Abstract
Iron (Fe) is an essential element for most living organisms because it acts as a cofactor for many proteins involved in various biological processes such as photosynthesis, respiration or nitrogen assimilation. Although iron is the fourth most abundant element in the earth's crust, its bioavailability for plants is very low. Two reasons explain this low bioavailability. First, iron is mainly present in soils in the form of insoluble hydroxides and oxides. Second, each additional pH unit in the soil lowers its bioavailability by a factor of 1000. Therefore, the acquisition of iron in alkaline soils (pH> 7) is particularly problematic for plants and they have developed different strategies to solubilize and up take this micronutrient. With the exception of Poaceae, the acquisition of iron by plants requires the reduction of Fe3+ into Fe2+ that is taken up by the roots. These two steps involve a ferric reductase and a high affinity transporter located at the plasma membrane of the root epidermis. The secretion of phenolic compounds such as coumarins has recently emerged as a crucial mechanism that facilitates the solubilisation of iron and thus plant iron acquisition. However, the molecular mechanisms associated with the secretion of coumarins into the soil, as well as their mode of action once secreted, have been little studied. My thesis project therefore consisted in studying the localization of fluorescent coumarins (i.e. esculin, fraxin and scopoline) in Arabidopsis thaliana roots in response to iron deficiency. Using bi-photon microscopy, I was able to show that coumarins were localized in the root endodermis, cortex and atrichoblasts of plants grown in iron deficiency conditions. The use of spectral microscopy has allowed me to show that fraxin is mainly localized at the cortex, while scopolin and esculin are present in the endodermis, the cortex and the atrichoblasts. These analyses allowed me to demonstrate that the secretion of scopoletin occurs exclusively via the root hairs, whereas the secretion of esculetin and fraxetin occurs through the whole root epidermis. In addition, I have demonstrated that the role of PDR9 was not restricted to the secretion of coumarins into the rhizosphere but also extended to their trafficking from the cortex to the root epidermis. A new transporter involved in the secretion of coumarins has also been identified. The study of the dynamics of coumarins revealed that these compounds are highly mobile, within the plant and in the soil. For instance, I have found that scopolin is transported from the roots to the leaves. Furthermore, I have demonstrated that once secreted into the soil, coumarins are taken up by the plant root in the form of FeIII-coumarin complexes (most probably via a transporter that remains to be identified and characterized) and that this mechanism participates to the acquisition process of iron by the plant. This later discovery opens a new avenue of exploration that will lead to a better understanding of the role of coumarins in plant iron nutrition.Key words: Arabidopsis thaliana, coumarins, iron, spectral microscopy, fraxetin, PDR9.