Plant cells at the organ surface use mechanical cues to activate a specific growth-control program.

During morphogenesis of multicellular organs, cells in distinct positions need to meet specific functional requirements to form the appropriate tissue type. For example, epidermal tissues need to form at the organ surface. In plants, the epidermis is widely considered essential for morphogenesis due to its role in both restricting and promoting growth. In the root, epidermal cells are partially covered by a protective root cap and partially positioned at the organ surface. Here, we propose that epidermal cells at the organ surface have unique requirements for growth control due to high mechanical tension, while covered epidermal cells are mechanically shielded by the root cap. We present in silico and in vivo evidence that plants use surface mechanical cues to activate a cell-type-specific growth-control program involving the small guanosine triphosphatase (GTPase) RAB-A5c, thereby allowing roots to maintain directional growth at the organ surface. Positional mechanical cues may thus be used to control expression of a sub-population of epidermal genes, linking gene regulation to surface-specific functional requirements.