Interactions between plants and rhizosphere microbes may play a critical role in the outcome of plant competition. Competitive interaction among plants may also be important for the development of rhizosphere soil communities. Microbes affect nutrient uptake (Tinker, 1976; Okon, 1982). Microorganisms may play important roles affecting plant competition acting as mutualistic or pathogenic plant associates (Allen and Allen, 1990). Rhizosphere microorganisms may affect plant growth directly (Woltz, 1978; Suslow and Schroth, 1982; Gaskins et al., 1985; Alstrom, 1987; Schippers et al., 1987) or indirectly by their effects on each other and the microscale alteration of soil nutrients. Plant performance may also be affected by competitive interactions between adjacent plants (Goldberg and Fleetwood, 1987; Gurevitch et al., 1990; Goldberg and Landa, 1991).
Another example of microbial diversity influencing plant growth is the investigation of deleterious rhizobacteria, which were discovered in the early 1980s. Investigations of deleterious rhizobacteria have led to changes in management practices for many crops (Schippers et al., 1987) and may lead to biological control of weed species (Kremer et al., 1990; Kennedy et al., 1991). Deleterious rhizobacteria that specifically inhibit various grass weeds, but do not affect the crop, have been isolated from soil (Kennedy et al., 1991; 1992; Kennedy, 1994) and inhibit plant growth by the production of plant-suppressive compounds (Tranel et al., 1993). These bacteria are excellent biological control agents, in part, because they are aggressive colonizers of the roots and residue, often constituting up to 95% of the total pseudomonads on the plant root (Stroo et al., 1988; Kennedy et al., 1992). Biocontrol is critical to sustainable agriculture systems, but a greater understanding of soil microbes and their ecology is needed before biological control can be implemented. There is a wealth of genetic material contained within the soil that may have potential in biotechnology programs; thus, diversity investigations will benefit more than just one area of science (Malik and Claus, 1987; Bull et al., 1992).
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