Maximum Siderophore Production Research Articles

EFFECT OF VARIOUS PHYSICOCHEMICAL PARAMETERS ON SIDEROPHORE PRODUCTION BY MARINE Pseudomonas aeruginosa MGPB31

Siderophores are low molecular weight compounds secreted by bacteria that 
 bind ferric iron with extremely high affinity. The siderophores help bacteria to 
 meet their iron needs under stress conditions as these compounds help in the 
 transport and storage of soluble form of iron in cells. In current study, we report 
 the effect of various physicochemical parameters that influence siderophore 
 production in marine Pseudomonas aeruginosa MGPB31. Siderophore 
 production was assayed using Chrome Azurol S (CAS) shuttle assay. The 
 maximum siderophore production (56% SU) was obtained in minimal M9 
 medium after 72 h incubation, followed by succinate medium (52% SU) and 
 King’s medium (51% SU). The M9 medium was then modified by replacing its 
 carbon, nitrogen and phosphate source. Enhanced siderophore production was 
 observed in modified medium comprising of glycerol (70% SU), tri ammonium 
 citrate (67% SU) and KH2PO4 (64% SU). The medium supplemented with iron 
 exhibited a significant reduction in siderophore production. The effect of 
 physical parameters like temperature, pH and agitation on siderophore 
 production revealed that the isolate exhibited maximum siderophore production 
 under static conditions in modified M9 medium devoid of iron at 25 ± 2°C and 
 neutral pH. The enhanced siderophore production obtained by marine P. 
 aeruginosa MGPB31 indicates its further applicability in fields of industry 
 agriculture and environment.

Investigation of the Siderophore Production and Associated Heavy Metal Accumulation Potential of Brevibacillus laterosporus 301/İK3-2

Siderophores are secondary metabolites released into the environment by various microorganisms, fungi, and plants to chelate iron from the surrounding environment. It is known that siderophores bind to other metals besides iron. Today, heavy metals, which are released as an undesirable result of industrial development, accumulate at high rates and pose a significant threat to biological living things. In this sense, remediation of heavy metal-contaminated sites is an urgent requirement. Siderophores are promising agents for the removal of heavy metals from natural habitats with the role of bioremediation. 
 In this study, the effect of heavy metals on the growth and siderophore production of Brevibacillus laterosporus was investigated. Maximum siderophore production was determined as 50 % at 48 h in the metal-free growth media. In addition, maximum siderophore production was determined for various heavy metals including 5 μM Hg2+, 0.5 mM Ni2+, 0.1 mM Co2+, and 2.5 μM Fe2+. Intracellular uptake of the mercury was also measured using optical emission spectroscopy and compared with siderophore production values of the B. laterosporus. The maximum biosorption of mercury was measured to be 40% in 5 μM Hg2+-containing media at 48 h of incubation. The results show that siderophore production is affected by uptake of various metals, and are usable for removing of heavy metals from environmental habitats.

Investigating the diversity of bacterial endophytes in maize and their plant growth-promoting attributes.

Maize is a widely known crop in cereals for its importance as staple food, and the endophytic bacteria which dwell within the maize tissues may have impact on the yield and quality of crop. In this study, endophytic bacteria associated with maize were characterized based on their plant growth promoting traits and identified at molecular level to provide valuable information for developing a microbial consortium to enhance the productivity of crop. The bacterial strains having multi-trait plant growth promoting attributes were studied quantitatively, and highest phosphate solubilization was shown by the isolate KL3E1 (573µg/mL), while K solubilization has reported maximum of 39mg/L by the isolate PdS3E1, IAA production was found maximum by the isolate LL3E1 (165μg/mL), and maximum siderophore production (93%) was shown by the isolate VaR3E1 and exopolysaccharide production was found highest by LS3E3 (65mg/L). The efficient isolates identified by 16S rRNA gene sequencing were found as Gordonia hongkongensis, Microbacterium hydrothermale, Kosakonia radicincitans, Kosakonia cowanii, Priestia megaterium, Priestia aryabhattai, Klebsiella pneumoniae, Cellulosimicrobium funkei, Bacillus licheniformis, Pantoea dispersa, Pseudomonas aeruginosa, and Methylorubrum populi. The results showed that these multi-trait plant growth promoting endophytic bacterial isolates could be used as bio inoculants for sustainable improvement of maize crop productivity.

Plant growth promoting endophytic Pseudomonas aeruginosa RD10 isolated from water hyacinth for phenol degradation

A total of twelve endophytic bacteria were isolated from the root of E. crassipes. All these bacterial strains were analyzed for phenol degradation potential. Twelve endophytic bacteria were subjected to polyphenol resistance analysis and all showed phenol tolerance up to 300 ppm. Among the twelve bacterial strains, the strain RD10 was resistant up to 800 ppm, and reduced growth were observed at higher concentrations. The plant growth-promoting properties of endophytic bacterial strains were analyzed. Among the isolated bacterial strain, only five strains produced ACC deaminase. IAA production was detected in seven bacterial isolates, while siderophore production was determined from only five bacterial strains. Among the isolated endophytes, the strain RD10 exhibited ACC deaminase, IAA, and siderophore activity in submerged fermentation. ACC deaminase production of 125.2 ± 4.8 m/ml was shown by strain RD10 and IAA production was 89.7 ± 5.1 μg/ml. Moreover, maximum siderophore production was observed (53.9 ± 4.9% siderophore units) in strain RD3. Catechol 2,3-dioxygenase activity of the bacterial strains was analyzed. Catechol 2,3 dioxygenase activity ranges from 0.03 ± 0.01 U/mg to 35.2 ± 1.1 U/mg. Phenol degradation by the isolated endophytes was studied and the initial phenol concentration was 500 ppm. The strain RD1 degraded 1.2 ± 0.2% phenol and more than 50% phenol degradation was observed in strains RD3, RD4, RD6, RD7, RD9, RD10, and RD11, respectively. The endophytic bacterial strain was Gram-negative, rodshaped and motile. It was non-spore-forming bacteria, capsulated and, showed a positive reaction to oxidase and catalase. In MacConkey agar medium, strain RD10 forms smooth and flat colonies, and the diameter of the colonies ranged from 2 to 3 mm. The colonies have an alligator skin-like appearance on top view and have regular margins. The strain RD10 was streaked on blood agar medium and the strain RD10 produced strong pigmentation on blood agar. Antibiotic sensitivity of strain RD10 against various antibiotics in terms of zone of inhibition was analyzed. The endophytic strain RD10 was sensitive to all selected antibiotics. Based on biochemical characters, pigment analysis and, molecular characterization, the strain was confirmed as P. aeruginosa RD10. In this study, P. aeruginosa RD10 was cultured at 400, 500 and, 600 mg/l phenol concentration for 96 h. Optimum growth was achieved at 500 mg/l concentration and an inhibitory effect was observed at 600 mg/l phenol concentration. The results confirmed that the selected strain can withstand and grow well at 500 mg/l phenol concentration. The strain RD10 was cultured at various incubation temperatures with 500 mg/l phenol concentration. Optimization of environmental factors can lead to maximum degradation of phenol. The results revealed that phenol degradation was influenced by temperature. The highest phenol degradation was achieved at 30 °C in Erlenmeyer flask culture and decreased at high temperatures. P. aeruginosa RD10 could degrade phenol at pH 5.5 (10.2 ± 0.5% removal), at pH 6.0, 15.4 ± 1.1% of the phenol was degraded after incubation for 72 h at 30 °C. Phenol removal (%) declined to 70.4 ± 2.9% at higher initial pH value (9.0).

Impact of Pseudomonas spp. on Plant Growth, Lytic Enzymes and Secondary Metabolites Production

Seven strains of Pseudomonas spp. were isolated from the south Gujarat region of India. Antifungal and bacterial activities of bacterial strains were evaluated against important plant pathogens in vitro, among them, PaRS was found most effective. The indole acetic acid production was recorded in all isolated Pseudomonas spp. Seed treatment with PaRS at 6 g/kg was significantly superior over other treatments for plant growth promotion (germination, shoot and root length, shoot and root weight, vigor index, and both shoot and root colonization) under standard roll towel method and pot conditions. The maximum siderophore production was observed in PaRS and medium production in PfRB, PaNS, and PfNC. PaNS and PaRS strains recorded strong HCN production but moderate production recorded in PaWP, PaWS, and PfNC. The maximum phosphate solubalization zone (22 mm) was found in PaRS. PaRS recorded maximum chitinase, ß-1,3-glucanase activity, hydrogen cyanide, and salicylic acid production as compared to other strains.

Application of Response Surface Methodology for Optimization of Siderophore Production

Background: In the present study a statistical model (Response Surface Methodology) was proposed for optimization of siderophore production by using Enterobacter hormaechei.Methods: The rhizospheric soil was used for isolation and isolates were screened for siderophore production by chrome-azurol S (CAS) assay. One potent isolate producing maximum siderophore was selected and characterized by 16S rRNA gene sequencing. The culture conditions were optimized for maximum siderophore production by using Central Composite Design. The response surface curves were used to predict the optimum levels of the factors affecting the yield of siderophore. Result: By using rhizospheric soil,eight isolates were obtained and one potent organism was identified as Enterobacter hormaechei subsp. oharae (Accession No. MT 775835) by BLAST. The maximum siderophore production (98%) was obtained in succinate medium and the optimum values of variables were found as pH 7, time 60 hrs, temp. 28°C and succinic acid conc. 0.40%. RSM was used to analyze the data by developing 3D surface plots and the residuals plots. ANOVA was used to determine regression coefficients.

Flux Balance Analysis for Media Optimization and Genetic Targets to Improve Heterologous Siderophore Production.

SummarySiderophores are small molecule metal chelators secreted in sparse quantities by their native microbial hosts but can be engineered for enhanced production from heterologous hosts like Escherichia coli. These molecules have been proved to be capable of binding heavy metals of commercial and/or environmental interest. In this work, we incorporated, as needed, the appropriate pathways required to produce several siderophores (anguibactin, vibriobactin, bacillibactin, pyoverdine, and enterobactin) into the base E. coli K-12 MG1655 metabolic network model to computationally predict, via flux balance analysis methodologies, gene knockout targets, gene over-expression targets, and media modifications capable of improving siderophore reaction flux. E. coli metabolism proved supportive for the underlying production mechanisms of various siderophores. Within such a framework, the gene deletion and over-expression targets identified, coupled with complementary insights from medium optimization predictions, portend experimental implementation to both enable and improve heterologous siderophore production. Successful production of siderophores would then spur novel metal-binding applications.

Zinc Ions Affect Siderophore Production by Fungi Isolated from the Panax ginseng Rhizosphere.

Although siderophore compounds are mainly biosynthesized as a response to iron deficiency in the environment, they also bind with other metals. A few studies have been conducted on the impact of heavy metals on the siderophore-mediated iron uptake by microbiome. Here, we investigated siderophore production by a variety of rhizosphere fungi under different concentrations of Zn²⁺ ion. These strains were specifically isolated from the rhizosphere of Panax ginseng (Korean ginseng). The siderophore production of isolated fungi was investigated with chrome azurol S (CAS) assay liquid media amended with different concentrations of Zn²⁺ (50 to 250 μg/ml). The percentage of siderophore units was quantified using the ultra-violet (UV) irradiation method. The results indicated that high concentrations of Zn²⁺ ion increase the production of siderophore in iron-limited cultures. Maximum siderophore production by the fungal strains was detected at Zn²⁺ ion concentration of 150 μg/ml except for Mortierella sp., which had the highest siderophore production at 200 μg/ml. One potent siderophore-producing strain (Penicillium sp. JJHO) was strongly influenced by the presence of Zn²⁺ ions and showed high identity to P. commune (100% using 18S-rRNA sequencing). The purified siderophores of the Penicillium sp. JJHO strain were chemically identified using UV, Fourier-transform infrared spectroscopy (FTIR), and matrix-assisted laser desorption/ionization time-of-flight mass spectrometer (MALDI-TOF-MS) spectra.

Optimization of Siderophore Production by Bacillus sp. PZ-1 and Its Potential Enhancement of Phytoextration of Pb from Soil.

In this study, the siderophore-producing characteristics and conditions of Bacillus sp. PZ-1 were investigated and the enhancement of siderophores on Pb uptake and translocation in Brassica juncea were determined. Results of single factor experiment showed that glucose, pH, and Pb(NO3)2 could stimulate PZ-1 growth and siderophore production. The maximum siderophore production of 90.52% siderophore units was obtained by response surface methodology optimization at the glucose concentration of 21.84 g/l, pH 6.18, and Pb(NO3)2 concentration of 245.04 μmol/l. The type of siderophore was hydroxamate and its concentration in the fermentation broth amounted to 32.24 μg/ml. Results of pot experiments indicated that the siderophores enhanced B. juncea to assimilate more Pb from soil with the uptake ratio from 1.04 to 2.74, and to translocate more Pb from underground to overground with the TF values from 1.21 to 1.48. The results revealed that Bacillus sp. PZ-1 could produce abundant siderophores and might be potentially used to augment the phytoextraction of Pb from soil.

Siderophore Producing Aspergillus spp as Bioinoculant for Enhanced Growth of Mung Bean

Two fungal species A. niger and A. parasiticus isolated from agriculture field were used in the present study. These two fungal isolates were studied for the siderophore production on the modified CAS agar medium. On daily observation it was observed that the growth was increased along with the production indicating the change in colour of CAS dye from blue to purplish red by A. niger and Yellow by A. parasiticus . Quantitative determination of siderophore production was further studied using liquid medium. After 10 days of incubation A. niger and A. parasiticus produce 80% and 73% units of siderophore. Chemical determination showed that both the fungal isolates showed hydroxamate type of siderophore. Optimization of type of inoculum showed that block inoculum produced maximum siderophore compared to loop and monosporic inoculation in both the fungal isolates. Neutral pH 7.0 showed maximum siderophore production in both the isolates. Decrease in iron concentration increases the siderophore production by the fungal isolates. Maximum siderophore production 80% and 74% units was observed in A. niger and A. parasiticus respectively when there was no addition of iron in the medium. Pot studies were carried out to observe the effect of fungal inoculum on mungbean plant. Inoculation with fungal isolates showed increase in number of leaves, number of branches, number of lateral roots, plant height, fresh weight and dry weight of plant compared to control. Thus both the fungal isolates proved to be potential bioinoculant for improving the growth of mungbean plant.

Siderophore production by Pseudomonas aeruginosa FP6, a biocontrol strain for Rhizoctonia solani and Colletotrichum gloeosporioides causing diseases in chilli

Siderophores are compounds secreted under low iron stress, which act as specific ferric iron chelating agents. Owing to their potential in the biological control of fungal phytopathogens, they may be used as an alternative strategy to chemical control. Pseudomonas aeruginosa FP6, previously isolated from rhizospheric soil samples was screened for its siderophore production on a chrome-azurol S agar plate. Change in the colour of the chrome-azurol S agar from blue to orange red confirmed the siderophore producing ability of P. aeruginosa FP6. The effects of various physicochemical parameters on siderophore production were studied. The maximum siderophore production was obtained in succinate medium (125 μM) followed by King's B medium (105 μM). The presence of sucrose and mannitol increased the siderophore production. Yeast extract proved to be the most suitable nitrogen source. Media supplemented with Pb2+, Mn2+ and Mg2+ showed appreciable siderophore production as well as growth of cultures. An increase in the iron concentration favoured growth but substantially reduced siderophore production. The strain when tested for its in-vitro antagonistic activity against Rhizoctonia solani and Colletotrichum gloeosporioides on King's B media, with and without FeCl3, showed a significant reduction in R. solani growth with FeCl3 supplementation compared to the control (without FeCl3), suggesting the role of siderophore mediated antagonism of R. solani. Antifungal activity was not influenced by FeCl3 in the case of C. gloeosporioides, suggesting the presence of other antagonistic mechanisms.

Evaluation of Siderophore Production and Antimicrobial Activity by Fluorescent Pseudomonas Diversity Associated with Rhizosphere of Apple and Pear

Replant problem is very serious problem which suppresses growth and yield of apple and pear in all major fruit growing areas of the world. Fluorescent Pseudomonas has potential to synthesize different secondary metabolites with diverse PGPR activities which enhance soil fertility and promote plant growth. Under this study, twenty-six Pseudomonas strains were isolated from the rhizosphere of apple and pear plants from their normal and replant sites. All the isolates were positive for catalase, oxidase, denitrification test, ledthinase test and tween 80 hydrolysis. They were further screened for antifungal and siderophore production. Maximum siderophore production was recorded in isolates AN-l-UHF, AN-3-UHF, PN-7-UHF and PN-13-UHF whereas, maximum antifungal activity against Dematophora sp., Fusarium oxysporum, Alternaria sp. and Pythium sp. was recorded in isolates AN-l-UHF, AN-3-UHF and PN-13-UHF. It was found that Pseudomonas sp. isolated from the replant sites showed comparatively less siderophore and antifungal activities. These potential isolates could be further used as biocontrol agents against various fungal pathogens in apple and pear orchards.

Exploring plant growth promoting potential of non rhizobial root nodules endophytes of Vigna radiata

Plant growth promoting (PGP) rhizobacteria exert beneficial effects and may establish as endophytes in their hosts. Here, plant growth promoting traits of 26 non rhizobial and one fungal endophyte previously isolated from Vigna radiata root-nodules were assessed for IAA and siderophore production, phosphate solubilization and hydrolytic enzymes production. Most bacterial endophytes improved seedling vigor index while fungal endophyte (Macrophomina phaseolina) lacked all PGP traits. Endophytes Ml, M10 and M15 were most influential in improving Seedling Vigor Index. Three endophytes having multiple PGP traits with maximum siderophore production: 46.77 μg mL−1 (Bacillus anthracis; Ml), IAA production: 10.81 μg mL−1 (Paenibacillus taichungensis; M10) and phosphate solubilization: 134.483 μg mL−1 (Paenibacillus xylanilyticus; M15) significantly increased root length (RL), shoot length (SL), number of lateral roots (NLR) and plant dry weight (DW) when inoculated/co inoculated with E. adhaerens (native rhizobia) to V. radiata in a small field trial. M10 inoculation produced longest RL while Ml when coinoculated with E. adhaerens produced highest SL and NLR. Ml inoculation or coinoculation was most effective in improving dry weight of mature plants. Most of the endophytes coinoculated with E. adhaerens improved growth parameters. We report that non rhizobial endophytes with PGP traits in combination with native rhizobia can be prospective candidates for use as biofertilizer.

Efficacy of indigenous plant growth promoting rhizobacteria on capsicum yield and soil health

Screening of plant growth promoting rhizobacteria (PGPR) was carried out by isolating 56 bacterial isolates from rhizosphere and roots of capsicum. The isolates were grouped on morphological similarities (RS1 to RS10), characterized biochemically and evaluated for various plant growth promoting (PGP) traits. Among all, six best isolates (RS2, RS3, RS4, RS7, RS8 and RS10) were evaluated for their effect on seed germination and seedling growth of capsicum under controlled conditions. Out of six, four best performers (RS2, RS3, RS4 and RS7) were evaluated for their effect on growth and yield of capsicum under net house conditions. Among which RS2 isolate possessed maximum P-solubilization (80.0%) and auxin production (29.67 μg/ml). However, RS7 isolate possessed maximum siderophore production (83.3%) and antagonism against Pythium spp. (41.7%), Phytophthora spp. (42.5%) and Colletotrichum spp. (40.1%). Seed bacterization with RS7 isolate (Bacillus sp.) showed maximum increase in shoot length (48.0%), shoot biomass (43.7%), root length (50.0%), root biomass (42.1%), yields (33.7%) and nutrient uptake N (40.0%), P (36.8%) and K (23.6%) over control which was statistically at par with treatment receiving RS2 isolate (Bacillus subtilis). Hence, the two bacterial isolates (RS2 and RS7) were strong to be used as biofertilizer and biocontrol agent for capsicum under mid-hill conditions of northwestern Himalayan region.

Optimization of MM9 Medium Constituents for Enhancement of Siderophoregenesis in Marine Pseudomonas putida Using Response Surface Methodology

Pseudomonas putida (CMMB2) was isolated from open ocean water of Gulf of Mannar. The isolate was identified based on 16S rRNA gene sequencing and phylogenetic analysis. Chrome azurol sulphonate assay confirms siderophore production by the isolate. Nature of siderophore produced by the isolate was found to be of mixed type. Siderophore production was found to be inversely proportional to iron concentration of the medium. Maximum siderophore production was observed with MM9 medium. Siderophore production was found to be influenced by different carbon, nitrogen and amino acid sources. Optimization of MM9 medium nutrient composition by response surface methodology (RSM) enhances siderophore production. Application of RSM is one of the strategic attempts in cost effective siderophore production process. Presence of aromatic ring in the siderophore with (C-O) and (C=C) stretching was ascertained by FTIR spectral analysis. Mass spectral analysis revealed the presence of chromophore in the pyoverdine siderophore. Cell free supernatant and purified siderophore was found to inhibit the growth of bacterial and fungal pathogens.

Fluorescent Pseudomonads as Plant Growth Promoting Rhizobacteria and Their Siderophoregenesis

Two fluorescent Pseudomonads, Pseudomonas fluorescens NCIM 5164 and Pseudomonas aeruginosa NCIM 2036 produced siderophores under iron limiting conditions. These Pseudomonas spps. were screened for siderophore production by three different tests commonly used to detect siderophores. Quantitative estimation of siderophores was done by the Chrome Azurol S assay (CAS), the universal test independent of siderophore type. P. fluorescens and P. aeruginosa produced 88% and 83% siderophore units respectively. The type of siderophore was detected by chemical as well as spectrophotometric assay. Pseudomonas fluorescens was found to produce hydroxamate type of siderophore and Pseudomonas aeruginosa produced both the hydroxamate (Pyoverdin) and phenol-catecholate (Pyochelin) siderophores. Evaluation study of siderophore showed that maximum siderophore production was obtained on succinate medium at pH 7.0. Both the Pseudomonas spps. were further tested as seed inoculants and found to be very effective in seed germination and plant growth promotion of Triticum aestivum and Apios americana plants under pot culture conditions. KeyWords - CAS, Catecholate, Hydroxamate, Pyoverdin, Pyochelin, Siderophores.

Chemical and structural characterization of hydroxamate siderophore produced by marine Vibrio harveyi

In the present study, 22 different bacteria were isolated from open ocean water from the Gulf of Mannar, India. Of the 22 isolates, 4 were identified as Vibrio spp. (VM1, VM2, VM3 and VM4) and found to produce siderophores (iron-binding chelators) under iron-limited conditions. Different media were found to have an influence on siderophore production. Maximum siderophore production was observed with VM1 isolate in MM9 salts medium at 48h of incubation. The isolate was confirmed as Vibrio harveyi based on 16S rRNA gene sequencing and phylogenetic analysis. Fourier-transform infrared (FTIR) and (1)H nuclear magnetic resonance (NMR) spectra revealed the hydroxamate nature of the siderophore produced. Further characterization of the siderophore revealed it to be of dihydroxamate nature, forming hexadentate ligands with Fe(III) ions. A narrow shift in ultraviolet (UV)-Vis spectrum was observed on photolysis due to ligand oxidation. Growth-promotion bioassay with Aeromonas hydrophila, Staphylococcus aureus and E.coli confirmed the iron-scavenging property of the siderophore produced by Vibrio harveyi.

Isolation and characterization of endophytic bacteria from endorhizosphere of sugarcane and ryegrass

The present study was conducted with a view to isolate and characterize endophytic bacterial diversity from endorhizosphere of sugarcane (Saccharum sp.) and rye grass (Lolium perenne). Eight bacterial isolates from sugarcane and seven from rye grass were obtained and identified as Azospirillum, Bacillus, E. coli and Pseudomonas. S1 and R1 (Azospirillum) sugarcane and ryegrass isolate exhibited maximum nitrogenase activity of 1221.0 and 1168.3 nM C 2H 4h -1 mg -1 protein respectively while the maximum IAA production was recorded in S5 (E. coli) and R7 (Bacillus) i.e. 19.3 and 20.0 mg ml -1 respectively. The maximum siderophore production was observed in S6 (E.coli) and R6 (Pseudomonas) i.e. 2.4 and 3.0 mg ml -1 respectively. Only S5 (E.coli) was observed to be solubilize phosphate (21 mg P). The endorhizosphere of sugarcane and rye grass exhibited endophytic bacterial diversity not only in terms of different types of isolates but also in terms of functional diversity.

Combat of iron-deprivation through a plant growth promoting fluorescent Pseudomonas strain GRP3A in mung bean ( Vigna radiata L. Wilzeck)

Microorganisms and plants sustain themselves under iron-deprived conditions by releasing siderophores. Among others, fluorescent pseudomonads are known to exert extensive biocontrol action against soil and root borne phytopathogens through release of antimicrobials and siderophores. In this study, production and regulation of siderophores by fluorescent Pseudomonas strain GRP3A was studied. Among various media tested, standard succinate medium (SSM) promoted maximum siderophore production of 56.59 mg l(-1). There were low levels of siderophore in complex media like King's B medium, trypticase soya medium and nutrient medium (41.27, 29.86 and 27.63 mg l(-1)), respectively. In defferrated SSM, siderophore level was quantified to be 68.74 mg l(-1). Supplementation with iron (FeCl3) resulted in decreased siderophore levels depending on concentration. Siderophore production was promoted by Zn2+ (78.94 mg l(-1)), Cu2+ (68.80 mg l(-1)) whereas Co2+ (57.33 mg l(-1)) and Fe3+ reduced siderophore production (37.44 mg l(-1) as compared to control (55.97 mg l(-1)). Strain GRP3A showed plant growth promotion under iron limited conditions.

Hydroxamate siderophore synthesis by Phialocephala fortinii, a typical dark septate fungal root endophyte.

The siderophore production of various isolates of Phialocephala fortinii was assessed quantitatively as well as qualitatively in batch assays under pure culture conditions at different pH values and iron(III) concentrations. We found a distinct effect of both of these parameters on siderophore synthesis and as well as on fungal growth. In comparative analyses of two of the isolates, maximum siderophore production was found at a pH in the range of pH 4.0 to 4.5 while, under the experimental conditions employed, the optimal concentration of ferric iron was determined to be between 20-40 microg iron (III) l(-1) (0.36-0.72 microM, respectively). HPLC analysis of the culture filtrate of most of the isolates of P. fortinii revealed the excretion of ferricrocin as main hydroxamate siderophore, followed by ferrirubin and ferrichrome C. The pattern of release of these three substances proved to be dependent on pH and iron(III) concentration of the culture medium, and to be specific for each isolate under investigation.