Root exudation of coumarins from soil-grown Arabidopsis thaliana in response to iron deficiency

In alkaline soils, iron (Fe) availability is limited and plants developed different strategies to overcome this constraint. Arabidopsis thaliana follows a reduction-based strategy, in which Fe is reduced at the root surface before plant uptake. Recent studies highlighted the role of phenylpropanoid-derived coumarins in the mobilisation and uptake of Fe in A. thaliana. In this study, we grew the A. thaliana wild-type Col-O and the f6'h1 mutant defective in phenylpropanoid-derived coumarin synthesis, on six contrasting soils and determined coumarin exudation rates and biochemical changes in the rhizosphere. On the natural alkaline soils, the f6'h1 mutant was not able to grow and growth could only be sustained with regular Fe supply. Root exudates were sampled hydroponically from soil-grown wild-type plants after washing roots. Scopoletin was the predominant coumarin found in the exudates of A. thaliana grown on the different soils. Exudation rates of scopoletin increased with decreasing bioavailability of Fe in soil, with the exception of the acidic soil where plants showed exudation rates similar to the rates measured in alkaline soils. This is the first study reporting exudation of coumarins from A. thaliana grown on a range of natural soils with varying Fe availability.