Metabolism of indole-3-acetic acid in rice: Identification and characterization of N-β- d-glucopyranosyl indole-3-acetic acid and its conjugates

Abstract

A search was made for conjugates of indole-3-acetic acid (IAA) in rice ( Oryza sativa) using liquid chromatography–electrospray ionization–tandem mass spectrometry (LC–ESI–MS/MS) in order to elucidate unknown metabolic pathways for IAA. N-β- d-Glucopyranosyl indole-3-acetic acid (IAA- N-Glc) was found in an alkaline hydrolysate of rice extract. A quantitative analysis of 3-week-old rice demonstrated that the total amount of IAA- N-Glc was equal to that of IAA. A LC–ESI–MS/MS-based analysis established that the major part of IAA- N-Glc was present as bound forms with aspartate and glutamate. Their levels were in good agreement with the total amount of IAA- N-Glc during the vegetative growth of rice. Further detailed analysis showed that both conjugates highly accumulated in the root. The free form of IAA- N-Glc accounted for 60% of the total in seeds but could not be detected in the vegetative tissue. An incorporation study using deuterium-labeled compounds showed that the amino acid conjugates of IAA- N-Glc were biosynthesized from IAA-amino acids. IAA- N-Glc and/or its conjugates were also found in extracts of Arabidopsis, Lotus japonicus, and maize, suggesting that N-glucosylation of indole can be the common metabolic pathway of IAA in plants.