Mitochondrial ferritin distribution in plant organs and its involvement in oxidative stress prevention

Ferritins are proteins able to store iron in a safe and soluble form (l). They are localized in the cytoplasm of mammalian cells whereas have a plastidial localization in the plant ones. Only recently this protein has been reported to be present in both animal (2) and plant (3) mitochondria. In this work, we initially examined the distribution of mitochondrial ferritin (MF) in different plant organs of pea. The plants were grown in hydroponic culture, in the dark, for 7 days and then up to 18 days under light-dark (16-8 h) cycles. The plants were grown either in the presence or in the absence of iron (supplied as Fe(SO4)2-citrate: 100 uM/500 uM). Purified mitochondria were isolated from roots (R), etiolated stems (S), and young (YL) and mature leaves (L). The MF was detected after SDS-PAGE and Western blotting using a polyclonal antibody raised against the pea seed ferritin. Figure 1 shows that the MF was detected in young roots and stems, grown without light, and, although to a lesser extent, in young leaves. In contrast, the MF was not detected in mature organs of plants grown in the presence of light even if iron was supplied. In order to verify the involvement of MF in oxidative stress prevention, we also examined the induction of lipoperoxidation, evaluated as malondialdehyde (MDA) formation, in purified mitochondria isolated from both young pea stems and mature leaves. Lipoperoxidation was induced by 10 uM iron in the presence of 200 uM ascorbate. Figure 2a shows that the level of lipoperoxidation was higher in mitochondria isolated from leaves that do not possess ferritin, than mitochondria isolated from etiolated stems. This decrease indicates that iron is taken up by the MF. Lipoperoxidation was also detected in purified mitochondria isolated from etiolated pea stems in the presence of either 200 uM or 5 mM ascorbate, but in the absence of iron. The increase of lipoperoxidation, when mitochondria were incubated with 5 mM ascorbate, indicates a release of iron from the MF (Figure 2b). These results confirm that MF plays a role in the prevention of oxidative stress in plant mitochondria, particularly in the initial stages of organ development. Therefore, MF is involved in the metabolism of iron by its release and/or acquisition that seems to be finely regulated by ascorbate concentration.