Patterns and symmetries in leaf development

The leaf is a coordinated mosaic of developmental domains, which are eviden t from leaf inception on the flanks of the apical meristem. The subdivision of the meristem into molecularly defined domains is regulated by the inter actions of a number of gene products and by receptor kinase-mediated signal s. The acquisition of symmetry axes in. the emerging leaf is a process coor dinated by hormones (such as auxin and cytokinins) and the expression of cl asses of genes (such. as the knox and the ARP, as 1/rs2/phan, genes). As wi th simple leaves, the architecture of compound leaves is defined by spatial /temporal gradients of regulatory gene functions: complexity results from t he interplay between leaf differentiation processes and genes maintaining a partial level of indeterminacy in the developing primordium. Boundaries be tween regions with different molecular addresses' are considered, in plants as in Drosophila, as organizing centres for lateral organ development.