Mycorrhizae:: a potential tool for plant protection but not a panacea

Arbuscular mycorrhizae are the plant symbiosis the most widely spread on the planet. These fungi, grouped in the phylum Glomeromycota, are distributed over all terrestrial ecosystems and found associated with the majority of land plants. To the well-known positive impacts of arbuscular mycorrhizae on plant yields should be added several other benefits such as a better survival rate of colonized plants, the maintenance of plant biodiversity, the improvement of soil microflora, and the reduction in harmful effects of both biotic and abiotic environmental stresses. Given such a panoply of benefits to plants and their environment, one could believe that mycorrhizae represent a panacea for solving problems related to plant production and plant protection. In fact, the "plant - mycorrhizae - pathogen - environment" complex constitutes a standard condition to be maintained or to be recovered in order to ensure the sustainability of the environment. The potential of mycorrhizae as a biocontrol agent globally covers five known mechanisms of interaction. Three of them concern the direct effect of symbiosis on plants. They are: 1) plant growth stimulation through an increased nutritive contribution and, consequently, better plant health; 2) the morphological transformation of the root system; and 3) the induction or suppression of defense mechanisms, and this mainly at the enzymatic level. Another mechanism concerns the pathogen: 4) through a direct competition with mycorrhizal fungi linked with nutrient availability and infection sites. Finally, mycorrhizae indirectly influence the soil structure and quality through: 5) the modification of the soil microflora and an increase in organic matter.