Rhizosphere microorganisms

E. A. Curl and B. Truelove, The Rhizosphere, Springer-Verlag, New York, 1986. R. Schonwilz and H. Ziegler, Interaction of maize roots and rhizosphere microorganisms, Z, Pflanzenerniihr. Bodenk. 752 217 (1989),... 

E. Bar-Ness, Y. Hadar, Y. Chen, V. Romheld, and H. Marschner, Short-term effects of rhizosphere microorganisms on Fe uptake from microbial sidcrophores by maize and oats. Plant Physiol. 100 45 (1992). 

Kraffczyk, G. Trolldenier, and H. Bcringcr, Soluble root exudates of maize Influence of potassium supply and rhizosphere microorganisms. Soil Biol. Biochem. 76 315 (1984). 

I. PRODUCTION OF ANTIBIOTICS. The production of secondary metabolites with antimicrobial properties has long been recognized as an important factor in disease suppression (see Chap. 7). Metabolites with biocontrol properties have been isolated from a large number of rhizosphere microorganisms, including the fluorescent pseudomonads (Table 2). Further discussion is not given here since this is the subject of recent reviews (122,123). 

A. B. Schippers, A. W. Bakker, and P. A. H. M. Bakker, Interactions of deleterious and beneficial rhizosphere microorganisms and the effect of cropping practices. Annual Review of Phytopathology 25 339 (1987). 

E. Liljeroth, J. A. Van Veen, and H. J. Miller, As.similate translocation to the rhizosphere of two wheat lines and subsequent utilization by rhizosphere microorganisms at two soil nitrogen concentrations. Soil Biology and Biochemistry 22 1015 (1990). 

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). 

F. O Gara, D. N. Dowling, and B. Boesten, Molecular Ecology of Rhizosphere Microorganisms, VCH, Weinheim, 1994. 

Early rhizosphere establishment is demonstrated in 2-3 day-old wheat plants when there is a shift towards a population of amino acid requiring bacteria (19). Maximum activity and numbers of rhizosphere microorganisms correlated with maximum vegetative plant development (20-22). Once established, the rhizosphere remains qualitatively similar, but quantitatively increases from seedling stage to maturity (23). After maturity the bacterial population reverts to a population similar to that in non-rhizospheric soils. 

L. L. Barton and J. Abadia, Iron Nutrition in Plants and Rhizospheric Microorganisms, Springer, Dordrecht, 2006, p. 477. 

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. 

Organic compounds released from sloughed-off root cells and tissues are a major carbon source for rhizosphere microorganisms but may indirectly have an impact as microbial metabolites on nutrient availability and on exclusion of toxic elements in the rhizosphere (Brimecombe et al., 2007). Continuous root turnover is a general feature of plant development, and insoluble root debris may comprise 50-90% of total rhizodeposition (Darrah, 1991). 

It has been hypothesized that rhizosphere microorganisms may accelerate the decomposition of native soil organic matter and also stimulate the dissolution of insoluble minerals in a way similar to that proposed for plants (pH, redox, and metal-complexation reactions see above). 

Owen, A. G., and Jones, D. L. (2001). Competition for amino acids between wheat roots and rhizosphere microorganisms and the role of amino acids in plants N acquisition. Soil Biol. Biochem. 33, 651-657. 

Varanini, Z., and Pinton, R. (2006). Plant-soil relationship Role of humic substances in Iron Nutrition. In Iron Nutrition in Plants and Rhizospheric Microorganisms, Barton, L. L., and Abadfa, J., eds., Springer-Verlag, Heidelberg, pp. 153-168. 

D Arcy Lameta, A., Jay, M. Study of soybean and lentil root exudates 3. Influence of soybean isoflavonoids on the growth of rhizobia and some rhizosphere microorganisms. Plant Soil 1987 101 269-272. 

Fate of Phenolic Allelochemicals in Soils - the Role of Soil and Rhizosphere Microorganisms... 

Because phenolic acid concentrations in soil solutions are determined not only by input processes (e.g., leaching, exudation, release of bound forms) but also by output processes (e.g., sorption, polymerization, utilization by microorganisms), simply determining soil solution concentrations over time cannot provide information on how any one of these processes may actually influence the soil solution concentrations of phenolic acids. The effects of each process must be characterized separately. The impact of soil or rhizosphere microorganisms, for example, could be estimated by coupling changes in soil solution concentrations of phenolic acids with the activity of soil or rhizosphere microorganisms that can utilize phenolic acids as a carbon source. This approach, however, assumes that all the other output process rates remain constant. 

INFLUENCE OF BULK-SOIL AND RHIZOSPHERE MICROORGANISMS ON PHYTOTOXICITY... 

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