TMK-mediated cell-surface auxin signaling orchestrates PIN polarity in patterning and gravitropism

The asymmetric distribution of the plant hormone auxin, directed by polar auxin transporters PIN-FORMED (PIN), is crucial for plant patterning and tropic responses. However, the mechanism establishing PIN polarity remained unclear. Two recent studies reveal a cell surface auxin signaling pathway mediated by transmembrane kinase (TMK) family receptor kinases. This pathway directly phosphorylates PIN proteins, regulating their polarity and activity. This creates a self-reinforcing positive feedback loop: auxin activates TMK, which polarizes PIN to enhance local auxin export, further amplifying the signal. This TMK–PIN module drives both pattern formation, by stabilizing initial auxin asymmetries, and rapid gravitropic responses by decisively amplifying gravity-induced auxin gradients. This discovery redefines auxin signaling, providing a unifying molecular principle for plant development and environmental adaptation.