Siderophores are low molecular weight protein molecules which chelate the free iron present in the soil and thus making the iron unavailable to the deleterious microorganism. Accordingly, a common strategy used by many pathogenic and saprophytic microorganism to tackle the problem of low iron bioavailability is the biosynthesis and excretion of high-affinity siderophores. Some specific strains of the fluorescent Pseudomonas sp. group exhibit plant growth promoting activity by producing extra cellular siderophore (microbial iron transport agents) which efficiently chelate environmental iron making it less available to certain native microflora. Authors studied an extra cellular compound with yellowish green fluorescence called as pyoverdin in Pseudomonas aeruginosa which have siderophore activity. They also stated that pyoverdin not only stimulated growth in iron deficient culture medium but also in defined medium containing trans-ferrin proteins and in human serum or plasma.
The siderophore pyoverdin secreted by Pseudomonas aeruginosa, besides acting as a iron-scavenger, also regulates the production of at least three virulence factors (exotoxin A, on endoprotease and pyoverdine), which are major contributors to the ability of this bacterium to cause disease and thus they proposed that this siderophore acts as a signaling molecule to control the production of secreted products. The chrome azurol S (CAS) assay is used to detect siderophore production by bacteria. Siderophores, including salicylic acid, pyochelin and pyoverdin, which chelate iron and other metals also contribute to disease suppression by conferring a competitive advantage to bio-control agents for the limited supply of essential trace minerals in natural habitats. Siderophores may indirectly stimulate the biosynthesis of other antimicrobial compounds by increasing the availability of these minerals to the bacteria. Siderophore and antibiotics may further function as a stress factor or signals including local and systemic host resistance.
Siderophores, particularly salicylic acid, have been implicated in the ability of certain stains to trigger induced resistance in plants and increasing their supply via inoculants may be advantageous. The excreted siderophore that scavenged ferric iron from the environment or host, the resulting iron-siderophore complex is reabsorbed by bacterial cells by a membrane associated AIP dependent transport system that often exhibits high substrate selectivity.In fungi the re-adsorption of iron siderophore complexes is mediated by the siderophore iron transport (SIT) family of the major facilitator protein super family. Several different mechanisms have been proposed for the recovery of ferric iron from the siderophore complex and reduction to the ferrous form for storage and utilization. Many siderophores are polypeptides that are biosynthesized by members of the Non-Ribosomal Peptide Synthetase (NRPS) multienzyme family, which is also responsible for the biosynthesis of the majority of microbial peptide antibiotics. The enzymology of NRPS catalyzed siderophore biosynthesis has been intensively studied over the last decade, and the biosynthetic mechanism for several types of structurally diverse peptide siderophore are now well understood.
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