Theoretical Model a- Viscoplastic Model for Cell Growth in Fission Yeast

We describe the fission yeast cell as a cylinder of radius R, and total length L. The intracellular turgor pressure P, is assumed to be homogeneous within the cell. The cell wall is also described as a homogeneous elastic material with Young’s modulus Ecw and thickness h, yielding a surface modulus, σcw = hEcw (Figure 1A). Localized secretion and deposition of new cell wall allows for fission yeast cell growth to occur exclusively at the tips over a domain of length R [1]. The longitudinal (along the cell long axis) tension in the walls arising from the internal turgor pressure is: h PRTP 2 = , (Eq. S1) In the spirit of the models developed for plants [2-5], we assume that growth appears effectively as a viscoplastic process, where the free growth rate v0 = dL /dt is proportional to the strain cwp ET / in the wall in excess of a threshold plastic strain ep. As growth occurs only in the cap of extension R, along the longitudinal axis:)(0 P c