Auxin Perception at the Plasma Membrane of Plant Cells: Recent Developments and Large Unknowns

Studies of membrane responses of tobacco protoplasts to auxin have demonstrated the existence of elementary response chains to auxins at the plasma membrane. In the presence of extracellular auxin molecules or auxin agonists, plasmamembrane auxin responsive proteins modulate the activity of a variety of electrogenic units, ultimately leading to changes in membrane potential. The plasma membrane auxin responsive proteins are immunologically related to ZmER-abpl, the major auxin-binding protein from maize. Evidence suggests a complex organization for the functional auxin perception units at the plasma membrane, comprising a soluble excreted auxin-binding protein associated to a transmembrane protein. Major unknowns concern the consequence(s) of the activation of the plasma membrane elementary chains of auxin perception on more integrated auxin responses. Strategies to answer the important question of whether the auxin effects result from the activation of a unique ubiquitous receptor or involve a multitargetted activation of independent receptors are discussed.