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In order to function in changing environmental conditions, all living organisms need to be equipped with two sets of seemingly contradictory mechanisms; these enable them to (1) integrate themselves in separation from the environment, and (2) sense and communicate with their immediate surrounding. During the course of evolution, several factors—both physical and chemical—have emerged as organismal integrators. Among these, gravity provides a major directional stimulus, while chemical compounds are usually used as internal integratory molecules (Bhalerao and Bennett 2003).

Cell walls are dynamic entities providing the link between cellular and organismal features of the plant. Being external to protoplasts, they provide not only the transportation pathways for signaling molecules, but also constitute a rich source of such molecules. Moreover, their spatial placement also makes them a specific “zone of the first contact” with the surrounding environment. Here, we provide a brief overview of the functions the walls play in plant signaling. We describe the physicochemical properties of the walls and discuss their impact on the type of signaling molecules transported via the apoplast, and the types of extracellular domains of receptor molecules. The role of the walls in the formation and maintenance of the structural and functional continuum between cell walls, the plasma membrane, and the cytoskeleton is then considered, especially with respect to mechanosensing, transduction of mechanical signals, and monitoring of cell wall integrity. Finally, a range of signaling molecules and their activities are reviewed.