In plants, the growth regulator indole-3-acetic acid (IAA) is found both free and conjugated to a variety of amino acids, peptides, and carbohydrates. IAA conjugated to leucine has effects in Arabidopsis thaliana similar to those of free IAA. The ilr1 mutant is insensitive to exogenous IAA-Leu and was used to positionally clone the Arabidopsis ILR1 gene. ILR1 encodes a 48-kilodalton protein that cleaves IAA-amino acid conjugates in vitro and is homologous to bacterial amidohydrolase enzymes. DNA sequences similar to that of ILR1 are found in other plant species.