Fe Acquisition by Coumarins; An Interplay between Ligand-Controlled and Reductive Dissolution

Recently it was established that plants employing
Strategy I Fe acquisition (all plants except for grasses)
strongly upregulate the exudation of coumarin compounds
under conditions of low Fe availability. This suggests that
coumarins, which have reducing and ligating properties, may
enhance the Fe concentration in soil solution. The mechanism
by which these compounds enhance Fe availability, as well as
how their effectiveness is affected by environmental
parameters (pH, PO2) had however remained unexplored.
These issues have been addressed in the current work.
A series of kinetic batch dissolution experiments were
carried out in which several coumarins (esculetin, scopoletin
and fraxetin) interacted with Fe(hydr)oxide minerals
(lepidocrocite, goethite and ferrihydrite), under oxic and
anoxic conditions, in presence and absence of the Fe(II)
scavenger Ferrozine and over a wide environmentally
relevant pH range (4 – 8.5). Fe(II) and Fe(III) solution
concentrations were measured over time and the
corresponding dissolution rates were determined.
At low pH values, exclusively Fe(II) was mobilized
through reductive dissolution. At circumneutral pH, both
Fe(II) and Fe(III) were mobilized, indicating the formation of
stable, soluble Fe-coumarin complexes for both Fe redox
species. Counterintuitively, under oxic conditions, and in the
presence of Ferrozine, the Fe(III) mobilization rate decreased.
This suggests Fe mobilization by coumarins at circumneutral
pH is autocatalyzed: the Fe(II) generated by surface Fe
reduction catalyzes the ligand-promoted dissolution of
Fe(III); removal of this Fe(II) from the mineral surface, either
through oxidation or through Fe(II) scavenging, eliminated
the catalysis. Our findings indicate an intricate interaction
between reductive and ligand-controlled dissolution in the
mechanisms by which coumarins enhance the bioavailability
of Fe to plants.