Cell walls expand through irreversible deformation under a turgor-driven cell wall stress in a process called “creep.” Creep involves the breaking of hydrogen bonds between cell wall polymers and the reversible and irreversible breakage of covalent bonds ( Cosgrove, 2005). The shape of the cell is bounded and defined by the cell wall and it follows that cell wall expansion must be differentially regulated around the cell to generate the final cell morphology. We therefore also put an emphasis on how certain tools and algorithms may be used to quantify and compare shapes. A largely neglected area in the field of plant cell shape is the usage of quantitative means to define shape. In this review we aim to dissect how simple and complex cell shapes, such as those of cylindrical hypocotyl cells and leaf epidermal pavement cells, respectively, are thought to be established. This model of cell shape formation was proposed by Paul Green in 1965 on the basis of observations of the growth of giant internodal cells of Nitella and is still relevant today ( Green, 1965). For changes in proportions to occur some parts of the cell surface must grow differently to other parts. To acquire their final shapes, cells must change their proportions while increasing in size. The walls in such cells are straight and the cells are largely iso-diametric, bearing little resemblance to the shapes commonly observed in mature organs. In immature organs where cell division is actively taking place, cells are small and the shapes of cells are largely defined by the most recent cell divisions where new cell walls have been laid down along previous division planes. Finally, we review recent examples of the use of novel imaging techniques and how they have contributed to our understanding of cell shape formation. We discuss some of the challenges in cell shape research with a particular emphasis on quantitative imaging and statistical analysis of shape in 2D and 3D, as well as novel developments in this area. These processes include cell wall synthesis, activity of the actin and microtubule cytoskeletons, in particular their regulation by microtubule associated proteins, actin-related proteins, GTP'ases and their effectors, as well as the recently-elucidated roles of plant hormone signaling and vesicular membrane trafficking. We summarize established concepts as well as recent additions to the understanding of how cells construct cell walls of a given shape and the underlying processes. In this review we summarize the current state of knowledge on cell shape formation in plants focusing on shape of simple cylindrical cells, as well as in complex multipolar cells such as leaf pavement cells and trichomes. Plant cell shape, seen as an integrative output, is of considerable interest in various fields, such as cell wall research, cytoskeleton dynamics and biomechanics. Max-Planck-Institute of Molecular Plant Physiology, Potsdam, Germany.
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