Root-microbe-soil interactions

Emphasis is not on compiling a comprehensive review, but rather on problems and potential for research in this area. Allelochemical sources, synthesis, metabolism, degradation, binding in soils, and mode of action are briefly presented and discussed with regard to root-microbe interactions. Data on these areas is accessed with recommendations and suggestions for further investigation. 

Mathesius, U., Conservation and divergence of signalling pathways between roots and soil microbes — the Rhizobium-legame symbiosis compared to the development of lateral roots, mycor-rhizal interactions and nematode-induced galls. Plant Soil, 255, 105, 2003. 

TRANSFORMATIONS AND DYNAMICS OF METALS AND METALLOIDS AS INFLUENCED BY SOIL-ROOT-MICROBE INTERACTIONS... 

As a recognition of the importance of biophysico-chemical processes of metals and metalloids in soil environments, we initiated the first volume of the lUPAC—Wiley book series Biophysico-Chemical Processes in Environmental Systems. This volume, which consists of 15 chapters, is organized into three parts Fundamentals of Biotic and Abiotic Interactions of Trace Metals and Metalloids with Soil Components Transformations and Dynamics of Metals and Metalloids as Influenced by Soil—Root—Microbe Interactions and Speciation, Mobility, and Bioavailability of Metals and Metalloids and Restoration of Contaminated Soils. 

The higher concentrations of phenolic acids required for a given percent inhibition between the two systems stem from the fact that nutrient cultures have a much more consistent environment than soil culture systems in that water, nutrients, and phenolic acids are evenly distributed in the treatment container and thus are readily available to interact with root surfaces. Soil systems, on the other hand, are much more complex heterogeneous environments in which roots must compete with a variety of soil sinks (e.g., clays, organic matter, and microbes) for water, nutrients, and phenolic acids. There is also mechanical resistance to the movement of water, nutrients, and phenolic acids and the growth of roots in soils. The slower development of inhibition after treatment and the slower recovery after phenolic acid depletion in soil systems is very likely related to the slower growth of seedlings in soil culture. 

E. A. Robleto, A. J. Scupham, and E. W. Triplett, Trifolitoxin production in Rhizohium eili strain CE3, increases competitiveness for rhizosphere colonization and root nodulation of pha.seolus vulgaris in soil. Mol. Plant Microbe Interact. 10 228-233 (1997). 

Such differences in the amount and type of rhizodeposition that occur on the root with time result in concomitant variations in microbial populations in the rhizosphere, both within the root (endorhizosphere), on the surface of the root (rhizoplane), and in the soil adjacent to the root (ectorhizosphere). The general microbial population changes and specific interaction of individual compounds from specific plants or groups of plants with individual microbial species are covered in detail elsewhere (Chap. 4). Consequently, this chapter is restricted to consideration of methodologies used to study carbon flow and microbial population dynamics in the rhizosphere, drawing on specific plant-microbe examples only when required. 

Root exudation and microbial action produce organic compounds with a range of composition and molecular weights. These compounds interact with the mineral particles, which also vary in size, shape, ciystallinity, and electric charge (Emerson et al. 1986). Interactions between soil mineral particles, organic matter and microbes can occur at many different size scales, because these materials have a large size range in soils (Fig. 7). 

Cordier C., Gianianzzi S., Gianinazzi-Perason V. Colonization patterns of root tissues by Phytophthora nicotianae var. parasitica related to reduced disease in mycorrhizal tomato. Plant Soil 1996 185 223-232. Cordier C., Pozo M.J. Barea J.M., Gianinazzi S. Gianinazzi-Pearson V. Cell defense responses associated with localized and systemic resistance to Phytophthora parasitica induced in tomato by an arbuscular mycorrhizal fungus. Mol Plant-Microbe Interactions 1998 11 1017-1028. 

Lambais M.R., Mehdy M.C. Suppression of endochitinase, (5-1, 3-endoglucanase and chalcone isomerase expression in bean vesicular-arbuscular mycorrhizal roots under different soil phosphate conditions. Mol Plant-Microbe Interactions 1993 6 75-83. 

The phytoremediation process may be viewed as a symbiotic process between plants and soil microbes that involved in phytoremediation (Lasat, 2002). Plant and bacterial interaction can enhance the effectiveness of phytoremediation technology because plants provide carbon and energy sources or root exudates in the rhizosphere that will support microbial community in the degradation and transformation of soil pollutants (Siciliano and Germida, 1998). In addition, the presence of soil microbes can increase the water solubility or bioavailability of pollutants in soils, which facilitates the uptake of pollutants by plants (Lasat, 2002 Siciliano and Germida, 1998). However, the specificity of the plant-bacteria interactions besides being much intricate is dependent upon soil and the aqueous conditions, which can alter contaminant... 

The bioavailability of a metal(loid) is controlled by its physicochemical speciation [e.g., Cr(OH)3 vs. HCrO4 ] in the soil environment and tlie biological (microbes, plant roots), physical (point of zero charge, soil moisture content), and chemical factors (pH, ionic strength, redox potential) interacting with metal(loid)s in soils (McBride, 1994 Sparks, 2003 Krishnamurti and Naidu, Chapter 11, this... 

In the rhizosphere, the kinds and concentrations of substrates are different from those in the bulk soil, because of root exudation. This leads to colonization by different populations of bacteria, fungi, protozoa, and nematodes. Plant-microbe interactions, in turn, affect physicochemical reactions in the rhizosphere. The total rhizosphere environment is governed by an interacting trinity of the soil, the plant. 

The differences between clones may depend on a combined effect of plant exudate and microbial effects on the exudate (Marschner, 1995). In studies under nonsterile conditions, rhizosphere microbes may alter the chemical composition of root exudates. Therefore, the differences between high and low metal soil condition as well as different metals in spiked soil can be due to toxic metal effects or effects resulting from an excess of chloride on microbes. When comparing various clones the differences in exudate composition could have been due to various microbe-clone relationships. One should, however, keep in mind that a microbe-plant relationship is present in real environment where we also find these metal-accumulation differences between clones. Whether the differences in rhizosphere processes are due to plants alone or a combination with microbial interactions has to be further investigated. 

The overall effect of plant-microbe interaction shows an increase in MBM in the rhizosphere, owing to the high supply of organic carbon by roots (Lynch and Whipps, 1990). As shown in Fig. 5, enhanced microbial activity was manifested by a steady increase in MBM. Horak (1982) demonstrated that mobilization of copper in the rhizosphere of peas should be a direct outcome of low-molecular-weight root exudates and of an indirect effect via microbial activity in the rhizosphere. The results of the present study show that changes in the major soil properties, including redox potential, DOC and microbial activities, are all in favor of a transformation of metals from less available to more available fractions, leading to variations in various metal fractions in the maize rhizosphere. 

Phenolic acids in soils occur either in a free state in the soil solution, reversibly sorbed to soil particles, fixed (irreversibly sorbed) very tightly to soil particles (e.g., recalcitrant organic matter, and clays), and/or on and in living and dead plant tissues/residues ( free , reversibly sorbed, and fixed). Of general interest to plant-plant allelopathic interactions are the free and reversibly sorbed states frequently referred to as the available fraction. Of particular interest is the active fraction of available phenolic acids, the fraction of available phenolic acids that actually interact with seeds, roots and microbes. Unfortunately we presently do not have a means of quantifying the active fraction, thus the focus on the available fraction. 

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