Utilization of microbial siderophores in iron acquisition by oat

Deciphering the role of non-Frankia nodular endophytes in alder through in vitro and genomic characterization

Endophytic bacterial populations are well-positioned to provide benefits to their host plants such as nutrient acquisition and plant hormone level manipulation. Actinorhizal plants such as alders are well known for their microbial symbioses that allow them to colonize harsh environments whether natural or anthropized. Although the nitrogen-fixing actinobacterium Frankia sp. is the main endophyte found in alder root nodules, other bacterial genera, whose roles remain poorly defined, inhabit this niche. In this study, we isolated a diverse panel of non-Frankia nodular endophytes (NFNE). Some NFNE were isolated from alders grown from surface-sterilized seeds and maintained in sterile conditions, suggesting these may have been seed-borne. In vitro testing of 24 NFNE revealed some possessed putative plant growth promotion traits. Their genomes were also sequenced to identify genes related to plant growth promotion traits. This study highlights the complexity of the alder nodular microbial community. It paves the way for further understanding of the biology of nodules and could help improve land reclamation practices that involve alders.

The Use of PGPB to Promote Plant Hydroponic Growth

Improvements to the world's food supply chain are needed to ensure sufficient food is produced to meet increasing population demands. Growing food in soilless hydroponic systems constitutes a promising strategy, as this method utilizes significantly less water than conventional agriculture, can be situated in urban areas, and can be stacked vertically to increase yields per acre. However, further research is needed to optimize crop yields in these systems. One method to increase hydroponic plant yields involves adding plant growth-promoting bacteria (PGPB) into these systems. PGPB are organisms that can significantly increase crop yields via a wide range of mechanisms, including stress reduction, increases in nutrient uptake, plant hormone modulation, and biocontrol. The aim of this review is to provide critical information for researchers on the current state of the use of PGPB in hydroponics so that meaningful advances can be made. An overview of the history and types of hydroponic systems is provided, followed by an overview of known PGPB mechanisms. Finally, examples of PGPB research that has been conducted in hydroponic systems are described. Amalgamating the current state of knowledge should ensure that future experiments can be designed to effectively transition results from the lab to the farm/producer, and the consumer.

Biosynthesis Pathways, Transport Mechanisms and Biotechnological Applications of Fungal Siderophores

Iron (Fe) is the fourth most abundant element on earth and represents an essential nutrient for life. As a fundamental mineral element for cell growth and development, iron is available for uptake as ferric ions, which are usually oxidized into complex oxyhydroxide polymers, insoluble under aerobic conditions. In these conditions, the bioavailability of iron is dramatically reduced. As a result, microorganisms face problems of iron acquisition, especially under low concentrations of this element. However, some microbes have evolved mechanisms for obtaining ferric irons from the extracellular medium or environment by forming small molecules often regarded as siderophores. Siderophores are high affinity iron-binding molecules produced by a repertoire of proteins found in the cytoplasm of cyanobacteria, bacteria, fungi, and plants. Common groups of siderophores include hydroxamates, catecholates, carboxylates, and hydroximates. The hydroxamate siderophores are commonly synthesized by fungi. L-ornithine is a biosynthetic precursor of siderophores, which is synthesized from multimodular large enzyme complexes through non-ribosomal peptide synthetases (NRPSs), while siderophore-Fe chelators cell wall mannoproteins (FIT1, FIT2, and FIT3) help the retention of siderophores. S. cerevisiae, for example, can express these proteins in two genetically separate systems (reductive and nonreductive) in the plasma membrane. These proteins can convert Fe (III) into Fe (II) by a ferrous-specific metalloreductase enzyme complex and flavin reductases (FREs). However, regulation of the siderophore through Fur Box protein on the DNA promoter region and its activation or repression depend primarily on the Fe availability in the external medium. Siderophores are essential due to their wide range of applications in biotechnology, medicine, bioremediation of heavy metal polluted environments, biocontrol of plant pathogens, and plant growth enhancement.

Serratia plymuthica MBSA-MJ1 Increases Shoot Growth and Tissue Nutrient Concentration in Containerized Ornamentals Grown Under Low-Nutrient Conditions

High fertilizer rates are often applied to horticulture crop production systems to produce high quality crops with minimal time in production. Much of the nutrients applied in fertilizers are not taken up by the plant and are leached out of the containers during regular irrigation. The application of plant growth promoting rhizobacteria (PGPR) can increase the availability and uptake of essential nutrients by plants, thereby reducing nutrient leaching and environmental contamination. Identification of PGPR can contribute to the formulation of biostimulant products for use in commercial greenhouse production. Here, we have identified Serratia plymuthica MBSA-MJ1 as a PGPR that can promote the growth of containerized horticulture crops grown with low fertilizer inputs. MBSA-MJ1 was applied weekly as a media drench to Petunia×hybrida (petunia), Impatiens walleriana (impatiens), and Viola×wittrockiana (pansy). Plant growth, quality, and tissue nutrient concentration were evaluated 8weeks after transplant. Application of MBSA-MJ1 increased the shoot biomass of all three species and increased the flower number of impatiens. Bacteria application also increased the concentration of certain essential nutrients in the shoots of different plant species. In vitro and genomic characterization identified multiple putative mechanisms that are likely contributing to the strain’s ability to increase the availability and uptake of these nutrients by plants. This work provides insight into the interconnectedness of beneficial PGPR mechanisms and how these bacteria can be utilized as potential biostimulants for sustainable crop production with reduced chemical fertilizer inputs.

Radiochemical Evidence for the Contribution of Chemotyped Siderophore Producing Bacteria Towards Plant Iron Nutrition

Fe deficiency is a major challenge that limits agricultural productivity and is a serious human health concern worldwide. Under iron-limiting conditions soil microorganisms produce siderophores, that chelates Fe³⁺ (ferric) and make it available to the plants. Selection of efficient siderophore producing bacteria and establishing their role in enhancing iron uptake is a strategic approach for improving plant nutrition. Hence 3 efficient isolates Pantoea agglomerans, Pseudomonas plecoglossida and Lactococcus lactis, selected from a repository of 154 bacteria, producing catecholate, hydroxamate and carboxylate siderophores, respectively, were assessed for Fe chelation efficiency using ⁵⁹Fe and their role in plant biometric parameters, Fe uptake and antioxidant enzymes with tomato (Strategy I) and wheat (Strategy II) test plants under hydroponic system. Cell-free siderophore preparation (Sid) improved plant parameters and iron nutrition more efficiently than bacterial inoculants. Pantoea agglomerans was proven best as its ⁵⁹Fe-bound siderophore complex showed the highest uptake of 4.25 and 1.59 Bq plant^-1 in wheat and tomato, respectively. Further, the Fe-starved plants (1 µm Fe-EDTA) showed around two-fold higher ⁵⁹Fe uptake than those raised under Fe-sufficient condition (100 µm Fe-EDTA). Results indicated that probably the bacterial mediated iron translocation in plants is Strategy III, complementing both Strategy I and II by facilitating higher availability of chelated Fe to plant reductases directly and/or through ligand exchange with phytosiderophores, respectively. This study highlights the need for integration of siderophore based formulations in INM strategies for enhancing plant iron content to address the Fe deficiency challenge of the soil and human nutrition.

Microbial Derived Compounds, a Step Toward Enhancing Microbial Inoculants Technology for Sustainable Agriculture

Sustainable agriculture remains a focus for many researchers, in an effort to minimize environmental degradation and climate change. The use of plant growth promoting microorganisms (PGPM) is a hopeful approach for enhancing plant growth and yield. However, the technology faces a number of challenges, especially inconsistencies in the field. The discovery, that microbial derived compounds can independently enhance plant growth, could be a step toward minimizing shortfalls related to PGPM technology. This has led many researchers to engage in research activities involving such compounds. So far, the findings are promising as compounds have been reported to enhance plant growth under stressed and non-stressed conditions in a wide range of plant species. This review compiles current knowledge on microbial derived compounds, taking a reader through a summarized protocol of their isolation and identification, their relevance in present agricultural trends, current use and limitations, with a view to giving the reader a picture of where the technology has come from, and an insight into where it could head, with some suggestions regarding the probable best ways forward.

Importance of the Rhizosphere Microbiota in Iron Biofortification of Plants

Increasing the iron content of plant products and iron assimilability represents a major issue for human nutrition and health. This is also a major challenge because iron is not readily available for plants in most cultivated soils despite its abundance in the Earth's crust. Iron biofortification is defined as the enhancement of the iron content in edible parts of plants. This biofortification aims to reach the objectives defined by world organizations for human nutrition and health while being environment friendly. A series of options has been proposed to enhance plant iron uptake and fight against hidden hunger, but they all show limitations. The present review addresses the potential of soil microorganisms to promote plant iron nutrition. Increasing knowledge on the plant microbiota and plant-microbe interactions related to the iron dynamics has highlighted a considerable contribution of microorganisms to plant iron uptake and homeostasis. The present overview of the state of the art sheds light on plant iron uptake and homeostasis, and on the contribution of plant-microorganism (plant-microbe and plant-plant-microbe) interactions to plant nutritition. It highlights the effects of microorganisms on the plant iron status and on the co-occurring mechanisms, and shows how this knowledge may be valued through genetic and agronomic approaches. We propose a change of paradigm based on a more holistic approach gathering plant and microbial traits mediating iron uptake. Then, we present the possible applications in plant breeding, based on plant traits mediating plant-microbe interactions involved in plant iron uptake and physiology.

Impact of Bacterial Siderophores on Iron Status and Ionome in Pea

Including more grain legumes in cropping systems is important for the development of agroecological practices and the diversification of protein sources for human and animal consumption. Grain legume yield and quality is impacted by abiotic stresses resulting from fluctuating availabilities in essential nutrients such as iron deficiency chlorosis (IDC). Promoting plant iron nutrition could mitigate IDC that currently impedes legume cultivation in calcareous soils, and increase the iron content of legume seeds and its bioavailability. There is growing evidence that plant microbiota contribute to plant iron nutrition and might account for variations in the sensitivity of pea cultivars to iron deficiency and in fine to seed nutritional quality. Pyoverdine (pvd) siderophores synthesized by pseudomonads have been shown to promote iron nutrition in various plant species (Arabidopsis, clover and grasses). This study aimed to investigate the impact of three distinct ferripyoverdines (Fe-pvds) on iron status and the ionome of two pea cultivars (cv.) differing in their tolerance to IDC, (cv. S) being susceptible and (cv. T) tolerant. One pvd came from a pseudomonad strain isolated from the rhizosphere of cv. T (pvd1T), one from cv. S (pvd2S), and the third from a reference strain C7R12 (pvdC7R12). The results indicated that Fe-pvds differently impacted pea iron status and ionome, and that this impact varied both according to the pvd and the cultivar. Plant iron concentration was more increased by Fe-pvds in cv. T than in cv. S. Iron allocation within the plant was impacted by Fe-pvds in cv. T. Furthermore, Fe-pvds had the greatest favorable impact on iron nutrition in the cultivar from which the producing strain originated. This study evidences the impact of bacterial siderophores on pea iron status and pea ionome composition, and shows that this impact varies with the siderophore and host-plant cultivar, thereby emphasizing the specificity of these plant-microorganisms interactions. Our results support the possible contribution of pyoverdine-producing pseudomonads to differences in tolerance to IDC between pea cultivars. Indeed, the tolerant cv. T, as compared to the susceptible cv. S, benefited from bacterial siderophores for its iron nutrition to a greater extent.

Genomic Diversity of Two Hydrocarbon-Degrading and Plant Growth-Promoting Pseudomonas Species Isolated from the Oil Field of Bóbrka (Poland)

Hydrocarbon-degrading bacteria are important resources for use in phytoremediation applications. Yet, for many hydrocarbonoclastic strains the genetic information regarding pollutant degradation and detoxification has not been thoroughly revealed. In this study, hydrocarbon-degrading bacteria were isolated from a long-term oil-polluted soil in Bóbrka, Poland. Pseudomonas spp. was the most dominant species. Of all 69 isolated strains tested in the laboratory using qualitative biochemical assays, 61% showed the capability to use diesel as sole carbon source, 33% could produce indole, 19% produced siderophores, 36% produced organic acids, and 54% were capable of producing 1-aminocyclopropane-1-carboxylate (ACC)-deaminase. From all morphologically and genetically different strains, two representative Pseudomonas spp., strain VI4.1 and VI4T1, were selected for genome sequencing. Genomic analyses indicated the presence of the full naphthalene dioxygenase operon (plasmid and chromosomal), of genes involved in the degradation of BTEX compounds (Benzene, Toluene, Ethylbenzene, Xylene) and alkanes (alkB gene) as well as the anthranilate degradation pathway (strain VI4T1) and terephthalate dioxygenase protein (strain VI4.1). Proton transfer reaction time-of-flight mass spectrometry (PTR-TOF-MS) analyses confirmed naphthalene and BTEX degradation within seven days. Motility, resistance to abiotic stresses, high and low temperatures, low pH, and salinity were confirmed at the genetic level and experimentally verified. The presence of multiple degradative and plant growth promotion genes, together with the in vitro experimental evidence, indicates the high value of these two strains and their potential use for sustainable site clean-up.

Bacteria Associated With a Commercial Mycorrhizal Inoculum: Community Composition and Multifunctional Activity as Assessed by Illumina Sequencing and Culture-Dependent Tools

The implementation of sustainable agriculture encompasses practices enhancing the activity of beneficial soil microorganisms, able to modulate biogeochemical soil cycles and to affect soil fertility. Among them, arbuscular mycorrhizal fungi (AMF) establish symbioses with the roots of most food crops and play a key role in nutrient uptake and plant protection from biotic and abiotic stresses. Such beneficial services, encompassing improved crop performances, and soil resources availability, are the outcome of the synergistic action of AMF and the vast communities of mycorrhizospheric bacteria living strictly associated with their mycelium and spores, most of which showing plant growth promoting (PGP) activities, such as the ability to solubilize phosphate and produce siderophores and indole acetic acid (IAA). One of the strategies devised to exploit AMF benefits is represented by the inoculation of selected isolates, either as single species or in a mixture. Here, for the first time, the microbiota associated with a commercial AMF inoculum was identified and characterized, using a polyphasic approach, i.e., a combination of culture-dependent analyses and metagenomic sequencing. Overall, 276 bacterial genera were identified by Illumina high-throughput sequencing, belonging to 165 families, 107 orders, and 23 phyla, mostly represented by Proteobacteria and Bacteroidetes. The commercial inoculum harbored a rich culturable heterotrophic bacterial community, whose populations ranged from 2.5 to 6.1 × 106 CFU/mL. The isolation of functional groups allowed the selection of 36 bacterial strains showing PGP activities. Among them, 14 strains showed strong IAA and/or siderophores production and were affiliated with Actinomycetales (Microbacterium trichotecenolyticum, Streptomyces deccanensis/scabiei), Bacillales (Bacillus litoralis, Bacillus megaterium), Enterobacteriales (Enterobacter), Rhizobiales (Rhizobium radiobacter). This work demonstrates for the first time that an AMF inoculum, obtained following industrial production processes, is home of a large and diverse community of bacteria with important functional PGP traits, possibly acting in synergy with AMF and providing additional services and benefits. Such bacteria, available in pure culture, could be utilized, individually and/or in multispecies consortia with AMF, as biofertilizers and bioenhancers in sustainable agroecosystems, aimed at minimizing the use of chemical fertilizers and pesticides, promoting primary production, and maintaining soil health and fertility.

Iron and Fungal Physiology: A Review of Biotechnological Opportunities

Iron is an essential inorganic micronutrient. Because of its low toxicity only a few studies have dealt with the importance of iron in fungal physiology. Most of the studies published so far focus on iron sequestration by animal fungal pathogens, iron uptake by mycorrhizal fungi, or iron redox activities by fungal wood degraders. However, a general overview on the relationship between fungal physiology and iron is still lacking. In this review we present a summary of the types of physiological activities that participate in iron homeostasis in fungi and how these activities can be used for the development of original biotechnological applications in relationship to iron-containing matrices. Concrete examples of biotechnological applications involving iron and fungi are also discussed. In the last part, a specific research project in biotechnology focusing on the use of fungi for the conservation of archaeological objects in iron is described in detail. This project aims at developing a new conservation-restoration method to preserve archaeological iron artifacts exploiting the ability of fungi to transform and uptake iron. Preliminary results obtained in this project regarding iron-reduction, iron uptake, and biogenic formation of iron minerals are presented and discussed.

Siderophores for molecular imaging applications

This review covers publications on siderophores applied for molecular imaging applications, mainly for radionuclide-based imaging. Siderophores are low molecular weight chelators produced by bacteria and fungi to scavenge essential iron. Research on these molecules has a continuing history over the past 50 years. Many biomedical applications have been developed, most prominently the use of the siderophore desferrioxamine (DFO) to tackle iron overload related diseases. Recent research described the upregulation of siderophore production and transport systems during infection. Replacing iron in siderophores by radionuclides, the most prominent Ga-68 for PET, opens approaches for targeted imaging of infection; the proof of principle has been reported for fungal infections using ⁶⁸Ga-triacetylfusarinine C (TAFC). Additionally, fluorescent siderophores and therapeutic conjugates have been described and may be translated to optical imaging and theranostic applications. Siderophores have also been applied as bifunctional chelators, initially DFO as chelator for Ga-67 and more recently for Zr-89 where it has become the standard chelator in Immuno-PET. Improved DFO constructs and bifunctional chelators based on cyclic siderophores have recently been developed for Ga-68 and Zr-89 and show promising properties for radiopharmaceutical development in PET. A huge potential from basic biomedical research on siderophores still awaits to be utilized for clinical and translational imaging.

Lotus japonicus plants of the Gifu B-129 ecotype subjected to alkaline stress improve their Fe(2+) bio-availability through inoculation with Pantoea eucalypti M91

Inoculation assays with Pantoea eucalypti M91 were performed on Lotus japonicus ecotype Gifu. Under alkaline conditions, this ecotype is characterized by the development of interveinal chlorosis of the apical leaves due to low mobilization of Fe(2+). Inoculation with P. eucalypti M91, a plant growth-promoting bacterial strain capable of producing pyoverdine-like and pyochelin-like siderophores under alkaline growth conditions, alters the root, resulting in a herringbone pattern of root branching. Additional features include improvement in Fe(2+) transport to the shoots, acidification of the hydroponic solution of the plant cultures, and an accompanying increase in the efficiency of the PSII parameters. In addition, there was an increase in the expression of the FRO1 and IRT1 genes, accompanied by a significant increase in FRO activity. Results showed that P. eucalypti M91 has a beneficial effect on the Fe acquisition machinery of Strategy I, as described for non-graminaceous monocots and dicots, suggesting its potential as an inoculant for legume crops cultivated in alkaline soils.

Screening and Evaluation of the Bioremediation Potential of Cu/Zn-Resistant, Autochthonous Acinetobacter sp. FQ-44 from Sonchus oleraceus L

The quest for new, promising and indigenous plant growth-promoting rhizobacteria and a deeper understanding of their relationship with plants are important considerations in the improvement of phytoremediation. This study focuses on the screening of plant beneficial Cu/Zn-resistant strains and assessment of their bioremediation potential (metal solubilization/tolerance/biosorption and effects on growth of Brassica napus seedlings) to identify suitable rhizobacteria and examine their roles in microbes-assisted phytoremediation. Sixty Cu/Zn-resistant rhizobacteria were initially isolated from Sonchus oleraceus grown at a multi-metal-polluted site in Shanghai, China. From these strains, 19 isolates that were all resistant to 300 mg⋅L^-1 Cu as well as 300 mg⋅L^-1 Zn, and could simultaneously grow on Dworkin-Foster salt minimal medium containing 1-aminocyclopropane-1-carboxylic acid were preliminarily selected. Of those 19 isolates, 10 isolates with superior plant growth-promoting properties (indole-3-acetic acid production, siderophore production, and insoluble phosphate solubilization) were secondly chosen and further evaluated to identify those with the highest bioremediation potential and capacity for bioaugmentation. Strain S44, identified as Acinetobacter sp. FQ-44 based on 16S rDNA sequencing, was specifically chosen as the most favorable strain owing to its strong capabilities to (1) promote the growth of rape seedlings (significantly increased root length, shoot length, and fresh weight by 92.60%, 31.00%, and 41.96%, respectively) under gnotobiotic conditions; (2) tolerate up to 1000 mg⋅L^-1 Cu and 800 mg⋅L^-1 Zn; (3) mobilize the highest concentrations of water-soluble Cu, Zn, Pb, and Fe (16.99, 0.98, 0.08, and 3.03 mg⋅L^-1, respectively); and (4) adsorb the greatest quantities of Cu and Zn (7.53 and 6.61 mg⋅g^-1 dry cell, respectively). Our findings suggest that Acinetobacter sp. FQ-44 could be exploited for bacteria-assisted phytoextraction. Moreover, the present study provides a comprehensive method for the screening of rhizobacteria for phytoremediation of multi-metal-polluted soils, especially those sewage sludge-amended soils contaminated with Cu/Zn.

Trichoderma Secondary Metabolites that Affect Plant Metabolism

Recently, there have been many exciting new developments relating to the use of Trichoderma spp. as agents for biocontrol of pathogens and as plant growth promoters. Several mechanisms have been proposed to explain the positive effects of these microorganisms on the plant host. One factor that contributes to their beneficial biological activities is related to the wide variety of metabolites that they produce. These metabolites have been found not only to directly inhibit the growth and pathogenic activities of the parasites, but also to increase disease resistance by triggering the system of defence in the plant host. In addition, these metabolites are also capable of enhancing plant growth, which enables the plant to counteract the disease with compensatory vegetative growth by the augmented production of root and shoot systems. This review takes into account the Trichoderma secondary metabolites that affect plant metabolism and that may play an important role in the complex interactions of this biocontrol agent with the plant and pathogens.

Intrinsic characteristics of Cr⁶⁺-resistant bacteria isolated from an electroplating industry polluted soils for plant growth-promoting activities

The Cr(6+)-resistant plant growth-promoting bacteria was isolated from soil samples that were collected from an electroplating industry at Coimbatore, India, that had tolerated chromium concentrations up to 500 mg Cr(6+)/L in Luria-Bertani medium. Based on morphology, physiology, and biochemical characteristics, the strain was identified as Bacillus sp. following the Bergey's manual of determinative bacteriology. Evaluation of plant growth-promoting parameters has revealed the intrinsic ability of the strain for the production of indole-3-acetic acid (IAA), siderophore, and solubilization of insoluble phosphate. Bacillus sp. have utilized tryptophan as a precursor for their growth and produced IAA (122 μg/mL). Bacillus sp. also exhibited the production of siderophore that was tested qualitatively using Chrome Azurol S (CAS) assay solution and utilized the insoluble tricalcium phosphate as the sole source of phosphate exhibiting higher rate of phosphate solubilization after 72 h of incubation (1.45 μg/mL). Extent of Cr(6+) uptake and accumulation of Cr(6+) in the cell wall of Bacillus sp. was investigated using atomic absorption spectrophotometer and scanning electron microscope-energy dispersive spectroscopy, respectively. The congenital capability of this Cr(6+)-resistant plant growth-promoting Bacillus sp. could be employed as bacterial inoculum for the improvement of phytoremediation in heavy metal contaminated soils.

Plant growth-promoting bacteria: mechanisms and applications

The worldwide increases in both environmental damage and human population pressure have the unfortunate consequence that global food production may soon become insufficient to feed all of the world's people. It is therefore essential that agricultural productivity be significantly increased within the next few decades. To this end, agricultural practice is moving toward a more sustainable and environmentally friendly approach. This includes both the increasing use of transgenic plants and plant growth-promoting bacteria as a part of mainstream agricultural practice. Here, a number of the mechanisms utilized by plant growth-promoting bacteria are discussed and considered. It is envisioned that in the not too distant future, plant growth-promoting bacteria (PGPB) will begin to replace the use of chemicals in agriculture, horticulture, silviculture, and environmental cleanup strategies. While there may not be one simple strategy that can effectively promote the growth of all plants under all conditions, some of the strategies that are discussed already show great promise.

Production of metabolites as bacterial responses to the marine environment

Bacteria in marine environments are often under extreme conditions of e.g., pressure, temperature, salinity, and depletion of micronutrients, with survival and proliferation often depending on the ability to produce biologically active compounds. Some marine bacteria produce biosurfactants, which help to transport hydrophobic low water soluble substrates by increasing their bioavailability. However, other functions related to heavy metal binding, quorum sensing and biofilm formation have been described. In the case of metal ions, bacteria developed a strategy involving the release of binding agents to increase their bioavailability. In the particular case of the Fe³⁺ ion, which is almost insoluble in water, bacteria secrete siderophores that form soluble complexes with the ion, allowing the cells to uptake the iron required for cell functioning. Adaptive changes in the lipid composition of marine bacteria have been observed in response to environmental variations in pressure, temperature and salinity. Some fatty acids, including docosahexaenoic and eicosapentaenoic acids, have only been reported in prokaryotes in deep-sea bacteria. Cell membrane permeability can also be adapted to extreme environmental conditions by the production of hopanoids, which are pentacyclic triterpenoids that have a function similar to cholesterol in eukaryotes. Bacteria can also produce molecules that prevent the attachment, growth and/or survival of challenging organisms in competitive environments. The production of these compounds is particularly important in surface attached strains and in those in biofilms. The wide array of compounds produced by marine bacteria as an adaptive response to demanding conditions makes them suitable candidates for screening of compounds with commercially interesting biological functions. Biosurfactants produced by marine bacteria may be helpful to increase mass transfer in different industrial processes and in the bioremediation of hydrocarbon-contaminated sites. Siderophores are necessary e.g., in the treatment of diseases with metal ion imbalance, while antifouling compounds could be used to treat man-made surfaces that are used in marine environments. New classes of antibiotics could efficiently combat bacteria resistant to the existing antibiotics. The present work aims to provide a comprehensive review of the metabolites produced by marine bacteria in order to cope with intrusive environments, and to illustrate how such metabolites can be advantageously used in several relevant areas, from bioremediation to health and pharmaceutical sectors.

Isolation and characterization of Ni mobilizing PGPB from serpentine soils and their potential in promoting plant growth and Ni accumulation by Brassica spp

The study was undertaken to assess the effects of Ni mobilizing bacteria on the plant growth and the uptake of Ni by Brassica juncea and Brassica oxyrrhina. Among a collection of Ni resistant bacterial strains isolated from the non-rhizosphere and rhizosphere soils of Alyssum serpyllifolium and Astragalus incanus at a serpentine site in Bragança, north-east of Portugal, nine strains were selected based on their ability to solubilize Ni in soil. Further assessment on plant growth-promoting parameters revealed the intrinsic ability of the Ni mobilizing strains to produce indole-3-acetic acid (IAA), siderophores, utilize 1-aminocyclopropane-1-carboxylic acid (ACC) as the sole N source and solubilize insoluble phosphate. All of the strains tested positive for IAA production and phosphate solubilization. In addition, all the strains, except SRS5 exhibited significant levels of siderophore production. Besides, five isolates showed positive for ACC deaminase activity. In pot experiments, inoculation of plants with Ni mobilizing strains increased the biomass of both B. juncea and B. oxyrrhina. Among the strains, Pseudomonas sp. SRI2, Psychrobacter sp. SRS8 and Bacillus sp. SN9 showed maximum increase in the biomass of the test plants. In addition, the strain SN9 significantly increased the Ni concentration in the root and shoot tissues of B. juncea and B. oxyrrhina. Further, a significantly positive correlation was observed between the bacterial Ni mobilization in soil and the total Ni uptake in both plant species. The findings, therefore, revealed that inoculation of Ni mobilizing plant growth-promoting bacterial strain SN9 increases the efficiency of phytoextraction directly by enhancing Ni accumulation in plant tissues and indirectly by promoting the shoot and root biomass of B. juncea and B. oxyrrhina.

Cadmium–Induced Siderophore Production by a High Cd-Resistant Bacterial Strain Relieved Cd Toxicity in Plants Through Root Colonization

This study focuses on the isolation and characterization of a high cadmium (Cd)-resistant bacterial strain, and possible exploitation of its Cd-accumulation and Cd-induced siderophore production property to improve plant growth in cadmium-contaminated soil through root colonization. The bacterial strain could tolerate up to 8 mM of Cd and could accumulate Cd intracellularly. The strain showed Cd-induced siderophore production maximally at 1.75 mM of Cd concentration under culture condition. It stimulated the growth of mustard and pumpkin plants in Cd-added soil through its establishment in rhizosphere. Through biochemical characterization and 16S rDNA sequence analysis, the strain KUCd1, as the name given to it, was identified as a strain of Pseudomonas aeruginosa.

Mechanisms of microbially enhanced Fe acquisition in red clover (Trifolium pratense L.)

Soil microorganisms may play an important role in plant Fe uptake from soils with low Fe bioavailability, but there is little direct experimental evidence to date. We grew red clover, an Fe-efficient leguminous plant, in a calcareous soil to investigate the role of soil microbial activity in plant Fe uptake. Compared with plants grown in non-sterlie (NS) grown plants, growth and Fe content of the sterile(s) grown plants was significantly inhibited, but was improved by foliar application of Fe EDTA, indicating that soil microbial activity should play an important role in plant Fe acquisition. When soil solution was incubated with phenolic root exudates from Fe-deficient red clover, a few microbial species thrived while growth of the rest was inhibited, suggesting that the Fe-deficient (-Fe) root exudates selectively influenced the rhizosphere's microbial community. Eighty six per cent of the phenolic-tolerant microbes could produce siderophore [the Fe(III) chelator] under -Fe conditions, and 71% could secrete auxin-like compounds. Interestingly, the synthetic and microbial auxins (MAs) significantly enhanced the Ferric reduction system, suggesting that MAs, in addition to siderophores, are important to plant Fe uptake. Finally, plant growth and Fe uptake in sterilized soil were significantly increased by rhizobia inoculation. Root Fe-EDTA reductase activity in the -Fe plant was significantly enhanced by rhizobia infection, and the rhizobia could produce auxin but not siderophore under Fe-limiting conditions, suggesting that the contribution of nodulating rhizobia to plant Fe uptake can be at least partially attributed to stimulation of turbo reductase activity through nodule formation and auxin production in the rhizosphere. Based on these observations, we propose as a model that root exudates from -Fe plants selectively influence the rhizosphere microbial community, and the microbes in turn favour plant Fe acquisition by producing siderophores and auxins.

Isolation and Functional Characterization of Siderophore-Producing Lead- and Cadmium-Resistant Pseudomonas putida KNP9

Heavy metals, being phytotoxic, cause growth inhibition and even plant death. Siderophore-producing bacterial strain KNP9 is growth promoting and has been isolated from Panki Power Plant, Kanpur, India. It simulated significant (p > 5%) root and shoot growth of mung bean to the extent of 16.48% and 28.80%, respectively in the presence of CdCl(2) (110 microM: ). However, the increase in root and shoot growth was 20% and 19.5%, respectively, in the presence of (CH(3)COO)(2)Pb (660 microM: ). Moreover, concentration of accumulated lead and cadmium in root and shoot was also reduced in the presence of this isolate ranging from 37.5 to 93.19%. A moderate reduction in chlorophyll content (39.14%) in the presence of 110 microM: CdCl(2) was rescued by bioinoculant KNP9. However, the 19.58% decrease in chlorophyll content in the case of lead acetate remained unchanged even in the presence of KNP9. Nevertheless, 16S ribosomal DNA (rDNA) sequencing identified KNP9 as a strain of Pseudomonas putida.

Solubilization of phosphates and micronutrients by the plant-growth-promoting and biocontrol fungus trichoderma harzianum rifai 1295-22

We investigated the capability of the plant-growth-promoting and biocontrol fungus Trichoderma harzianum Rifai 1295-22 (T-22) to solubilize in vitro some insoluble or sparingly soluble minerals via three possible mechanisms: acidification of the medium, production of chelating metabolites, and redox activity. T-22 was able to solubilize MnO2, metallic zinc, and rock phosphate (mostly calcium phosphate) in a liquid sucrose-yeast extract medium, as determined by inductively coupled plasma emission spectroscopy. Acidification was not the major mechanism of solubilization since the pH of cultures never fell below 5.0 and in cultures containing MnO2 the pH rose from 6.8 to 7.4. Organic acids were not detected by high-performance thin-layer chromatography in the culture filtrates. Fe2O3, MnO2, Zn, and rock phosphate were also solubilized by cell-free culture filtrates. The chelating activity of T-22 culture filtrates was determined by a method based on measurement of the equilibrium concentration of the chrome azurol S complex in the presence of other chelating substances. A size exclusion chromatographic separation of the components of the culture filtrates indicated the presence of a complexed form of Fe but no chelation of Mn. In liquid culture, T. harzianum T-22 also produced diffusible metabolites capable of reducing Fe(III) and Cu(II), as determined by the formation of Fe(II)-Na2-bathophenanthrolinedisulfonic acid and Cu(I)-Na2-2, 9-dimethyl-4,7-diphenyl-1,10-phenanthrolinedisulfonic acid complexes. This is the first report of the ability of a Trichoderma strain to solubilize insoluble or sparingly soluble minerals. This activity may explain, at least partially, the ability of T-22 to increase plant growth. Solubilization of metal oxides by Trichoderma involves both chelation and reduction. Both of these mechanisms also play a role in biocontrol of plant pathogens, and they may be part of a multiple-component action exerted by T-22 to achieve effective biocontrol under a variety of environmental conditions.

Evidence that a deferrioxamine B degrading enzyme is a serine protease

Siderophores are organic biomolecules synthesized by a wide variety of microbes. The molecules sequester ferric ion from environments where it is present at extremely low concentrations. Siderophores are of consequence with respect to microbial nutrition, pathogenicity, virulence, and microbe-plant interactions. How siderophores are degraded and returned to the carbon and nitrogen cycles is not well understood. The catalytic activity of an enzyme from a bacterium that degrades the siderophore deferrioxamine B has been examined. While the degradation of deferrioxamine B is sensitive to sulfhydryl and metal moiety inhibitors, the data presented is most consistent with the hypothesis that the enzyme uses a hydroxyl moiety (serine peptidase) to catalyze the degradation of deferrioxamine B. If sulfhydryl and metal inhibitors are simultaneously present at concentrations that when alone only partially inhibit the enzyme, the enzyme is unable to catalyze deferrioxamine B dissimilation. Analysis of the inhibitor experiments conducted led to the conclusion that the deferrioxamine B degrading enzyme is a serine-peptidase-like enzyme that needs calcium ions and sulfhydryl groups to be fully activated or stabilized. The knowledge of the catalytic moieties of the enzyme will be exploited to purify the enzyme.

The Role of Ligand Exchange in the Uptake of Iron from Microbial Siderophores by Gramineous Plants

The siderophore rhizoferrin, produced by the fungus Rhizopus arrhizus, was previously found to be as an efficient Fe source as Fe-ethylenediamine-di(o-hydroxphenylacetic acid) to strategy I plants. The role of this microbial siderophore in Fe uptake by strategy II plants is the focus of this research. Fe-rhizoferrin was found to be an efficient Fe source for barley (Hordeum vulgare L.) and corn (Zea mays L.). The mechanisms by which these Gramineae utilize Fe from Fe-rhizoferrin and from other chelators were studied. Fe uptake from 59Fe-rhizoferrin, 59Fe-ferrioxamine B, 59Fe-ethylenediaminetetraacetic acid, and 59Fe-2[prime]-deoxymugineic acid by barley plants grown in nutrient solution at pH 6.0 was examined during periods of high (morning) and low (evening) phytosiderophore release. Uptake and translocation rates from Fe chelates paralleled the diurnal rhythm of phytosiderophore release. In corn, however, similar uptake and translocation rates were observed both in the morning and in the evening. A constant rate of the phytosiderophore's release during 14 h of light was found in the corn cv Alice. The results presented support the hypothesis that Fe from Fe-rhizoferrin is taken up by strategy II plants via an indirect mechanism that involves ligand exchange between the ferrated microbial siderophore and phytosiderophores, which are then taken up by the plant. This hypothesis was verified by in vitro ligand-exchange experiments.

The enhancement of plant growth by free-living bacteria

The ways in which plant growth promoting rhizobacteria facilitate the growth of plants are considered and discussed. Both indirect and direct mechanisms of plant growth promotion are dealt with. The possibility of improving plant growth promoting rhizobacteria by specific genetic manipulation is critically examined.Key words: plant growth promoting rhizobacteria, PGPR, bacterial fertilizer, soil bacteria.

Short-term effects of rhizosphere microorganisms on fe uptake from microbial siderophores by maize and oat

Effects of rhizosphere microorganisms on Fe uptake by oat (Avena sativa) and maize (Zea mays) were studied in short-term (10 h) nutrient solution experiments. Fe was supplied either as microbial siderophores (pseudobactin [PSB] or ferrioxamine B [FOB]) or as phytosiderophores obtained as root exudates from barley (epi-3-hydroxy-mugineic acid [HMA]) under varied population densities of rhizosphere microorganisms (axenic, uninoculated, or inoculated with different microorganism cultures). When maize was grown under axenic conditions and supplied with FeHMA, Fe uptake rates were 100 to 300 times higher compared to those in plants supplied with Fe siderophores. Fe from both sources was taken up without the involvement of an extracellular reduction process. The supply of FeHMA enhanced both uptake rate and translocation rate to the shoot (more than 60% of the total uptake). However, increased density of microorganisms resulted in a decrease in Fe uptake rate (up to 65%), presumably due to microbial degradation of the FeHMA. In contrast, when FeFOB or FePSB was used as the Fe source, increased population density of microorganisms enhanced Fe uptake. The enhancement of Fe uptake resulted from the uptake of FeFOB and FePSB by microorganisms adhering to the rhizoplane or living in the free space of cortical cells. The microbial apoplastic Fe pool was not available for root to shoot transport or, thus, for utilization by the plants. These results, in addition to the low uptake rate under axenic conditions, are in contrast to earlier hypotheses suggesting the existence of a specific uptake system for Fe siderophores in higher plants. The bacterial siderophores PSB and FOB were inefficient as Fe sources for plants even when supplied by stem injection. It was concluded that microorganisms are involved in degradation processes of microbial siderophores, as well as in competition for Fe with higher plants.

Iron uptake by plants from microbial siderophores : a study with 7-nitrobenz-2 oxa-1,3-diazole-desferrioxamine as fluorescent ferrioxamine B analog

The synthetically produced fluorescent siderophore NBD-desferrioxamine B (NBD-DFO), an analog of the natural siderophore ferrioxamine B, was used to study iron uptake by plants. Short-term (10-hour) (55)Fe uptake rates by cotton (Gossypium spp.) and maize (Zea mays L.) plants from the modified siderophore were similar to those of the natural one. In longer-term uptake experiments (3 weeks), both siderophore treatments resulted in similar leaf chlorophyll concentration and dry matter yield. These results suggest that the synthetic derivative acts similarly to the natural siderophore. The NBD-DFO is fluorescent only when unferrated and can thus be used as a probe to follow iron removal from the siderophore. Monitoring of the fluorescence increase in a nutrient solution containing Fe(3+)-NBD-DFO showed that iron uptake by plants occurs at the cell membrane. The rate of iron uptake was significantly lower in both plant species in the presence of antibiotic agent, thus providing evidence for iron uptake by rhizosphere microbes that otherwise could have been attributed to plant uptake. Confocal fluorescence microscopy revealed that iron was taken up from the complex by cotton plants, and to a much lesser extent by maize plants. The active cotton root sites were located at the main and lateral root tips. Significant variations in the location and the intensity of the uptake were noticed under nonaxenic conditions, which suggested that rhizosphere microorganisms play an important role in NBD-DFO-mediated iron uptake.

Iron uptake and molecular recognition in Pseudomonas putida: receptor mapping with ferrichrome and its biomimetic analogs

The presence of an Fe(3+)-ferrichrome uptake system in fluorescent Pseudomonas spp. was demonstrated, and its structural requirements were mapped in Pseudomonas putida with the help of biomimetic ferrichrome analogs. Growth tests, 55Fe3+ uptake, and competition experiments demonstrated that the synthetic L-alanine derivative B5 inhibits the action of ferrichrome but does not facilitate Fe3+ transport, while the enantiomeric D-Ala derivative B6 fails to compete with ferrichrome. Contraction of the molecule's envelope by replacing L-Ala by glycine provided a synthetic carrier, B9, which fully simulates ferrichrome as a growth promoter. Sodium azide inhibited 55Fe3+ uptake of the Gly derivative B9, suggesting an active transport process. These data demonstrate the chiral discrimination of the ferrichrome receptor and its sensitivity to subtle structural changes. They further confirm that receptor binding is a necessary but not sufficient condition for Fe3+ uptake to occur and suggest that binding to the receptor and transport proteins might rely on different recognition patterns.

Differential Siderophore Utilization and Iron Uptake by Soil and Rhizosphere Bacteria

The differential availabilities of the hydroxamate siderophores ferrioxamine B (FOB) and ferrichrome (FC) and the pseudobactin siderophores St3, 7NSK 2 , and WCS 358 as sources of Fe for soil and rhizosphere bacteria were studied. About 20% of the total bacterial CFU from the rhizospheres of four plant species were able to use FOB as the sole Fe source in an Fe-deficient medium, while about 12, 10, 2, and > 1% were able to use FC and pseudobactins 7NSK 2 , St3, and WCS 358, respectively. Of the 165 colonies isolated from plates containing pseudobactins, 64 were able to use the pseudobactin on which they were isolated as the sole Fe source in pure culture. Cross-feeding tests showed that almost all of these 64 strains were also able to use at least one of the other siderophores studied (pseudobactin, FOB, or FC). Pseudomonas putida StS2, Pseudomonas maltophilia 7NM1, and Vibrio fluvialis WS1, which were originally isolated on pseudobactins St3, 7NSK 2 , and WCS 358, respectively, were selected for their ability to grow with pseudobactin St3 as the sole Fe source. They incorporated 55 Fe 3+ mediated by pseudobactin St3 at various rates (71.5, 4, and 23 pmol/min/mg [dry weight] of cells, respectively). Similarly, P. putida St3 was shown to incorporate 55 Fe 3+ mediated by FOB and FC. We suggest that the ability of bacteria to utilize a large variety of siderophores confers an ecological advantage.

Utilization of microbial siderophores by mycorrhizal and non-mycorrhizal pine roots

In iron-deficient conditions, most bacteria and fungi are known to release siderophores, iron-chelating compounds. Most plants do not produce siderophores, but seem to use microbial siderophores as iron sources. Although ectomycorrhizal fungi have been found to release siderophores in pure culture, little research has addressed production by mycorrhizas and the consequences for plant iron nutrition. The objectives of this study were to determine the effect of an ectomycorrhizal fungus [Pisolithus tinctorius (Pers.) Coker and Couch] on the utilization of siderophore (ferrioxamine B) by slash pine (Pinus elliottii Engelm.) roots grown under iron-deficient or iron-sufficient conditions. Experiments were conducted with excised roots and whole seedlings. Uptake of ⁵⁵ Fe from ferrioxamine B was lower by mycorrhizal roots than non-mycorrhizal roots. Growth under iron-deficient conditions had little effect on iron uptake by non-mycorrhizal roots but increased the uptake by mycorrhizal roots. Uptake of iron from a non-purified siderophore isolated from a pure culture of P. tinctorius was also lower by mycorrhizal roots. The uptake of iron was not dependent on the pH of the uptake solution. Differential responses could be attributed to different mechanisms of iron uptake between fungal cells and root cells. However, the higher iron content of mycorrhizal roots may indicate a negative feedback effect on uptake.