Mitochondrial ferritin is truly a functional iron-storage protein in cucumber roots

Intracellular Fe trafficking must satisfy both chloroplastic and mitochondrial demands for Fe without allowing its accumulation, in dangerous, redox-active forms, in the plant organelles. The Fe storage protein ferritin is involved in such intracellular control of Fe trafficking and homeostasis in both organelles. The characterization of Arabidopsis atfer4 heterotrophic cells as well as atfer4 plants, both knock-out for the ferritin isoform targeted also to mitochondria, unveiled a putative signalling role of ferritin, beside its known role as Fe store. These results prompted us to further investigate the functional role of mitochondrial ferritin in plants. We could show the presence of the monomer ferritin in mitochondria of cucumber roots, which is strictly dependent on the Fe supply in growth medium; we could also detect, in cucumber mitochondria, the 24-mer ferritin complex, which binds Fe(III) and whose presence is also strictly dependent on the Fe supply. Such results indicate that mitochondrial ferritin is a functional iron-storage protein in cucumber roots. The functional interaction, in mitochondria, between ferritin and the mitochondrial iron chaperone protein frataxin is currently under investigation in our laboratories