Auxins are a class of compounds that are key regulators of plant cell growth. The exact mechanism of action of auxin-induced growth is unknown but involves many rapid cellular changes to prepare the cell to expand under turgor pressure. One of the effects of auxin is an increase in cell wall synthesis by stimulating the secretion of wall precursors. Molecular genetic evidence for an auxin receptor mediating wall synthesis or any other requisite aspect of auxin-induced growth is lacking. However, auxin-binding protein 1 (ABP1) has properties that make it a candidate receptor mediating auxin-induced growth. The effect of constitutive overexpression of maize ABP1 was examined using cell lines to avoid the problem of counterselection of ABP1 expression during whole plant regeneration after transformation. We found two lines overexpressing ABP1 and in both lines, cells are larger, contain thick cell walls, and have the morphology of transfer cells. Most importantly, these three features are dependent on the presence of auxin; in its absence, ABP1-transformed cells have the nontransformed control cell phenotype. Auxin dependence for ABP1 action, taken together with its well-characterized auxin binding, fullfill the criteria of auxin receptor function.

Thus, ABP1 mediates at least some of the cell wall changes associated with auxin-regulated cell growth and is proposed to be an auxin receptor.