Growth and development of plants are regulated by signalling substances such as hormones. In plants, interactions between hormonal pathways represent crucial factors that govern their action. As the molecular basis for such hormonal cross-talk remains largely unknown, we will investigate the underlying mechanisms with a special focus on regulation of postembryonic organogenesis. We consider lateral root formation in Arabidopsis as an ideally suited model system, because it encompasses fundamental aspects of plant growth and development, such as dedifferentiation, re-entry into the cell cycle, coordinated cell divisions and differentiation. Furthermore, in lateral root formation, these processes are controlled by multiple hormonal pathways. In our proposal, we will focus on four main research directions. 1. Convergence of hormonal pathways on transport-dependent auxin distribution upstream of lateral root formation. Here, we want to identify key points in which auxin and other signalling pathways converge during lateral root formation and the molecular components involved in the process. 2. Role of auxin-cytokinin interaction in lateral root formation. Molecular events involved in auxin-cytokinin regulated lateral root formation will be studied by transcriptome analysis. 3. Identification of components of hormonal cross-talk by genetic approaches. Using lateral root formation as a model, we will perform mutant screens that will specifically target interactions between selected hormonal pathways. The spectrum of identified molecular components will be further expanded by a chemical genomics approach. 4. Formulation of general models for hormonal regulation of organogenesis. The acquired knowledge on molecular networks and their mutual interactions will be used to mathematically model lateral root development and to extrapolate them also on other developmental situations.