"In animals, the formation and growth of new organs cease at the completion of development; in plants, the body is continuously built throughout the plant lifespan. In addition, in plants there is virtually no cell migration due to rigid cell walls: this provides a unique opportunity for dissecting the relations between pattern formation and final organ structure.Plant post-embryonic development takes place in localized regions called meristems. In the root of Arabidopsis thaliana, stem cells in the apical region of the meristem self-renew and produce daughter cells that differentiate in the distal meristem transition zone. To ensure root growth, the rate of cell differentiation must equal the rate of generation of new cells. Recent work of the proponent has shown that maintenance of the Arabidopsis root meristem size - and consequently root growth - is controlled by the interaction between two hormones in the meristem transition zone: cytokinin, which promotes cell differentiation, and auxin, which promotes cell division, and has unveiled the regulatory circuit underlying this interaction.Scope of this proposal is to clarify how the cytokinin/auxin interaction influences the activities of the entire root meristem, thus ensuring a balance between cell differentiation and cell division.In particular, the work proposed aims to:- Clarify how the cytokinin/auxin interaction is perceived in the transition zone to bring about synchronous differentiation of all cell files of the distal meristem.- Understand how the cytokinin/auxin interaction maintains a balance between cell differentiation in the (distal) transition zone and cell division in the (apical) stem cell niche.- Unveil other inputs and regulatory circuits that interact with the cytokinin/auxin regulatory circuit in controlling root growth.These goals will be achieved via the identification of genes involved in these processes. Hypothesis-driven experiments will allow verifying the role of candidate genes, and genetic and genome-wide approaches will allow identifying new genes.In these activities, we will take advantage of state-of-the-art techniques, including tissue- and cell-specific gene expression techniques, analysis of microarrays from cell lines isolated via cell sorter, light microscopy and confocal imaging techniques, as well as classic genetic, physiological and pharmacological approaches.The experimental results of these research lines will be integrated into computational models with the ultimate goal of describing the mechanisms of root growth and its control"