Growth directions and stiffness across cell layers determine whether tissues stay smooth or buckle

Nature exhibits organs of various shapes and forms, ranging from smooth to undulated morphologies. How cells coordinate their growth to produce smoothly shaped leaves and leaf-like organs such as sepals remains unclear. We identified a mutant, as2-7D, that exhibits ectopic expression of ASYMMETRIC LEAVES 2 (AS2) on the outer epidermis. Our analysis reveals that ectopic AS2 expression causes the outer epidermis of as2-7D sepals to buckle during early stages of sepal development. Buckling is caused by conflicting cell growth directions and unequal tissue stiffness across the epidermal layers. Overexpression of cyclin-dependent kinase (CDK) inhibitor Kip-related protein 1 (KRP1) in as2-7D aligns the growth directions of the outer epidermal cells along the longitudinal axis, increases the overall stiffness of the outer epidermis, and restores sepal smoothness. Further experiments suggest that buckling promotes the convergence of auxin efflux transporter protein PIN-FORMED 1 (PIN1) to initiate pointed outgrowths. Thus, we show that growth along the longitudinal axis during early developmental stages and comparable stiffness across both epidermal layers of Arabidopsis thaliana sepals are essential for smoothness, as seen in the wild type. Our findings suggest that in addition to molecular cues influencing tissue mechanics, tissue mechanics can also modulate molecular signals, giving rise to well-defined shapes.