Auxin is arguably the most important phytohormone found in the plant kingdom. The hormone is required for a multitude of growth control functions, including gravitropism and phototropism. Auxin homeostatic control is achieved in plants by a process of conjugation in which auxin is inactivated by being bound to another molecule, such as an amino acid or sugar. Auxin can be kept in a large, inactive pool by this method of chemical \u27‘conjugation” and the amide or ester bonds hydrolyzed as needed to provide “free” active hormone to the plant. The IAR3 gene family is highly conserved in Plantae and function as auxin conjugate hydrolases. We have investigated the substrate recognition and activity of a new homologue to the IAR3 family, PpIAR31, which we isolated from a species of moss, Physcomitrella patens. This is the first such enzyme isolated from moss and may allude to how the gene family originally evolved. We found that PpIAR31 is able to recognize and hydrolyze several forms of auxin conjugates as substrates (e.g. IAA-Alanine, IBA-Alanine, and IPA-Alanine), but was less efficient at cutting bonds on auxins with larger, more hydrophobic amino acids (e.g. IAA-leucine, and IAA-Phenylalanine). The genetic distances of the four hydrolases (PpIAR31, -32, - 33, and -34) detected in the moss genome positioned them structurally closer to bacterial than plant hydrolases. Evidence from codon usage and Principal Coordinate Analyses provides support that these enzymes may have originated in Plantae by at least one Horizontal Gene Transfer event from soil bacteria into early moss
