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RESEARCH PRODUCT

Implication of pyoverdines in the interactions of fluorescent pseudomonads with soil microflora and plant in the rhizosphere

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Soils are known to be oligotrophic environments whereas soil microflora is mostly heterotrophic in such way that microbial growth in soil is mainly limited by the scarce sources of readily available organic compounds (Wardle 1992). Therefore, in soils, microflora is mostly in stasis (fungistasis/bacteriostasis) (Lockwood 1977). In counterpart, plants are autotrophic organisms responsible for the primary production resulting from the photosynthesis. A significant part of photosynthetates are released from plant roots to the soil through a process called rhizodeposition. These products, i.e. the rhizodeposits, are made of exudates, lysates, mucilage, secretions and dead cell material, as well as gases including respiratory CO2 and ethylene. Depending on plant species, age and environmental conditions, rhizodeposits can account for up to 40% of net fixed carbon (Lynch and Whipps 1990). On average, 17% of net fixed carbon appears to be released by the roots (Nguyen 2003). This significant release of organic compounds by plant roots in soil oligotrophic environments is then expected to affect strongly the heterotrophic microflora located closely to the roots. Indeed, one century ago, Hiltner (1904) observed an increased proliferation of heterotrophic bacteria in contact with the roots. This author proposed to call rhizosphere the volume of soil surrounding roots in which the microflora is influenced by these roots. Since then, further studies have shown that living roots modify the biological and physicochemical properties of rhizospheric soil determining the rhizosphere effect (Curl and Truelove 1986; Lemanceau and Heulin 1998; Lynch 1990; Rovira 1965). Rhizodeposition affects the soil microflora and especially leads to (i) an increase of its density (Clark 1949; Rovira 1965), biomass (Barber and Lynch 1977) 8 Implication of Pyoverdines in the Interactions of Fluorescent Pseudomonads with Soil Microflora and Plant in the Rhizosphere