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

In plants, intracellular Fe trafficking must satisfy both chloroplastic and mitochondrial demands for Fe without allowing its accumulation in the organelles, in dangerous, redox-active forms. The Fe storage protein ferritin is involved in such intracellular control of Fe trafficking and homeostasis in both organelles; indeed it can be targeted to chloroplasts (as known since years) but also to mitochondria [1-3]. The recent characterization of Arabidopsis atfer4 mutants knock-out for the ferritin isoform targeted also to mitochondria [2,3], unveiled a putative signalling role of ferritin, beside its known role as Fe store; the fact that Fe trafficking is affected in heterotrophic atfer4 mutant cells [2], but not in atfer4 plants [3], posed the question whether the AtFer4 role observed in heterotrophic cells unveils an ancestral role of mitochondrial ferritin [3] whose relevance might have undergone a replacement of control of Fe homeostasis by other regulatory pathways, during the evolution of green plants. We therefore tested if the ferritin localized in mitochondria is truly functional as iron-store in green plants, i.e. if the ferritin monomers can truly assemble into the multimer “cage” able to store Fe(III). For that, cucumber seedlings were grown in hydroponic culture under different conditions of Fe supply (Fe deficiency, sufficiency or excess), and mitochondria enriched fractions were extracted from their roots. Ferritin accumulates in mitochondria of Fe-sufficient as well as Fe-excess roots; also, such ferritin indeed forms the expected multimer complexes associated with Fe(III) accumulation, as shown from native gel electrophoresis coupled with Fe staining. In both protein analyses (PAGE and native gels) the detected signals are stronger in Fe-excess enriched fractions. We therefore conclude that ferritin is truly functional, in cucumber mitochondria, as Fe store,. The functional interaction, in mitochondria, between ferritin and frataxin, in currently under investigation in Arabidopsis.