![]() ![]() ![]() pombe, new cell wall is added and remodeled at the growing cell tip(s), and turgor pressure provides force for elongation. Bottom: the circumferential motion of cytoplasmic MreB polymers (purple) is dependent on cell-wall synthesis, suggesting that MreB tracks represent new glycan strands (green) that have been added into the old wall with peptide crosslinks (red). Recent evidence suggests that insertion occurs in bursts (green patches) and is coordinated by the bacterial actin homolog MreB. coli, new cell wall is inserted along the cylindrical midcell (shaded green region) and not at the poles (cyan). (a) Top: in rod-shaped bacteria such as E. Mechanisms underlying rod-shaped cell-wall growth. Studies on the morphogenesis of rods will provide a conceptual and experimental framework that can then be applied to more complex shapes. In this review, we describe our current understanding of mechanisms for forming rod-like shapes, and speculate on possible evolutionary advantages of this particular shape. These contrasting mechanisms suggest that a rod-like shape may have independently evolved multiple times. tumefaciens, Corynebacterium glutamicum) grow by insertion of new wall material at cell tips (Figure 1b). pombe, plant pollen tubes, and certain other bacteria ( A. subtilis (Gram-positive) grow by inserting cell wall material along the length of the cylindrical portion of the cell (Figure 1a). Interestingly, different species build rods in distinct ways. Thus, key elements of walled cell morphogenesis include the physical properties of the cell wall and the processes responsible for its synthesis and remodeling, and the balance of forces between cell-wall extension and turgor pressure ultimately shapes the cell. In the intact cell, a large difference in osmotic pressure across the cytoplasmic membrane (turgor) provides a force that expands the elastic cell wall, analogous to pressure inflating a balloon. When the cell wall is removed, cells lose their shape conversely, isolated cell walls largely retain the shape of the cell. The wall can be regarded as a thin shell of fibrous, viscoelastic material. One speculation is that the shape and aspect ratio of these rods may have particular evolutionary advantages.Ī common feature of all walled cells is that the cell wall and turgor pressure give the cell its shape. pombe even have similar aspect ratios (length is approximately four times width), despite a nearly 100-fold difference in volume and qualitatively different spatial patterns of growth. Well-studied examples include bacteria ( Escherichia coli, Bacillus subtilis, Agrobacterium tumefaciens), fungi (fission yeast Schizosaccharomyces pombe, Aspergillus nidulens), and plants (pollen tubes, stem and root axis epidermal cells in Arabidopsis thaliana). ![]() The rod, a radially symmetric cylinder with rounded ends, represents a relatively simple geometry that is ubiquitous in unicellular walled organisms. Here, we explore how cells generate rod-like shapes. Comparisons among organisms with a common shape may help to reveal general principles that dictate shape determination and its evolutionary origins. For instance, in addition to traditional molecular cell biology, it will be important to understand the role of cellular mechanics, the material properties of cells, their microenvironment, and evolutionary constraints. ![]() Addressing these questions will require the integration of biology, physics, and chemistry. How are cell shapes generated? Are there reasons why cells have adopted certain shapes over others? Although many shape-determining factors have been identified across divergent organisms, mere characterization of individual cellular components has not revealed how shape is determined, nor has it provided much insight into the context under which these shapes evolved. How nanometer-scale molecular components construct micron-scale cells of specific shapes and sizes remains an outstanding question in biology. ![]()
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