Cellular and molecular aspects of iron metabolism in plants

Iron is an essential element for plant metabolism because of its redox properties. Its long distance and intracellular trafficking require specialized proteins and low molecular mass chelates because of its insolubility and toxicity in presence of oxygen. Iron deficiency induces various morphological and biochemical changes. They include root hair morphogenesis, differentiation of rhizodermal cells into transfer cells, yellowing of leaves and ultrastructural disorganisation of chloroplasts and mitochondria, as well as increased synthesis of organic acids and phenolics, and activation of root systems responsible for an enhanced iron uptake capacity. Upon iron resupply, these alterations disappeared within few days and a transient accumulation of the iron storage protein ferritin in the plastids is one of the early events in this process. Iron excess can also occur in plants where it elicits an oxidative stress leading to necrotic spots in the leaves. Induction of ferritin synthesis is again an early event of the plant response to this iron toxicity. Plant hormones such as auxin, abscisic acid and ethylene, as well as reactive oxygen intermediates play an important role in the transduction pathways, allowing plants to respond to these iron-deficiency and excess stresses. Similarities and differences among the various mechanisms responsible for iron uptake and storage in mammals, higher plants and yeast are outlined. Relationships between iron and copper metabolism are also indicated.