Catalytic Promiscuity of Rice 2-Oxoglutarate/Fe(II)-Dependent Dioxygenases Supports Xenobiotic Metabolism

The rice 2-oxoglutarate/Fe(II)-dependent dioxygenase HIS1 mediates the catalytic inactivation of five distinct {beta}-triketone herbicides (bTHs). During a search for potential inhibitors of HIS1, we found that it mediates the hydroxylation of trinexapac-ethyl (TE) in the presence of Fe2+ and 2-oxoglutarate. TE is a plant growth regulator that blocks gibberellin biosynthesis, and we observed that its addition to culture medium induced growth retardation of rice seedlings in a concentration-dependent manner. Similar treatment with hydroxylated TE revealed that hydroxylation greatly attenuated the inhibitory effect of TE on plant growth. Forced expression of HIS1 in a rice his1 mutant also reduced its sensitivity to TE compared with that of the nontransformant. These results indicated that HIS1 metabolizes TE and thereby markedly reduces its ability to slow plant growth. Furthermore, testing of five HIS1-related proteins (HSLs) of rice revealed that OsHSL2 and OsHSL4 also metabolize TE in vitro. HSLs from wheat and barley also showed such activity. In contrast, OsHSL1, which shares the highest amino acid sequence identity with HIS1 and metabolizes the bTH tefuryltrione, did not manifest TE-metabolizing activity. Site-directed mutagenesis of OsHSL1 informed by structural models showed that substitution of three amino acids with the corresponding residues of HIS1 conferred TE-metabolizing activity similar to that of HIS1. Our results thus reveal a catalytic promiscuity of HIS1 and its related enzymes that supports xenobiotic metabolism in plants.