Figure 1
Conceptual model of possible interplay between two ligands (X,Y) with different binding affinities for metals (M). The model is drawn after the cartoon proposed by McRose and co-workers of high Fe affinity (Y) and low Fe affinity (X) siderophore interaction2. Formation of a M–X complex strips M from the soil aggregate and brings the metal into solution. The metal is then exchanged between X and Y, Y transports M towards cell surface whilst X is free to return to the soil aggregate. McRose identified X as a ligand with low affinity and Y as a ligand with high affinity. To confirm the possibility of a synergistic interaction between weak and strong organic ligands with different metal binding affinities during acquisition and transport in the rhizosphere, we need to identify the physicochemical conditions in the solution that enable the exchange of metal ions from one ligand to the other, the so-called ligand exchange point (LEP), and show that distinct different stabilty windows exist. Identifying stabilty windows is also crucial to answer the uncertainty if metal–ligand complexes are stable at the relevant interfaces in the rhizosphere, i.e., soil organisms (e.g., roots, bacteria, fungi) and soil aggregates (e.g., minerals, organic matter).
