Plant cell walls.

Plants are able to generate large leaf surfaces that act as two-dimensional solar panels with a minimum investment in building material, thanks to a hydrostatic skeleton. This requires high intracellular pressures (up to 1 MPa), which depend on the presence of strong cell walls. The walls of growing cells (also called primary walls), are remarkably able to reconcile extreme tensile strength (up to 100 MPa) with the extensibility necessary for growth. All walled organisms are confronted with this dilemma - the need to balance strength and extensibility - and bacteria, fungi and plants have evolved independent solutions to cope. In this Primer, we discuss how plant cells have solved this problem, allowing them to support often very large increases in volume and to develop a broad variety of shapes (Figure 1A,B,D). This shape variation reflects the targeted deposition of wall material combined with local variations in cell-wall extensibility, processes that remain incompletely understood. Once the cell has reached its final size, it can lay down secondary wall layers, the composition and architecture of which are optimized to exert specific functions in different cell types (Figure 1E-G). Such functions include: providing mechanical support, for instance, for fibre cells in tree trunks or grass internodes; impermeabilising and strengthening vascular tissue to resist the negative pressure of the transpiration stream; increasing the surface area of the plasma membrane to facilitate solute exchange between cells (Figure 1C); or allowing important elastic deformation, for instance, to support the opening and closing of stomates. Specialized secondary walls, such as those constituting seed mucilage, are stored in a dehydrated form in seedcoat epidermis cells and show rapid swelling upon hydration of the seed. Other walls, in particular in reserve tissues, can accommodate large amounts of storage polysaccharides, which can be easily mobilized as a carbon source. Here we will discuss some general principles underlying wall architecture and wall growth that have emerged from recent studies, as well as future questions for investigation (Box 1).