  One of the central problems in animal and plant developmental biology is
deciphering how chemical and mechanical signals interact within a tissue to
produce organs of defined size, shape and function. Cell walls in plants impose
a unique constraint on cell expansion since cells are under turgor pressure and
do not move relative to one another. Cell wall extensibility and constantly
changing distribution of stress on the wall are mechanical properties that vary
between individual cells and contribute to rates of expansion and orientation
of cell division. How exactly cell wall mechanical properties influence cell
behavior is still largely unknown. To address this problem, a novel,
subcellular element computational model of growth of stem cells within the
multilayered shoot apical meristem (SAM) of Arabidopsis thaliana is developed
and calibrated using experimental data. Novel features of the model include
separate, detailed descriptions of cell wall extensibility and mechanical
stiffness, deformation of the middle lamella and increase in cytoplasmic
pressure generating internal turgor pressure. The model is used to test novel
hypothesized mechanisms of formation of the shape and structure of the growing,
multilayered SAM based on WUS concentration of individual cells controlling
cell growth rates and layer dependent anisotropic mechanical properties of
subcellular components of individual cells determining anisotropic cell
expansion directions. Model simulations also provide a detailed prediction of
distribution of stresses in the growing tissue which can be tested in future
experiments.
