Mechanisms governing interaction between multicellular organisms and microbes are central for understanding pathogenesis, symbiosis and the function of ecosystems. We propose to address these mechanisms by pioneering an interdisciplinary approach for understanding cellular signalling, response processes and organ development. The challenge is to determine factors synchronising three processes, organogenesis, infection thread formation and bacterial infection, running in parallel to build a root nodule hosting symbiotic bacteria. We aim to exploit the unique possibilities for analysing endocytosis of bacteria in model legumes and to develop genomic, genetic and biological chemistry tools to break new ground in our understanding of carbohydrates in plant development and plant-microbe interaction. Surface exposed rhizobial polysaccharides play a crucial but poorly understood role in infection thread formation and rhizobial invasion resulting in endocytosis. We will undertake an integrated functional characterisation of receptor-ligand mechanisms mediating recognition of secreted polysaccharides and subsequent signal amplification. So far progress in this field has been limited by the complex nature of carbohydrate polymers, lack of a suitable experimental model system where both partners in an interaction could be manipulated and lack of corresponding methods for carbohydrate synthesis, analysis and interaction studies. In this context our legume model system and the discovery that the legume Nod-factor receptors recognise bacterial lipochitin-oligosaccharide signals at their LysM domains provides a new opportunity. Combined with advanced bioorganic chemistry and nanobioscience approaches this proposal will engage the above mentioned limitations.