UC Riverside

Stem cell maintenance in multilayered shoot apical meristems (SAMs) of plants requires regulation of cell growth and division. Exactly how the milieu of chemical and mechanical signals interact in the SAM to regulate cell division plane orientation is not well understood. In this work, simulations using a 2D multiscale mathematical model are combined with experiments to suggest and test three hypothesized mechanisms for the regulation of cell division plane orientation and the direction of anisotropic cell expansion. Simulations predict that in the Apical corpus, WUSCHEL and cytokinin regulate the direction of anisotropic cell expansion, and cells divide according to tensile stress on the cell wall. In the Basal corpus, model simulations suggest dual roles for WUSCHEL and cytokinin in regulating the direction of anisotropic cell expansion and cell division plane orientation. This is supported by a detailed analysis of experimental images upon manipulation of WUSCHEL and cytokinin. Moreover, simulations predict that this layer-specific mechanism maintains the experimentally observed shape and structure of the SAM and the WUSCHEL distribution in the tissue. The 2D model is then extended to include expansion and division of cells out-of-plane to form a pseudo-3D (P3D) model. Both models include boundary conditions which represent tension experienced by the SAM epidermis. The P3D model maintains the epidermal cell monolayer crucial for development in wildtype SAMs. By comparing the behavior of the P3D and 2D models, it is shown that tension-guided cell division plane orientation acts to regulate cell and tissue shape distributions.