Sterol gene expression in Arabidopsis

Sterols are recognized as playing vital roles in plant development, as has been demonstrated from mutational studies. The study of the hydra mutants of Arabidopsis provides new tools to dissect sterol function in development at the molecular genetic level. Both HYDl and HYD2 genes encode sterol biosynthetic enzymes and their mutants exhibit pleiotropic phenotypes including defective embryogenesis, defective cell shape, polarity and patterning, multiple cotyledons and short roots and hypocotyls. The fact that neither hyd1 nor hyd2 are rescued by the exogenous application of BRs suggests other sterol-dependent signaling defects in these mutants. Interestingly, HYD genes are not expressed to detectable levels in all the tissues that show developmental defects. The aim of the work described in this thesis was to determine whether there is a non-cell autonomous sterol-dependent signal required for the correct growth and development of the root. The approach was a) to silence HYD gene expression in specific cell types of the root using RNAi; and b) to activate HYD gene expression in specific root cell types in a null hyd mutant background. It was found that attempts to silence the HYD1 and HYD2 genes in the root meristem by RNAi-mediated gene silencing had a small effect on root meristem function and patterning in individual transgenic lines for HYD1 gene repression. However, by expressing the HYD1 gene in cells of the root meristem of the hyd1 mutant, there was a dramatic effect on the development of the root. These data are consistent with the view that sterols function in a non-cell autonomous way to regulate root development in plants