Characterisation of Members of the HD-Zip I and DREB/ERF Transcription Factor Families and their Functions in Plant Stress Responses

Abstract: Plants have to adapt to various environmental conditions. In comparison with other eukaryotes, plant genomes encode relatively many transcription factors, and have families of transcription factors only found in plants, two such are the large AP2/ERF (APETALA2/Ethylene Responsive Factor) and HD-Zip (Homeodomain leucine zipper) families. Members of these families play roles in plant development, responses to stress and other environmental stimuli. This thesis describes the functions of members of both these families. Two AP2/ERF proteins of the DREB/ERF (Drought Responsive Element Binding/Ethylene Responsive Factor) subfamily, AtERF38 and AtERF39 show a negative effect on growth in transgenic over-expression lines. They are regulated by stress and confer stress tolerance, which suggests a function as negative growth regulators during stress. The 14 Oryza sativa (rice) HD-Zip I genes were described and a phylogenetic analysis of the HD-Zip I genes in Arabidopsis thaliana, rice and Craterostigma plantagineum defined four clades that were conserved between mono and dicots, but also showed that four clades had most likely been lost in either lineage. The phylogeny was supported by conserved intron/exon patterning and the duplication history of the rice genome. The expression patterns in different organs indicate a non-conserved regulation of genes within and between clades. Drought stress regulation was conserved within two clades, but not between the clades. Further analysis on the function of four HD-Zip I genes in Arabidopsis, ATHB5, -6, -7 and -12 showed an involvement in the transcriptional regulation of ABA receptors and signalling components. These are the first described direct target genes for ATHB7 and ATHB12. It was also shown that there exists a direct cross-regulation between these four HD-Zips. The effect of ATHB5, -6, -7 and -12 would be as negative regulators of ABA signalling, desensitising the plant to constant levels of ABA, and enabling sensitivity to changes in stress level.