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Morphogenesis of walled cells back

 

Fungal hyphae
Hyphal growth of Neurospora crassa. Membrane is stained. Adapted from P.C. Hickey and N.D. Read, Biology of Living Fungi (2003).

How are the size and shape of a cell determined? We address this question in the case of walled cells, both prokaryotes and eukaryotes, as the existence of a cell wall defines better the problem. Walled cells can grow either globally, via the uniform addition of wall material to the preexisting cell wall, a process termed diffuse growth, or locally, via tip growth, which consists of the assembly and subsequent expansion of new cell wall in the apical region of the cell. Tip growth is broadly used by many organisms, ranging from plants (pollen tubes and root hairs) to fungi (fungal hyphae and fission and budding yeast), and even bacteria, to explore and interact with their environment.

Tip growth
Sketch of the apical region of a growing pollen tube (example of tip-growing cell) showing how secretory vesicles (circles) are transported along actin filaments (red) toward the apex, where they secrete the new cell wall material in the preexisting cell wall (gray).

Most recent cellular morphogenesis studies have focused on the building blocks of the cytoskeleton and the cell wall, as well as the signalling pathways that control and are controlled by these molecular interactions. Indeed, a detailed understanding of all molecular components and pathways involved in controlling cell size and shape is essential to obtain a complete description of cell morphogenesis. However, as important as the molecular view is, at the mesoscopic scale the assembly and extension of the cell wall are strongly constrained by the laws of physics. No matter how complex intracellular processes and signaling pathways may be, the morphogenesis of the cell must obey certain incontrovertible laws at the mesoscopic scale, such as force balance and mass conservation. This mesoscopic approach is complementary to the microscopic one, highlighting the relevant physical parameters that determine cell shape and size at the mesoscopic scale and, thereby identifying those magnitudes that cells need to regulate in order to modulate growth and shape the cell.

Using tip growth as a motivating example, we study the interplay between growth and mechanics in shaping a walled cell. We describe the observed irreversible expansion of the cell wall during growth as the extension of an inhomogeneous viscous fluid shell under the action of turgor pressure, fed by a material source in the neighborhood of the growing tip. This allows us to determine theoretically the radius of the cell and its growth velocity in terms of the turgor pressure and the secretion rate and rheology of the cell wall material. We derive simple scaling laws for the geometry of the cell and find that a single dimensionless parameter, which characterizes the relative roles of cell wall assembly and expansion, is sufficient to explain the observed variability in shapes of tip-growing cells. More generally, our description provides a framework to understand cell growth and remodeling in plants (pollen tubes, root hairs, etc.), fungi (hyphal growth) and bacteria in the context of both tip growth and diffuse growth.

download pdf Shape and dynamics of tip-growing cells. O. Campàs and L. Mahadevan. Current Biology 19, 2102-07 (2009).
download SI

downloadStrategies for cell shape control in tip-growing cells. O. Campàs, E. Rojas, J. Dumais and L. Mahadevan. American J. of Botany 99, 1577-82 (2012).