Protein involved in the auxin signaling pathway (e.g. transport and signal transduction) that regulates many aspects of plant growth and development (e.g. caulogenesis, rhizogenesis, tropisms, nodulation). The major form of this phytohormone is indole-3-acetic acid (IAA) that can be synthesized both from tryptophan (Trp) using Trp-dependent pathways and from an indolic Trp precursor via Trp-independent pathways. Plants can also obtain IAA by b-oxidation of indole-3-butyric acid (IBA), a second endogenous auxin, or by hydrolysing IAA conjugates, in which IAA is linked to amino acids, sugars or peptides.