Reduction rhizosphere

Modifiers Modification of the rhizosphere soil with (e.g., protons, reductants)... 

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.

A. W. Bakker and B. Schippers, Microbial cyanide production in the rhizosphere in relation to potato yield reduction and Pseudomonas spp-mediated plant growth-stimulation, Soil Biology and Biochemistry 19 451 (1987). 

Reciprocally, the growth on single C source significantly decreases the bacterial diversity. For example, in the rhizosphere soil of potato, a dramatic reduction in the number of ribotypes was found by temperature gradient gel electrophoresis (TGGE) after 48 h of incubation with single C source substrate in Biolog microplate wells (I46). 

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. 

K. Posta, H. Mar.schner, and V. Romheld, Manganese reduction in the rhizosphere of mycorrhizal and non-mycorrhizal maize, Mycorrhiza 5 119 (1994). 

Zhang TC, Pang H. Applications of microelectrode techniques to measure pH and oxidation-reduction potential in rhizosphere. Soil Environ. Sci. Technol. 1999 33 1293-1299. 

Bouldin DR. 1966. Speculations on the oxidation-reduction status of the rhizosphere of rice roots in submerged soils. In IAEA Technical Report 655. Vienna IAEA, 128-139. 

Bashan, Y. Levanony, H. (1988). Adsorption of the rhizosphere bacterium Azospirillum brasilense Cd to soil, sand and peat particles.Journal of General Microbiology, 134,1811-20. Baxter, R. M. (1990). Reductive dechlorination of certain chlorinated organic compounds... 

Because of the large demand for 02 by roots and rhizosphere microorganisms, a large change in the soil s redox potential can be expected. Most redox processes are coupled with the release (oxidation) or consumption (reduction) of protons and therefore may cause pH changes also. 

Recent data show that Fe-WEHS could be used by dicots via a reduction-based mechanism more efficiently than other Fe-chelates that may be present in the rhizosphere, such as Fe-phytosiderophores and Fe-citrate (Cesco et al., 2006). It has been shown that Fe-WEHS could also be used by barley plants (strategy II) via a mechanism possibly involving ligand exchange between phytosiderophores and WEHS (Cesco et al., 2002). 

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