The HDZip Class I Transcription Factors in Arabidopsis thaliana Characterisation of HDZip Genes Involved in the Mediation of Environmental Signals

Sammanfattning: Homeodomain leucine zipper (HDZip) proteins constitute a large family of transcription factors characterised by the presence of a DNA-binding homeodomain and an adjacent leucine zipper motif, which mediates protein-dimer formation. The HDZip genes of Arabidopsis have been divided into four classes, HDZip I-IV. This thesis describes the characterisation and phylogeny of the class I HDZip genes and focuses on the expression and function of four HDZip I genes, ATHB5, -6, -7 and -16.The phylogenetic analyses of the 17 HDZip I sequences defined six subclasses, supported by the intron patterning and the traced duplication history of the genes. The members within each subclass showed diversification in expression, suggesting that the cis regulatory regions of the closely related genes have undergone evolutionary changes. However, similarities in the gene expression patterns between genes also exist and external factors like the availability of water and quality of light directs the expression of a subset of HDZip I genes. Expression analyses revealed that the plant hormone abscisic acid (ABA) is involved as a systemic signal for the salt or osmoticum induced ATHB7 expression, whereas light signals mediated through the blue light photoreceptors was found to direct the expression of ATHB6.Phenotypic analyses of plants with altered levels of ATHB6 or ATHB16 suggested that these paralogous genes encode proteins with very similar functions. ATHB16 was shown to act as a negative regulator of leaf cell expansion, as a suppressor of the flowering time sensitivity to photoperiod and as a positive regulator of blue light dependent inhibition of hypocotyl growth. A similar role for ATHB6 in the regulation of hypocotyl elongation was recorded. Further, analyses of multiple loss-of-function plants demonstrated that ATHB5, -6 and -16 function at least in part redundantly in mediating light effects on hypocotyl elongation.