Functional characterization of the pointed cotyledon subclass of HDZip genes in Arabidopsis thaliana

Abstract: Genes encoding homeodomain leucine zipper, HDZip, transcription factors constitute a large gene family in Arabidopsis thaliana. In this thesis the isolation and characterization of four HDZip genes (ATHB3, -13, -20 and -23) is described. These genes are similar in sequence and form a distinct subclass within the HDZip gene family. Since the genes cause similar alterations in cotyledon shape when expressed constitutively, we refer to the members of this subclass as pointed cotyledon HDZip genes. To determine the biological functions of the genes, the phenotypes of plants constitutively expressing the genes have been analysed. Each of the genes specifically inhibits lateral cell expansion in cotyledons and leaves and thereby causes them to be abnormally narrow. Detailed expression analysis shows that only ATHB23 is expressed in the entire leaf and cotyledon from early stages of development while ATHB20 is predominantly expressed in the root cortex. ATHB13 is expressed in basal parts of mature leaves and floral organs and ATHB3 in root and stem cortex. The ATHB13 protein acts within a signalling pathway that mediates a response to sucrose that specifically regulates the expression of specific sugar-regulated genes. ATHB3 specifically inhibits primary root development without affecting the development of secondary roots when constitutively expressed. Reduced expression of ATHB3 by antisense suppression results in increased expression of ATHB13, indicating that ATHB3 acts as a repressor of ATHB13 expression in the wild type.This thesis also reports the isolation of seven new genes of HDZip class I and reviews available functional information on the genes in this class. One conclusion is that HDZip I proteins that are closely related phylogenetically are also functionally related, in most cases. Seven different mutations in HDZip I genes were also identified. The lack of phenotypic deviations from wild type of these mutants suggests that these HDZip proteins act in a redundant fashion in the plant.