Function of Siderophores in the Plant Rhizosphere

Siderophores are iron chelating agents secreted by microorganisms and graminaceous plants in response to iron deficiency. The nature and the rate of release of siderophores differ among plant species and even cultivars [26,41,42].

These compounds are important for iron nutrition and are also speculated to function in the ecology of microorganisms in the plant rhizosphere. Under aerated conditions, at neutral to alkaline pH, inorganic iron is extremely insoluble. In such conditions, plants and microorganisms rely absolutely on iron uptake from organic complexes or iron that has been solubilized by siderophores and organic compounds from root exudates. Organic acids secreted by plant roots dissolve iron as a specific response to iron deficiency [26]. Graminaceous plants initially release organic acids in the rhizosphere, but as the plant becomes more iron stressed, these are followed by increased production of highly efficient chelators, called phytosiderophores, secreted in localized zones behind the root tips [56].

Recently, exchange of metals between siderophores and phytosiderophores has been proposed as a primary mechanism for plant use of microbial siderophores [57-58]. It has also been shown that microbial siderophores may strip iron from phytosiderophores [59]. The partitioning of metals between different types of siderophores and other iron complexes depends on the stability constants as well as the concentration of each chelator and the ability of the chelators to attack the surface of iron minerals and undergo exchange.

The competition for iron between plant and microorganisms involves very complex interactions that depend on a number of factors. For example, differences in the level of siderophore production by all the competing microorganisms; the chemical stabilities of various siderophores and other chelators with iron; their resistance to degradation; and the ability of different siderophores in the soil solution may interact through ligand exchange.

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