Microorganisms associated with the rhizosphere have been demonstrated to increase root exudation through production of plant hormones and by physically damaging the roots (45). These actions create a nutrient-rich rhizosphere zone that is naturally colonized by many beneficial or sometimes pathogenic microorganisms. Bacteria and fungi have a considerable impact on plant growth, development, and productivity. The numerous inter­actions between bacteria, fungi, and roots may have beneficial, harmful, or neutral effects on the plant, the outcome being dependent on the type of symbiont interaction and the soil conditions (46). “Helper bacteria” may promote the formation of fungal communities in decaying biomass litter (47). Little is known about the ecology of these helper bacteria; however, an analogy to the role played by bacteria present in mycorrhizal fungi can be made (47). In the rhizosphere of plants, and perhaps in the decaying biomass pile, fungi are always accompanied by another important group of microorganisms, the bacteria which also prosper in the organic-rich environment (mostly sugars, amino acids, and organic acids) released from the roots and mycorrhizal fungi. Among them, there is a large cate­gory of growth-promoting rhizobacteria that influence plant growth directly or indirectly by releasing a variety of compounds, from mineral nutrients, to phytohormones, and an­timicrobial compounds. Rhizobacteria have been demonstrated to promote plant growth directly through production of plant hormones such as auxins (48), gibberellins (49), and ethylene (46). Production of indole-3-ethanol or indole-3-acetic acid (IAA), compounds belonging to the auxins, have been reported for several bacterial genera, such as Frankia (50-52), Klebsiella and Enterobacter (53), and Bacillus (49). Most importantly, for rhizobac — teria to act beneficially, they must be able to efficiently colonize and multiply in the plant rhizosphere.

Electron microscopic studies and molecular methods revealed large bacterial populations associated with mycorrhizal roots and extraradical hyphae (54-56). In some cases, bacterial endosymbionts were discovered in fungal hyphae (55,57). The establishment of mycorrhizas on roots is affected by the microbial populations of the rhizosphere, and especially by some bacteria, which can have either a positive or a negative effect on mycorrhiza formation. Garbaye (47, 58) defined a new bacterial category, the mycorrhization helper bacteria that strongly promoted ectomycorrhiza formation. FreyKlett and coworkers (59) suggested that these helper bacteria stimulate the growth of fungal mycelia, thus increasing the probability of a root-mycelium encounter. Some mycorrhiza helper bacteria were noncultivable, and were identified only by molecular ecological methods.