Role of Pseudomonas putida Indoleacetic Acid in Development of the Host Plant Root System

Many plant-associated bacteria synthesize the phytohormone indoleacetic acid (IAA). While IAA produced by phytopathogenic bacteria, mainly by the indoleacetamide pathway, has been implicated in the induction of plant tumors, it is not clear whether IAA synthesized by beneficial bacteria, usually via the indolepyruvic acid pathway, is involved in plant growth promotion. To determine whether bacterial IAA enhances root development in host plants, the ipdc gene that encodes indolepyruvate decarboxylase, a key enzyme in the indolepyruvic acid pathway, was isolated from the plant growth-promoting bacterium Pseudomonas putida GR12-2 and an IAA-deficient mutant constructed by insertional mutagenesis. The canola seedling primary roots from seeds treated with wild-type P. putida GR12-2 were on average 35 to 50% longer than the roots from seeds treated with the IAA-deficient mutant and the roots from uninoculated seeds. In addition, exposing mung bean cuttings to high levels of IAA by soaking them in a suspension of the wild-type strain stimulated the formation of many, very small, adventitious roots. Formation of fewer roots was stimulated by treatment with the IAA-deficient mutant. These results suggest that bacterial IAA plays a major role in the development of the host plant root system.     

Utilization of the plant hormone indole-3-acetic acid for growth by Pseudomonas putida strain 1290

We have isolated from plant surfaces several bacteria with the ability to catabolize indole-3-acetic acid (IAA). One of them, isolate 1290, was able to utilize IAA as a sole source of carbon, nitrogen, and energy. The strain was identified by its 16S rRNA sequence as Pseudomonas putida. Activity of the enzyme catechol 1,2-dioxygenase was induced during growth on IAA, suggesting that catechol is an intermediate of the IAA catabolic pathway. This was in agreement with the observation that the oxygen uptake by IAA-grown P. putida 1290 cells was elevated in response to the addition of catechol. The inability of a catR mutant of P. putida 1290 to grow at the expense of IAA also suggests a central role for catechol as an intermediate in IAA metabolism. Besides being able to destroy IAA, strain 1290 was also capable of producing IAA in media supplemented with tryptophan. In root elongation assays, P. putida strain 1290 completely abolished the inhibitory effect of exogenous IAA on the elongation of radish roots. In fact, coinoculation of roots with P. putida 1290 and 1 mM concentration of IAA had a positive effect on root development. In coinoculation experiments on radish roots, strain 1290 was only partially able to alleviate the inhibitory effect of bacteria that in culture overproduce IAA. Our findings imply a biological role for strain 1290 as a sink or recycler of IAA in its association with plants and plant-associated bacteria.     

Contribution of Indole-3-Acetic Acid Production to the Epiphytic Fitness of Erwinia herbicola

Erwinia herbicola 299R produces large quantities of indole-3-acetic acid (IAA) in culture media supplemented with l-tryptophan. To assess the contribution of IAA production to epiphytic fitness, the population dynamics of the wild-type strain and an IAA-deficient mutant of this strain on leaves were studied. Strain 299XYLE, an isogenic IAA-deficient mutant of strain 299R, was constructed by insertional interruption of the indolepyruvate decarboxylase gene of strain 299R with the xylE gene, which encodes a 2,3-catechol dioxygenase from Pseudomonas putida mt-2. The xylE gene provided a useful marker for monitoring populations of the IAA-deficient mutant strain in mixed populations with the parental strain in ecological studies. A root bioassay for IAA, in which strain 299XYLE inhibited significantly less root elongation than strain 299R, provided evidence that E. herbicola produces IAA on plant surfaces in amounts sufficient to affect the physiology of its host and that IAA production in strain 299R is not solely an in vitro phenomenon. The epiphytic fitness of strains 299R and 299XYLE was evaluated in greenhouse and field studies by analysis of changes in the ratio of the population sizes of these two strains after inoculation as mixtures onto plants. Populations of the parental strain increased to approximately twice those of the IAA-deficient mutant strain after coinoculation in a proportion of 1:1 onto bean plants in the greenhouse and onto pear flowers in field studies. In all experiments, the ratio of the population sizes of strain 299R and 299XYLE increased during periods of active growth on plant tissue but not when population sizes were not increasing with time.

Many plant-associated bacteria have the ability to produce the plant growth regulator indole-3-acetic acid (IAA) (5, 9, 25, 33). IAA is involved in diseases caused by gall- and knot-forming bacterial species (33); however, its role in other bacteria remains undefined. It is unclear whether these bacteria produce IAA during colonization of plant surfaces and whether this metabolite is beneficial to the bacteria during their growth and survival in the phyllosphere. The production of IAA may enable bacteria to detoxify tryptophan analogues present on plant surfaces (15), to downregulate genes involved in plant defense responses (33), or to inhibit the development of the hypersensitive response by plants (26). We recently demonstrated that the ipdC gene, which encodes the indolepyruvate decarboxylase of Erwinia herbicola (Pantoea agglomerans) 299R and which is involved in the indolepyruvate pathway for IAA synthesis in this epiphytic strain (2), is osmoresponsive and plant inducible (3). We hypothesized that the secretion of IAA may modify the microhabitat of epiphytic bacteria by increasing nutrient leakage from plant cells; enhanced nutrient availability may better enable IAA-producing bacteria to colonize the phyllosphere and may contribute to their epiphytic fitness (1).Few studies have attempted to determine the ecological significance of IAA production in pathogenic bacteria. Varvaro and Martella (31) showed that IAA-deficient mutants of Pseudomonas syringae pv. savastanoi, obtained by selection for resistance to α-methyltryptophan, were reduced in their ability to colonize and survive on olive leaf surfaces. The survival of an α-methyltryptophan-resistant IAA-deficient mutant of P. syringae pv. savastanoi in knots also was affected, its population declining more rapidly than that of the parental strain when inoculated alone into oleander leaf tissue (28). The importance of IAA production in bacterial colonization of bean leaves was also tested with the brown spot pathogen P. syringae pv. syringae and an IAA-deficient mutant derived by insertional mutagenesis (21). Although no difference in the survival of the parental and mutant strains on bean leaves was observed in the greenhouse, a small difference in their behavior was apparent in experiments conducted in a mist chamber (21). There have been no studies of the role of IAA production in plant-associated bacteria that do not cause disease.IAA biosynthesis is not essential for bacterial growth and survival, since IAA-deficient mutants grow as well as their IAA-producing parental strain in vitro (2, 29). Large differences in the epiphytic behaviors of IAA-producing bacteria and isogenic IAA-deficient mutants consequently would not be expected. Even small contributions of IAA production to epiphytic fitness could account for the common presence of this phenotype in epiphytic bacteria (19). Measurements of changes in the ratio of two strains following coinoculation, a common approach in ecological studies, can allow the detection of even small differences in the competitive behaviors of two organisms. This approach can detect much smaller differences in behavior between closely related species than comparison of populations of these species when present singly in separate habitats (16). In this study, we tested the role of IAA in the epiphytic fitness of E. herbicola by comparing the relative changes in the population sizes of the parental and IAA-deficient mutant strains with time after their inoculation onto plants in both controlled and field environments.     

Comparison of growth,exogenous auxin sensitivity,and endogenous indole-3-acetic acid content in roots ofHordeum vulgare L. and an agravitropic mutant

The chemically induced barley (Hordeum vulgare L.) mutation, agr, was found to be a simple recessive trait resulting in agravitropic roots and normal gravitropic shoots. The total seedling root growth was similar for mutant and wild-type roots, although the mutant had fewer roots per seed and greater elongation per root. Although the concentration of exogenous indole-3-acetic acid (IAA) required to reduce root growth by 50% (GR50) was 12 times greater for the agravitropic mutant, agravitropic and gravitropic roots were equally sensitive to exogenous applications of 2,4-dichlorophenoxyacetic acid (2,4-D) and naphthalene acetic acid (NAA). Root IAA contents, determined by high-pressure liquid chromatography (HPLC), were not different for gravitropes and agravitropes. The greater root elongation rates, lack of sensitivity to exogenous IAA, and normal endogenous IAA levels indicate that auxin-controlled growth regulation may be altered in the mutant.     

Discovery of a bacterial gene cluster for catabolism of the plant hormone indole 3-acetic acid

The isolation and annotation of an 8994-bp DNA fragment from Pseudomonas putida 1290, which conferred upon P. putida KT2440 the ability to utilize the plant hormone indole 3-acetic acid (IAA) as a sole source of carbon and energy, is described. This iac locus (for indole 3-acetic acid catabolism) was identified through analysis of a plasposon mutant of P. putida 1290 that was no longer able to grow on IAA or indole 3-acetaldehyde and was unable to protect radish roots from stunting by exogenously added IAA. The iac locus consisted of 10 genes with coding similarity to enzymes acting on indole or amidated aromatics and to proteins with regulatory or unknown function. Highly similar iac gene clusters were identified in the genomes of 22 bacterial species. Five of these, i.e. P. putida GB-1, Marinomonas sp. MWYL1, Burkholderia sp. 383, Sphingomonas wittichii RW1 and Rhodococcus sp. RHA1, were tested to confirm that bacteria with IAA-degrading ability have representatives in the Alpha-, Beta- and Gammaproteobacteria and in the Actinobacteria. In P. putida 1290, cat and pca genes were found to be essential to IAA-degradation, suggesting that IAA is channeled via catechol into the beta-ketoadipate pathway. Also contributing to the IAA degrading phenotype were genes involved in tricarboxylate cycling, gluconeogenesis, and carbon/nitrogen sensing.     

Interactions between Plants and Epiphytic Bacteria Regarding Their Auxin Metabolism

Proofs of different kind are presented of the existence of highly active bacteria producing IAA from tryptophan on plant surfaces and in plant homogenates. Both homogenates and washing solutions of nonsterile pea plant parts are active in producing IAA from tryptophan. Activity is much enhanced by the addition of glucose or by preincubating the preparations; it is abolished by sterile filtration, by some bactericidic and bacteriostatic substances, by chloramphenicol, streptomycin, and albucid (penicillin being only partly effective). Preparations of sterile plants do not produce IAA from tryptophan within the detection limit of the Salkowski test. The bacteria are even present on seed surfaces, in the air, and in aceton or ammonium sulfate precipitations of homogenates. Main products of the bacterial tryptophan conversion, as demonstrated by paper chromatography, are indolepyruvic acid, indoleacetic acid, indolecarboxaldehyde, and indolecarboxylic acid. In presence of glucose indolepyruvic acid is by far dominating. Many hitherto known results about tryptophan conversion to IAA by higher plants are likely to be falsified by epiphytic bacteria.     

Effect of Wild-Type and Mutant Plant Growth-Promoting Rhizobacteria on the Rooting of Mung Bean Cuttings

Mung bean cuttings were dipped in solutions of wild type and mutant forms of the plant growth-promoting rhizobacterium Pseudomonas putida GR12-2 and then incubated for several days until roots formed. The bacteria P. putida GR12-2 and P. putida GR12-2/aux1 mutant do not produce detectable levels of the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase, whereas P. putida GR12-2/acd36 is an ACC deaminase minus mutant. All bacteria produce the phytohormone indole-3-acetic acid (IAA), and P. putida GR12-2/aux1 overproduces it. Treatment of cuttings with the above-mentioned bacteria affected the rates of ethylene production in the cuttings in a way that can be explained by the combined effects of the activity of ACC deaminase localized in the bacteria and bacterial produced IAA. P. putida GR12-2 and P. putida GR12-2/acd36-treated cuttings had a significantly higher number of roots compared with cuttings rooted in water. In addition, the wild type influenced the development of longer roots. P. putida GR12-2/aux1 stimulated the highest rates of ethylene production but did not influence the number of roots. These results are consistent with the notion that ethylene is involved in the initiation and elongation of adventitious roots in mung bean cuttings. Received October 21, 1998; accepted January 3, 1999     

Catalase activity and the survival of Pseudomonas putida, a root colonizer, upon treatment with peracetic acid

Peracetic acid is used as a sterilant in several industrial settings. Cells of a plant-colonizing bacterium, Pseudomonas putida in liquid suspension, were more sensitive to killing by peracetic acid when they lacked a major catalase activity, catalase A. Low doses of peracetic acid induced promoter activity of the gene encoding catalase A and increased total catalase specific activity in cell extracts. Microbes present in native agricultural soils rapidly degraded the active oxygen species present in peracetic acid. The simultaneous release of oxygen was consistent with a role for catalase in degrading the hydrogen peroxide that is part of the peracetic acid-equilibrium mixture. Amendment of sterilized soils with wild-type P. putida restored the rate of degradation of peracetic acid to a higher level than was observed in the soils amended with the catalase A-deficient mutant. The association of the bacteria with the plant roots resulted in protection of the wild-type as well as the catalase-deficient mutant from killing by peracetic acid. No differential recovery of the wild-type and catalase A mutant of P. putida was observed from roots after the growth matrix containing the plants was flushed with peracetic acid.     

The auxin-signaling pathway is required for the lateral root response of Arabidopsis to the rhizobacterium Phyllobacterium brassicacearum

Plant root development is highly responsive both to changes in nitrate availability and beneficial microorganisms in the rhizosphere. We previously showed that Phyllobacterium brassicacearum STM196, a plant growth-promoting rhizobacteria strain isolated from rapeseed roots, alleviates the inhibition exerted by high nitrate supply on lateral root growth. Since soil-borne bacteria can produce IAA and since this plant hormone may be implicated in the high nitrate-dependent control of lateral root development, we investigated its role in the root development response of Arabidopsis thaliana to STM196. Inoculation with STM196 resulted in a 50% increase of lateral root growth in Arabidopsis wild-type seedlings. This effect was completely abolished in aux1 and axr1 mutants, altered in IAA transport and signaling, respectively, indicating that these pathways are required. The STM196 strain, however, appeared to be a very low IAA producer when compared with the high-IAA-producing Azospirillum brasilense sp245 strain and its low-IAA-producing ipdc mutant. Consistent with the hypothesis that STM196 does not release significant amounts of IAA to the host roots, inoculation with this strain failed to increase root IAA content. Inoculation with STM196 led to increased expression levels of several IAA biosynthesis genes in shoots, increased Trp concentration in shoots, and increased auxin-dependent GUS staining in the root apices of DR5::GUS transgenic plants. All together, our results suggest that STM196 inoculation triggers changes in IAA distribution and homeostasis independently from IAA release by the bacteria.     

Chromosaponin I specifically interacts with AUX1 protein in regulating the gravitropic response of Arabidopsis roots

We have found that chromosaponin I (CSI), a gamma-pyronyl-triterpenoid saponin isolated from pea (Pisum sativum L. cv Alaska), specifically interacts with AUX1 protein in regulating the gravitropic response of Arabidopsis roots. Application of 60 microM CSI disrupts the vertically oriented elongation of wild-type roots grown on agar plates but orients the elongation of agravitropic mutant aux1-7 roots toward the gravity. The CSI-induced restoration of gravitropic response in aux1-7 roots was not observed in other agravitropic mutants, axr2 and eir1-1. Because the aux1-7 mutant is reduced in sensitivity to auxin and ethylene, we examined the effects of CSI on another auxin-resistant mutant, axr1-3, and ethylene-insensitive mutant ein2-1. In aux1-7 roots, CSI stimulated the uptake of [(3)H]indole-3-acetic acid (IAA) and induced gravitropic bending. In contrast, in wild-type, axr1-3, and ein2-1 roots, CSI slowed down the rates of gravitropic bending and inhibited IAA uptake. In the null allele of aux1, aux1-22, the agravitropic nature of the roots and IAA uptake were not affected by CSI. This close correlation between auxin uptake and gravitropic bending suggests that CSI may regulate gravitropic response by inhibiting or stimulating the uptake of endogenous auxin in root cells. CSI exhibits selective influence toward IAA versus 1-naphthaleneacetic acid as to auxin-induced inhibition in root growth and auxin uptake. The selective action of CSI toward IAA along with the complete insensitivity of the null mutant aux1-22 toward CSI strongly suggest that CSI specifically interacts with AUX1 protein.     

A rice tryptophan deficient dwarf mutant, tdd1, contains a reduced level of indole acetic acid and develops abnormal flowers and organless embryos

Indole-3-acetic acid (IAA) plays a critical role in many aspects of plant growth and development; however, complete pathways of biosynthesis, localization and many aspects of functions of IAA in rice remain unclear. Here, we report the analysis of a rice tryptophan- (Trp-) and IAA-deficient mutant, tryptophan deficient dwarf1 ( tdd1 ) , which is embryonic lethal because of a failure to develop most organs during embryogenesis. Regenerated tdd1 plants showed pleiotropic phenotypes: dwarfing, narrow leaves, short roots and abnormal flowers. TDD1 encodes a protein homologous to anthranilate synthase β-subunit, which catalyses the first step of the Trp biosynthesis pathway and functions upstream of Trp-dependent IAA biosynthesis. TDD1-uidA and DR5-uidA expression overlapped at many sites in WT plants but was lacking in tdd1 , indicating that TDD1 is involved in auxin biosynthesis. Both Trp and IAA levels in flowers and embryos were much lower in tdd1 than in wild type (WT). Trp feeding completely rescued the mutant phenotypes and moderate expression of OsYUCCA1 , which encodes a key enzyme in Trp-dependent IAA biosynthesis, also rescued plant height and root length, indicating that the abnormal phenotypes of tdd1 are caused predominantly by Trp and IAA deficiency. In tdd1 embryos, the expression patterns of OSH1 and OsSCR , which mark the presumptive apical region and the L2 layer, respectively, are identical to those in WT, suggesting a possibility either that different IAA levels are required for basic pattern formation than for organ formation or that an orthologous gene compensates for TDD1 deficiency during pattern formation.     

Isolation and Characterization of Mutants of the Plant Growth-Promoting Rhizobacterium Pseudomonas putida GR12-2 That Overproduce Indoleacetic Acid

Following transposon Tn5 mutagenesis of the plant growth-promoting rhizobacterium Pseudomonas putida GR12-2, mutants that were able to grow in the presence of the tryptophan analog 5-fluorotryptophan were selected. Seven of the 50 5-fluorotryptophan-resistant mutants overproduced the phytohormone indoleacetic acid (IAA). Of these seven mutants, the highest level of IAA was observed with strain P. putida GR12-2/aux1, which produced four times the amount of indoleacetic acid synthesized by the wild-type strain. Strain P. putida GR12-2/aux1, in contrast to the wild type, lost the ability to stimulate the elongation of the roots of canola seedlings under gnotobiotic conditions. The growth rate, siderophore production, and 1-aminocyclopropane-1-carboxylate deaminase activity of mutant strain P. putida GR12-2/aux1 were identical to those of the wild-type strain. The role of IAA in the mechanism of plant growth stimulation by P. putida GR12-2 and other plant growth-promoting rhizobacteria is discussed.     

Auxin production by bacteria associated with orchid roots

Bacteria associated with the roots of greenhouse tropical orchids were shown to produce indole-3-acetic acid (IAA) and to excrete it into the culture liquid. The presence and activity of IAA were demonstrated colorimetrically, by thin-layer chromatography, and by biotests. The associated bacteria varied in their ability to excrete indole compounds (1-28 microg/ml nutrient broth). Addition of tryptophan to the growth medium enhanced phytohormone production. Upon addition of 200 microg/ml tryptophan, the bacteria isolated from Dendrobium moschatum roots (Sphingomonas sp. 18, Microbacterium sp. 23, Mycobacterium sp. 1, Bacillus sp. 3, and Rhizobium sp. 5) produced 50.2, 53.1, 92.9, 37.6, and 60.4 microg IAA/ml respectively, while the bacteria isolated from Acampe papillosa roots (Sphingomonas sp. 42, Rhodococcus sp. 37, Cellulomonas sp. 23, Pseudomonas sp. 24, and Micrococcus luteus) produced 69.4, 49.6, 53.9, 31.0, and 39.2 microg IAA/ml. Auxin production depended on cultivation conditions and on the growth phase of the bacterial cultures. Treatment of kidney bean cuttings with bacterial culture liquid promoted formation of a "root brush" with location height 7.4- to 13.4-fold greater than the one in the control samples. The ability of IAA-producing associated bacteria to act as stimulants of the host plant root development is discussed.     

Basipetal auxin transport is required for gravitropism in roots of Arabidopsis

Auxin transport has been reported to occur in two distinct polarities, acropetally and basipetally, in two different root tissues. The goals of this study were to determine whether both polarities of indole-3-acetic acid (IAA) transport occur in roots of Arabidopsis and to determine which polarity controls the gravity response. Global application of the auxin transport inhibitor naphthylphthalamic acid (NPA) to roots blocked the gravity response, root waving, and root elongation. Immediately after the application of NPA, the root gravity response was completely blocked, as measured by an automated video digitizer. Basipetal [(3)H]IAA transport in Arabidopsis roots was inhibited by NPA, whereas the movement of [(14)C]benzoic acid was not affected. Inhibition of basipetal IAA transport by local application of NPA blocked the gravity response. Inhibition of acropetal IAA transport by application of NPA at the root-shoot junction only partially reduced the gravity response at high NPA concentrations. Excised root tips, which do not receive auxin from the shoot, exhibited a normal response to gravity. The Arabidopsis mutant eir1, which has agravitropic roots, exhibited reduced basipetal IAA transport but wild-type levels of acropetal IAA transport. These results support the hypothesis that basipetally transported IAA controls root gravitropism in Arabidopsis.     

Utilization of the Plant Hormone Indole-3-Acetic Acid for Growth by Pseudomonas putida Strain 1290

We have isolated from plant surfaces several bacteria with the ability to catabolize indole-3-acetic acid (IAA). One of them, isolate 1290, was able to utilize IAA as a sole source of carbon, nitrogen, and energy. The strain was identified by its 16S rRNA sequence as Pseudomonas putida. Activity of the enzyme catechol 1,2-dioxygenase was induced during growth on IAA, suggesting that catechol is an intermediate of the IAA catabolic pathway. This was in agreement with the observation that the oxygen uptake by IAA-grown P. putida 1290 cells was elevated in response to the addition of catechol. The inability of a catR mutant of P. putida 1290 to grow at the expense of IAA also suggests a central role for catechol as an intermediate in IAA metabolism. Besides being able to destroy IAA, strain 1290 was also capable of producing IAA in media supplemented with tryptophan. In root elongation assays, P. putida strain 1290 completely abolished the inhibitory effect of exogenous IAA on the elongation of radish roots. In fact, coinoculation of roots with P. putida 1290 and 1 mM concentration of IAA had a positive effect on root development. In coinoculation experiments on radish roots, strain 1290 was only partially able to alleviate the inhibitory effect of bacteria that in culture overproduce IAA. Our findings imply a biological role for strain 1290 as a sink or recycler of IAA in its association with plants and plant-associated bacteria.     

Competitiveness in root colonization by Pseudomonas putida requires the rpoS gene

The rpoS gene in Pseudomonas putida was essential for plant root colonization under competitive conditions from other microbes. The RpoS- mutant survived less well than the wild-type strain in culture medium, and unlike the wild-type, failed to colonize the roots in a peat matrix containing an established diverse microflora. The RpoS-deficient P. putida isolate was generated by insertion of a glucuronidase-npt cassette into the rpoS gene. The RpoS mutant had dose-dependent increased sensitivity to oxidative stress and produced Mn-superoxide dismutase activity earlier than the parent. While extracts from wild-type P. putida stationary-phase cells contained three isozymes of catalase (CatA, CatB, and CatC), the sigma38-deficient P. putida lacked CatB. These results are consistent with previous findings that CatB is induced in stationary-phase.     

Pathways of IAA production from tryptophan by plants and by their epiphytic bacteria: Metabolism of indolepyruvic acid and indolelactic acid

Metabolites of indolepyruvic acid and indolelactic acid were investigated using 2 systems: a bacterial (pea stem homogenates containing the epiphytic bacteria) and a plant system (pea stem sections under sterile conditions). The products of spontaneous indolepyruvic acid decomposition in aqueous solution and during chromatography were investigated, too. Biological indolepyruvic acid conversion yields, besides those substance amounts which occur spontaneously, indoleacetic acid, indoleethanol (tryptophol) and (only in the sterile plant system) indoleacetaldehyde. An inhibitor extract from pea stems decreases the indoleacetic acid and increases the indoleethanol and indoleacetaldehyde gain. Indolelactic acid is not metabolized in the sterile plant sections. Indolelactic acid oxidation by the bacteria-containing homogenate yields indolepyruvic acid and is inhibited by the inhibitor extract.     

Pathways of IAA Production from Tryptophan by Plants and by Their Epiphytic Bacteria: A Comparison

Metabolites of tryptophan were investigated using 2 systems: a bacterial (Peastem homogenates containing the epiphytic bacteria) and a plant system (pea stem sections under sterile conditions). The plant system produces: indolepyruvic acid (IPyA), indoleacetaldehyde (IAAld) indoleacetic acid (IAA), indoleethanol (tryptophol, IAAol), indolecarboxylie acid (ICA), indolecarboxaldehyde (ICAld). Bacteria produce additionally: indoleactic acid (ILA), tryptamine (TNH₂) and the unknown Xb and Yb, but IAAld was not detected. A nonacidic inhibitor extract from pea stems decreases the gain of IAA, IPyA, ILA, Yb. It increases the gain of IAAld, IAAol, TNH₂, Xb, and (only in the bacterial system) ICA and ICAld. Three sites of inhibitor action are suggested, namely the steps Try → IPyA, TNH₂→ IAAld, IAAld → IAA.     

IAA production during germination of Orobanche spp. seeds

Broomrapes (Orobanche spp.) are parasitic plants, whose growth and development fully depend on the nutritional connection established between the parasite and the roots of the respective host plant. Phytohormones are known to play a role in establishing the specific Orobanche-host plant interaction. The first step in the interaction is seed germination triggered by a germination stimulant secreted by the host-plant roots. We quantified indole-3-acetic acid (IAA) and abscisic acid (ABA) during the seed germination of tobacco broomrape (Orobanche ramosa) and sunflower broomrape (O. cumana). IAA was mainly released from Orobanche seeds in host-parasite interactions as compared to non-host-parasite interactions. Moreover, germinating seeds of O. ramosa released IAA as early as 24 h after the seeds were exposed to the germination stimulant, even before development of the germ tube. ABA levels remained unchanged during the germination of the parasites' seeds. The results presented here show that IAA production is probably part of a mechanism triggering germination upon the induction by the host factor, thus resulting in seed germination.