Cloning and characterization of a locus encoding an indolepyruvate decarboxylase involved in indole-3-acetic acid synthesis in Erwinia herbicola.

Erwinia herbicola 299R synthesizes indole-3-acetic acid (IAA) primarily by the indole-3-pyruvic acid pathway. A gene involved in the biosynthesis of IAA was cloned from strain 299R. This gene (ipdC) conferred the synthesis of indole-3-acetaldehyde and tryptophol upon Escherichia coli DH5 alpha in cultures supplemented with L-tryptophan. The deduced amino acid sequence of the gene product has high similarity to that of the indolepyruvate decarboxylase of Enterobacter cloacae. Regions within pyruvate decarboxylases of various fungal and plant species also exhibited considerable homology to portions of this gene. This gene therefore presumably encodes an indolepyruvate decarboxylase (IpdC) which catalyzes the conversion of indole-3-pyruvic acid to indole-3-acetaldehyde. Insertions of Tn3-spice within ipdC abolished the ability of strain 299R to synthesize indole-3-acetaldehyde and tryptophol and reduced its IAA production in tryptophan-supplemented minimal medium by approximately 10-fold, thus providing genetic evidence for the role of the indolepyruvate pathway in IAA synthesis in this strain. An ipdC probe hybridized strongly with the genomic DNA of all E. herbicola strains tested in Southern hybridization studies, suggesting that the indolepyruvate pathway is common in this species. Maximum parsimony analysis revealed that the ipdC gene is highly conserved within this group and that strains of diverse geographic origin were very similar with respect to ipdC.     

Physiological evidence for differently regulated tryptophan-dependent pathways for indole-3-acetic acid synthesis in Azospirillum brasilense

Disruption of ipdC, a gene involved in indole-3-acetic acid (IAA) production by the indole pyruvate pathway in Azospirillum brasilense Sp7, resulted in a mutant strain that was not impaired in IAA production with lactate or pyruvate as the carbon source. A tryptophan auxotroph that is unable to convert indole to tryptophan produced IAA if tryptophan was present but did not synthesise IAA from indole. Similar results were obtained for a mutant strain with additional mutations in the genes ipdC and trpD. This suggests the existence of an alternative Trp-dependent route for IAA synthesis. On gluconate as a carbon source, IAA production by the ipdC mutant was inhibited, suggesting that the alternative route is regulated by catabolite repression. Using permeabilised cells we observed the enzymatic conversion of tryptamine and indole-3-acetonitrile to IAA, both in the wild-type and in the ipdC mutant. IAA production from tryptamine was strongly decreased when gluconate was the carbon source.     

Growth and indole-3-acetic acid biosynthesis of Azospirillum brasilense Sp245 is environmentally controlled

Batch and fed batch cultures of Azospirillum brasilense Sp245 were conducted in a bioreactor. Growth response, IAA biosynthesis and the expression of the ipdC gene were monitored in relation to the environmental conditions (temperature, availability of a carbon source and aeration). A. brasilense can grow and produce IAA in batch cultures between 20 and 38 degrees C in a standard minimal medium (MMAB) containing 2.5 gl(-1)l-malate and 50 microgml(-1) tryptophan. IAA synthesis requires depletion of the carbon source from the growth medium in batch culture, causing growth arrest. No significant amount of IAA can be detected in a fed batch culture. Varying the concentration of tryptophan in batch experiments has an effect on both growth and IAA synthesis. Finally we confirmed that aerobic growth inhibits IAA synthesis. The obtained profile for IAA synthesis coincides with the expression of the indole-3-pyruvate decarboxylase gene (ipdC), encoding a key enzyme in the IAA biosynthesis of A. brasilense.     

Characterization of phenylpyruvate decarboxylase, involved in auxin production of Azospirillum brasilense

Azospirillum brasilense belongs to the plant growth-promoting rhizobacteria with direct growth promotion through the production of the phytohormone indole-3-acetic acid (IAA). A key gene in the production of IAA, annotated as indole-3-pyruvate decarboxylase (ipdC), has been isolated from A. brasilense, and its regulation was reported previously (A. Vande Broek, P. Gysegom, O. Ona, N. Hendrickx, E. Prinsen, J. Van Impe, and J. Vanderleyden, Mol. Plant-Microbe Interact. 18:311-323, 2005). An ipdC-knockout mutant was found to produce only 10% (wt/vol) of the wild-type IAA production level. In this study, the encoded enzyme is characterized via a biochemical and phylogenetic analysis. Therefore, the recombinant enzyme was expressed and purified via heterologous overexpression in Escherichia coli and subsequent affinity chromatography. The molecular mass of the holoenzyme was determined by size-exclusion chromatography, suggesting a tetrameric structure, which is typical for 2-keto acid decarboxylases. The enzyme shows the highest kcat value for phenylpyruvate. Comparing values for the specificity constant kcat/Km, indole-3-pyruvate is converted 10-fold less efficiently, while no activity could be detected with benzoylformate. The enzyme shows pronounced substrate activation with indole-3-pyruvate and some other aromatic substrates, while for phenylpyruvate it appears to obey classical Michaelis-Menten kinetics. Based on these data, we propose a reclassification of the ipdC gene product of A. brasilense as a phenylpyruvate decarboxylase (EC 4.1.1.43).     

Comparative in situ analysis of ipdC-gfpmut3 promoter fusions of Azospirillum brasilense strains Sp7 and Sp245

Inoculation of wheat roots with Azospirillum brasilense results in an increase of plant growth and yield, which is proposed to be mainly due to the bacterial production of indole-3-acetic acid in the rhizosphere. Field inoculation experiments had revealed more consistent plant growth stimulation using A. brasilense strain Sp245 as compared with the strain Sp7. Therefore, the in situ expression of the key gene ipdC (indole-3-pyruvate decarboxylase) was examined in these two strains. Within the ipdC promoter of strain Sp245 a region of 150 bases was identified, which was missing in strain Sp7. Thus, three different translational ipdC promoter fusions with gfpmut3 were constructed on plasmid level: the first contained the part of the Sp245 promoter region homologous to strain Sp7, the second was bearing the complete promoter region of Sp245 including the specific insertion and the third comprised the Sp7 promoter region. By comparing the fluorescence levels of these constructs after growth on mineral medium with and without inducing amino acids, it could be demonstrated that ipdC expression in A. brasilense Sp245 was subject to a stricter control compared with strain Sp7. Microscopic detection of these reporter strains colonizing the rhizoplane documented for the first time an in situ expression of ipdC.     

Evidence for production of the phytohormone indole-3-acetic acid by cyanobacteria

The ability of cyanobacteria to produce the phytohormone indole-3-acetic acid (IAA) was demonstrated. A colorimetric (Salkowski) screening of 34 free-living and symbiotically competent cyanobacteria, that represent all morphotypes from the unicellular to the highly differentiated, showed that auxin-like compounds were released by about 38% of the free-living as compared to 83% of the symbiotic isolates. The endogenous accumulation and release of IAA were confirmed immunologically (ELISA) using an anti-IAA antibody on 10 of the Salkowski-positive strains, and the chemical authenticity of IAA was further verified by chemical characterization using gas chromatography-mass spectrometry in Nostoc PCC 9229 (isolated from the angiosperm Gunnera) and in Nostoc 268 (free-living). Addition of the putative IAA precursor tryptophan enhanced IAA accumulation in cell extracts and supernatants. As the genome of the symbiotically competent Nostoc PCC 73102 contains homologues of key enzymes of the indole-3-pyruvic acid pathway, a transaminase and indolepyruvate decarboxylase (IpdC), the putative ipdC gene from this cyanobacterium was cloned and used in Southern blot analysis. Out of 11 cyanobacterial strains responding positively in the Salkowski/ELISA test, ipdC homologues were found in 4. A constitutive and possibly tryptophan-dependent production of IAA via the indole-3-pyruvic acid pathway is therefore suggested. The possible role of IAA in cyanobacteria in general and in their interactions with plants is discussed.     

T-DNA gene 5 of Agrobacterium modulates auxin response by autoregulated synthesis of a growth hormone antagonist in plants.

Oncogenes carried by the transferred DNA (T-DNA) of Agrobacterium Ti plasmids encode the synthesis of plant growth factors, auxin and cytokinin, and induce tumour development in plants. Other T-DNA genes regulate the tumorous growth in ways that are not yet understood. To determine the function of T-DNA gene 5, its coding region was expressed in Escherichia coli. Synthesis of the gene 5 encoded protein (26 kDa) correlated with a 28-fold increase in conversion of tryptophan to indole-3-lactate (ILA), an auxin analogue. Expression of chimeric gene 5 constructs in transgenic tobacco resulted in overproduction of ILA that enhanced shoot formation in undifferentiated tissues and increased the tolerance of germinating seedlings to the inhibitory effect of externally supplied auxin. Promoter analysis of gene 5 in plants revealed that its expression was inducible by auxin and confined to the vascular phloem cells. cis-regulatory elements required for auxin regulation and phloem specific expression of gene 5 were mapped to a 90 bp promoter region that carried DNA sequence motifs common to several auxin induced plant promoters, as well as a binding site for a nuclear factor, Ax-1. ILA was found to inhibit the auxin induction of the gene 5 promoter and to compete with indole-3-acetic acid (IAA) for in vitro binding to purified cellular auxin binding proteins. It is suggested therefore that ILA autoregulates its own synthesis and thereby modulates a number of auxin responses in plants.     

Indole-3-acetic acid homeostasis in transgenic tobacco plants expressing the Agrobacterium rhizogenes rolB gene

The rolB gene of the plant pathogen Agrobacterium rhizogenes has an important role in the establishment of hairy root disease in infected plant tissues. When expressed as a single gene in transgenic plants the RolB protein gives rise to effects indicative of increased auxin activity. It has been reported that the RolB product is a β-glucosidase and proposed that the physiological and developmental alterations in transgenic plants expressing the rolB gene are the result of this enzyme hydrolysing bound auxins, in particular (indole-3-acetyl)-β-D-glucoside (IAGluc), and thereby bringing about an increase in the intracellular concentration of indole-3-acetic acid (IAA). Using tobacco plants as a test system, this proposal has been investigated in detail. Comparisons have been made between the RolB phenotype and that of IaaM/iaaH transformed plants overproducing IAA. In addition, the levels of IAA and IAA amide and IAA ester conjugates were determined in wild-type and transgenic 35S-rolB tobacco plants and metabolic studies were carried out with [¹³C₆]IAA [2′-¹⁴C]IAA, [¹⁴C]IAGluc, [5-³H]-2-o-(indole-3-acetyl)-myo-inositol and [¹⁴C]indole-3-acetylaspartic acid. The data obtained demonstrate that expression of the rolB encoded protein in transgenic tobacco does not produce a phenotype that resembles that of IAA over producing plants, does not alter the size of the free IAA pool, has no significant effect on the rate of IAA metabolism, and, by implication, appears not to influence the overall rate of IAA biosynthesis. Furthermore, the in vivo hydrolysis of IAGluc, and that of the other IAA conjugates that were tested, is not affected. On the basis of these findings, it is concluded that the RolB phenotype is not the consequence of an increase in the size of the free IAA pool mediated by an enhanced rate of hydrolysis of IAA conjugates.     

Cloning and characterization of a root sunflower peroxidase gene putatively involved in cell elongation

A cDNA-encoding a peroxidase (Helianthus annuus POX (HaPOX)1) was isolated and characterized from the roots of sunflower seedlings. This gene exhibited homology with other peroxidases from several higher-plant species, and its expression in the root growth was particularly abundant during cell expansion. To elucidate the precise functions of HaPOX1 in sunflower root, we examined its expression pattern in response to several plant growth regulators. Expression of HaPOX1 is down-regulated by abscisic acid (ABA), whereas indole-3-acetic acid (IAA) induced its expression. These results suggest that HaPOX1 expression is differentially regulated by phytohormonal components of signaling cascades. Since IAA appears to participate in the regulation of HaPOX1 expression, we postulate that the peroxidase encoded by HaPOX1 may be involved in the reactions that promote cell elongation during the early stage of root growth.