Rhizospheric organic compounds in the soil-microorganism-plant system: their role in iron availability

Poor iron (Fe) availability in soil represents one of the most important limiting factors of agricultural production and is closely linked to physical, chemical and biological processes within the rhizosphere as a result of soil–microorganism–plant interactions. Iron shortage induces several mechanisms in soil organisms, resulting in an enhanced release of inorganic (such as protons) and organic (organic acids, carbohydrates, amino acids, phytosiderophores, siderophores, phenolics and enzymes) compounds to increase the solubility of poorly available Fe pools. However, rhizospheric organic compounds (ROCs) have short half-lives because of the large microbial activity at the soil–root interface, which might limit their effects on Fe mobility and acquisition. In addition, ROCs also have a selective effect on the microbial community present in the rhizosphere. This review aims therefore to unravel these complex dynamics with the objective of providing an overview of the rhizosphere processes involved in Fe acquisition by soil organisms (plants and microorganisms). In particular, the review provides information on (i) Fe availability in soils, including mineral weathering and Fe mobilization from soil minerals, ligand and element competition and plant-microbe competition; (ii) microbe–plant interactions, focusing on beneficial microbial communities and their association with plants, which in turn influences plant mineral nutrition; (iii) plant–soil interactions involving the metabolic changes triggered by Fe deficiency and the processes involved in exudate release from roots; and (iv) the influence of agrochemicals commonly used in agricultural production systems on rhizosphere processes related to Fe availability and acquisition by crops.