Mechanics of cell growth and tissue architecture in plants

Plant development entails cell division, cell growth and shaping, and the differentiation of meristematic cells into highly specialized cell types. Differentiation and cell shape are closely linked and involve the regulation of the mechanical properties of the cell wall. The mechanisms leading to the generation of the diverse array of shapes and functionalities found in plant tissues are perplexing and poorly understood. In my Ph.D. research, I investigated the mechanical principles underlying the formation of wavy leaf pavement cells. Further, I studied the putative mechanical advantage that emerges from the interlocking patterns. Epidermal pavement cells consist of two parallel periclinal walls connected by vertical anticlinal walls. At the borders, wavy pavement cells make interlocking indentations and protrusions. 3D confocal micrographs of cotyledons stained with cellulose-specific fluorophores revealed a significant accumulation of cellulose at the sites of indentation on the periclinal walls extending down the anticlinal walls. Staining the cotyledon samples with COS488 also suggested a higher abundance of de-esterified pectin at these sites. I developed finite element models of the cell wall deformation and simulated the biochemical inhomogeneities by assigning alternately stiffened regions across and along the periclinal walls on two sides of an anticlinal wall. It was observed that the non-deforming stiffened regions develop into sites of indentations when the internal pressure stretches the cell wall. The model also suggested higher stresses to associate with the neck regions. The model results indicate that a positive feedback loop between stress and cell wall stiffness could generate wavy shapes starting from infinitesimally small differences in cell wall stiffness or stress. Further, the model suggests that stochastic buckling events can initiate the cell shaping process. It has been long hypothesized that the interlocking pattern of pavement cells improves cell-cell adhesion and thus the tensile strength of the epidermis. Stretching to rupture the leaf samples of wild-type Arabidopsis or any1 mutant with reduced cell waviness did not show any cell detachment upon failure. However, I hypothesized the undulating cell borders could enhance the resistance of the epidermis against the propagation of damage. I observed a considerable anisotropy in the tear behavior of onion epidermis parallel and perpendicular to the cells’ main axis. Tears along the cell lines propagated without much resistance resulting in brittle failure of the tissue. This was observed to originate from tears propagating by cell detachment. Perpendicular to the cells’ main axis, tears had considerable difficulty in propagating. Fracturing the leaf samples with wavy epidermal cells, I noticed the cracks propagated in both the cell borders and the cell wall intermittently. I hypothesized that this pattern of fracture in the epidermis with wavy cells indicates an increase in the fracture toughness. To untangle the influence of material anisotropy from the cell geometry on fracture toughness, I designed macroscopic physical models of the epidermis by laser engraving the cell patterns on polymethylmethacrylate. This way, the isotropic material would allow studying only the effect of cell geometry. While fracturing the control specimens with no engraving and the specimens with longitudinally placed cells demonstrated a brittle fracture, fractures transverse to cell lines in the onion cell patterns or across the Arabidopsis cotyledon wavy cell pattern showed an increased fracture toughness. I suggest the wavy shape of pavement cells in the epidermis results from the alternate placement of stiffer regions in the cell wall, a process that can initiate from a stochastic stress anisotropy due to buckling. Further, these shapes increase the fracture toughness of the plant epidermis protecting it against the spread of damage; an ingenious defense mechanism at the most exposed surfaces.

Keyphrases: Cellulose, Plant cell shape, Morphogenesis, Cell mechanics, Finite element analysis