Floral MADS-box genes control how flower parts develop. Petunia petals have different layers that make up their shape. Mutants with a gene called PhDEF in the outer layer have small tubes, while mutants with PhDEF in the inner layer have small, pale petals. The gene PhDEF helps activate the color production pathway in petals. This study shows that where PhDEF is active in petunia petals affects how the tube and limb parts grow, revealing how different layers contribute to petal shape.