Inactivity of Oxidation Products of Indole-3-acetic Acid on Ethylene Production in Mung Bean Hypocotyls

The suggestion that indole-3-acetic acid (IAA)-stimulated ethylene production is associated with oxidative degradation of IAA and is mediated by 3-methyleneoxindole (MOI) has been tested in mung bean (Phaseolus aureus Roxb.) hypocotyl segments. While IAA actively stimulated ethylene production, MOI and indole-3-aldehyde, the major products of IAA oxidation, were inactive. Tissues treated with a mixture of intermediates of IAA oxidation, obtained from a 1-hour incubation of IAA with peroxidase, failed to stimulate ethylene production. Furthermore, chlorogenic acid and p-coumaric acid, which are known to interfere with the enzymic oxidation of IAA to MOI, had no effect on IAA-stimulated ethylene production. Other oxidation products of IAA, including oxindole-3-acetic acid, indole-3-carboxylic acid, (2-sulfoindole)-3-acetic acid, and dioxindole-3-acetic acid, were all inactive. 1-Naphthaleneacetic acid was as active as IAA in stimulating ethylene production but was decarboxylated at a much lower rate than IAA, suggesting that oxidative decarboxylation of auxins is not linked to ethylene production. These results demonstrate that IAA-stimulated ethylene production in mung bean hypocotyl tissue is not mediated by MOI or other associated oxidative products of IAA.     

The Identification of Indole-3-Acetic Acid and Indole-3-Acetamide in the Hypocotyls of Japanese Cherry

Both indole-3-acetamide (IAM) and indole-3-acetic acid (IAA)were identified in extracts of the hypocotyls of Japanese cherryby GC/MS. Exogenous IAA and IAM promoted the elongation of segmentsof these hypocotyls and the effect of IAA applied together withIAM was the same as that of IAA alone. (Received July 29, 1992; Accepted October 19, 1992)     

Regulation of Auxin-induced Ethylene Production in Mung Bean Hypocotyls: Role of 1-Aminocyclopropane-1-Carboxylic Acid

Ethylene production in mung bean hypocotyls was greatly increased by treatment with 1-aminocyclopropane-1-carboxylic acid (ACC), which was utilized as the ethylene precursor. Unlike auxin-stimulated ethylene production, ACC-dependent ethylene production was not inhibited by aminoethoxyvinylglycine, which is known to inhibit the conversion of S-adenosylmethionine to ACC. While the conversion of methionine to ethylene requires induction by auxin, the conversion of methionine to S-adenosylmethionine and the conversion of ACC to ethylene do not. It is proposed that the conversion of S-adenosylmethionine to ACC is the rate-limiting step in the biosynthesis of ethylene, and that auxin stimulates ethylene production by inducing the synthesis of the enzyme involved in this reaction.     

Indole-3-Pyruvic Acid as an Intermediate in the Conversion of Tryptophan to Indole-3-Acetic Acid. I. Some Characteristics of Tryptophan Transaminase from Mung Bean Seedlings

Seedlings of mung bean (Phaseolus aureus) contain a soluble enzyme capable of converting l-tryptophan to indole-3-pyruvic acid by transamination. The concentration of the enzyme is highest in the stem meristem and primary leaves and lowest in the roots. The enzyme was purified 28.6 fold by ammonium sulphate precipitation, Sephadex G-200 filtration, and electrophoresis. The isoelectric point of the enzyme protein was pH 6.6. The optimum pH and temperature for the catalytic conversion were ca. 8.5 and 53°C respectively. Using l -tryptophan and α-ketoglutarate as substrates Km was found to be 3.3 × 10^?4 M and the activation energy 18,270 cal per mole. The enzyme converted only the l -form of tryptophan, phenylalanine, tyrosine, and histidine. Out of 13 other l -amino acids tested 8 could be transaminated. Eight α-keto acids tested could all be used as substrates. High efficiency of an α-keto acid as an amino group acceptor agreed usually with high efficiency of the corresponding amino acid as a donor. The pari ß-methyl-α-ketoisovaleric acid and isoleucine was an exception to that rule. Addition of pyridoxalphosphate to the reaction mixture was not needed. The indole-3-pyruvic acid formed in the reaction was trapped and partly stabilized as its borate complex and measured spectrophotometrically at 327 nm. The keto acid formed was further identified by chromatography of its 2,4-dinitrophenylhydrazone in 4 solvent systems. When using α-keto-glutaric acid as a substrate, the glutamic acid produced was determined by the glutamate dehydrogenase method. The sensitivity of the assay permits enzyme determinations in extracts from 5 mg leaves or 100 mg roots.     

Changes in the Level of [C]Indole-3-Acetic Acid and [C]Indoleacetylaspartic Acid during Root Formation in Mung Bean Cuttings

Changes in the levels of [¹⁴C]indole-3-acetic acid (IAA) and [¹⁴C]indole-acetylaspartic acid (IAAsp) were examined during adventitious root formation in mung bean (Vigna radiata [L.] R. Wilcz. `Berken') stem cuttings. IAAsp was identified by GC-MS as the primary conjugate in IAA-treated cuttings. During root formation in IAA-treated cuttings, the level of [¹⁴C]IAAsp increased rapidly the first day and then declined; [¹⁴C]IAA was rapidly metabolized and not detected after 12 hours.     

Longitudinal Gradients of Indole-3-Acetic Acid and Abscisic Acid in the Hypocotyl of Etiolated Bean Seedlings

The longitudinal distribution of abscisic acid (ABA) and indole-3-aceticacid (IAA) along the hypocotyl of 5-d-old etiolated Phaseolusvulgaris L. cv. Limburg seedlings was measured. IAA was analysedby the L-methyl-indole--pryone assay (2-MIP) and ABA by electroncapture gas chromatography (ECD-GC). Length and width of theinner parenchyma cells, growth rate and protein content werealso measured. Cell expansion occurred predominantly in a region20 mm below the centre of the hook where elongation rate wasmaximal and where protein concentration decreased rapidly withdistance from the hook. The ratio between ABA and IAA was constant along the lengthof the hypocotyl. On a fresh weight basis the concentrationof both growth substances was maximal in the upper (youngest)part, decreased in slightly older sections where cell expansionwas proceeding and was smallest in the basal regions where cellexpansion was complete. However, when expressed on a proteinbasis the concentration gradient of the hormones was the reverseof that described on a fresh weight basis. Key words: IAA, ABA, hypocotyl, etiolated, bean     

Distribution of Indole-3-Acetic Acid in the Apoplast and Symplast of Squash (Cucurbita maxima) Hypocotyls

The concentration of endogenous IAA was higher in an apoplasticsolution (2.3xl0^–7M) than in a symplastic solution (0.5x 10^–7 M) obtained from segments of etiolated squash (Cucurbitamaxima Duch.) hypocotyls. Exogenously applied IAA (10^–5M) increased the level of IAA in both the apoplastic and thesymplastic solution. The concentration of IAA in the apoplasticsolution increased to 75% of the concentration of exogenousIAA in 4 h, but that in the symplastic solution increased onlyto 23% of the concentration of exogenous IAA. These resultsdemonstrate that the concentration of endogenous IAA is higherin the apoplast than in the symplast of squash hypocotyls, andthey suggest that IAA exerts its physiological effects, at leastto some extent, from the outside of cells. (Received September 20, 1996; Accepted January 10, 1997)     

Production of the Phytohormone Indole-3-Acetic Acid by Estuarine Species of the Genus Vibrio

Strains of Vibrio spp. isolated from roots of the estuarine grasses Spartina alterniflora and Juncus roemerianus produce the phytohormone indole-3-acetic acid (IAA). The colorimetric Salkowski assay was used for initial screening of IAA production. Gas chromatography-mass spectroscopy (GC-MS) was then employed to confirm and quantify IAA production. The accuracy of IAA quantification by the Salkowski assay was examined by comparison to GC-MS assay values. Indole-3-acetamide, an intermediate in IAA biosynthesis by the indole-3-acetamide pathway, was also identified by GC-MS. Multilocus sequence typing of concatenated 16S rRNA, recA, and rpoA genes was used for phylogenetic analysis of environmental isolates within the genus Vibrio. Eight Vibrio type strains and five additional species-level clades containing a total of 16 environmental isolates and representing five presumptive new species were identified as IAA-producing Vibrio species. Six additional environmental isolates similar to four of the Vibrio type strains were also IAA producers. To our knowledge, this is the first report of IAA production by species of the genus Vibrio or by bacteria isolated from an estuarine environment.Estuaries along the east coast of temperate North America are ecologically valuable, productive systems dominated by only a few species of plants. Spartina alterniflora (smooth cord grass; hereinafter referred to as Spartina) is a keystone species responsible for very high rates of primary production in Atlantic coast marshes and is a major contributor to the global cycling of several elements (10, 14, 15, 35, 38, 39, 45). Juncus roemerianus (black needle rush; hereinafter referred to as Juncus) is a common subdominant species (28) residing in areas of higher elevation, lower salinity, and less frequent tidal inundation. The roots of these macrophytes are associated with a diverse assemblage of microorganisms, including N₂-fixing and sulfate-reducing bacteria, which greatly contribute to their productivity (30, 31).The phytohormone indole-3-acetic acid (IAA) is the most commonly occurring naturally produced auxin and the most thoroughly studied plant growth regulator. IAA directs several aspects of plant growth and development (37), including the induction and regulation of a variety of processes: e.g., cell division, root extension, vascularization, apical dominance, and tropisms (6, 32). The effects of IAA on plant root tissue are concentration dependent and can be species specific. Responses to increasing IAA concentrations advance from the stimulation of primary root tissue to the development of lateral and adventitious roots and finally to the complete cessation of root growth (1, 6, 16, 29, 32, 37, 44).Many microorganisms interact with and affect their environment through the production and transudation of signal compounds (17). The findings of numerous studies (see, e.g., references 8, 23, 25, and 37) demonstrate that a variety of plant-associated terrestrial bacteria produce and exude IAA. Auxin synthesis by cyanobacteria has also been reported previously (40). IAA is thought to reduce the integrity of plant cell walls by upregulating the production of cellulases and hemicelluloses, resulting in the leakage of some simple sugars and other nutrients that would benefit root-associated microorganisms (17). Likewise, root growth would be an advantage to resident bacteria due to the increased availability of root exudates and root surface for growth. Microorganisms that produce IAA can influence the host plant and function as pathogens, symbionts, or growth regulators, depending on how their IAA production influences the concentration of the plant''s endogenous IAA pool and on the sensitivity of the plant to auxin. Organisms such as Erwinia chrysanthemi, Pseudomonas savastanoi, and Agrobacterium tumefaciens are phytopathogens of many host plants (11, 21, 23, 46). Other organisms, including Azospirillum brasilense and Pseudomonas putida GR12-2, have proven beneficial to plants, and many IAA producers have been shown to stimulate increases in root mass and/or length (20, 37, 44).The aim of the present study was to assess IAA synthesis by Vibrio strains isolated from the roots of highly productive salt marsh grasses. The Salkowski assay was used to perform an initial screening for the presence of IAA, gas chromatography-mass spectroscopy (GC-MS) verified and quantified IAA production, and multilocus sequence typing (MLST) analysis classified all isolates within the genus Vibrio.     

Metabolism of Indole-3-Acetic Acid in Arabidopsis

The metabolism of indole-3-acetic acid (IAA) was investigated in 14-d-old Arabidopsis plants grown in liquid culture. After ruling out metabolites formed as an effect of nonsterile conditions, high-level feeding, and spontaneous interconversions, a simple metabolic pattern emerged. Oxindole-3-acetic acid (OxIAA), OxIAA conjugated to a hexose moiety via the carboxyl group, and the conjugates indole-3-acetyl aspartic acid (IAAsp) and indole-3-acetyl glutamate (IAGlu) were identified by mass spectrometry as primary products of IAA fed to the plants. Refeeding experiments demonstrated that none of these conjugates could be hydrolyzed back to IAA to any measurable extent at this developmental stage. IAAsp was further oxidized, especially when high levels of IAA were fed into the system, yielding OxIAAsp and OH-IAAsp. This contrasted with the metabolic fate of IAGlu, since that conjugate was not further metabolized. At IAA concentrations below 0.5 μm, most of the supplied IAA was metabolized via the OxIAA pathway, whereas only a minor portion was conjugated. However, increasing the IAA concentrations to 5 μm drastically altered the metabolic pattern, with marked induction of conjugation to IAAsp and IAGlu. This investigation used concentrations for feeding experiments that were near endogenous levels, showing that the metabolic pathways controlling the IAA pool size in Arabidopsis are limited and, therefore, make good targets for mutant screens provided that precautions are taken to avoid inducing artificial metabolism.The plant hormone IAA is an important signal molecule in the regulation of plant development. Its central role as a growth regulator makes it necessary for the plant to have mechanisms that strictly control its concentration. The hormone is believed to be active primarily as the free acid, and endogenous levels are controlled in vivo by processes such as synthesis, oxidation, and conjugation. IAA has been shown to form conjugates with sugars, amino acids, and small peptides. Conjugates are believed to be involved in IAA transport, in the storage of IAA for subsequent use, in the homeostatic control of the pool of the free hormone, and as a first step in the catabolic pathways (Cohen and Bandurski, 1978; Nowacki and Bandurski, 1980; Tuominen et al., 1994; Östin et al., 1995; Normanly, 1997). It is generally accepted that in some species conjugated IAA is the major source of free IAA during the initial stages of seed germination (Ueda and Bandurski, 1969; Sandberg et al., 1987; Bialek and Cohen, 1989), and there is also evidence that in some plants (but not all; see Bialek et al., 1992), the young seedling is entirely dependent on the release of free IAA from conjugated pools until the plant itself is capable of de novo synthesis (Epstein et al., 1980; Sandberg et al., 1987).The function of conjugated IAA during vegetative growth is somewhat less clear. It has been shown that conjugated IAA constitutes as much as 90% of the total IAA in the plant during vegetative growth (Normanly, 1997). However, the role of the IAA conjugates at this stage of the plant''s life cycle remains unknown. Analysis of endogenous IAA conjugates in vegetative tissues has revealed the presence of a variety of different compounds, including indole-3-acetyl-inositol, indole-3-acetyl-Ala, IAAsp, and IAGlu (Anderson and Sandberg, 1982; Cohen and Baldi, 1983; Chisnell, 1984; Cohen and Ernstsen, 1991; Östin et al., 1992). Studies of vegetative tissues have indicated that IAAsp, one of the major conjugates in many plants, is the first intermediate in an irreversible deactivation pathway (Tsurumi and Wada, 1986; Tuominen et al., 1994; Östin, 1995). Another mechanism that is believed to be involved in the homeostatic control of the IAA pool is catabolism by direct oxidation of IAA to OxIAA, which has been shown to occur in several plant species (Reinecke and Bandurski, 1983; Ernstsen et al., 1987).One area in the study of IAA metabolism in which our knowledge is increasing is the analysis of the homeostatic controls of IAA levels in plants. It has been possible, for instance, to increase the levels of IAA in transgenic plants expressing iaaM and iaaH genes from Agrobacterium tumefaciens. Analysis of these transgenic plants has indicated that plants have several pathways that can compensate for the increased production of IAA (Klee et al., 1987; Sitbon, 1992). It is expected that future studies using now-available genes will provide further insight into IAA metabolism. For example, a gene in maize encoding IAA-Glc synthetase has been identified, and several genes (including ILR1, which may be involved in hydrolysis of the indole-3-acetyl-Leu conjugate) have been cloned from Arabidopsis (Szerszen et al., 1994; Bartel and Fink, 1995). Furthermore, Chou et al. (1996) identified a gene that hydrolyzes the conjugate IAAsp to free IAA in the bacterium Enterobacter aggloremans.Because of its small genome size, rapid life cycle, and the ease of obtaining mutants, Arabidopsis is increasingly used as a genetic model system to investigate various aspects of plant growth and development. IAA signal transduction is also being investigated intensively in Arabidopsis in many laboratories (Leyser, 1997). Mutants with altered responses to externally added auxins or IAA conjugates have been identified in Arabidopsis. The identified mutants are either signal transduction mutants such as axr1-4 (Lincoln et al., 1990), or have mutations in genes involved in auxin uptake or transport, such as aux1 and pin1 (Okada et al., 1991; Bennett et al., 1996). A few mutants that are unable to regulate IAA levels or are unable to hydrolyze IAA conjugates, sur1-2 and ilr1, respectively, have also been identified (Bartel and Fink, 1995; Boerjan et al., 1995). To our knowledge, no mutant that is auxotrophic for IAA has been identified to date, which may reflect the redundancy in IAA biosynthetic pathways or the lethality of such mutants.In spite of the work reported thus far, many aspects of the metabolism of IAA in Arabidopsis require further investigation, because few details of the processes involved in IAA regulation are known. This lack of knowledge puts severe constraints on genetic analysis of IAA metabolism in Arabidopsis. For example, it is essential to have prior knowledge of IAA metabolism to devise novel and relevant screens with which to identify mutants of IAA metabolism. We have sought to address this issue by identifying the metabolic pathways involved in catabolism and conjugation under conditions that minimally perturb physiological processes. In this investigation we studied the conjugation and catabolic pattern of IAA by supplying relatively low levels of labeled IAA and identifying the catabolites and conjugates by MS. Different feeding systems were tested to optimize the application of IAA and to avoid irregularities in metabolism attributable to culturing, feeding conditions, or microbial activity. It is well documented that IAA metabolism is altered according to the amount of exogenous auxin applied; therefore, we placed special emphasis on distinguishing between catabolic routes that occur at near-physiological concentrations and those that occur at the high auxin concentrations commonly used in mutant screens.     

Oxidation of Indole-3-Acetic Acid to Oxindole-3-Acetic Acid by an Enzyme Preparation from Zea mays

Indole-3-acetic acid is oxidized to oxindole-3-acetic acid by Zea mays tissue extracts. Shoot, root, and endosperm tissues have enzyme activities of 1 to 10 picomoles per hour per milligram protein. The enzyme is heat labile, is soluble, and requires oxygen for activity. Cofactors of mixed function oxygenase, peroxidase, and intermolecular dioxygenase are not stimulatory to enzymic activity. A heat-stable, detergent-extractable component from corn enhances enzyme activity 6- to 10-fold. This is the first demonstration of the in vitro enzymic oxidation of indole-3-acetic acid to oxindole-3-acetic acid in higher plants.     

Effect of 2,4-Dinitrophenol on Auxin-induced Ethylene Production and Auxin Conjugation by Mung Bean Tissue

Auxin-induced ethylene production by mung bean (Phaseolus mungo L.) hypocotyl segments was markedly inhibited by 2,4-dinitrophenol regardless of whether or not kinetin was present. Uptake of indoleacetic acid-2-¹⁴C was also inhibited in the presence of 2,4-dinitrophenol. Segments treated only with indoleacetic acid rapidly converted indoleacetic acid into indole-3-acetylaspartic acid with time whereas kinetin suppressed indoleacetic acid conjugation. Formation of indole-3-acetylaspartic acid was significantly reduced when 2,4-dinitrophenol was present. The suppression of indoleacetic acid conjugation by kinetin and 2,4-dinitrophenol appeared to be additive, and the free indoleacetic acid level in segments treated with 2,4-dinitrophenol in the presence of indoleacetic acid or indoleacetic acid plus kinetin was remarkably higher than in corresponding segments which received no 2,4-dinitrophenol.     

The Relationship between Levels of Endogenous Gibberellins and the Response of Vigna Hypocotyls to Exogenous Indole-3-Acetic Acid

Segments excised from the upper and the lower parts of cowpea(Vigna unguiculata L.) hypocotyls were compared in terms oftheir responses to exogenous indole-3-acetic acid (IAA) in relationto their endogenous levels of gibberellin. Growth of the segmentswas measured continuously during xylem perfusion with a lineardifferential transformer. IAA induced a burst of elongationin the upper segments but only slight promotion of growth inthe lower segments. Treatment with uniconazole, a potent inhibitorof the biosynthesis of gibberellins, reduced the responsivenessof the upper segments to exogenous IAA to about one half ofthe control value. Pre-perfusion with GA₃ of such segments fortwo hours prior to application of IAA, partially restored theresponsiveness to IAA. Analysis by GC/MS identified GA₁, GA₄,GA₉ GA₂₀ and GA₅₁ as native gibberellins in the hypocotyls ofcowpea seedlings. Analysis by GC/SIM also showed that the physiologicallyactive gibberellins (GA₁ and GA₄) were located mainly in theupper part of the hypocotyl and the treatment with uniconazolemarketly reduced the endogenous level of gibberellins thereto less than 11% of the control level. These results suggestthat levels of endogenous gibberellins possibly control theresponse to IAA in these segments. (Received May 12, 1994; Accepted November 15, 1994)     

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.     

Role of Brassinosteroids, Ethylene, Abscisic Acid, and Indole-3-Acetic Acid in Mango Fruit Ripening

Rapid ripening of mango fruit limits its distribution to distant markets. To better understand and perhaps manipulate this process, we investigated the role of plant hormones in modulating climacteric ripening of ??Kensington Pride?? mango fruits. Changes in endogenous levels of brassinosteroids (BRs), abscisic acid (ABA), indole-3-acetic acid (IAA), and ethylene and the respiration rate, pulp firmness, and skin color were determined at 2-day intervals during an 8-day ripening period at ambient temperature (21?±?1°C). We also investigated the effects of exogenously applied epibrassinolide (Epi-BL), (+)-cis, trans-abscisic acid (ABA), and an inhibitor of ABA biosynthesis, nordihydroguaiaretic acid (NDGA), on fruit-ripening parameters such as respiration, ethylene production, fruit softening, and color. Climacteric ethylene production and the respiration peak occurred on the fourth day of ripening. Castasterone and brassinolide were present in only trace amounts in fruit pulp throughout the ripening period. However, the exogenous application of Epi-BL (45 and 60?ng?g^?1 FW) advanced the onset of the climacteric peaks of ethylene production and respiration rate by 2 and 1?day, respectively, and accelerated fruit color development and softening during the fruit-ripening period. The endogenous level of ABA rose during the climacteric rise stage on the second day of ripening and peaked on the fourth day of ripening. Exogenous ABA promoted fruit color development and softening during ripening compared with the control and the trend was reversed in NDGA-treated fruit. The endogenous IAA level in the fruit pulp was higher during the preclimacteric minimum stage and declined during the climacteric and postclimacteric stages. We speculate that higher levels of endogenous IAA in fruit pulp during the preclimacteric stage and the accumulation of ABA prior to the climacteric stage might switch on ethylene production that triggers fruit ripening. Whilst exogenous Epi-BL promoted fruit ripening, endogenous measurements suggest that changes in BRs levels are unlikely to modulate mango fruit ripening.     

Enhancement of Indole-3-Acetic Acid Photodegradation by Vitamin B6

Dear Editor, The plant hormone indole-3-acetic acid （IAA） has long been used in plant culture media for practical applications and sci- entific inquiries. The use of IAA is complicated by the fact that IAA is a photo-labile compound. In Murashige and Skoog （MS） plant media （Murashige and Skoog, 1962）, the concen- trations of salts and mineral nutrients are known to hasten the photodegradation of IAA under white light （Dunlap and Robacker, 1988）. This degradation can be virtually eliminated by the use of a yellow-colored light filter that removes UV, violet, and some of the blue wavelengths from the incident light （Stasinopoulos and Hangarter, 1990）. However, the use of yellow light clearly affects the quality of light that the plants under study receive. In addition to applications in plants, IAA has been used in human health applications.     

Identification of Indole-3-Acetic Acid in the Basidiomycete Schizophyllum commune

Indole-3-acetic acid (IAA) was detected in the ether extracts of culture filtrates of indigotin-producing strains of the basidiomycete Schizophyllum commune. Several solvents, known to give distinctly different R_F values for IAA, and 3 location reagents gave identical results with synthetic IAA and IAA found in the extract. Confirmation was obtained by the Avena straight growth test, split pea test, and ultraviolet absorption spectrum.