Fig. 3

Physiological relevant stress gradient can induce distinguishable transverse microtubule bands. (A) Schematic of the stress pattern gradient pertinent to a finite element model of trichome morphogenesis. Magnitude of stress decreases from left to right, along the direction parallel to cell long axis while the stress is highly anisotropic and predominant in the transverse direction, indicated by the ellipses. The lower panels show an unfolded stress pattern based on the model results. The unfolded meshed stress pattern is mapped onto the simulation domain. (B) Representative confocal image showing microtubule localization in stage 4 young trichome branches (left panel). Transverse band is prominent near the cell apex. The MDZ is highlighted and marked in yellow. Scale bar: 10 μm. Right panel shows the intensity profile from the apex to the base in the image shown in the left panel. (CN) A catastrophe-inducing boundary condition is implemented on the two short edges of the simulation domain (30 μm x10 µm). Catastrophe-inducing boundary: when a growing microtubule encounters a boundary, it switches to a shrinkage state. The long edges are assumed to have periodic boundary conditions. MT band formation along the transverse direction is noticeable in cases with a stress gradient. (C, G, K) Early-time (20 min) and (D, H, L) steady-state (100 min) morphology of network in stress free (C, D), stress gradient (G, H), and stress gradient with isotropic stress in the tip zone (K, L, 2 μm tip zone marked by black bar). (E, I, M) Local order parameter of microtubules in subregions of cases in D, H, L respectively. For this, the network was divided into fifteen subregions, incrementally separated by 2 μm along the x direction. Blue curve indicates a mean value, and shaded region represents standard deviation (n = 10). (F, J, N) Normalized intensity of MT segments (tubulin dimers) as a function of the distance from the tip (left boundary), which is nearest to apex of a real cell. (F) For a homogenous network, the distribution of MT segments is relatively even along the cell long axis. (J) For a network with stress gradient, the distribution of MT segments peaks near the boundary and gradually decreases along the cell long axis. (N) With an isotropic stress zone near the tip (black bar), the distribution is similar to (J) except there is a decline in the intensity of microtubules in the zone. The distribution is consistent with the right panel in B