Iron rhizospheric

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.

J. Gerke, W. Romer, and A. Jungk, The excretion of citric and malic acid by proteoid roots of Liipinus aihus L. effects on soil solution concentrations of phosphate, iron, and aluminium in the proteoid rhizosphere samples of an Oxisol and a Luvi.sol. Z. Pktnzenernaehr. Bodenk. I57 2S9 (1994). 

H. F. Bienfait, H. J. Lubberding, P. Heutink, L. Linder, J. Visser, R. Kaptain and K. Dijkstra, Rhizosphere aeidilication by iron deficient bean plants the role of trace amounts of divalent metal ions. Plant Physiol. 90 359 (1989). 

D. L. Jones. P. Darrah, and L. V. Kochian, Critical evaluation of organic acid mediated iron dissolution in the rhizosphere and its potential role in root iron uptake. Plant Soil 180 57 (1996). 

V. Romheld and H. Marschner. Mobilization of iron in the rhizosphere of different plant species. Adv. Plant Niitr. 2 155 (1986). 

D. E. Crowley and D. Gries, Modeling of iron availability in plant rhizosphere. Biochemistry of Metal Micronutrients in the Rhizosphere (J. A. Manthey, D. E. Crowley, and D. G. Luster eds.), Lewis Publishers, Boca Raton, Florida, 1994, p. 199. 

Despite the wealth of information on siderophores, there is still considerable debate as to how they function in the plant rhizosphere and the degree to which they accumulate in soils. Much of this debate has been due to inadequate methodology for detecting siderophores at microsite locations in the rhizosphere and the lack of analytical methods for in situ study of the interaction of siderophores and other iron mobilizing substances. Using simplified systems in the laboratory, it is possible to examine many different scenarios as to how siderophores might function. Yet, for the most part, there is still almost no information... 

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). 

Siderophore production by rhizosphere bacteria that are culturable on agar media has been estimated by plating out colonies on an indicator agar medium containing chrome azurol, a weak iron-chelating complex that changes color upon... 

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. 

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. 

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