Mobile microRNAs hit the target

MicroRNAs (miRNAs) are negative regulators of gene expression in eukaryotic organisms, whereas small interfering RNAs (siRNAs) guide host-cell defence against viruses, transposons and transgenes. A key issue in plant biology is whether miRNAs act only in cells in which they are formed, or if, like siRNAs, they also function after passive diffusion or active transportation into other cells. Recent reports show that miRNAs are indeed able to move between plant cells to direct developmental programming of gene expression. In both leaf and root development, miRNAs establish intercellular gradients of gene expression that are essential for cell and tissue differentiation. Gradients in gene expression also play crucial roles in animal development, and there is strong evidence for intercellular movement of miRNAs in animals. Thus, intercellular movement of miRNAs may be crucial to animal developmental biology as well as plants.