TMK1 phosphorylates and stabilizes PIN2 to reinforce auxin flux during root gravitropism

Plant growth depends on tightly coordinated auxin signaling and directional auxin transport, yet the molecular feedback mechanism that directly links these processes during root gravitropism has remained mechanistically unresolved. The recent study by Rodriguez et al. (Cell, 2025) reveals a novel cell-surface auxin signaling pathway. It is shown that gravity perception-induced initial auxin asymmetry activates transmembrane kinase 1 (TMK1) in the lower side cells of the root. The activated TMK1 then interacts with pin-formed 2 (PIN2) and phosphorylates its hydrophilic loop, thereby stabilizing the PIN2 protein. This asymmetric distribution of PIN2 further enhances the auxin flow on the lower side, thus forming a self-reinforcing positive feedback loop that drives force for root tip gravitropic bending. This study provides an updated perspective on the auxin signal and transport feedback, signifying a new advancement in our comprehension of the mechanisms underlying plant adaptive growth. • Gravity-induced auxin asymmetry activates the ABP1/ABL3–TMK1 signaling module at the root cell surface. • TMK1 phosphorylates and stabilizes PIN2, establishing asymmetric auxin transport during root gravitropism. • The phosphorylation-based feedback converts auxin gradients into sustained directional flow for adaptive root bending.