INRA-CMSE, Laboratoire de Recherches sur la Flore Pathogene du Sol, 17 rue Sully, 21034 Dijon, Cedex, France

It has long been established that the plant affects the rnicrobial populations inhabiting the rhizosphere, In this area, the density of the rnicroflora is higher and the rnicrobial populations are different from those of the bulk soil. Both differences have been related to the large amounts of organic compounds released by the roots and are called the rhizosphere eftect. Over the last decade, the increasing concern for the impact of the human activity, including agriculture, on the living beings has stimulated numerous studies on the microbial diversity. Several studies have focused on the effect of the plant on the soil-borne microflora and more specifically on that of the rhizosphere on the populations of fluorescent pseudomonads. These bacteria are often considered as a good model for studying the interactions between the plant and the non symbiotic microflora because their are well represented in the rhizosphere and they may contribute to degrading toxic compounds and to improving the plant growth and health. These researches have benefited from the progress achieved in the methods used to characterise the genotype and the phenotype of the microbial populations.

Using these methods, we confirmed the effect of the on the microbial distribution. Soil-borne populations were shown to differ from the rhizospheric populations. which suggests that the plant selects specific bacterial populations. The selection exerted by the plant toward fluorescent pseudomonads appears to be related to their basal metabolism. Indeed, they are able to assimilate specific organic compounds (electron donors), and to mobilize Fe+++ and to reduce nitrogen oxides (electron acceptors). Bacterial traits implicated in the selection achieved by the plant were not related to plasmid DNA. The plant selection was not clearly associated with any specific species nor biovar. The selection was shown to be at least partly, plant specific. The populations associated with two plant species could be distinguished on the basis of their ability to assimilate discriminating compounds. Selection by the plant also depends on the type of soil. Populations of fluorescent pseudomonads associated with a given plant species were shown to vary according to the soil in which the plant is cultivated. This variation was both ascribed to differences between the indigenous populations of the two soils and to the influence of the soil type on the interactions between the plant and the rnicroflora [1,2,3].

The interaction between the host plant and the fluorescent pseudomonads also depends on the rhizospheric microflora. Bacterial populations associated with mycorhizas and mycorrhizosphere were shown to differ from those of the adjacent bulk soil. This difference was again related to the ability of the selected populations to assimilate specific organic compounds [4]. The selection of the fluorescent pseudomonads by the root is also influenced by the presence of necrosis due to the infection by pathogenic fungi (Gaeumannomyces graminis var.tritici) [5,6]. This influence could be due to a modification of the root.

In addition to these trophic relationships, other relationships can occur in the bacterial selection by the plant. Membrane characteristics of endophytic populations were shown to be distinct from the populations from the root surface [7]. Populations tolerant to toxic compounds released by the plant would benefit from a competitive advantage compared to the susceptible populations. The possible implication of signal molecules from the plant could also be proposed. Such molecules have been shown to play an important role in the host-specificity of the Rhizobiaceae. This hypothesis is consistent with the recent demonstration of an environmental stimulus which influence the root colorization by a strain of P.fluorescens through a two component system [8].

1. Lemanceau P. Samson R, Alabouvette C, 1988 Agronomie 8,243-249.
2. Lemanceau P, Corberand T., Gardan L., Latour X., Laguerre G., Beoutgras J-M, Alabouvette C., 1995 Applied and Environmental Microbiology 61, 1004-1012.
3. Latour X., Corberand T., Laguerre G., Lemanceau P., 1996 Applied and Environmental Microbiology 62 2449-2456.
4. Frey P, Frey-Khai P., Garbayo J., Berge O., Heulin T., 1997 Applied and Environmental Microbiology 63 1852-1860.
5. Sarnguet A., Lucas P., Guiltherm A.Y., Ce Vos P., 1997, Proceedings of the 4th Workshop on PGPR, Sapporo, Japan.
6. Raaijmakers JM., Weller DM., 1998 Molecular Plant-Microbe Interactions 11, 144-152.
7. Van Peer R., Punte HLM, de Weger LA., Schippers B., 1990, Applied and Environmental Microbiology 56, 2462-2470.