Auxin represents a key signal in plants, regulating almost every aspect of their growth and development. Major breakthroughs have been made dissecting the molecular basis of auxin transport, perception, and response. In contrast, how plants control the metabolism and homeostasis of the major form of auxin in plants, indole-3-acetic acid (IAA), remains unclear. In this paper, we initially describe the function of the Arabidopsis thaliana gene DIOXYGENASE FOR AUXIN OXIDATION 1 (AtDAO1). Transcriptional and translational reporter lines revealed that AtDAO1 encodes a highly root-expressed, cytoplasmically localized IAA oxidase. Stable isotope-labeled IAA feeding studies of loss and gain of function AtDAO1 lines showed that this oxidase represents the major regulator of auxin degradation to 2-oxoindole-3-acetic acid (oxIAA) in Arabidopsis. Surprisingly, AtDAO1 loss and gain of function lines exhibited relatively subtle auxin-related phenotypes, such as altered root hair length. Metabolite profiling of mutant lines revealed that disrupting AtDAO1 regulation resulted in major changes in steady-state levels of oxIAA and IAA conjugates but not IAA. Hence, IAA conjugation and catabolism seem to regulate auxin levels in Arabidopsis in a highly redundant manner. We observed that transcripts of AtDOA1 IAA oxidase and GH3 IAA-conjugating enzymes are auxin-inducible, providing a molecular basis for their observed functional redundancy. We conclude that the AtDAO1 gene plays a key role regulating auxin homeostasis in Arabidopsis, acting in concert with GH3 genes, to maintain auxin concentration at optimal levels for plant growth and development.Arabidopsis thaliana | IAA degradation | oxidase | dioxygenase | root hair elongation D istinct indole-3-acetic acid (IAA) conjugation and degradation pathways operate to maintain optimal auxin concentrations for plant growth and developmental processes. There are three major forms of auxin conjugates identified in diverse plants: ester-linked IAA-sugar conjugates, amide-linked IAA-amino acid conjugates, and amide-linked IAA peptide and protein conjugates (reviewed in ref.

Phosphate (P) is an essential macronutrient for plant growth. Roots employ adaptive mechanisms to forage for P in soil. Root hair elongation is particularly important since P is immobile. Here we report that auxin plays a critical role promoting root hair growth in Arabidopsis in response to low external P. Mutants disrupting auxin synthesis (taa1) and transport (aux1) attenuate the low P root hair response. Conversely, targeting AUX1 expression in lateral root cap and epidermal cells rescues this low P response in aux1. Hence auxin transport from the root apex to differentiation zone promotes auxin-dependent hair response to low P. Low external P results in induction of root hair expressed auxin-inducible transcription factors ARF19, RSL2, and RSL4. Mutants lacking these genes disrupt the low P root hair response. We conclude auxin synthesis, transport and response pathway components play critical roles regulating this low P root adaptive response.

The hormone auxin is a key regulator of plant growth and development, and great progress has been made understanding auxin transport and signaling. Here, we show that auxin metabolism and homeostasis are also regulated in a complex manner. The principal auxin degradation pathways in Arabidopsis include oxidation by Arabidopsis thaliana gene DIOXYGENASE FOR AUXIN OXIDATION 1/2 (AtDAO1/2) and conjugation by Gretchen Hagen3s (GH3s). Metabolic profiling of dao1-1 root tissues revealed a 50% decrease in the oxidation product 2-oxoindole-3-acetic acid (oxIAA) and increases in the conjugated forms indole-3-acetic acid aspartic acid (IAA-Asp) and indole-3-acetic acid glutamic acid (IAA-Glu) of 438-and 240-fold, respectively, whereas auxin remains close to the WT. By fitting parameter values to a mathematical model of these metabolic pathways, we show that, in addition to reduced oxidation, both auxin biosynthesis and conjugation are increased in dao1-1. Transcripts of AtDAO1 and GH3 genes increase in response to auxin over different timescales and concentration ranges. Including this regulation of AtDAO1 and GH3 in an extended model reveals that auxin oxidation is more important for auxin homoeostasis at lower hormone concentrations, whereas auxin conjugation is most significant at high auxin levels. Finally, embedding our homeostasis model in a multicellular simulation to assess the spatial effect of the dao1-1 mutant shows that auxin increases in outer root tissues in agreement with the dao1-1 mutant root hair phenotype. We conclude that auxin homeostasis is dependent on AtDAO1, acting in concert with GH3, to maintain auxin at optimal levels for plant growth and development.T he plant hormone auxin regulates a myriad of processes in plant growth and development (1). Although significant progress has been made in understanding the molecular basis of auxin transport, perception, and response, the control of auxin metabolism and homeostasis via conjugation and degradation remains less well-studied.Several forms of auxin conjugates have been identified in plants, including ester-linked indole-3-acetic acid (IAA)-sugar conjugates and amide-linked IAA-amino acid conjugates (2). In Arabidopsis thaliana, the Gretchen Hagen3 (GH3) family of auxin-inducible acyl amido synthetases has been shown to convert IAA to IAA-amino acids (3). Most amino acid IAA conjugates are believed to be inactive, and some, such as indole-3-acetic acid aspartic acid (IAA-Asp) and indole-3-acetic acid glutamic acid (IAA-Glu), can also be further metabolized (4-6). The conversion of IAA to indole-3-acetic acid glucose (IAA-glc) is catalyzed by the uridine diphosphate glucosyltransferase UGT84B1 (UGT) (7). The oxidized form of IAA, 2-oxindole-3-acetic acid (oxIAA), has been identified as a major IAA catabolite in Arabidopsis (4, 6, 8) and can be further metabolized by conjugation to glucose (9). oxIAA has been shown to be an irreversible IAA catabolite that has very little biological activity compared with IAA and is not transported via the polar auxin ...

The original version of this Article omitted the following from the Acknowledgements: ‘We also thank DBT-CREST BT/HRD/03/01/2002.’ This has been corrected in both the PDF and HTML versions of the Article.