Iron is an essential element for all living organisms, although it
can be very toxic to cell. This depends on its high reactivity with
hydrogen peroxide (Fenton reaction) which leads to the generation of hydroxyl radical. In plants, chloroplasts and mitochondria are two of the major sites of hydrogen peroxide generation. To prevent this risk, two strategies can be employed, namely scavenging of hydrogen peroxide or sequestration of iron. Ferritins are a class of ubiquitous proteins able to store iron, thus providing a mechanism to control its homeostasis. In plant cells, ferritins have been found only in chloroplasts. In this work we tried to verify whether plant mitochondria also contain these iron-storage proteins. It was found that the proteins of mitochondria, isolated from etiolated pea stems, cross-reacted with a polyclonal antibody raised against pea seed ferritin. The band identified corresponded to a protein with a molecular mass of 29 kDa, a value which is identical to that of ferritin. These mitochondria were devoid of contaminations from other types of membranes and, in particular, from plastids, as evaluated by marker enzyme determinations and electron microscopy observations. In addition, by means of a speci®c net program (www.inra.fr/servlets/WebPredotar), it was shown that a `putative ferritin' of Arabidopsis thaliana, could be present in mitochondria with a score of 0.974. Therefore, to con®rm the above result on pea mitochondria and the latter observation, the proteins of mitochondria from A. thaliana cell cultures were cross-reacted with pea ferritin antibody. Again, the reaction involved a protein of about 29 kDa, thus confirming the presence of ferritins also in this type of plant material. These results show clear evidence that plant mitochondria possess ferritins and, therefore, unveil new mechanisms of prevention of oxidative stress in plant cells.
