Iron acquisition by coumarin-type siderophores

Vanessa Paffrath, Ricardo F.H Giehl

Iron (Fe) is an essential micronutrient for plants. Although highly abundant in soils, its availability to plants is strongly decreased in calcareous soils which represent around one third of all cultivated soils worldwide. Under such conditions, Fe solubility is low, which limitis Fe uptake by plants and results in leaf chlorosis, impaired plant growth, and decreased yield and quality of harvested parts of crop plants.

Plants have evolved two basic strategies to acquire sparingly available Fe. Graminaceous plant species employ a chelation based-mechanism that relies on the synthesis and secretion of mugineic acid-type phytosiderophores, which can solubilize Fe(lll) by forming complexes with high affinity. In these plants, the whole Fe(lll)-phytosiderophore complex is then taken up by roots. By contrast, in dicotyledonous plant species Fe(lll) must be reduced prior to uptake by root-expressed Fe(ll) transporters.

Recently, our research group has found that dicotyledonous plants also release small molecules with Fe-chelating and reducing properties (Schmid et al. , 2014, Plant Physiol.; Rajniak et al., 2018, Nature Chem. Biol.). In several of these species, these molecules consist of a class of courmarin-type siderophores derived from the phenylpropanoid pathway and that are thought to assist the membrane-bound Fe(lll) reduction mechanism by solubilizing and reducing Fe(lll) from insoluble sources. Although the identity of major enzymes involved with the synthesis of these coumarins has been identified, the precise biochemical mechanisms underlying coumarin-mediated Fe acquistion still remain unclear.

In this project we use Arabidopsis thaliana as a model organism to further inverstigate the role of coumarin-type siderophores in Fe acquisiton. Our goal is to characterize the compositon of root exudates under different Fe-limiting conditions and assess which of the exuded components exhibit Fe chelation and/or reduction properties, as well as to identify further molecular players involved in the synthesis and release of coumarins in Fe-deficient plants. Our approaches include the collection of root exudates of different Arabidopsis mutants and over-expression lines, the analysis of coumarins by LC-MS/MS and vitro Fe(lll)-mobilization/reduction assays.