Smooth Elongation of Pavement Cells Induced by RIC1 Overexpression Leads to Marginal Protrusions of the Cotyledon in Arabidopsis thaliana.

The interdigitated pavement cell shape is suggested to be mechanically rational at both the cellular and tissue levels, but the biological significance of the cell shape is not fully understood. In this study, we explored the potential importance of the jigsaw puzzle-like cell shape for cotyledon morphogenesis in Arabidopsis. We used a transgenic line overexpressing a Rho-like GTPase-interacting protein, ROP-INTERACTIVE CRIB MOTIF-CONTAINING PROTEIN 1 (RIC1), which causes simple elongation of pavement cells. Computer-assisted microscopic analyses, including virtual reality observation, revealed that RIC1 overexpression resulted in abnormal cotyledon shapes with marginal protrusions, suggesting that the abnormal organ shape might be explained by changes in the pavement cell shape. Microscopic, biochemical and mechanical observations indicated that the pavement cell deformation might be due to reduction in the cell wall cellulose content with alteration of cortical microtubule organization. To examine our hypothesis that simple elongation of pavement cells leads to an abnormal shape with marginal protrusion of the cotyledon, we developed a mathematical model that examines the impact of planar cell growth geometry on the morphogenesis of the organ that is an assemblage of the cells. Computer simulations supported experimental observations that elongated pavement cells resulted in an irregular cotyledon shape, suggesting that marginal protrusions were due to local growth variation possibly caused by stochastic bias in the direction of cell elongation cannot be explained only by polarity-based cell elongation, but that an organ-level regulatory mechanism is required.