Rhizosphere importance

H. Marschner, V. Romheld, W. J. Horst, and P. Martin, Root-induced changes in the rhizosphere importance for the mineral nutrition of plants. Z. Pflanzenernarhr. Boclenk. /49 441 (1986). 

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. 

Mirleau P, R Wogelius, A Smith, MA Kertesz (2005) Importance of organosulfur utilization for survival of Pseudomonas putida in soil and rhizosphere. Appl Environ Microbiol 71 6571-6577. 

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... 

Leigh MB, JS Fletcher, X Fu, FJ Schmitz (2002) Root turnover an important source of microbial substrates in rhizosphere remediation of recalcitrant metabolites. Environ Sci Technol 36 1579-1583. 

As mentioned before and in Chaps. 4 and 6, the concentration of rhizode-position decreases as the distance from the rhizoplane increases, whereas the opposite generally occurs for the concentration of any plant nutrient in soil. In this context, the role of rhizospheric soil, rather than that of the bulk soil, is crucial for plant nutrition. It has also to be considered that very different situations can occur depending on the type of nutrient (24) and the nutritional status of plants (see Chap. 3) furthermore, different portions of the root system are characterized by differential nutrient-specific rates of uptake (25). All the above statements point to the necessity of reconsidering the concept of plant nutrient availability giving more importance to the situation occurring in the soil surrounding the root. 

This chapter considers the various types of root products with a potential functional role in the usually tough environment of soil. Only direct effects of immediate benefit to plant growth—e.g., an increase in nutrient solubility—are considered here. Although root products of a plant species may have a direct effect on important groups of soil organisms, such as rhizobia and mycorrhizae. their effect on the plant is not immediate these and aspects related to microbial activity in the rhizosphere are not considered here (see Chaps. 4, 7, and 10). For an extensive and recent review of the microorganisms in the rhizosphere, the reader is referred to Bowen and Rovira (23). 

S. J. Grayston, D. Vaughan, D. Jones. Rhizosphere carbon flow in trees, in comparison with annual plants the importance of root exudation and its impact on microbial activity and nutrient availability. Appl. Soil Ecol. 5 29 (1996). 

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). 

Further progress may derive from a more accurate definition of the chemical and physical properties of the humic substances present at the rhizosphere and how they interact with the root-cell apoplast and the plasma membrane. An interaction with the plasma membrane H -ATPase has already been observed however this master enzyme may not be the sole molecular target of humic compounds. Both lipids and proteins (e.g., carriers) could be involved in the regulation of ion uptake. It therefore seems necessary to investigate the action of humic compounds with molecular approaches in order to understand the regulatory aspects of the process and therefore estimate the importance of these molecules as modulators of the root-soil interaction. 

Recurrent is the lack of adequate techniques to assess carbon flows through the plants and microbes into soil organic matter (151). Most important is the development of techniques and protocols to separate rhizosphere from nonrhizosphere soil as well as possibly to facilitate analyses of soil carbon dynamics. The use of carbon isotopes, and, where possible, application of double labeling with C and C, seems inevitable in order to separate the contribution of different substrates to the formation of the soil organic matter pool and to get to an understanding of the ecological advantage of exudates and rhizodeposits. 

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