Iron metabolism in soybean grown in calcareous soil is influenced by plant growth-promoting rhizobacteria: a functional analysis

Iron deficiency results in severe yield losses, particularly in calcareous soils. Recent evidences suggest that biofertilizers with plant growth-promoting rhizobacteria (PGPR) may be an efficient strategy for enhancing iron (Fe) nutrition in legumes. This work aimed at evaluating the capacity of PGPR strains to enhance Fe uptake-related processes in soybean grown in calcareous soil. From the studied 24 PGPR, Sphingobium fuliginis ZR 1–6 and Pseudomonas jessenni ZR 3–8 strains were selected for the inoculation experiment based on their in vitro ability to produce indole-3-acetic acid, 1-aminocyclopropane-1-carboxylic acid deaminase, siderophores, and organic acids, to tolerate high pH, and to reduce Fe3+. The effect of bacterial inoculation on improving Fe uptake was tested using each isolate alone or combined and through the evaluation of several morphological, physiological, and molecular parameters. Inoculation with S. fuliginis showed beneficial effects particularly at the root level by the improvement of ferric chelate activity (111%) and FRO2 expression (646%), resulting in increased Fe root content (62%). Inoculation with P. jessenii increased Zn and Mn concentrations in the trifoliates (463% and 51%, respectively), decreased Zn concentration in the roots (88%), and increased the expression of FER4 in the trifoliates (5260%). Combined inoculation of both strains fostered Fe accumulation in the trifoliates and increased the expression of IRT1 and FER4 genes, indicating an improved capacity of Fe translocation to the shoots. These results suggest that inoculation with selected PGPR strains could be effective in improving Fe uptake and accumulation in soybean grown under Fe-deficient conditions.