Plant pathogens such as bacteria and fungi compete for space or nutrients and this may be a key feature for biocontrol mechanism (Whipps 2001) in rhizosphere. The iron requirement of the plant influences the rhizosphere microbial community structure (Yang and Crowley 2000). Iron is abundant but its bioavailability in the soil is limited due to the insoluble form of ferric hydroxide.
Micromolar concentration of iron is required by bacteria and fungi for their growth. Therefore they evolve high affinity iron uptake system the siderophore and a membrane–bound system for transport and utilization of chelated iron (Neilands and Nakamura 1991). Siderophore (Greek: “iron carrier”) are low molecular weight compounds that chelates ferric ion and serve as a vehicle to transport iron into the cell and are produced by bacteria and fungi growing under iron stress. The role of these compounds is to scavenge iron from the environment and make it available to microbial cell. Transport of iron into the cell is mediated by specific membrane receptor. Siderophore mediated competition for iron deprive pathogen of iron resulting in decrease incidence of disease. These bacterial iron chelators sequester the limited supply of iron available in the rhizosphere making it unavailable to pathogenic fungi, thereby restricting their growth (O’Sullivan and O’Gara 1992; Loper and Henkels 1999). Certain plant growth-promoting pseudomonads inhibit deleterious and pathogenic rhizosphere bacteria and fungi by producing siderophores (Buyer and Sikora 1990; Kloepper et al., 1980). Microorganisms growing under aerobic condition need iron for various metabolic functions, such as reduction of oxygen for synthesis of ATP, formation of heme etc. (Neilands 1995). Ferric-siderophore complexes on being recognized are bound to outer receptor proteins and iron is transported into the microbial cell where it becomes available for metabolic function.
Interesting aspect is diverse organisms can use same type of siderophore (Handelsman and Stabb 1996). The fluorescent pseudomonads produce a class of siderophores known as pseudobactins which are structurally complex iron-binding molecules. Plants too can acquire iron from certain pseudobactins (Duijff et al., 1994 a, b). The siderophore pseudobactin 358 (PSB358) produced by Pseudomonas putida WCS358 is involved in suppression of fusarial wilt disease of carnation (Lemanceau et al., 1992). Matthijs et al. (2007) studied thioquinolobactin siderophore produced by Pseudomonas which showed anti-Pythium activity. Pseudomonas putida and Pseudomonas fluorescens are soil inhabitants, and certain strains promote plant growth and health by suppressing diseases caused by soil borne pathogens. The capacity of these strains to produce pyoverdines is linked, in some cases, with disease suppression (Loper and Buyer 1991).
Role of the pyoverdine siderophore produced by many Pseudomonas species biologically control Pythium and Fusarium species (Loper and Buyer 1991). Pseudomonas aeruginosa 7NSK2 produces three siderophores: the yellow-green fluorescent pyoverdin, the salicylate derivative pyochelin, and salicylic acid. Role of pyochelin and pyoverdin was shown to suppress Pythium causing damping–off of tomato by Pseudomonas aeruginosa 7NSK2 (Buysens et al., 1996).
Pseudomonas aeruginosa needs iron to sustain growth. Two siderophores pyoverdine and pyochelin are produced by this bacterium characterized by high and low affinities for iron respectively. P. aeruginosa is able to utilize different siderophores from other microorganisms (Cornelis and Dingemans 2013). Some siderophore can be used only by the bacteria that produce them (Ongena et al., 1999) whereas, Pseudomonas spp. can utilize siderophore produced by many different bacteria and fungi (Loper and Henkels 1999). Various environmental factors can also influence the quantity of siderophores produced (Duffy and Défago 1999). Siderophore is an important aspect of microorganisms. The solubilization of iron of different composition present in soil is brought about by bacteria and fungi by dissolution and chelation through organic acid or siderophores.
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