The Beneficial Effect of Mycorrhizae on N Utilization by the Host-Plant: Myth or Reality?

Nitrogen represents the major element in plants. This element is present in most of the basic constituents: purines, pyrimidines, and amino acids, and is important for the autotrophic capacity of the plants, being part of the light harvesting molecules, e.g., chlorophyll. De facto, N deficiency in plants leads rapidly to chlorosis and stunted growth. Plants being sessile organisms, they are completely dependent on the N availability in the soil solution for their growth and productivity. In soil, a harsh competition exists for N acquisition between the microorganisms (mainly bacteria and fungi) and the plants. However, plant species have developed various strategies of N acquisition. These strategies are including microorganism partners into a symbiotic association. First, legumes are associated with bacteria to assimilate aerial N. A second form of mutualism is the association between plants and mycorrhizal fungi. The fungus and the plant are associated underground forming a close bound between the hyphae and the root cells, the so-called mycorrhizal root. These mycorrhizal roots are characterized by the presence of an extensive network of mycelium developing out of the root and exploring the soil. Therefore, the fungal partner considerably increases the volume of soil that is exploited compared to the nonmycorrhizal root system (Rousseau et al. 19994). The mycelium also develops between the root cells inside the root cortex. All mineral and nutrient exchanges between the host cell (carbohydrate release) and the fungal cells (phosphorus and nitrogen release) are thought to take place in this fungal‐plant interface (Smith and Read 1997). Melin and Nilsson (1952) were the first to demonstrate the N uptake by the extra radical mycelium and its transfer to the host plant. Since this pioneering study, a considerable amount of work has been devoted to the capacities of mycorrhizal symbiosis to modify the access to the various N-forms from the soil, including the uptake and assimilation of N by the partners. The main purpose of

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