Negative regulator of the auxin response. Not localized under normal conditions, but found in the cytoplasm and in the nucleus when auxin is added. Expressed in cotyledons, hypocotyls, roots, newly developing leaves and shoot apical meristem. Not detected in flowers, siliques or mature leaves. Expressed during the primary developmental stages. Up-regulated by auxin. Knock-down mutants have no visible phenotype in the absence of exogenous auxin, but are more sensitive to auxin-induced inhibition of root elongation and initiation of lateral roots.