Like all seed plants, the body of the model plant Arabidopsis thaliana consists of two distinct systems, the root and the shoot. The shoot system contains the shoot apical meristem (SAM) harboring a population of plant stem cells. During the vegetative growth phase the population of stem cells in the SAM divides asymmetrically and produces the cells that will eventually form new organs. Leaves are initiated at the periphery of the SAM in a circular fashion. Proper establishment of the ad/abaxial (top/bottom) axis is of great importance because shape and size of the outgrowing leaf blade highly depends on the juxtaposition of ad/abaxial tissue. Several genes have been isolated that are involved in specifying either adaxial or abaxial cell fate. These genes often act as master regulators. As a postdoctoral fellow I have generated transgenic Arabidopsis plants expressing inducible versions of some of these master regulators. Using microarrays, we have identified target genes of these master regulators in Arabidopsis. In order to validate unknown or hypothetical proteins which are targets of these master regulators I am proposing experiments directed to identify evolutionary conserved signaling pathways. Therefore I have established transgenic Cardamine plants, which are closely related to Arabidopsis, expressing these inducible master regulators. Transformation of a more distant plant species, Aquilegia, is currently in progress. Using a combination of microarray and next generation sequencing approaches we aim to identify target genes of these master regulators in Cardamine and Aquilegia plants. The identification of these target genes will allow us to study evolutionary conserved signaling modules as well as to define unique signaling pathways. These investigations will be complemented by protein evolution studies of the master regulators and by analyzing cis-element evolution of the target genes they control. The identification of conserved and unique signaling pathways in a diverse range of species will further our understanding of how the three-dimensional shape of leaves is controlled and modulated at the molecular level.