Catalyzes the synthesis of indole-3-acetic acid (IAA)-amino acid conjugates, providing a mechanism for the plant to cope with the presence of excess auxin. Strongly reactive with Glu, Gln, Trp, Asp, Ala, Leu, Phe, Gly, Tyr, Met, Ile and Val. Little or no product formation with His, Ser, Thr, Arg, Lys, or Cys. Also active on pyruvic and butyric acid analogs of IAA, PAA and the synthetic auxin naphthaleneacetic acid (NAA). The two chlorinated synthetic auxin herbicides 2,4-D and 3,6-dichloro-o-anisic acid (dicamba) cannot be used as substrates (PubMed:15659623). Involved in auxin signal transduction. Inhibits shoot and hypocotyl cell elongation, and lateral root cell differentiation in light (PubMed:11169197). Optimum pH is 9.0. No activity at or below pH 7.5. Expressed in cotyledons, stipules, true leaves, hypocotyls, and all parts of the roots. Not detected in flowers. By auxin. Not regulated by light. The gain-of-function mutant dfl1-D (T-DNA tagging) has a short hypocotyl under blue, red and far-red light, but not in darkness, shows an exaggerated dwarf phenotype in the adult plant caused by the inhibition of cell elongation in shoots, and inhibition of the lateral root growth without any reduction of primary root length. Belongs to the IAA-amido conjugating enzyme family.