ECOREMEDIATION. COOPERATION BETWEEN PLANTS AND SOIL MICROORGANISMS, MOLECULAR ASPECTS AND LIMITS

In terrestrial ecosystems, higher plants are organized in two quite different spatial parts: a) a photosynthetic compartment, developed in the atmosphere, under light, and b) a soil compartment, specialized in the absorption of water and mineral salts, in the dark. An intense flux of water, of mineral salts and of organic compounds passes from the soil to the leaves. Small non-charged organic molecules with a medium lipophilicity can move passively in the transpiration stream, from soil water to the transpirating leaves. The composition of the soil solution is regulated by adsorption on the clay-humus complex and by exchanges with the microbial biomass. In the course of evolution, co-occuring microbes and plants have both developed, or maintained antitoxic properties allowing them to transform a large range of molecules, which are not among the group of essential compounds needed for microbial nor plant life. These antitoxic biochemical capabilities are generally not induced by the presence of a toxic property, but often only by foreign, unknown molecules. The case of phytoalexins seems to be an exception to this rule. Organic molecules reaching the soil are first submitted to microbial transformation, and plants absorb a mixture of molecules, of limited log P, either unchanged or partly transformed by microbes. During plant necrosis, the molecules which had been previously absorbed, and which are partly transformed by plant enzymes, are released into the soil and new transformations occur at this level, finally giving a complete mineralization. These mechanisms are illustrated by the case of two pesticides, atrazine and fipronil.