage of cycloartenol synthase (CAS, EC 5.4.99.8), an oxidosqualene cyclase that catalyzes the first committed step in the sterol pathway of plants. BY-2 cells were treated with RO 48-8071, a potent inhibitor of oxidosqualene cyclization. Short-term treatments (24 h) resulted in accumulation of oxidosqualene with no changes in the final sterol products. Interestingly, long-term treatments (6 days) induced down-regulation in gene expression not only of CAS but also of the SMT2 gene coding sterol methyltransferase 2 (EC 2.1.1.41). This explains some of the increase in 24-methyl sterols at the expense of the 24-ethyl sterols stigmasterol and sitosterol. In our alternative strategy, CAS gene expression was partially blocked by using an inducible artificial microRNA. The limited effectiveness of this approach might be explained by some dependence of the machinery for RNAi formation on an operating MVA/sterol pathway. For comparison we checked the effect of RO 48-8071 on a green cell suspension of Arabidopsis and on seedlings, containing a small spectrum of triterpenes besides phytosterols. Triterpenes remained essentially unaffected, but phytosterol accumulation was clearly diminished.