We have recently discovered in Arabidopsis thaliana a novel plasma membrane receptor kinase (THESEUS1 or THE1), which plays a role in monitoring the integrity of the cell wall. In this proposal we will further investigate (1) the ligand : how does this receptor kinase detect the cell wall perturbation ? (2) the signal transduction : how is the signal translated into responses: i.e. changes in gene expression, cell wall composition and growth rate ? (3) the biological function : under which environmental or developmental circumstances this response plays an adaptive role ? The phenotype of mutants of closely-related THE1 family members also indicates a role for cell wall recognition in pollentube-embryosac interaction and cell morphogenesis. The identification of THE1 paves the way for the study of the communication of the plant cell with its extracellular matrix and thus opens up a novel field in plant biology.