Sterols regulate development and gene expression in Arabidopsis

Sterols are important not only for structural components of eukaryotic cell membranes but also for biosynthetic precursors of steroid hormones. In plants, the diverse functions of sterol-derived brassinosteroids (BRs) in growth and development have been investigated rigorously, yet little is known about the regulatory roles of other phytosterols. Recent analysis of Arabidopsis fackel (fk) mutants and cloning of the FK gene that encodes a sterol C-14 reductase have indicated that sterols play a crucial role in plant cell division, embryogenesis, and development: Nevertheless, the molecular mechanism underlying the regulatory role of sterols in plant development has not been revealed. In this report, we demonstrate that both sterols and BR are active regulators of plant development and gene expression. Similar to BR, both typical (sitosterol and stigmasterol) and atypical (8, 14-diene sterols accumulated in fk mutants) sterols affect the expression of genes involved in cell expansion and cell division. The regulatory function of sterols in plant development is further supported by a phenocopy of the fk mutant using a sterol C-14 reductase inhibitor, fenpropimorph. Although fenpropimorph impairs cell expansion and affects gene expression in a dose-dependent manner, neither effect can be corrected by applying exogenous BR. These results provide strong evidence that sterols are essential for normal plant growth and development and that there is likely a BR-independent sterol response pathway in plants. On the basis of the expression of endogenous FK and a reporter gene FK::beta-glucuronidase, we have found that FK is up-regulated by several growth-promoting hormones including brassinolide and auxin, implicating a possible hormone crosstalk between sterol and other hormone-signaling pathways.