Figure 2: A mechanical model of fission yeast growth.

(a) Elastic deformation of the cell wall following cell plasmolysis. The diagonal line indicates a ratio of 1:2 between the meridional and circumferential strains. Top: fluorescence of a Qdot-labelled cell before (left) and after (right) plasmolysis (Note: here and in other images contrast has been inverted for clarity). (b) Measured wall element displacements at OEs using fluorescent Qdots. Top left: temporal projection of a Qdot-labelled cell during growth. (c) Canonical wall expansion profile at the OE. The meridional and circumferential strain rates were inferred from the best fit of the wall displacement field shown in b. (d) Elastic properties space. Blue dots and ellipse: normalized Young’s modulus (E/P) and Poisson’s ratio (ν) inferred from the data points in a. White circle and error bars: best material properties inferred from the whole-cell simulations of e and f. Background colour map: fit between the canonical OE expansion profile and the growth simulations (j) for each pair [ν, E/P] (dark red: best fit). White level curve: sub-region of the space yielding a predicted expansion profile within the 95% confidence of the observed profile (star: point closest to the experimental elastic properties). (e) Contour of a plasmolysed cell (left) used as input to the elastic shell model. Numerical inflation of the plasmolysed cell yields a turgid cell geometry (blue) very similar to the observed cell (background: Qdot-labelled cell). (f) Simulation of the septum-NE transition with the relaxed septum shown as a dashed line. (g) Predicted NE curvature following the septum-NE transition (compare with Fig. 1b). (h) Morphogenetic model of cell ends where both wall incorporation and elastic deformation contribute. (i) Simulation of cell growth using the experimental wall areal expansion as growth input and mechanical build-up of circumferential anisotropy. Left: schematic of cell end curvature evolution through time at the simulated OE/NE (top/bottom). Right: simulated curvature kymographs for the OE/NE (top/bottom; compare with Fig. 1d). (j) Comparison of the canonical expansion profile (left) and the predicted expansion from the simulations (right). The areal expansion profiles (colour map) and expansion anisotropy (ellipses) are predicted precisely by the model.