Quantitative analysis of indole-3-acetic acid metabolites in Arabidopsis

Indoleacetic acid, a product of transferred DNA, inhibits vir gene expression and growth of Agrobacterium tumefaciens C58

Agrobacterium tumefaciens induces crown gall tumors by transferring a piece of its tumor-inducing plasmid into plant cells. This transferred DNA encodes the synthesis of indole acetic acid (IAA) and cytokinin, and their overproduction results in tumor formation. The transfer is initiated by a two-component regulatory system, VirA/G recognizing plant signal molecules in the plant rhizosphere and activating a regulon on the tumor-inducing plasmid, which is required for the processing and transfer of DNA and protein. Although a great deal is known about vir gene activation, nothing is known about whether or how the vir gene regulon is inactivated after plant cell transformation. Presumably, just as a mechanism exists for activating the vir gene regulon only when a plant is in the immediate environment, a mechanism should exist for inactivating the same regulon once it has fulfilled its mission to transferred DNA into plant cells. We now show that IAA inactivates vir gene expression by competing with the inducing phenolic compound acetosyringone for interaction with VirA. IAA does not inhibit the vir genes in cells containing a constitutive sensor virA locus, which does not require any signal molecules to become phosphorylated. At higher concentrations, IAA inhibits the growth of Agrobacterium and many other plant-associated bacteria but not the growth of bacteria that occupy other ecological niches. These observations provide the missing link in the cycle of vir gene activation and inactivation.

Arabidopsis Aux/IAA genes are involved in brassinosteroid-mediated growth responses in a manner dependent on organ type

We examined whether auxin/indole-3-acetic acid (Aux/IAA) proteins, which are key players in auxin-signal transduction, are involved in brassinosteroid (BR) responses. iaa7/axr2-1 and iaa17/axr3-3 mutants showed aberrant BR sensitivity and aberrant BR-induced gene expression in an organ-dependent manner. Two auxin inhibitors were tested in terms of BR responses. Yokonolide B inhibited BR responses, whereas p-chlorophenoxyisobutyric acid did not inhibit BR responses. DNA microarray analysis revealed that 108 genes were up-regulated, while only eight genes were down-regulated in iaa7. Among the genes that were up- or down-regulated in axr2, 22% were brassinolide-inducible genes, 20% were auxin-inducible genes, and the majority were sensitive neither to BR nor to auxin. An inhibitor of BR biosynthesis, brassinazole, inhibited auxin induction of the DR5-GUS gene, which consists of a synthetic auxin-response element, a minimum promoter, and a beta-glucuronidase. These results suggest that Aux/IAA proteins function in auxin- and BR-signaling pathways, and that IAA proteins function as the signaling components modulating BR sensitivity in a manner dependent on organ type.

Simultaneous determination of multiple phytohormones in plant extracts by liquid chromatography-electrospray tandem mass spectrometry

A rapid multiresidue method to quantify three different classes of plant hormones has been developed. The reduced concentrations of these metabolites in real samples with complex matrixes require sensitive techniques for their quantification in small amounts of plant tissue. The method described combines high-performance liquid chromatography with electrospray ionization tandem mass spectrometry. Deuterium-labeled standards were added prior to sample extraction to achieve an accurate quantification of abscisic acid, indole-3-acetic acid, and jasmonic acid in a single run. A simple method of extraction and purification involving only centrifugation, a partition against diethyl ether, and filtration was developed and the analytical method validated in four different plant tissues, citrus leaves, papaya roots, barley seedlings, and barley immature embryos. This method represents a clear advantage because it extensively reduces sample preparation and total time for routine analysis of phytohormones in real plant samples.

Cell cycle progression in the pericycle is not sufficient for SOLITARY ROOT/IAA14-mediated lateral root initiation in Arabidopsis thaliana

To study the mechanisms behind auxin-induced cell division, lateral root initiation was used as a model system. By means of microarray analysis, genome-wide transcriptional changes were monitored during the early steps of lateral root initiation. Inclusion of the dominant auxin signaling mutant solitary root1 (slr1) identified genes involved in lateral root initiation that act downstream of the auxin/indole-3-acetic acid (AUX/IAA) signaling pathway. Interestingly, key components of the cell cycle machinery were strongly defective in slr1, suggesting a direct link between AUX/IAA signaling and core cell cycle regulation. However, induction of the cell cycle in the mutant background by overexpression of the D-type cyclin (CYCD3;1) was able to trigger complete rounds of cell division in the pericycle that did not result in lateral root formation. Therefore, lateral root initiation can only take place when cell cycle activation is accompanied by cell fate respecification of pericycle cells. The microarray data also yielded evidence for the existence of both negative and positive feedback mechanisms that regulate auxin homeostasis and signal transduction in the pericycle, thereby fine-tuning the process of lateral root initiation.

Characterization of tryptophan-overproducing potato transgenic for a mutant rice anthranilate synthase alpha-subunit gene (OASA1D)

Potato plants (Solanum tuberosum cv. May Queen) transgenic for OASA1D, which encodes a point mutant of an alpha-subunit of rice (Oryza sativa) anthranilate synthase (AS, EC 4.1.3.27), were generated in order to determine the effects of the mutant gene on levels of free tryptophan (Trp) and AS activity in this important crop. Expression of OASA1D in potato induced a 2- to 20-fold increase in the amount of free Trp. This increase was likely due to a reduction in the sensitivity of AS containing the mutant alpha-subunit to feedback inhibition by Trp. Nontargeted metabolite profiling by high-performance liquid chromatography coupled with ultraviolet photodiode array detection as well as targeted profiling by liquid chromatography coupled with mass spectrometry revealed no marked changes in the levels of other metabolites, with the exception of indole-3-acetic acid (IAA), in the transgenic plants. The level of IAA in the upper part of the shoot was increased by a factor of 8.3-39, depending on the transgenic lines, with no detectable effect on plant growth or development. The effects of transformation thus appeared limited to the biosynthesis of Trp and IAA, with the overall metabolic network in potato being virtually unaffected. These results suggest that transformation with OASA1D may prove effective for the breeding of crops with an increased level of free Trp.

Purification and determination of plant hormones auxin and abscisic acid using solid phase extraction and two-dimensional high performance liquid chromatography

A method for separation and purification of plant hormones auxin and abscisic acid based on mixed mode reversed-phase anion-exchange solid phase extraction and two-dimensional HPLC was developed. Two-dimensional HPLC in "heart cutting" mode was very efficient in the purification of these two hormones. Its purification power is high enough to allow reliable on-line quantification of both hormones even with non-selective detectors.

Purification of 3-indolylacetic acid by solid phase extraction

This paper deals with the use of solid-phase extraction (SPE) for the extraction and purification of 3-indolylacetic acid (IAA), the main and most important representative of the plant growth regulators auxins. The procedure is based on the use of C18-SPE columns in two steps. In the first one, raw extract in methanol:water (4:1) is applied on the column in order to remove neutral ballast. In the second step the eluate is diluted with water to a final methanol concentration of 20% (to decrease the elution strength of the solvent) and acidified with formic acid to a final concentration of 1% (to convert the IAA from the anionic to the neutral form). Neutral IAA is then retained on the second SPE column, eluted by (acidified) methanol, and subjected to final analysis. Scintillation counting of tritium labeled IAA standard was used for the investigation and optimization of the SPE procedure. Gas chromatography with mass spectrometric detection was suggested as a convenient method for routine determination of IAA after its methylation with diazomethane. Overall recovery of the procedure was estimated as 89-94% and a physiological level of IAA equal to 0.48 nmol/g (84 ng/g) fresh weight was found using an optimized SPE-GC-MS method.

Auxin and light control of adventitious rooting in Arabidopsis require ARGONAUTE1

Adventitious rooting is a quantitative genetic trait regulated by both environmental and endogenous factors. To better understand the physiological and molecular basis of adventitious rooting, we took advantage of two classes of Arabidopsis thaliana mutants altered in adventitious root formation: the superroot mutants, which spontaneously make adventitious roots, and the argonaute1 (ago1) mutants, which unlike superroot are barely able to form adventitious roots. The defect in adventitious rooting observed in ago1 correlated with light hypersensitivity and the deregulation of auxin homeostasis specifically in the apical part of the seedlings. In particular, a clear reduction in endogenous levels of free indoleacetic acid (IAA) and IAA conjugates was shown. This was correlated with a downregulation of the expression of several auxin-inducible GH3 genes in the hypocotyl of the ago1-3 mutant. We also found that the Auxin Response Factor17 (ARF17) gene, a potential repressor of auxin-inducible genes, was overexpressed in ago1-3 hypocotyls. The characterization of an ARF17-overexpressing line showed that it produced fewer adventitious roots than the wild type and retained a lower expression of GH3 genes. Thus, we suggest that ARF17 negatively regulates adventitious root formation in ago1 mutants by repressing GH3 genes and therefore perturbing auxin homeostasis in a light-dependent manner. These results suggest that ARF17 could be a major regulator of adventitious rooting in Arabidopsis.

Metabolic Profiling of Tryptophan-overproducing Rice Calli that Express a Feedback-insensitive α Subunit of Anthranilate Synthase

The profile of aromatic metabolites in calli was compared between wild-type rice (Oryza sativa cv. Nipponbare) and tryptophan-overproducing transgenic rice lines that express a gene (OASA1D) for a feedback-insensitive alpha subunit of anthranilate synthase. Metabolic profiling by high-performance liquid chromatography coupled with photodiode array detection of ultraviolet absorbance revealed a total of 71 peaks in both wild-type and transgenic calli. Only a limited effect on the pattern of major aromatic compounds was observed in tryptophan-accumulating transgenic rice lines, with the exception of an approximately 80-fold increase in the amount of tryptophan. Expression of OASA1D induced relatively small changes in several minor metabolites. One of the minor metabolites whose abundance was increased by OASA1D expression was purified and identified as a previously unknown indole-alkaloid glucoside. The levels of free and conjugated forms of indole-3-acetic acid (IAA), a plant hormone derived from the tryptophan biosynthetic pathway, were determined separately by liquid chromatography and tandem mass spectrometry (LC-MS/MS). The amounts of both free IAA and its conjugates were increased in the transgenic calli, suggesting that the activity of anthranilate synthase or the concentration of tryptophan (or both) is an important determinant of IAA biosynthesis.

Characterization of an Arabidopsis enzyme family that conjugates amino acids to indole-3-acetic acid

Substantial evidence indicates that amino acid conjugates of indole-3-acetic acid (IAA) function in auxin homeostasis, yet the plant enzymes involved in their biosynthesis have not been identified. We tested whether several Arabidopsis thaliana enzymes that are related to the auxin-induced soybean (Glycine max) GH3 gene product synthesize IAA-amino acid conjugates. In vitro reactions with six recombinant GH3 enzymes produced IAA conjugates with several amino acids, based on thin layer chromatography. The identity of the Ala, Asp, Phe, and Trp conjugates was verified by gas chromatography-mass spectrometry. Insertional mutations in GH3.1, GH3.2, GH3.5, and GH3.17 resulted in modestly increased sensitivity to IAA in seedling root. Overexpression of GH3.6 in the activation-tagged mutant dfl1-D did not significantly alter IAA level but resulted in 3.2- and 4.5-fold more IAA-Asp than in wild-type seedlings and mature leaves, respectively. In addition to IAA, dfl1-D was less sensitive to indole-3-butyric acid and naphthaleneacetic acid, consistent with the fact that GH3.6 was active on each of these auxins. By contrast, GH3.6 and the other five enzymes tested were inactive on halogenated auxins, and dfl1-D was not resistant to these. This evidence establishes that several GH3 genes encode IAA-amido synthetases, which help to maintain auxin homeostasis by conjugating excess IAA to amino acids.

A novel auxin conjugate hydrolase from wheat with substrate specificity for longer side-chain auxin amide conjugates

This study investigates how the ILR1-like indole acetic acid (IAA) amidohydrolase family of genes has functionally evolved in the monocotyledonous species wheat (Triticum aestivum). An ortholog for the Arabidopsis IAR3 auxin amidohydrolase gene has been isolated from wheat (TaIAR3). The TaIAR3 protein hydrolyzes negligible levels of IAA-Ala and no other IAA amino acid conjugates tested, unlike its ortholog IAR3. Instead, TaIAR3 has low specificity for the ester conjugates IAA-Glc and IAA-myoinositol and high specificity for the conjugates of indole-3-butyric acid (IBA-Ala and IBA-Gly) and indole-3-propionic-acid (IPA-Ala) so far tested. TaIAR3 did not convert the methyl esters of the IBA conjugates with Ala and Gly. IBA and IBA conjugates were detected in wheat seedlings by gas chromatography-mass spectrometry, where the conjugate of IBA with Ala may serve as a natural substrate for this enzyme. Endogenous IPA and IPA conjugates were not detected in the seedlings. Additionally, crude protein extracts of wheat seedlings possess auxin amidohydrolase activity. Temporal expression studies of TaIAR3 indicate that the transcript is initially expressed at day 1 after germination. Expression decreases through days 2, 5, 10, 15, and 20. Spatial expression studies found similar levels of expression throughout all wheat tissues examined.

A family of auxin-conjugate hydrolases that contributes to free indole-3-acetic acid levels during Arabidopsis germination

Auxins are hormones important for numerous processes throughout plant growth and development. Plants use several mechanisms to regulate levels of the auxin indole-3-acetic acid (IAA), including the formation and hydrolysis of amide-linked conjugates that act as storage or inactivation forms of the hormone. Certain members of an Arabidopsis amidohydrolase family hydrolyze these conjugates to free IAA in vitro. We examined amidohydrolase gene expression using northern and promoter-beta-glucuronidase analyses and found overlapping but distinct patterns of expression. To examine the in vivo importance of auxin-conjugate hydrolysis, we generated a triple hydrolase mutant, ilr1 iar3 ill2, which is deficient in three of these hydrolases. We compared root and hypocotyl growth of the single, double, and triple hydrolase mutants on IAA-Ala, IAA-Leu, and IAA-Phe. The hydrolase mutant phenotypic profiles on different conjugates reveal the in vivo activities and relative importance of ILR1, IAR3, and ILL2 in IAA-conjugate hydrolysis. In addition to defective responses to exogenous conjugates, ilr1 iar3 ill2 roots are slightly less responsive to exogenous IAA. The triple mutant also has a shorter hypocotyl and fewer lateral roots than wild type on unsupplemented medium. As suggested by the mutant phenotypes, ilr1 iar3 ill2 imbibed seeds and seedlings have lower IAA levels than wild type and accumulate IAA-Ala and IAA-Leu, conjugates that are substrates of the absent hydrolases. These results indicate that amidohydrolases contribute free IAA to the auxin pool during germination in Arabidopsis.

IAR4, a gene required for auxin conjugate sensitivity in Arabidopsis, encodes a pyruvate dehydrogenase E1alpha homolog

The formation and hydrolysis of indole-3-acetic acid (IAA) conjugates represent a potentially important means for plants to regulate IAA levels and thereby auxin responses. The identification and characterization of mutants defective in these processes is advancing the understanding of auxin regulation and response. Here we report the isolation and characterization of the Arabidopsis iar4 mutant, which has reduced sensitivity to several IAA-amino acid conjugates. iar4 is less sensitive to a synthetic auxin and low concentrations of an ethylene precursor but responds to free IAA and other hormones tested similarly to wild type. The gene defective in iar4 encodes a homolog of the E1alpha-subunit of mitochondrial pyruvate dehydrogenase, which converts pyruvate to acetyl-coenzyme A. We did not detect glycolysis or Krebs-cycle-related defects in the iar4 mutant, and a T-DNA insertion in the IAR4 coding sequence conferred similar phenotypes as the originally identified missense allele. In contrast, we found that disruption of the previously described mitochondrial pyruvate dehydrogenase E1alpha-subunit does not alter IAA-Ala responsiveness or confer any obvious phenotypes. It is possible that IAR4 acts in the conversion of indole-3-pyruvate to indole-3-acetyl-coenzyme A, which is a potential precursor of IAA and IAA conjugates.

Auxin regulation of cytokinin biosynthesis in Arabidopsis thaliana: a factor of potential importance for auxin-cytokinin-regulated development

One of the most long-lived models in plant science is the belief that the long-distance transport and ratio of two plant hormones, auxin and cytokinin, at the site of action control major developmental events such as apical dominance. We have used in vivo deuterium labeling and mass spectrometry to investigate the dynamics of homeostatic cross talk between the two plant hormones. Interestingly, auxin mediates a very rapid negative control of the cytokinin pool by mainly suppressing the biosynthesis via the isopentenyladenosine-5'-monophosphate-independent pathway. In contrast, the effect of cytokinin overproduction on the entire auxin pool in the plant was slower, indicating that this most likely is mediated through altered development. In addition, we were able to confirm that the lateral root meristems are likely to be the main sites of isopentenyladenosine-5'-monophosphate-dependent cytokinin synthesis, and that the aerial tissue of the plant surprisingly also was a significant source of cytokinin biosynthesis. Our demonstration of shoot-localized synthesis, together with data demonstrating that auxin imposes a very rapid regulation of cytokinin biosynthesis, illustrates that the two hormones can interact also on the metabolic level in controlling plant development, and that the aerial part of the plant has the capacity to synthesize its own cytokinin independent of long-range transport from the root system.

Metabolic fate of jasmonates in tobacco bright yellow-2 cells

Jasmonic acid and methyl jasmonate play an essential role in plant defense responses and pollen development. Their levels are temporarily and spatially controlled in plant tissue. However, whereas jasmonate biosynthesis is well studied, metabolic pathways downstream of jasmonic acid are less understood. We studied the uptake and metabolism of jasmonic acid and methyl jasmonate in tobacco (Nicotiana tabacum) Bright Yellow-2 suspension culture. We found that upon uptake, jasmonic acid was metabolized to its Glc and gentiobiose esters, and hydroxylation at C-11 or C-12 occurred. Free hydroxylated jasmonates were the preferential fraction of the culture medium. Upon hydrolysis of methyl jasmonate to jasmonic acid, a similar set of conversions occurs. In contrast to jasmonic acid, none of its derivatives interfere with the G2/M transition in synchronized tobacco Bright Yellow-2 cells.

Brassinolide induces IAA5, IAA19, and DR5, a synthetic auxin response element in Arabidopsis, implying a cross talk point of brassinosteroid and auxin signaling

Despite numerous physiological studies addressing the interactions between brassinosteroids (BRs) and auxins, little is known about the underlying molecular mechanisms. We studied the expression of IAA5 and IAA19 in response to treatment with indole acetic acid (IAA) or brassinolide (BL), the most active BR. Exogenous IAA induced these genes quickly and transiently, whereas exogenous BL induced them gradually and continuously. We also found that a fusion of DR5, a synthetic auxin response element, with the GUS (beta-glucuronidase) gene was induced with similar kinetics to those of the IAA5 and IAA19 genes in response to both IAA and BL treatment of transgenic plants. These results suggest that the IAA genes are induced by BL, at least in part, via the activation of the auxin response element. Endogenous IAA levels per gram fresh weight did not increase when seedlings of Arabidopsis wild type (WT) or the BR-deficient mutant det2 were treated with BL. Furthermore, the levels of IAA transcripts were lower in the det2 mutant than in the WT, even though endogenous IAA levels per gram fresh weight were higher in the det2 mutant than in the WT. In conclusion, the lack of evidence for auxin-mediated activation of early auxin-inducible genes in response to BL suggests that the BR and auxin signaling pathways independently activate the transcriptional system of the IAA and DR5-GUS genes.

A method for profiling classes of plant hormones and their metabolites using liquid chromatography-electrospray ionization tandem mass spectrometry: an analysis of hormone regulation of thermodormancy of lettuce (Lactuca sativa L.) seeds

A highly selective and sensitive method for the simultaneous analysis of several plant hormones and their metabolites is described. The method combines high-performance liquid chromatography (HPLC) with positive and negative electrospray ionization-tandem mass spectrometry (ESI-MS/MS) to quantify a broad range of chemically and structurally diverse compounds. The addition of deuterium-labeled analogs for these compounds prior to sample extraction permits accurate quantification by multiple reaction monitoring (MRM). Endogenous levels of abscisic acid (ABA), abscisic acid glucose ester (ABA-GE), 7'-hydroxy-abscisic acid (7'-OH-ABA), phaseic acid (PA), dihydrophaseic acid (DPA), indole-3-acetic acid (IAA), indole-3-aspartate (IAAsp), zeatin (Z), zeatin riboside (ZR), isopentenyladenine (2iP), isopentenyladenosine (IPA), and gibberellins (GA)1, GA3, GA4, and GA7 were determined simultaneously in a single run. Detection limits ranged from 0.682 fmol for Z to 1.53 pmol for ABA. The method was applied to the analysis of plant hormones and hormonal metabolites associated with seed dormancy and germination in lettuce (Lactuca sativa L. cv. Grand Rapids), using extracts from only 50 to 100 mg DW of seed. Thermodormancy was induced by incubating seeds at 33 degrees C instead of 23 degrees C. Germinating seeds transiently accumulated high levels of ABA-GE. In contrast, thermodormant seeds transiently accumulated high levels of DPA after 7 days at 33 degrees C. GA1 and GA3 were detected during germination, and levels of GA1 increased during early post-germinative growth. After several days of incubation, thermodormant seeds exhibited a striking transient accumulation of IAA, which did not occur in seeds germinating at 23 degrees C. We conclude that hormone metabolism in thermodormant seeds is surprisingly active and is significantly different from that of germinating seeds.

Comprehensive chemical derivatization for gas chromatography-mass spectrometry-based multi-targeted profiling of the major phytohormones

In the present investigation we report selection of the N-methyl-N-(tert.-butydimethylsilyl)trifluoroacetamide (MTBSTFA) reagent as the most comprehensive derivatization protocol among 17 tested reactions covering trifluoroacetylation, pentafluorobenzylation, methylations, and trimethylsilylations. MTBSTFA allowed easy and robust tert.-butyldimethylsilyl derivatization of 1-aminocyclopropane-1-carboxylic acid, indole-3-acetic acid, (+/-)-jasmonic acid, salicylic acid, (+/-)-abscisic acid, meta-topolin, and trans-zeatin. Detection limits as analysed by selected ion monitoring quadrupole GC-MS were 0.2, 0.01, 1.0, 0.02, 0.3, 0.3, and 0.9 pmol of injected substance, respectively. Analysis of gibberellic acid A3, trans-zeatin riboside and (+/-)-abscisic acid-beta-D-glucopyranosyl ester was best when coupled by splitting extracts and trimethysilylation. The MTBSTFA derivatization protocol was optimised, and validated. The preparation was insensitive to 2% residual water and to < or = 1 day storage at room temperature. The final scheme was highly reproducible and successfully applied to extracts from approximately 300 mg (fresh mass) of tobacco (Nicotiana tabacum) root and Arabidopsis thaliana seedling.

Over-expression of an Arabidopsis gene encoding a glucosyltransferase of indole-3-acetic acid: phenotypic characterisation of transgenic lines

An analysis of the multigene family of Group 1 glucosyltransferases (UGTs) of Arabidopsis thaliana revealed a gene, UGT84B1, whose recombinant product glucosylated indole-3-acetic acid (IAA) in vitro. Transgenic Arabidopsis plants constitutively over-expressing UGT84B1 under the control of the CaMV 35S promoter have been constructed and their phenotype analysed. The transgenic lines displayed a number of changes that resembled those described previously in lines in which auxin levels were depleted. A root elongation assay was used as a measure of auxin sensitivity. A reduced sensitivity of the transgenic lines compared to wild-type was observed when IAA was applied. In contrast, application of 2,4-dichlorophenoxyacetic acid (2,4-D), previously demonstrated not to be a substrate for UGT84B1, led to a wild-type response. These data suggested that the catalytic specificity of the recombinant enzyme in vitro was maintained in planta. This was further confirmed when levels of IAA metabolites and conjugates were measured in extracts of the transgenic plants and 1-O-IAGlc was found to be elevated to approximately 50 pg mg-1 FW, compared to the trace levels characteristic of wild-type plants. Surprisingly, in the same extracts, levels of free IAA were also found to have accumulated to some 70 pg mg-1 FW compared to approximately 15 pg mg-1 FW in extracts of wild-type plants. Analysis of leaves at different developmental stages revealed the auxin gradient, typical of wild-type plants, was not observed in the transgenic lines, with free IAA levels in the apex and youngest leaves at a lower level compared to wild-type. In total, the data reveal that significant changes in auxin homeostasis can be caused by overproduction of an IAA-conjugating enzyme.

Characterization of a family of IAA-amino acid conjugate hydrolases from Arabidopsis

The mechanisms by which plants regulate levels of the phytohormone indole-3-acetic acid (IAA) are complex and not fully understood. One level of regulation appears to be the synthesis and hydrolysis of IAA conjugates, which function in both the permanent inactivation and temporary storage of auxin. Similar to free IAA, certain IAA-amino acid conjugates inhibit root elongation. We have tested the ability of 19 IAA-l-amino acid conjugates to inhibit Arabidopsis seedling root growth. We have also determined the ability of purified glutathione S-transferase (GST) fusions of four Arabidopsis IAA-amino acid hydrolases (ILR1, IAR3, ILL1, and ILL2) to release free IAA by cleaving these conjugates. Each hydrolase cleaves a subset of IAA-amino acid conjugates in vitro, and GST-ILR1, GST-IAR3, and GST-ILL2 have K(m) values that suggest physiological relevance. In vivo inhibition of root elongation correlates with in vitro hydrolysis rates for each conjugate, suggesting that the identified hydrolases generate the bioactivity of the conjugates.