Genes encoding homeodomain-leucine zipper proteins in Arabidopsis thaliana

Abstract: A new class of transcription factors, the homeodomain-leucine zipper (HD-Zip) proteins,have been identified in Arabidopsis thaliana. These proteins contain a homeodomainfollowed by a leucine-zipper motif, a combination so far only found in plants. Sixdifferent genes (ATHB-genes for Arabidopsis thaliana homeobox) which encode proteinsin this class, have been isolated in this study. The HD-Zip class of Arabidopsis includesmore than 20 members in total, the majority ate of unknown function. Analysis ofmRNA expression, shows the isolated genes to be expressed at various levels during thevegetative stage of the plant life cycle.The first gene isolated, ATHB-3, showed a high level of expression in the cortexcells of the root and stem. The function of this gene is unknown. The functions of twoother genes, ATHB-6 and ATHB-7 were found to be involved in the control of vegetativegrowth, in response to water availability. ATHB-6 and ATHB-7 are induced by waterdeficit as well as by the plant hormone abscisic acid (ABA). The expression of ATHB-7was reduced in the ABA insensitive, abi1 mutant background indicating that ATHB-7functions downstream of ABI1 in a signal transduction pathway of an ABA mediateddrought stress response. Transgenic Arabidopsis plants, expressing ATHB-7 at highlevels show a reduction in growth elongation consistent with ATHB-7 regulating cell-elongation in response to drought. An ATHB-6 promoter-marker gene fusion intransgenic Arabidopsis showed ATHB-6 to be expressed in the developing anddifferentiating cells of the leaf and root, suggesting ATHB-6 to be a developmentalregulator in these tissues.Taken together, the results presented in this study show that plants, like animals, usehomeobox genes as transcription factors to control different developmental processes. Incontrast to animal homeobox genes, which act in the control of embryonal development,the plant homeobox genes of the HD-Zip class seem to be important regulators of plantdevelopment and differentiation in response to environmental factors.