A pathway of auxin action, from perception to response, has not been completely defined, as yet. However, there are now good evidences that auxin-binding protein ABP1, which is associated with a membrane docking protein is a G-protein-coupled receptor. The signal transduction pathway appear to involve activation of phospholipase A2 leading to the production of lipid second messengers which activate the plasma membrane H+-ATPase by a protein kinase-dependent mechanism. Increasing evidences now suggest that MAP kinase pathway is involved in mediating auxin responses.