To the editor:

The paper by Goto et al.1 in your March issue highlights overexpression of the iron storage protein ferritin in transgenic plants as an approach for treating iron deficiency in humans. Goto et al. overexpress iron in seeds, achieving a threefold increase in iron content in seeds, levels comparable to experiments in my laboratory in which ferritin was overexpressed in leaves2. Another group has shown that oral administration of plant ferritin can alleviate iron deficiency anemia in rats3. These findings have promulgated the idea that iron-fortified transgenic plants overexpressing ferritin could be used to mitigate iron deficiency in the human diet1.

Iron uptake, however, is not determined only by levels of iron storage proteins. Complex interactions between plant and soil within the rhizosphere also profoundly influence iron content. Solid phases controlling iron solubility in soils, chemical speciation of iron in solution, importance of redox in the solubilization of iron, and the role of synthetic and natural chelates in transport processes that occur near roots are among soil-dependent factors determining iron bioavailability4. In addition, plant iron uptake mechanisms are intimately associated with loading processes of other metals, some of them being potentially toxic for humans. Ferritin overaccumulation in transgenic tobacco leaves leads to excessive iron sequestration and the activation of iron transport systems, as revealed by an increase in root ferric reductase activity2. Independently, it has been reported that iron-deficiency activation of the IRT1 ferrous iron transporter was most likely responsible for cadmium loading of pea plants5. In grasses, iron(III) uptake occurs through phytosiderophores of the mugineic acid family; this transport system, activated under the above conditions, is also capable of transporting metals such as zinc and probably copper6.

In the light of these findings, the behavior of transgenic plants overexpressing ferritin with regard to iron and heavy-metal loading under various soil conditions should be addressed in future experiments. Only after the risks of toxic metal loading have been deemed acceptable can such plants be introduced into the human food chain.