Role of Beneficial Microsymbionts on the Plant Performance and Plant Fitness

The number ofmicrobes in the rhizosphere of higher plants ismuch higher in comparison to bulk soil (Hiltner 1904). Due to this positive rhizosphereeffect, there are active and passive physical and chemical changes in the root system of the higher plants, which may have a great impact both on their nutrient status and their growth (Biro 2003). Among the microbes present in the rhizosphere, the microsymbiont bacteria and fungi are the most important for the plant growth and development. Their role and capacity for the biological nitrogen fixation and phosphorousmobilisation is quite established (Barea et al. 2002). The translocation of macroand micronutrients in these zones is influenced by the enhanced microbial activities. The effects aremediatedbydirect transfer of nutrients fromplant by the increased root system, and also by improving the competitiveness of higher plants in the nutrient uptake. Among the beneficial microbes, the associative and symbiotic N2 bacteria and the arbuscular mycorrhizal fungi (AMF) are the most common in the rhizosphere of higher plants. Artificial seed and soil inoculation techniques are used as a simple application of a mixed nodule extracts, or as a soil-root mixture (Hiltner 1904). However, the introduced microbes usually enter in competition with the native microflora in the soil (Graham 1992). The negative effects of the abiotic environmental stress factors (temperature, drought, acidity etc.) are also common (Graham 1992; Bayoumi et al. 1995). The final influence of any microbial inoculation in the rhizosphere, therefore, is the result of the complex interactions between the plants, the rhizosphere inhabitants and the different microbial and environmental components involved (Postma et al. 1989). The antagonistic and synergistic behaviours, of the beneficial microsymbionts is a crucial step considering the plant growth and their sustainability (Hoflich 1993).