The mechanics of tip growth morphogenesis: what we have learned from rubber balloons

Abstract

If you can't demonstrate it with balloons, it's probably not important anyway.Stephen A. Wainwright, quoted in [Vogel 2003].Morphogenesis of plant, fungal, and bacterial cells depends heavily on surface mechanics and in particular on the stiff wall that surrounds these cells. In this paper, we show that tubular rubber balloons offer a useful physical model of tip growth morphogenesis. In particular, the balloons reproduce accurately the inhomogeneity and anisotropy of surface expansion observed during tip growth. Comparison between the two systems has led to a simple model of tip growth that assumes linear constitutive relations with inhomogeneous material properties. The strain rate profile predicted by the model is a surprisingly good fit to the data given the model's simplicity. We suggest that a meridional gradient of compliance or extensibility is the key mechanical feature that explains the similar strain rate profiles in tip-growing cells across broad taxonomic groups as well as in rubber balloon analogs.

Perspectives on biological growth and remodeling

“…Thus plant cells offer a striking example of a stress-dependent growth process of the kind described above. Although few studies have focused on the constitutive behavior of the growing cell wall, other mechanical aspects of plant cell expansion have been analyzed in detail (Lockhart, 1969; Green et al, 1971; Sellen, 1983; Zhu & Boyer, 1992, Goriely and Tabor, 2003; Dumais et al, 2006; Bernal et al, 2007). In particular, pressurized shell models with simple constitutive relations can predict cell surface expansion with surprising accuracy (Sellen, 1983; Dumais et al, 2006; Bernal et al, 2007).…”

Not-so-tip-growth

“…Although structural analyses have failed to reveal any special cellular features that may help maintain this unusual expansion pattern, mechanical analyses do suggest that the apparent wall viscosity or stiffness is reduced in the annulus of high expansion rate. 5,7 The reduced wall viscosity could be the result of increased activity of wall loosening agents such as expansins 8,9 or reduced activity of pectin methylesterases (PME). An alternative explanation is that the annulus of high surface expansion is experiencing higher rates of secretion of uncross-linked wall material.…”

How and why cells grow as rods

“…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. 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 [ 3 ]. Interestingly, different species build rods in distinct ways.…”