Siderophores are small organic molecules produced by microorganisms under iron-limiting conditions which enhance the uptake of iron to the microorganisms. In environment, the ferric form of iron is insoluble and inaccessible at physiological pH (7.35-7.40). Under this condition, microorganisms synthesize siderophores which have high affinity for ferric iron. These ferric iron-siderophore complexes are then transported to cytosol. In cytosol, the ferric iron gets reduced into ferrous iron and becomes accessible to microorganism. In recent times, siderophores have drawn much attention due to its potential roles in different fields. Siderophores have application in microbial ecology to enhance the growth of several unculturable microorganisms and can alter the microbial communities. In the field of agriculture, different types of siderophores promote the growth of several plant species and increase their yield by enhancing the Fe uptake to plants. Siderophores acts as a potential biocontrol agent against harmful phyto-pathogens and holds the ability to substitute hazardous pesticides. Heavy-metal-contaminated samples can be detoxified by applying siderophores, which explicate its role in bioremediation. Siderophores can detect the iron content in different environments, exhibiting its role as a biosensor. In the medical field, siderophore uses the "Trojan horse strategy" to form complexes with antibiotics and helps in the selective delivery of antibiotics to the antibiotic-resistant bacteria. Certain iron overload diseases for example sickle cell anemia can be treated with the help of siderophores. Other medical applications of siderophores include antimalarial activity, removal of transuranic elements from the body, and anticancer activity. The aim of this review is to discuss the important roles and applications of siderophores in different sectors including ecology, agriculture, bioremediation, biosensor, and medicine.

Microbial siderophores and their potential applications: a review

Endophytes are non-pathogenic microbes living inside plants. We asked whether endophytic species were conserved in the agriculturally important plant genus Zea as it became domesticated from its wild ancestors (teosinte) to modern maize (corn) and moved from Mexico to Canada. Kernels from populations of four different teosintes and 10 different maize varieties were screened for endophytic bacteria by culturing, cloning and DNA fingerprinting using terminal restriction fragment length polymorphism (TRFLP) of 16S rDNA. Principle component analysis of TRFLP data showed that seed endophyte community composition varied in relation to plant host phylogeny. However, there was a core microbiota of endophytes that was conserved in Zea seeds across boundaries of evolution, ethnography and ecology. The majority of seed endophytes in the wild ancestor persist today in domesticated maize, though ancient selection against the hard fruitcase surrounding seeds may have altered the abundance of endophytes. Four TRFLP signals including two predicted to represent Clostridium and Paenibacillus species were conserved across all Zea genotypes, while culturing showed that Enterobacter, Methylobacteria, Pantoea and Pseudomonas species were widespread, with γ-proteobacteria being the prevalent class. Twenty-six different genera were cultured, and these were evaluated for their ability to stimulate plant growth, grow on nitrogen-free media, solubilize phosphate, sequester iron, secrete RNAse, antagonize pathogens, catabolize the precursor of ethylene, produce auxin and acetoin/butanediol. Of these traits, phosphate solubilization and production of acetoin/butanediol were the most commonly observed. An isolate from the giant Mexican landrace Mixteco, with 100% identity to Burkholderia phytofirmans, significantly promoted shoot potato biomass. GFP tagging and maize stem injection confirmed that several seed endophytes could spread systemically through the plant. One seed isolate, Enterobacter asburiae, was able to exit the root and colonize the rhizosphere. Conservation and diversity in Zea-microbe relationships are discussed in the context of ecology, crop domestication, selection and migration.

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Conservation and Diversity of Seed Associated Endophytes in Zea across Boundaries of Evolution, Ethnography and Ecology

Schwyn and Neiland developed a universal siderophore assay using chrome azurol S(CAS) and hexadecyltrimethylammonium bromide (HDTMA) as indicators. In Schwyn and Neiland’s original paper, the procedure given for making CAS agar is written in general terms and can be difficult to follow, especially for an individual who has limited experience making more complex media. Here, we give a step-by-step protocol for making the CAS agar plates, and we discuss how this media can be incorporated in a comprehensive project in a microbiology lab course for biology majors.

Use of Blue Agar CAS Assay for Siderophore Detection.

Plant growth promoting rhizobacteria produce chemical compounds with different benefits for the plant. Among them, HCN is recognized as a biocontrol agent, based on its ascribed toxicity against plant pathogens. Based on several past studies questioning the validity of this hypothesis, we have re-addressed the issue by designing a new set of in vitro experiments, to test if HCN-producing rhizobacteria could inhibit the growth of phytopathogens. The level of HCN produced by the rhizobacteria in vitro does not correlate with the observed biocontrol effects, thus disproving the biocontrol hypothesis. We developed a new concept, in which HCN does not act as a biocontrol agent, but rather is involved in geochemical processes in the substrate (e.g. chelation of metals), indirectly increasing the availability of phosphate. Since this scenario can be important for the pioneer plants living in oligotrophic alpine environments, we inoculated HCN producing bacteria into sterile mineral sand together with germinating plants and showed that the growth of the pioneer plant French sorrel was increased on granite-based substrate. No such effect could be observed for maize, where plantlets depend on the nutrients stored in the endosperm. To support our concept, we used KCN and mineral sand and showed that mineral mobilization and phosphate release could be caused by cyanide in vitro. We propose that in oligotrophic alpine environments, and possibly elsewhere, the main contribution of HCN is in the sequestration of metals and the consequential indirect increase of nutrient availability, which is beneficial for the rhizobacteria and their plant hosts.

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Hydrogen Cyanide in the Rhizosphere: Not Suppressing Plant Pathogens, but Rather Regulating Availability of Phosphate.

Zinc is an imperative micronutrient required for optimum plant growth. Zinc solubilizing bacteria are potential alternatives for zinc supplementation and convert applied inorganic zinc to available forms. This study was conducted to screen zinc solubilizing rhizobacteria isolated from wheat and sugarcane, and to analyze their effect on wheat growth and development. Fourteen exo-polysaccharides producing bacterial isolates of wheat were identified and characterized biochemically as well as on the basis of 16S rRNA gene sequences. Along these, 10 identified sugarcane isolates were also screened for zinc solubilizing ability on five different insoluble zinc sources. Out of 24, five strains, i.e., EPS 1 (Pseudomonas fragi), EPS 6 (Pantoea dispersa), EPS 13 (Pantoea agglomerans), PBS 2 (E. cloacae) and LHRW1 (Rhizobium sp.) were selected (based on their zinc solubilizing and PGP activities) for pot scale plant experiments. ZnCO3 was used as zinc source and wheat seedlings were inoculated with these five strains, individually, to assess their effect on plant growth and development. The effect on plants was analyzed based on growth parameters and quantifying zinc content of shoot, root and grains using atomic absorption spectroscopy. Plant experiment was performed in two sets. For first set of plant experiments (harvested after 1 month), maximum shoot and root dry weights and shoot lengths were noted for the plants inoculated with Rhizobium sp. (LHRW1) while E. cloacae (PBS 2) increased both shoot and root lengths. Highest zinc content was found in shoots of E. cloacae (PBS 2) and in roots of P. agglomerans (EPS 13) followed by zinc supplemented control. For second set of plant experiment, when plants were harvested after three months, Pantoea dispersa (EPS 6), P. agglomerans (EPS 13) and E. cloacae (PBS 2) significantly increased shoot dry weights. However, significant increase in root dry weights and maximum zinc content was recorded for Pseudomonas fragi (EPS 1) inoculated plants, isolated from wheat rhizosphere. While maximum zinc content for roots was quantified in the control plants indicating the plant's inability to transport zinc to grains, supporting accelerated bioavailability of zinc to plant grains with zinc solubilizing rhizobacteria.

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N. Cabirol

Papers

O-CAS, a fast and universal method for siderophore detection