Auxin perception and signal transduction

The action of auxin on whole plants is very complex, but we are starting to understand how some of the earliest events are signalled in single cells. There is now good evidence that auxin induces rapid events at the plasma membrane by binding to a population of the auxin-binding protein ABP1, which is associated with a membrane-spanning docking protein, possibly a G-protein-coupled receptor (GPCR). ABP1 is targeted to the endoplasmic reticulum (ER) lumen, but it does not appear to bind auxin within the ER and its function (if any) in this location is unknown. It is also not known how the protein reaches the cell surface, but it is possible that it is exported together with its docking protein. Binding of auxin causes a conformational change affecting the C-terminus of ABP1 and it is likely that this change serves to activate the receptor at the plasma membrane. The signal transduction pathway appears to involve activation of phospholipase A(2) (PLA(2)) leading to the production of lipid second messengers which activate the plasma membrane proton ATPase (H+-ATPase) by a phosphorylation-dependent mechanism. Branch points exist that could potentially lead from this pathway to responses in the nucleus, but there is not yet any firm evidence that ABP1 is involved in such responses. Since intracellular auxin concentrations are correlated with sensitivity in some cases, it is possible that there is also a site of auxin perception inside the cell.