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

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.