Iron Toxicity and Its Relation to Nitrogen and Phosphorus Availability in Ectomycorrhizal Fungi

Iron (Fe) is essential for the maintenance of processes such as respiration and photosynthesis, thus being an important element for the plant production of cultivated or natural species. However, the combination of Fe and the acidity of acid rain creates favourable environments for the occurrence of oxy-reduction reactions, which transform Fe3+ into soluble forms (Fe2+), increasing the availability of this metal in the soil, causing direct and indirect effects on plant growth, development, and productivity. Fe can also affect the availability of other nutrients in the soil like nitrogen (N) and phosphorus (P), directly influencing plant growth. At an optimum concentration, Fe can increase the availability of N, while in high concentrations, it does not alter N content. However, Fe has a high affinity for P and in acid soils this metal is commonly found in high concentrations, thus Fe usually binds to P, making this macronutrient unavailable for uptake by the plant, thereby negatively affecting the plant’s metabolism. To promote the recovery of environments contaminated by toxic levels of Fe, the use of mycorrhizal associations, such as ectomycorrhizae, is considered a promising practice. Ectomycorrhizal (ECM) fungi developed three Fe absorption mechanisms to prevent its insolubility and toxicity: acidification of the medium, reduction of ferric to ferrous form, and secretion of iron-chelating molecules (melanins and siderophores). However, for host plants to obtain the benefits of fungi under conditions of Fe toxicity, it is necessary to carry out in vitro studies, using several sources of this metal. It is also extremely important to analyse the relationship of Fe with other nutrients in the medium, mainly N and P, since ECM fungi have intra and interspecific variations, and their growth is influenced by several abiotic factors.