Plant root systems occupy the soil horizon richest in SOM where the live, senescent and dead roots provide substrate materials for microbial growth.
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The rhizosphere has been termed the region of soil under the influence of the plant roots and in which there is a proliferation of soil microorganisms. In addition, two other areas of the roots that are colonised by microbes, the rhizoplane (the root surface) and the endorhizosphere (the interior of the root). The concept of the root cortex as being part of a microbial continuum extending from the soil associated with the roots to the root cortical tissues has been introduced (Old & Nicolson, 1982).
The number of soil microorganisms in the vicinity of the root has been found to decrease with the distance from the root tissue. Bacterial coverage of the root has been estimated to be in the range of 5 to 10% (Rovira, 1979). Distribution of bacterial cells on the root surface appears to be uneven and related to plant microbe contact rather than microbial mobility (Howie & Cook, 1985; Dijkstra et al., 1987; Misaghi et al., 1992). Bacterial cells have rarely been found associated with the rapidly growing apical tip, with the first area to be colonized being just beyond the zone of elongation. Weller (1984) monitored the distribution of a fluorescent pseudomonad that was suppressive to the causal agent, Gaeumannomyces graminis var. tritici of take-all of wheat after its introduction as a coating on the seed. A population gradient of bacterial colonisation was demonstrated along the roots with highest populations at the base of the root and the population declining towards the root tip. A number of studies have consistently identified high densities of microbial cells around cell junctions that have been attributed to an abundant supply of root exudates and an association with dead epidermal cells (Rovira,
1979; Bennet & Lynch, 1981).
Howie & Cook (1985) investigated the role of cell motility in pseudomonads with flagella and non-motile mutants in the colonisation of roots. N o
significant differences in motile and non-motile populations were found, indicating that bacterial distribution was dependent on root elongation in the absence of the downward movement of water. Dijkstra et al. (1987) considered that the differing bacterial growth rates, in association with passive displacement of cells via elongation of the cells at the root surface could account for the differing densities of colonization of wheat roots between
Pseudomonas fluorescens and Bacillus subtilis, which were determined to be
107 c.f.u. cm-1 root for P flourescens and 105 c.f.u. cm-1 root for B.subtilis.
Microbial densities in the rhizosphere environment generally increase with the age of the plant roots (Campbell & McDonald, 1989).
Protozoan population levels also increase in the vicinity of the root and in response to elevated numbers of bacteria, R/S ratios of up to 10 have been observed. Their predation activities are not only thought to affect the microbial populations but also to release minerals back into the soil that might be of benefit to the plant. If the plant is nitrogen or phosphorous limited, as it may well be in most natural ecosystems, the photosynthate produced is probably sufficient for both and it may not matter if some is lost through the roots provided this produces a marginal increase in phosphorous and nitrogen uptake (Campbell & McDonald, 1989). Circumstantial evidence of enhanced deposition of the plant photosynthate via the production of root exudates in natural soils as opposed to in vitro conditions has been observed (Lynch,
1982).
Microbial interactions in the rhizosphere and on the root surface (rhizoplane)
The root environment has been characterised as a site of intense microbial competition for nutrients (Fravel, 1988; Weller, 1988). Much interest has
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focused on the microbial interactions occurring in the rhizosphere and the rhizoplane with a view to the manipulating the root microflora to either suppress phytopathogens (Biological control) (Weller, 1988) or to enhance plant growth by the introduction of Plant Growth Promoting Rhizobacteria (PGPR) (Kloepper et al., 1980; 1989). Considerable inconsistency has been associated with experiments on biological control (Howie & Echandi, 1983) and much speculation still surrounds the reasons for a correlation between reduced disease incidence and increased crop yields. Production of antibiotics by inoculants (Fravel, 1988) and aggressive colonization of available niches or rhizosphere competence (Weller, 1988) have frequently been attributed to the successful establishment and activity of inoculants in the rhizosphere. One of the most successful biological control strategies involved the use of a non- tumourgenic Agrobacterium radiobacter K84 strain to control crown gall disease, caused by the related but virulent Agrobacterium tumefaciens (Moore, 1985). Control operates via the production of Agrocin by K84, this bacteriocin is taken up into the susceptible strains by a permease encoded by the tumour-forming (Ti) plasmid of A.tumefaciens (Engler et al., 1975), the requirement for this plasmid encoded gene means that the bacterocin is selectively targeted at A.tumefaciens.
Enhanced yields in the presence of PGPR have been attributed to the extracellular production of siderophores, which efficiently complex environmental iron making it unavailable to certain components of the soil microflora (Kloepper et al., 1980). Increased yields achieved through inoculation of plants with PGPR were mimicked by the action of a yellow- green fluorescent siderophore isolated from Pfluorescens and named pseudobactin (Kloepper et al„ 1980). Many PGPR have been identified to produce plant growth promoting substances such as auxins and gibberellins
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(Brown, 1972; 1974). Positive correlations between production of these compounds in vitro with enhanced crop yields and altered root or plant morphology have been reported. However, the significance of these plant microbe interactions remains to be established (Brown, 1974).
Analysis of the organic materials found on or in association with the roots reveal an assortment o f soluble molecules that include amino, aliphatic and aromatic acids and amides and sugars. In addition to these are a range of complex insoluble structures such as cellulose, lignin, protein and chitin. With the simple sugars and amino and organic acids common to all rhizospheres and the more complex compounds particular to certain plant rhizospheres. A number of these compounds have particular biostatic and biocidal properties. Pathways for the release of plant assimilates from roots include the leakage or diffusion of molecules across cell membranes, root secretions and extrusions. Root caps and tips are sites of active exudation, releasing mucilaginous secretions in addition to root cap and root tip cells. The main root axis predominantly releases soluble and diffusible material and some mucigel (a mixture of polysaccharides). Mucigel has been identified to be the dominant excretory product of the roots and was found to account for 80% of the carbon lost from wheat roots, aside from losses due to respiration. Labelled 14C tracer studies revealed that 11% of plant assimilate is acquired for microbial respiration and 2% ends up as SOM (Paul & Clark, 1989).
In addition to the enhanced amounts of organic matter available in the rhizosphere, other environmental gradients may occur. In general, the water potential surrounding the roots is usually not that different from un-planted soil, provided that there is continuity between the soil water and the plant root, and the water enters uniformly over the root surface. However, if there is a
local water stress in the rhizosphere these assumptions may not be valid and water uptake may be concentrated into a small proportion o f the total root which is in direct contact with the soil. Certain groups of bacteria and fungi will tolerate reduced water potentials. There may also be compaction of soil around the roots, generated as the root pushes through the soil and contributing to a restricted flow of water. Gas exchange in the rhizosphere may be different, for instance in well aerated soils there may be a slight elevation of C 0 2 levels and depletion of oxygen. However, as the soil becomes waterlogged or even wet, the situation alters as diffusion of 0 2 and C 0 2 is slow through the films of water surrounding the roots and soil pores.