Characterisation of two morphogenesis mutants in Arabidopsis thaliana
In this thesis is described the characterisation of two morphogenesis mutants of Arabidopsis thaliana, with the aim of furthering our understanding of signalling in development. A thorough phenotypic analysis of both mutants is presented, with particular attention paid to the seedling root. The results of genetic analysis of hydra2 are also presented. The hydra mutants are characterised by a pleiotropic phenotype, with defective embryonic and seedling cell patterning, morphogenesis and root growth. The HYDRA1 gene encodes a A8-A7 sterol isomerase, while HYDRA2 encodes a sterol C14 reductase, previously identified as the FACKEL gene product. Seedlings mutant for each gene are similarly defective in the concentrations of the three major Arabidopsis sterols. Reporter gene analysis showed mis-expression of the hormonally-regulated DR5, IAA2 and ACSl promoters and of the epidermal cell file-specific GL2 promoter in the mutants. The mutants also exhibit enhanced responses to auxin. The phenotypes can be partially rescued by inhibition of auxin and ethylene signalling, but not by exogenous brassinosteroids. Evidence of abnormal activity of hormone-related membrane-bounds proteins and of altered membrane permeability to auxin is presented. A model in which correct sterol profiles are required for regulated auxin and ethylene signalling through effects on membrane function is proposed.