Postdoctoral Research Associate, Roeder Lab Cornell University Ithaca, New York
Body of Abstract: Organ size and shape robustness is a remarkable property that is not well understood. To address how organ shape reproducibility is achieved, we screened for variable organ size and shape (vos) mutants using Arabidopsis sepals as the model organ of interest. The vos3 mutant exhibits random ectopic lumps and outgrowths on the sepal outer epidermis (abaxial). Mutation underlying vos3 maps to a point mutation in the upstream region of ASYMMETRIC LEAVES 2 (AS2), a key adaxial specific transcription factor involved in adaxial identity specification, resulting in ectopic expression of AS2 on the sepal abaxial surface. We show that in initial stages, ectopic expression of AS2 on vos3 abaxial surface disrupts cell growth patterning, causing mechanical stress mediated buckling. Our mathematical model predicts that overgrowth and reduced stiffness of the abaxial vs adaxial surface can in theory, cause buckling. Outer surface cells of vos3 grow sideways compared to wild type, leading to the initial buckle along the proximal-distal direction. Using osmotic treatments, we found that outer epidermis of vos3 is also significantly softer compared to the inner surface, whereas both epidermal layers exhibit similar stiffness in wild type. Thus, both conditions for buckling are met in vos3 and not in wild type sepals. Also, overexpression of KRP1, a cyclin dependent kinase inhibitor, in vos3 both limits the sideways growth and severely reduces buckling, supporting the model. We also find PIN forms ectopic convergence sites in vos3 located to the buckles, where outgrowths start forming. To test whether auxin is necessary, we live imaged vos3 in the presence of NPA, a polar auxin transport inhibitor. While NPA treated vos3 samples show no difference in buckling, the outgrowth formation is largely reduced. Thus, buckling is sufficient to create PIN convergence sites which initiates outgrowths, suggesting buckling as a mechanism driving plant development.