Rhizosphere iron reduction

Figure 7 Mixld for iron (Fe) deficiency induced changes in root physiology and rhizo-sphere chemistry associated with Fc acquisition in strategy I plants. (Modified froin Ref. 1.) A. Stimulation of proton extru.sion by enhanced activity of the plasnialemma ATPase —> Felll solubilization in the rhizospherc. B. Enhanced exudation of reductanls and chela-tors (carhoxylates. phenolics) mediated by diffusion or anion channels Pe solubilization by Fein complexation and Felll reduction. C. Enhanced activity of plasma membrane (PM)-bound Felll reductase further stimulated by rhizosphere acidificalion (A). Reduction of FolII chelates, liberation of Fell. D. Uptake of Fell by a PM-bound Fell transporter.

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). 

Recent studies have further examined the iron stress response of pseudomonads using an iron-regulated, ice-nucleation gene reporter (inaZ) for induction of the iron stress response (17,18,84). This particular reporter system was developed by Loper and Lindow (85) for study of microbial iron stress on plant surfaces but was later employed in soil assays. In initial. studies, cells of Pseudomonas fluorescens and P. syringae that contained the pvd-inaZ fusion were shown to express iron-responsive ice-nucleation activity in the bean rhizosphere and phyllosphere. Addition of iron to leaves or soil reduced the apparent transcription of the pvd-inaZ reporter gene, as shown by a reduction in the number of ice nuclei produced. 

As summarized by Jacobson (1994), biological Fe(III) reduction will be more important than chemical reduction when amorphous Fe(IIl) oxides are plentiful and continually regenerated, or H2S production is low relative to the Fe(III) concentration. This first condition is likely to be met in the rhizosphere where radial O2 loss drives Fe oxide formation. The second condition will be met in low-salinity wetlands, or in saline systems with mineral (i.e., iron-rich) sediments. However, even chemical reduction of Fe(III) is ultimately due to microbes since the H2S that reduces the Fe is the result of a biological process, SO4" reduction (Megonigal et al., 2004). 

---