Does the exudation of coumarins from Fe-deficient, soil-grown Brassicaceae species play a significant role in plant Fe nutrition?

Abstract

The availability of iron (Fe) is highly limited for plant uptake in calcareous soils. To overcome this challenge, plants have developed different strategies of Fe mobilization. Non-gramineous species employ a reduction-based strategy (strategy I), which is assisted by the release of Fe(III)-chelating and -reducing coumarins. In this study, the coumarin release by three strategy-I plant species of the Brassicaceae family, i.e. Brassica napus L., Raphanus sativus L. and Sinapis alba L., on calcareous soil was examined using the RHIZOtest approach. With this experimental setup, the plants were first grown hydroponically in a cylindrical growth unit for 12 days, which was then subsequently brought in contact with calcareous, Fe deficient soil for another 9 days. Root exudates were collected in both experimental phases. Total carbon (C) release, coumarin exudation rates, trace metal plant uptake as well as trace metal mobilization were determined in response to Fe deficiency after 3, 6 and 9 days of soil exposure. High total C exudation rates indicated a clear stress response upon Fe deficiency during the soil contact period. Six different coumarins were detected, with scopoletin being predominantly released by all three plant species during the hydroponic stage with zero Fe supply. The exudation rate of all detected coumarins was significantly lower during soil exposure and represented only a very small fraction of the total organic compounds released. Although the role of coumarins in Fe mobilization is very complex and still not completely understood, it seems that coumarins are less important in Fe mobilization by the three investigated plant species when grown on soil compared to phytosiderophore-releasing strategy-II plants. Our data further indicate that plants growing on a Fe-deficient soil release less coumarins than in an artificial nutrient solution setup with zero Fe supply, which highlights the importance of determining root exudation rates from soil-grown plants, since data from hydroponic studies are likely not representative for soil-grown plants.