Siderophores phytosiderophores

Recently, exchange of metals between siderophores and phytosiderophores has been proposed as a primary mechanism for plant use of microbial siderophores (20,21). Conversely, it has also been shown that microbial siderophores... 

IV. IRON UPTAKE MECHANISMS FROM SIDEROPHORES AND PHYTOSIDEROPHORES... 

In the rhizosphere, microorganisms utilize either organic acids or phytosiderophores to transport iron or produce their own low-molecular-weight metal chelators, called siderophores. There are a wide variety of siderophores in nature and some of them have now been identified and chemically purified (54). Pre.sently, three general mechanisms are recognized for utilization of these compounds by microorganisms. These include a shuttle mechanism in which chelators deliver iron to a reductase on the cell surface, direct uptake of metallated siderophores with destructive hydrolysis of the chelator inside the cell, and direct uptake followed by reductive removal of iron and resecretion of the chelator (for reviews, see Refs. 29 and 54). 

These data strongly suggest that siderophore production by root-colonizing microorganisms is induced only at a level neeessary to supplement that which is not provided by phytosiderophores and organic acids released during the plant iron stress response. Thus, the plant iron stress response may control iron availability to microorganisms in the rhizosphere. 

D. E. Crowley. V. Romheld, H. Marschner, and P. J. Szaniszlo. Root-microbial effects on plant iron uptake from siderophores and phytosiderophores. Plant Soil 142 1 (1992). 

Some metals may need to be mobilized from the environment to make them bioavailable. Iron in particular must be rendered more soluble to be accessible for uptake. Microorganisms and some plants have evolved with secreted ligands known as siderophores (or phytosiderophores). These ligands bind Fe + with extraordinary affinity. For example, a complex of the siderophore enterobactin with ferric iron has a formal stability constant of 10 (19). Once siderophores compete with other environmental ligands for iron, the ferric iron-siderophore complex then binds to specific transport proteins at the microbial... 

Siderophores produced by plants and soil microorganisms may play an important role in the complexation and weathering of Fe-bearing minerals. Several bacteria and fungi produce siderophores in order to enhance Fe solubility. Certain grasses also excrete phytosiderophores under Fe-deficient conditions the affinity of phytosiderophores for Fe (III) is less than that of microbial siderophores. All siderophores contain the anionic reactive group (R—CO—NO—), which enables these compounds to be extremely effective chelators of Fe. 

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