Hladik, P. | bioRxiv Science

bioRxiv2026

Comprehensive characterisation of IAA inactivation pathways reveals the impact of glycosylation on auxin metabolism and plant development

Together with biosynthesis and transport, inactivation regulates the concentration of indole-3-acetic acid (IAA), a key auxinic compound with a myriad of functions in plant development. Main inactive IAA metabolites are categorised into oxidised forms and ester- or amide-linked conjugates. DIOXYGENASE FOR AUXIN OXIDATION1 (DAO1) and DAO2, 2-oxoglutarate and iron-dependent dioxygenases, contribute to IAA oxidative inactivation in collaboration with group II GRETCHEN HAGEN3 (GH3) IAA-amido synthetases, while a group of UDP-glycosyltransferases (UGTs) conjugate IAA to sugars. To study the IAA inactivation routes, we generated combinatorial mutants between all group II GH3s (gh3oct) and DAO1 or DAO2, as well as between the DAOs and main UGTs. In vivo [13C6]IAA feeding experiments traced the metabolic fate of the exogenously applied IAA, supporting the main IAA inactivation pathway, in which DAO acts downstream of GH3s. They also indicated that UGT-mediated IAA glycosylation is more important than previously assumed for modulating IAA levels and plant development. Our metabolic and transcriptomic data further revealed that gh3oct may still produce some GH3 activity, explaining previous reported phenotypic inconsistencies. Our data additionally suggest that other not yet identified metabolic activities might play a role in IAA overproducing plants, and that the premature downregulation of flowering time integrators like FLOWERING LOCUS C (FLC) likely underlies the early flowering of gh3oct and gh3oct dao1 plants.

plant biology↗

bioRxiv2024

Phenylacetic acid metabolism in land plants: novel pathways and metabolites

In recent years, substantial progress has been made in exploring auxin conjugation and metabolism, primarily aiming at indole-3-acetic acid (IAA). However, the metabolic regulation of another key auxin, phenylacetic acid (PAA), remains largely uncharacterized. Here, we provide a comprehensive exploration of PAA metabolism in land plants. Through LC-MS screening across multiple plant species and their organs, we identified four previously unreported endogenous PAA metabolites: phenylacetyl-leucine (PAA-Leu), phenylacetyl-phenylalanine (PAA-Phe), phenylacetyl-valine (PAA-Val), and phenylacetyl-glucose (PAA-glc). Enzyme assays, genetic evidence, crystal structures, and docking studies demonstrate that PAA and IAA share core metabolic machinery, revealing a complex regulatory network that maintains auxin homeostasis. Furthermore, our study of PAA conjugation with amino acids and glucose suggests limited compensatory mechanisms within known conjugation pathways, pointing to the existence of alternative metabolic routes in land plants. These insights advance our knowledge of auxin-specific metabolic networks and highlight the unique complexity within plant hormone regulation.

plant biology↗