Investigation into sterol signalling in Arabidopsis

The hydra sterol mutants (hydral and fk(^hyd2)) phenotypes are characterised by short thickened roots and a shoot consisting of a mass of indistinct leaves. At the cellular level, cell patterning is disorganised and cell shape irregular, hydra sterol mutants are not phenotypically rescued by application of extrogeneous sterols (Lindsey et al, 2003) and have auxin and ethylene signalling defects but no defects in biosynthesis (Souter et al., 2002).It is not known at the mechanistic level how the hydra phenotype is generated. The disruption in sterol biosynthesis and subsequent altered sterol profile may lead to a loss of sterol-based signals required for development (Schrick et al, 2000), or disruption of other hormone signalling pathways (Souter et al. 2002, 2004); or some other mechanism. To determine whether sterol biosynthesis is required in specific cell types, we expressed the wild type HYDRA1 and HYDRA2 genes respectively under tissue specific promoters in the relevant hydra backgrounds and looked for evidence of phenotypic rescue. The analysis included examination of GFP expression in UAS enhancer trap lines, quantification of root length, examining the root tip cellular structure and characterization of cellular defects in mature plants using microscopy and tissue staining. Phenotypic rescue occurred in all lines analysed, however there were differences in the extent of phenotypic rescue under different promoters and in different independent transgenic lines. Where the same promoters were used, there was a difference in the degree of rescue in hydral to fk(^hyd2). Fk(^hyd2) displayed partial rescue whereas hydral displayed almost complete restoration to wild type phenotype fk(^hyd2) is known to have the more severe phenotype of the two mutants, this result may indicate the product of C-14 reductase has a critical role in plant development. The major conclusion is correct sterol biosynthesis is not required in all root tissues for correct plant development.