Perspective on the biotechnological production of bacterial siderophores and their use

Unlocking the genomic potential of Red Sea coral probiotics

The application of beneficial microorganisms for corals (BMC) decreases the bleaching susceptibility and mortality rate of corals. BMC selection is typically performed via molecular and biochemical assays, followed by genomic screening for BMC traits. Herein, we present a comprehensive in silico framework to explore a set of six putative BMC strains. We extracted high-quality DNA from coral samples collected from the Red Sea and performed PacBio sequencing. We identified BMC traits and mechanisms associated with each strain as well as proposed new traits and mechanisms, such as chemotaxis and the presence of phages and bioactive secondary metabolites. The presence of prophages in two of the six studied BMC strains suggests their possible distribution within beneficial bacteria. We also detected various secondary metabolites, such as terpenes, ectoines, lanthipeptides, and lasso peptides. These metabolites possess antimicrobial, antifungal, antiviral, anti-inflammatory, and antioxidant activities and play key roles in coral health by reducing the effects of heat stress, high salinity, reactive oxygen species, and radiation. Corals are currently facing unprecedented challenges, and our revised framework can help select more efficient BMC for use in studies on coral microbiome rehabilitation, coral resilience, and coral restoration.

Purification and Characterization of Desferrioxamine B of Pseudomonas fluorescens and Its Application to Improve Oil Content, Nutrient Uptake, and Plant Growth in Peanuts

Microorganisms produce siderophores, which are low-molecular-weight iron chelators when iron availability is limited. The present analyzed the role of LNPF1 as multifarious PGPR for improving growth parameters and nutrient content in peanut and soil nutrients. Such multifarious PGPR strains can be used as effective bioinoculants for peanut farming. In this work, rhizosphere bacteria from Zea mays and Arachis hypogaea plants in the Salem area of Tamil Nadu, India, were isolated and tested for biochemical attributes and characteristics that stimulate plant growth, such as the production of hydrogen cyanide, ammonia (6 µg/mL), indole acetic acid (76.35 µg/mL), and solubilizing phosphate (520 µg/mL). The 16S rRNA gene sequences identified the isolate LNPF1 as Pseudomonas fluorescens with a similarity percentage of 99% with Pseudomonas sp. Isolate LNPF1 was evaluated for the production of siderophore. Siderophore-rich supernatant using a Sep Pack C18 column and Amberlite-400 Resin Column (λmax 264) produced 298 mg/L and 50 mg/L of siderophore, respectively. The characterization of purified siderophore by TLC, HPLC, FTIR, and 2D-NMR analysis identified the compound as desferrioxamine, a hydroxamate siderophore. A pot culture experiment determined the potential of LNPF1 to improve iron and oil content and photosynthetic pigments in Arachis hypogaea L. and improve soil nutrient content. Inoculation of A. hypogea seeds with LNPF1 improved plant growth parameters such as leaf length (60%), shoot length (22%), root length (54.68%), fresh weight (47.28%), dry weight (37%), and number of nuts (66.66) compared to the control (untreated seeds). This inoculation also improved leaf iron content (43.42), short iron content (38.38%), seed iron (46.72%), seed oil (31.68%), carotenoid (64.40%), and total chlorophyll content (98.%) compared to control (untreated seeds). Bacterized seeds showed a substantial increase in nodulation (61.65%) and weight of individual nodules (95.97) vis-à-vis control. The results of the present study indicated that P. fluorescens might be utilized as a potential bioinoculant to improve growth, iron content, oil content, number of nuts and nodules of Arachishypogaea L., and enrich soil nutrients.

Evaluation of Bacillus isolates as a biological control agents against soilborne phytopathogenic fungi

Pesticides, used in agriculture to control plant diseases, pose risks to the environment and human health. To address this, there's a growing focus on biocontrol, using microorganisms instead of chemicals. In this study, we aimed to identify Bacillus isolates as potential biological control agents. We tested 1574 Bacillus isolates for antifungal effects against pathogens like Botrytis cinerea, Fusarium solani, and Rhizoctonia solani. Out of these, 77 isolates formed inhibition zones against all three pathogens. We then investigated their lytic enzyme activities (protease, chitinase, and chitosanase) and the production of antifungal metabolites (siderophore and hydrogen cyanide). Coagulase activity was also examined to estimate potential pathogenicity in humans and animals. After evaluating all mechanisms, 19 non-pathogenic Bacillus isolates with significant antifungal effects were chosen. Molecular identification revealed they belonged to B. subtilis (n = 19) strains. The 19 native Bacillus strains, demonstrating strong antifungal effects in vitro, have the potential to form the basis for biocontrol product development. This could address challenges in agricultural production, marking a crucial stride toward sustainable agriculture.

Microbiome convergence enables siderophore-secreting-rhizobacteria to improve iron nutrition and yield of peanut intercropped with maize

Intercropping has the potential to improve plant nutrition as well as crop yield. However, the exact mechanism promoting improved nutrient acquisition and the role the rhizosphere microbiome may play in this process remains poorly understood. Here, we use a peanut/maize intercropping system to investigate the role of root-associated microbiota in iron nutrition in these crops, combining microbiome profiling, strain and substance isolation and functional validation. We find that intercropping increases iron nutrition in peanut but not in maize plants and that the microbiota composition changes and converges between the two plants tested in intercropping experiments. We identify a Pseudomonas secreted siderophore, pyoverdine, that improves iron nutrition in glasshouse and field experiments. Our results suggest that the presence of siderophore-secreting Pseudomonas in peanut and maize intercropped plays an important role in iron nutrition. These findings could be used to envision future intercropping practices aiming to improve plant nutrition.

Characterization and Statistical Optimization of Enterobatin Synthesized by Escherichia coli OQ866153

Microorganisms produce siderophores, which are secondary metabolites with a high affinity for iron. Siderophores have received significant attention due to their diverse applications in ecological and clinical research. In this study, siderophores production by Escherichia coli OQ866153 was optimized using two-stage statistical approach involving Plackett-Burman design (PBD) and response surface methodology (RSM) using central composite design (CCD). Out of 23 variables, succinate, tryptophan, Na2HPO4, CaCl2, agitation, and KH2PO4 were found to have the most significant effect on siderophores production in the first optimization stage with the highest SU% of 43.67%. In the second stage, RSM using CCD was utilized, and the optimal conditions were determined to be 0.3 g/l succinate, 0 g/l tryptophan, 6 g/l Na2HPO4, 0.1 g/l CaCl2, 150 RPM agitation, and 0.6 g/l KH2PO4, resulting in a maximum siderophore units (SU%) of 89.13%. The model was significant, as indicated by the model f-value of 314.14 (p-value = 0.0004) and coefficient of determination R2 of 0.9950. During validation experiments, the obtained maximum SU% was increased up to 87.1472%, which was two times as the value obtained under ordinary conditions (46.62%). The produced siderophores were purified and characterized using 1H, 13C NMR, IR spectroscopy. The obtained results indicated that the compound was enterobactin and entABCDEF genes were further detected in Escherichia coli OQ866153 extracted DNA. To our knowledge, this is the first report of statistical optimization for enterobactin synthesis by an E. coli strain isolated from a clinical source in Egypt.

Iron status and root cell morphology of Arabidopsis thaliana as modified by a bacterial ferri-siderophore

We previously provided evidence for the contribution of pyoverdine to the iron nutrition of Arabidopsis. In the present article, we further analyze the mechanisms and physiology of the adaptations underlying plant iron nutrition through Fe(III)-pyoverdine (Fe(III)-pvd). An integrated approach combining microscopy and nanoscale secondary ion mass spectrometry (NanoSIMS) on plant samples was adopted to localize pyoverdine in planta and assess the impact of this siderophore on the plant iron status and root cellular morphology. The results support a possible plant uptake mechanism of the Fe(III)-pvd complex by epidermal root cells via a non-reductive process associated with the presence of more vesicles. Pyoverdine was transported to the central cylinder via the symplastic and/or trans-cellular pathway(s), suggesting a possible root-to-shoot translocation. All these processes led to enhanced plant iron nutrition, as previously shown. Overall, these findings suggest that bacterial siderophores contribute to plant iron uptake and homeostasis.

Optimization of Siderophore Production in Three Marine Bacterial Isolates along with Their Heavy-Metal Chelation and Seed Germination Potential Determination

Siderophores are low-molecular-weight and high-affinity molecules produced by bacteria under iron-limited conditions. Due to the low iron (III) (Fe+3) levels in surface waters in the marine environment, microbes produce a variety of siderophores. In the current study, halophilic bacteria Bacillus taeanensis SMI_1, Enterobacter sp., AABM_9, and Pseudomonas mendocina AMPPS_5 were isolated from marine surface water of Kalinga beach, Bay of Bengal (Visakhapatnam, Andhra Pradesh, India) and were investigated for siderophore production using the Chrome Azurol S (CAS) assay. The effect of various production parameters was also studied. The optimum production of siderophores for SMI_1 was 93.57% siderophore units (SU) (after 48 h of incubation at 30 °C, pH 8, sucrose as carbon source, sodium nitrate as nitrogen source, 0.4% succinic acid), and for AABM_9, it was 87.18 %SU (after 36 h of incubation period at 30 °C, pH 8, in the presence of sucrose, ammonium sulfate, 0.4% succinic acid). The maximum production of siderophores for AMPPS_5 was 91.17 %SU (after 36 h of incubation at 35 °C, pH 8.5, glucose, ammonium sulfate, 0.4% citric acid). The bacterial isolates SMI_1, AABM_9, and AMPPS_5 showed siderophore production at low Fe+3 concentrations of 0.10 µM, 0.01 µM, and 0.01 µM, respectively. The SMI_1 (73.09 %SU) and AMPPS_5 (68.26 %SU) isolates showed siderophore production in the presence of Zn+2 (10 µM), whereas AABM_9 (50.4 %SU) exhibited siderophore production in the presence of Cu+2 (10 µM). Additionally, these bacterial isolates showed better heavy-metal chelation ability and rapid development in seed germination experiments. Based on these results, the isolates of marine-derived bacteria effectively produced the maximum amount of siderophores, which could be employed in a variety of industrial and environmental applications.

Hydrogel capsules as new approach for increasing drying survival of plant biostimulant gram-negative consortium

Several plant growth-promoting bacteria (PGPB) are gram-negative, and their cell viability is affected during the bio-inoculant production. Hence, formulation-drying processes provide challenges that limit the adoption of these beneficial microorganisms in sustainable agricultural production. Among delivery system strategies for gram-negative PGPB, the encapsulating cells in biopolymeric materials are emerging as a promising alternative. This research aims to evaluate the effect of additives and crosslinking agents on the survival of the consortium of Herbaspirillum frisingense AP21, Azospirillum brasilense D7, and Rhizobium leguminosarum T88 in hydrogel capsules. Three crosslinkers and diverse potential drying protectors were tested. Calcium gluconate provides notable consortium survival advantages regarding colony-forming units (CFUs) (losses of up to 4 log CFU) compared to calcium lactate and calcium chloride (up to 6 log CFU). Additives such as skimmed milk, whey protein, and Gelita® EC improve the recovery of viable cells after the drying process, demonstrating an increase in cell survival of the three bacteria by up to 4 log CFU. The combination of these substances into a capsule prototype extends the storage stability of bacterial consortium up to 3 months at 18 ± 2 °C. This study expands the knowledge for formulating gram-negative PGPB consortium, regarding the crosslinker and drying protector relationship on encapsulation processes with drying survival and further storage stability performance. KEY POINTS: • Hydrogel immobilization formulation approach for PGPB consortium • Enhancing drying survival of gram-negative PGPB consortium • Increasing storage stability of PGPB consortium at 18 °C.

Plant growth promoting activities of Pseudomonas sp. and Enterobacter sp. isolated from the rhizosphere of Vachellia gummifera in Morocco

The Moroccan endemic Vachellia gummifera grows wild under extreme desert conditions. This plant could be used as an alternative fodder for goats, and camels, in order to protect the Argan forests against overgrazing in Central and Southwestern Moroccan semiarid areas. With the aim to improve the V. gummifera population's density in semiarid areas, we proposed its inoculation with performing plant growth-promoting bacteria. Hence, 500 bacteria were isolated from the plant rhizosphere. From these, 291 isolates were retained for plant growth-promoting (PGP) activities assessment. A total of 44 isolates showed the best phosphates solubilization potential, as well as siderophore and auxin production. The combination of REP-PCR (repetitive extragenic palindromic-polymerase chain reaction) fingerprinting, PGP activities, and phenotypic properties, allowed the selection of three strains for the inoculation experiments. The three selected strains' 16S rRNA sequencing showed that they are members of the Enterobacter and Pseudomonas genera. The inoculation with three strains had diverse effects on V. gummifera growth parameters. All single and combined inoculations improved the plant shoot weight by more than 200%, and the root length by up to 139%, while some combinations further improved protein and chlorophyll content, thereby improving the plant's forage value. The three selected strains constitute an effective inoculum for use in the arid and semiarid zones of southern Morocco.

Importance of microbial amphiphiles: interaction potential of biosurfactants, amyloids, and other exo-polymeric-substances

Microorganisms produce a diverse group of biomolecules having amphipathic nature (amphiphiles). Microbial amphiphiles, including amyloids, bio-surfactants, and other exo-polymeric substances, play a crucial role in various biological processes and have gained significant attention recently. Although diverse in biochemical composition, these amphiphiles have been reported for common microbial traits like biofilm formation and pathogenicity due to their ability to act as surface active agents with active interfacial properties essential for microbes to grow in various niches. This enables microbes to reduce surface tension, emulsification, dispersion, and attachment at the interface. In this report, the ecological importance and biotechnological usage of important amphiphiles have been discussed. The low molecular weight amphiphiles like biosurfactants, siderophores, and peptides showing helical and antimicrobial activities have been extensively reported for their ability to work as quorum-sensing mediators. While high molecular weight amphiphiles make up amyloid fibers, exopolysaccharides, liposomes, or magnetosomes have been shown to have a significant influence in deciding microbial physiology and survival. In this report, we have discussed the functional similarities and biochemical variations of several amphipathic biomolecules produced by microbes, and the present report shows these amphiphiles showing polyphyletic and ecophysiological groups of microorganisms and hence can `be replaced in biotechnological applications depending on the compatibility of the processes.

Suppression of tomato wilt by cell-free supernatants of Acinetobacter baumannii isolates from wild cacao from the Colombian Amazon

Tomato vascular wilt caused by Fusarium oxysporum f. sp. lycopersici (Fol) is one of the most limiting diseases of this crop. The use of fungicides and varieties resistant to the pathogen has not provided adequate control of the disease. In this study, siderophore-producing bacteria isolated from wild cocoa trees from the Colombian Amazon were characterized to identify prominent strategies for plant protection. The isolates were taxonomically classified into five different genera. Eight of the fourteen were identified as bacteria of the Acinetobacter baumannii complex. Isolates CBIO024, CBIO086, CBIO117, CBIO123, and CBIO159 belonging to this complex showed the highest efficiency in siderophore synthesis, producing these molecules in a range of 91-129 µmol/L deferoxamine mesylate equivalents. A reduction in disease severity of up to 45% was obtained when plants were pretreated with CBIO117 siderophore-rich cell-free supernatant (SodSid). Regarding the mechanism of action that caused antagonistic activity against Fol, it was found that plants infected only with Fol and plants pretreated with SodSid CBIO117 and infected with Fol showed higher levels of PR1 and ERF1 gene expression than control plants. In contrast, MYC2 gene expression was not induced by the SodSid CBIO117 application. However, it was upregulated in plants infected with Fol and plants pretreated with SodSid CBIO117 and infected with the pathogen. In addition to the disease suppression exerted by SodSid CBIO117, the results suggest that the mechanism underlying this effect is related to an induction of systemic defense through the salicylic acid, ethylene, and priming defense via the jasmonic acid pathway.

Engineering Siderophore Biosynthesis and Regulation Pathways to Increase Diversity and Availability

Siderophores are small metal chelators synthesized by numerous organisms to access iron. These secondary metabolites are ubiquitously present on Earth, and because their production represents the main strategy to assimilate iron, they play an important role in both positive and negative interactions between organisms. In addition, siderophores are used in biotechnology for diverse applications in medicine, agriculture and the environment. The generation of non-natural siderophore analogs provides a new opportunity to create new-to-nature chelating biomolecules that can offer new properties to expand applications. This review summarizes the main strategies of combinatorial biosynthesis that have been used to generate siderophore analogs. We first provide a brief overview of siderophore biosynthesis, followed by a description of the strategies, namely, precursor-directed biosynthesis, the design of synthetic or heterologous pathways and enzyme engineering, used in siderophore biosynthetic pathways to create diversity. In addition, this review highlights the engineering strategies that have been used to improve the production of siderophores by cells to facilitate their downstream utilization.

Metabolically versatile psychrotolerant Antarctic bacterium Pseudomonas sp. ANT_H12B is an efficient producer of siderophores and accompanying metabolites (SAM) useful for agricultural purposes

BACKGROUND

Bacterial siderophores are chelating compounds with the potential of application in agriculture, due to their plant growth-promoting (PGP) properties, however, high production and purification costs are limiting factors for their wider application. Cost-efficiency of the production could be increased by omitting purification processes, especially since siderophores accompanying metabolites (SAM) often also possess PGP traits. In this study, the metabolism versatility of Pseudomonas sp. ANT_H12B was used for the optimization of siderophores production and the potential of these metabolites and SAM was characterized in the context of PGP properties.

RESULTS

The metabolic diversity of ANT_H12B was examined through genomic analysis and phenotype microarrays. The strain was found to be able to use numerous C, N, P, and S sources, which allowed for the design of novel media suitable for efficient production of siderophores in the form of pyoverdine (223.50-512.60 μM). Moreover, depending on the culture medium, the pH of the siderophores and SAM solutions varied from acidic (pH < 5) to alkaline (pH > 8). In a germination test, siderophores and SAM were shown to have a positive effect on plants, with a significant increase in germination percentage observed in beetroot, pea, and tobacco. The PGP potential of SAM was further elucidated through GC/MS analysis, which revealed other compounds with PGP potential, such as indolic acetic acids, organic acids, fatty acids, sugars and alcohols. These compounds not only improved seed germination but could also potentially be beneficial for plant fitness and soil quality.

CONCLUSIONS

Pseudomonas sp. ANT_H12B was presented as an efficient producer of siderophores and SAM which exhibit PGP potential. It was also shown that omitting downstream processes could not only limit the costs of siderophores production but also improve their agricultural potential.

A Four-Step Platform to Optimize Growth Conditions for High-Yield Production of Siderophores in Cyanobacteria

In response to Iron deprivation and in specific environmental conditions, the cyanobacteria Anabaena flos aquae produce siderophores, iron-chelating molecules that in virtue of their interesting environmental and clinical applications, are recently gaining the interest of the pharmaceutical industry. Yields of siderophore recovery from in vitro producing cyanobacterial cultures are, unfortunately, very low and reach most of the times only analytical quantities. We here propose a four-step experimental pipeline for a rapid and inexpensive identification and optimization of growth parameters influencing, at the transcriptional level, siderophore production in Anabaena flos aquae. The four-steps pipeline consists of: (1) identification of the promoter region of the operon of interest in the genome of Anabaena flos aquae; (2) cloning of the promoter in a recombinant DNA vector, upstream the cDNA coding for the Green Fluorescent Protein (GFP) followed by its stable transformation in Escherichia Coli; (3) identification of the environmental parameters affecting expression of the gene in Escherichia coli and their application to the cultivation of the Anabaena strain; (4) identification of siderophores by the combined use of high-resolution tandem mass spectrometry and molecular networking. This multidisciplinary, sustainable, and green pipeline is amenable to automation and is virtually applicable to any cyanobacteria, or more in general, to any microorganisms.

Insight into the Global Negative Regulation of Iron Scavenger 7-HT Biosynthesis by the SigW/RsiW System in Pseudomonas donghuensis HYS

7-Hydroxytropolone (7-HT) is a unique iron scavenger synthesized by Pseudomonas donghuensis HYS that has various biological activities in addition to functioning as a siderophore. P. donghuensis HYS is more pathogenic than P. aeruginosa toward Caenorhabditis elegans, an observation that is closely linked to the biosynthesis of 7-HT. The nonfluorescent siderophore (nfs) gene cluster is responsible for the orderly biosynthesis of 7-HT and represents a competitive advantage that contributes to the increased survival of P. donghuensis HYS; however, the regulatory mechanisms of 7-HT biosynthesis remain unclear. This study is the first to propose that the ECF σ factor has a regulatory effect on 7-HT biosynthesis. In total, 20 ECF σ factors were identified through genome-wide scanning, and their responses to extracellular ferrous ions were characterized. We found that SigW was both significantly upregulated under high-iron conditions and repressed by an adjacent anti-σ factor. RNA-Seq results suggest that the SigW/RsiW system is involved in iron metabolism and 7-HT biosynthesis. Combined with the siderophore phenotype, we also found that SigW could inhibit siderophore synthesis, and this inhibition can be relieved by RsiW. EMSA assays proved that SigW, when highly expressed, can directly bind to the promoter region of five operons of the nfs cluster to inhibit the transcription of the corresponding genes and consequently suppress 7-HT biosynthesis. In addition, SigW not only directly negatively regulates structural genes related to 7-HT synthesis but also inhibits the transcription of regulatory proteins, including of the Gac/Rsm cascade system. Taken together, our results highlight that the biosynthesis of 7-HT is negatively regulated by SigW and that the SigW/RsiW system is involved in mechanisms for the regulation of iron homeostasis in P. donghuensis HYS. As a result of this work, we identified a novel mechanism for the global negative regulation of 7-HT biosynthesis, complementing our understanding of the function of ECF σ factors in Pseudomonas.

Machine learning-assisted medium optimization revealed the discriminated strategies for improved production of the foreign and native metabolites

The composition of medium components is crucial for achieving the best performance of synthetic construction in genetically engineered cells. Which and how medium components determine the performance, e.g., productivity, remain poorly investigated. To address the questions, a comparative survey with two genetically engineered Escherichia coli strains was performed. As a case study, the strains carried the synthetic pathways for producing the aromatic compounds of 4-aminophenylalanine (4APhe) or tyrosine (Tyr), common in the upstream but differentiated in the downstream metabolism. Bacterial growth and compound production were examined in hundreds of medium combinations that comprised 48 pure chemicals. The resultant data sets linking the medium composition to bacterial growth and production were subjected to machine learning for improved production. Intriguingly, the primary medium components determining the production of 4PheA and Tyr were differentiated, which were the initial resource (glucose) of the synthetic pathway and the inducer (IPTG) of the synthetic construction, respectively. Fine-tuning of the primary component significantly increased the yields of 4APhe and Tyr, indicating that a single component could be crucial for the performance of synthetic construction. Transcriptome analysis observed the local and global changes in gene expression for improved production of 4APhe and Tyr, respectively, revealing divergent metabolic strategies for producing the foreign and native metabolites. The study demonstrated that ML-assisted medium optimization could provide a novel point of view on how to make the synthetic construction meet the designed working principle and achieve the expected biological function.

Establishment and Validation of a New Analysis Strategy for the Study of Plant Endophytic Microorganisms

Amplicon sequencing of bacterial or fungal marker sequences is currently the main method for the study of endophytic microorganisms in plants. However, it cannot obtain all types of microorganisms, including bacteria, fungi, protozoa, etc., in samples, nor compare the relative content between endophytic microorganisms and plants and between different types of endophytes. Therefore, it is necessary to develop a better analysis strategy for endophytic microorganism investigation. In this study, a new analysis strategy was developed to obtain endophytic microbiome information from plant transcriptome data. Results showed that the new strategy can obtain the composition of microbial communities and the relative content between plants and endophytic microorganisms, and between different types of endophytic microorganisms from the plant transcriptome data. Compared with the amplicon sequencing method, more endophytic microorganisms and relative content information can be obtained with the new strategy, which can greatly broaden the research scope and save the experimental cost. Furthermore, the advantages and effectiveness of the new strategy were verified with different analysis of the microbial composition, correlation analysis, inoculant content test, and repeatability test.

Effect of Indole-3-Acetic Acid on Tomato Plant Growth

Plant growth-promoting bacteria have several abilities to promote plant growth and development. One of these skills is the synthesis of indole-3-acetic acid (IAA), which mainly promotes root and shoot development. The bacteria Bacillus subtilis and Azospirillum brasilense have been widely used in agriculture with this function. However, little is known about whether the joint inoculation of these bacteria can reduce plant development by the excess of IAA produced as a result of the joint inoculation. The objective of the present study was to verify the effect of IAA on the inoculation of B. subtilis and A. brasilense in three tomato genotypes. The Micro-Tom genotype without mutation for IAA synthesis, Entire, has high sensitivity to IAA, and the diageotropic genotype (dgt) has low sensitivity to IAA. The results show that the plant parameter most sensitive to microbial inoculation is the number of roots. No treatment increased the shoot dry mass parameters for the Micro-Tom genotype and dgt, root dry mass for the Micro-Tom genotype, plant height for the Micro-Tom and Entire genotypes, root area and root volume for the genotype dgt. The Azm treatment reduced plant height compared to the control in the dgt, the BS + Azw and BS + Azm treatments in the Micro-Tom genotype and the Azw + Azm treatment in the dgt genotype reduced the plant diameter compared to the control. BS and BS + Azw reduced the number of roots in the Micro-Tom. The results strongly support that the mixture of B. subtilis and A. brasilense can reduce some parameters of plant development; however, this effect is possibly an interference in the mode of action of growth promotion of each isolate and is not related to an excess of IAA produced by the bacteria.

Optimization and identification of siderophores produced by Pseudomonas monteilii strain MN759447 and its antagonism toward fungi associated with mortality in Dalbergia sissoo plantation forests

Siderophore-positive bacteria present in the rhizosphere and in bulk soil assist plants by either inhibiting phytopathogen proliferation or increasing plant growth. The bacterial diversity of the Shisham forest ecosystem in the Tarai region of the Western Himalayas was studied and used for siderophore production, taking into account the large-scale dieback and wilt-induced mortality in Dalbergia sissoo (common name: shisham) plantation forests and the importance of soil microbes in tree health. In addition, Pseudomonas, Burkholderia, and Streptomyces were prominent siderophore-positive bacteria in Shisham forests. Pseudomonas species are known for their remarkable siderophore-producing ability. Bacterial siderophores inhibit pathogen growth by rapidly lowering the number of ferric ions in the rhizosphere. The Pseudomonas monteilii strain MN759447 was isolated from a D. sissoo plantation forest at the Agroforestry Research Centre, Pantnagar, Uttarakhand (28°58'N 79°25'E/28.97°N 79.41°E). It produces a significant number of siderophore units (80.36% in total). A two-stage optimization of growth factors was attempted in the strain MN759447 for better siderophore recovery. In the first-stage single-factor experiment, among the five variables studied, only pH, NH₄NO₃ concentration, and Fe concentration affected siderophore synthesis. In the second stage, an optimization of pH, NH₄NO₃ concentration, and Fe concentration for improved growth and enhanced siderophore production was carried out using a Box-Behnken design with response surface methodology. By using LC-MS, two derivatives of pseudomonine, salicylic acid, and kynurenic acid were detected as siderophores in the purified XAD-2 methanol extract of the P. monteilii strain MN759447. In addition to siderophore production, the P. monteilii strain MN759447 also exhibited a broad range of antagonistic activity against Aspergillus calidoustus (65%), Fusarium oxysporum (41.66%), Talaromyces pinophilus (65%), and Talaromyces verruculosus (65.1%) that are linked to sissoo mortality. To our knowledge, this is the first report on siderophore-producing bacteria isolated, identified, and characterized from the D. sissoo Roxb. forest habitat. This strain can also be developed as a commercial product.

Genome mining strategies for metallophore discovery

Many bacteria use small-molecule chelators called metallophores to acquire trace metals from their environment. These molecules play a central role in interactions between bacteria, plants, and animals. Hence, knowing their full diversity is key to combatting infectious diseases as well as harnessing beneficial microbial communities. Metallophore discovery has been streamlined by advances in genome mining, where genomes are scanned for genes involved in metallophore biosynthesis. This review highlights recent trends and advances in predicting the presence and structure of metallophores based solely on genomic information. Recent work suggests new families of metallophores remain hidden from current homology-based approaches. Their discovery will require new genome mining approaches that move beyond biosynthesis to consider metallophore transporters, regulation, and evolution.