What makes plants different? Principles of extracellular matrix function in 'soft' plant tissues.

An overview of the biomechanic and morphogenetic function of the plant extracellular matrix (ECM) in its primary state is given. ECMs can play a pivotal role in cellular osmo- and volume-regulation, if they enclose the cell hermetically and constrain hydrostatic pressure evoked by osmotic gradients between the cell and its environment. From an engineering viewpoint, such cell walls turn cells into hydraulic machines, which establishes a crucial functional differences between cell walls and other cellular surface structures. Examples of such hydraulic machineries are discussed. The function of cell walls in the control of pressure, volume, and shape establishes constructional evolutionary constraints, which can explain aspects commonly considered typical of plants (sessility, autotrophy). In plants, 'cell division' by insertion of a new cell wall is a process of internal cytoplasmic differentiation. As such it differs fundamentally from cell separation during cytokinesis in animals, by leaving the coherence of the dividing protoplast basically intact. The resulting symplastic coherence appears more important for plant morphogenesis than histological structure; similar morphologies are realized on the basis of distinct tissue architectures in different plant taxa. The shape of a plant cell is determined by the shape its cell wall attains under multiaxial tensile stress. Consequently, the development of form in plants is achieved by a differential plastic deformation of the complex ECM in response to this multiaxial force (hydrostatic pressure). Current concepts of the regulation of these deformation processes are briefly evaluated.