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

From smooth to buckled, nature exhibits organs of various shapes and forms. How cellular growth patterns produce smooth organ shapes like those of leaves and sepals remains unclear. Here we show that unidirectional growth and comparable stiffness across both epidermal layers of Arabidopsis sepals are essential for smoothness. We identified a mutant with ectopic ASYMMETRIC LEAVES 2 (AS2) expression that exhibits buckles on the outer epidermis due to conflicting growth directions and unequal epidermal stiffnesses. Aligning growth direction and increasing stiffness restores smoothness. Furthermore, buckling generates outgrowth by influencing auxin efflux transporter protein PIN-FORMED 1 polarity, indicating buckling can initiate organogenesis. 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.