Synchrotron X-rays for Metals in Plants, Soils, and Environmental Systems: Early Science and Community Building at XLEAP

Root-secreted Coumarins Increase Phosphorus Bioavailability to Plants through Reductive Dissolution of Iron

Aug 5, 2026, 1:45 PM

15m

Bradfield 101 (Cornell University)

Oral presentation Plant-soil interactions and biogeochemical cycling Plant–soil interfaces and biogeochemical cycling

Speaker

Christopher Schuler (Massachusetts Institute of Technology)

Description

The essential nutrient phosphorus often has low bioavailability in soils because of adsorption to iron minerals. This can be especially problematic in agricultural soils, where large amounts of “legacy” phosphorus accumulate. Eudicot plants are known to access iron by secreting redox-active organic molecules, e.g. coumarins, from their roots. However, the ability of these metabolites to desorb phosphorus from the surfaces of iron minerals has not been explored. To test this, we reacted phosphate-doped ferrihydrite with fraxetin, a widely produced coumarin, across soil-relevant pH and oxygen gradients. Fraxetin solubilized both phosphorus and iron at every tested pH in anoxic experiments but was ineffective at high pH in the presence of oxygen. We also used P XANES to determine whether this reaction changed the bonding environment of phosphorus; after 48 hours, no changes were detected. Next, we tested differences in tissue nutrient concentrations between coumarin-producing wild type Arabidopsis thaliana and a coumarin-deficient A. thaliana mutant to determine whether our abiotic results translated to enhanced iron and phosphorus bioavailability. Plants were grown in potting soil amended with varying concentrations of ferrihydrite, to tune phosphorus availability, and calcium carbonate, to tune iron availability. Coumarin production improved tissue iron content in high pH soils; in low pH soils, coumarin production increased tissue phosphorus content. We also used XRF microprobe analysis to investigate the distribution and speciation of iron and phosphorus in the rhizosphere of coumarin-producing and coumarin-null plants. Phosphorus-rich iron phases were concentrated on the outside of roots relative to bulk soil, suggesting that root mucilage has an important role in entraining these nutrient sources in accessible locations. This work shows that coumarins increase phosphorus bioavailability via the reductive dissolution of iron minerals, enhancing our understanding of the strategies plants employ to access soil phosphorus.