Plant growth-promoting rhizobacteria (PGPR) are soil bacteria that are able to colonize rhizosphere and to enhance plant growth by means of a wide variety of mechanisms like organic matter mineralization, biological control against soil-borne pathogens, biological nitrogen fixation, and root growth promotion. A very interesting feature of PGPR is their ability of enhancing nutrient bioavailability. Several bacterial species have been characterized as P-solubilizing microorganisms while other species have been shown to increase the solubility of micronutrients, like those that produce siderophores for Fe chelation. The enhanced amount of soluble macro- and micronutrients in the close proximity of the soil-root interface has indeed a positive effect on plant nutrition. Furthermore, several pieces of evidence highlight that the inoculation of plants with PGPR can have considerable effects on plant at both physiological and molecular levels (e.g., induction of rhizosphere acidification, up- and downregulation of genes involved in ion uptake, and translocation), suggesting the possibility that soil biota could stimulate plants being more efficient in retrieving nutrients from soil and coping with abiotic stresses. However, the molecular mechanisms underlying these phenomena, the signals involved as well as the potential applications in a sustainable agriculture approach, and the biotechnological aspects for possible rhizosphere engineering are still matters of discussion

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Plants release a multitude of organic compounds into the rhizosphere, some of which are flavonoids. These products of secondary metabolism are mainly studied for their antioxidant properties and for their role in the establishment of rhizobium-legume symbiosis; however, it has been recently demonstrated that flavonoids can also affect nutrient availability through soil chemical changes. This review will give an overview of the types and amounts of flavonoids released by roots of different plant species, as well as summarize the available knowledge on root exudation mechanisms. Subsequently, factors influencing their release will be reported, and the methodological approaches used in the literature will be critically described. Finally, the direct contribution of plant-borne flavonoids on the nitrogen, phosphorous and iron availability into the rhizosphere will be discussed

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

Plants produce and release in the surrounding soil, the so-called rhizosphere, a vast variety of secondary metabolites. Among them, flavonoids are the most studied, mainly for their role in the establishment of rhizobiumlegume symbiosis; on the other hand, some studies highlight that they are also important in the plant strategies to acquire nutrients from the soil, for example, by acting on its chemistry. The scope of this review is to give a quick overview on the types and amounts of plant-released flavonoids in order to focus on their effects on soil activities that in turn can influence nutrient availability and so plant mineral nutrition; emphasis is given to the different nutrient cycles, soil enzyme, and soil bacteria activities, and their influence on soil macrofauna and roots of other plants. Finally, the possible outcome of the climate change on these processes is discussed.