<i>N</i> -glucosylation of indole-3-acetyl amino acids modulates auxin metabolism and growth traits in <i>Oryza sativa</i>

Dynamic regulation of auxin (indole-3-acetic acid; IAA) levels is crucial for proper plant growth and development and is finely regulated through biosynthesis, transport, metabolic inactivation, and signal transduction. While <i>O</i>-glucosylation of IAA is well established, the physiological significance of <i>N</i>-glucosylation pathways acting on IAA-amino acid conjugates remains largely unexplored. Here, we identify a UDP-glucosyltransferase in rice (<i>Oryza sativa</i>), designated IAAspGT, that catalyzes the <i>N</i>-glucosylation of indole-3-acetyl (IA) amino acid conjugates. Functional analysis revealed that natural allelic variants of IAAspGT differ markedly in enzymatic activity, with the high-activity allele prevalent in <i>indica</i> and the low-activity allele in <i>japonica</i> cultivars. Mutational analysis identified key residues within the glucose-accepting substrate-binding domain that account for this variation. A metabolic perturbation experiment demonstrated that IAAspGT competes with the DAO-mediated oxidation, thereby affecting active auxin levels. Plants harboring the high-activity allele exhibited enhanced root elongation under nutrient-deficient conditions and altered growth allocation between the panicle and other above-ground organs. Haplotype analysis indicated that the high-activity allele is ancestral and widely retained in the wild relatives. These findings establish IA-amino acid <i>N</i>-glucosylation as a previously overlooked metabolic branch that modulates auxin availability and contributes to environmental responses and growth plasticity in rice.