Accumulation du fer dans les cellules végétales au niveau des nucléoles, Ligne LUCIA. Deciphering Iron Loading in Seeds and Discovery of a New Pool of Iron in the Nucleolus with Synchrotron Radiation X-ray Fluorescence
Résumé
IRON SHINES UNDER LUCIA’S LIGHT
Iron (Fe) is an essential element for all living organisms, being the main cofactor of electron transfer proteins of photosynthesis, respiration and biosynthetic pathways. The control of Fe distribution to the different subcellular compartments (chloroplasts, mitochondria…) is crucial to maintain the metabolic activity of plant cells, particularly in conditions of Fe deficiency. One main challenge is thus to decipher how Fe is delivered to each compartment, where it is localized and what is its chemical environment. In this context, there is a specific need for analytical approaches to unravel two key aspects of Fe homeostasis: high-resolution imaging and in situ speciation.
We have used seeds from pea (Pisum sativum) as a model to study the process of Fe transport and localization, since these seeds are particularly Fe-rich and their size is compatible with the constraints of synchrotron beam lines. Thanks to the access to the LUCIA beamline, we have been able to characterize the chemical forms of Fe that are delivered to the embryo during the seed filling process and we have obtained crucial information on the subcellular localization of Fe. We have shown by EXAFS (Extended X-ray Absorption Fine Structure, LUCIA) and mass spectrometry that Fe is delivered to the developing embryo as ferric (FeIII) complexes with citrate and malate (1). This information was pivotal to further dissect how Fe is imported to the embryo. Indeed, we have unravelled the central role of ascorbic acid in the reduction of the Fe(III) complexes for the transport of Fe(II) by embryos. In the cells, we have discovered with μXRF imaging the existence of an unexpected pool of Fe located in a subcellular structure that was proven to correspond to the nucleolus, raising the question of the role of Fe in this specific compartment (2, 3).
In conclusion, the results obtained with the LUCIA beam line have led us to important discoveries in plant biology, paving the way for new mechanisms and roles of iron in plants.
REFERENCES
1. Grillet L, Ouerdane L, Flis P, Hoang M, Isaure M-P, Lobinski R, Curie C, Mari S (2014) Ascorbate efflux as a new strategy for iron reduction and transport in plants. J. Biol. Chem 289 : 2515-2525
2. Roschzttardtz H, Grillet L, Isaure M-P, Conejero G, Ortega R, Curie C. Mari S (2011) Plant cell nucleolus as a hot spot for iron. J. Biol. Chem. 286: 27863-27866
3. Grillet L, Mari S, Schmidt W (2014) Iron in seeds – Loading pathways and subcellular localization. Front. Plant Sci., 4 : 535-538