Abstract
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.
Highlights
Iron plays a vital role in the growth and development of living organisms, and it is one of the most abundant elements found on earth [1]
Two possible mechanisms by which plants could obtain Fe from microbial siderophores have been suggested: (i) high redox potential of microbial siderophores can be reduced by the donation of ferrous in the transport system, and (ii) microbial ferric ions are transported in plant root through the apoplast where the reduction of siderophore takes place [187], with consequent ferrous accumulation in the apoplast leading to a high concentration of Fe (II) in the root [188], and (iii) siderophores of microbial origin can chelate Fe from soils and perform ligand exchange with phytosiderophores [189]
Siderophores play a significant role in the iron homeostasis of fungi, which are similar to bacteria and plants for the mobilization of extracellular iron
Summary
Iron plays a vital role in the growth and development of living organisms, and it is one of the most abundant elements found on earth [1]. Siderophores primarily scavenge iron through complex formation with other metals such as molybdenum and cobalt [19] These compounds promote plant growth and play an important role in pathogen biocontrol [20] and bioremediation of metal-polluted environments [21]. Fungi usually produce hydroxamate and carboxylate siderophore types, which have been primarily studied in Aspergillus species. Many fungi can produce more than one siderophore type, especially under low iron availability. Aspergillus fumigatus often produces a hydroxamate siderophore and triacetyl fusarine C (TAFC) for tapping extracellular iron [22]. An older strain (48 h) yielded acetylated TAFC due to breakdown and uptake of fusigen [24] Another fungus, Wolfiporia cocos, known as a brown-rot fungus, may produce different types of catecholate siderophores [25]. It is still not fully known yet to what extent this is possible
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