The rhizosphere

Conditions in the rhizosphere, the cylinder of soil that surrounds the plant root at a distance of up to 2 - 5 mm (Curl and Truelove, 1986) can be very different from those in the bulk soil. This is the local environment from which the root takes up nutrients, excretes inorganic and organic species, and in which there is shedding and decomposition of parts of the root surface. The pH and the microbial population can both deviate from those in the bulk soil, the latter showing a population density 2-20 times higher. The roots of many plant families are associated with particular fungi, mycor-rhizae, which are very important for the mineral nutrition of plants (Tinker and Gilden, 1983 Marschner et al., 1986, Streit and Stumm, 1993). 

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A recent suggestion has been to use plants to stimulate the microbial degradation of the hydrocarbon (hydrocarbon phytoremediation). This has yet to receive clear experimental verification, but the plants are proposed to help deUver air to the soil microbes, and to stimulate microbial growth in the rhizosphere by the release of nutrients from the roots. The esthetic appeal of an active phytoremediation project can be very great. 

It will also be important to understand the rhizosphere ecology around the roots of metal accumulating plants fully. Maximizing the bioavailabihty of the contaminant metals in this zone may require the optimization of the microbial communities, or perhaps the addition of soil amendments. There are early indications that such intervention may be beneficial (88), but research in this area is at a very early stage. 

Walton BT, Anderson TA. 1990. Microbial degradation of trichloroethylene in the rhizosphere Potential application to biological remediation of waste sites. Appl Environ Microbiol 56 1012-1016. 

Establishment of inoculated Azospirillum spp. in the rhizosphere and in roots of field grown wheat and sorghum. Plant Soil 90 35-46. 

Molina L, C Ramos, E Dnqne, MC Ronchel, JM Garcia, L Lyke, JL Ramos (2000) Snrvival of Pseudomonas putida KT2440 in soil and in the rhizosphere of plants nnder greenhonse and environmental conditions. Soil Biol Biochem 32 315-321. 

These results may be particularly relevant in the context of bioremediation, and lend support to the potential role of the rhizosphere and of plants (Chapters 12 and 14, Part 4). 

There has been considerable interest in the use of plants for bioremediation and this merits a rather extensive discussion. Plants can play an important role in bioremediation for several reasons (1) they can transport contaminants from the soil, (2) they can metabolize the contaminants after uptake, or (3) they can produce exudes that support microbial activity for degradation of the contaminants. In addition, bacteria can produce metabolites that counter the effect of toxins produced by fungi, and serve as biocontrol agents that diminish the need for the application of agrochemicals. Plant exudates play an important role in supporting the growth and activity of bacteria that carry out the degradation of contaminants in the rhizosphere and rhizoplane (the external surface of roots... 

Brazil GM, L Kenefick, M Callanan, A Haro, V de Lorenzo, DN Dowling, F O Gara (1995) Constrnction of a rhizosphere psendomonad with potential to degrade polychlorinated biphenyls and detection of bph gene expression in the rhizosphere. Appl Environ Microbiol 61 1946-1952. 

Notz R, M Maurhofer, H Dubach, D Haas, G Defago (2002) Fusaric acid-producing strains of Fusarium oxysporum alter 2,4-diacetylphloroglucinol biosynthetic gene expression in Pseudomonas fluorescens CHAO and in the rhizosphere of wheat. Appl Environ Microbiol 68 2229-2235. 

The Rhizosphere as a Site of Biochemical Interactions Among Soil Components, Plants, and Microorganisms... 

The rhizosphere lacks physically precise delimitation (18). The volume of rhizosphere depends on the rate of exudation and impact utilization of rhizodeposits (Chap. 6). The spatial and temporal distribution of exudates as well as their metabolism is related to the concentration of CO (Chap. 6). However, according to Darrah (Chap. 11), the layer of soil where microbial growth is affected by exudates can be 1-2 mm wide. 

Both H-thymidine incorporation and radiolabeled leucine incorporation techniques have been recently used to determine bacterial activity and growth in the rhizosphere of barley seedling (28), Bacteria were initially released from the rhizosphere using homogenization and centrifugation before adding the labeled substrates. The cell incorporation rate was twice as high in the rhizosphere than in bulk soil. In addition, both the leucine and thymidine incorporation rates increased with the distances from the root tip (28). 

Any bacterial species living in a mixed microbial population, such as that of the rhizosphere, may encounter not only the molecular signal produced by a cell of the. same species but also molecular signals produced by cells of different species. The situation is made more complex by the presence of plant molecular signals, and by the fact that the same AHL molecule can be used to regulate the... 

It is well known that chemical compo.sition of rhizosphere solution can affect plant growth. Particularly, uptake of nutrients may be considerably influenced by the ionic concentration of the rhizosphere solution (40). Despite the difficulty of defining the exact concentration of ions in the rhizosphere surrounding each root (or even root portion), it has been unequivocally demonstrated that plants have evolved mechanisms to cope with the uneven distribution of ions in the root surrounding in order to provide adequate supply of each essential nutrient (41). These mechanisms include expression of transporter genes in specific root zones or cells and synthesis of enzymes involved in the uptake and assimilation of nutrients (40,43). Interestingly, it has been shown that specific isoforms of the H -ATPase are expressed in the plasma membrane of cell roots it has been proposed that the expression of specific isoforms in specific tissues is relevant to nutrient (nitrate) acquisition (44) and salt tolerance (45). 

Molecular analysis of the interaction between plants, microbes, and soil components may help us understand the causal relationships of events taking plaee in the rhizosphere. Nevertheless, due to the necessity to simplify the experimental approaches, we still do not have the complete picture that takes into ae-count the relative weight of each factor. 

R. W. Hedges, and E. Messens, Genetic aspects of rhizo.sphere interactions. The Rhizosphere (J. M. Lynch, ed.). John Wiley, Chichester, 1990, p. 129. 

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