Iron homeostasis in plants: Sensing and signaling pathways

Since iron is both an essential element as well as a potential toxin, it is a nutrient which on the one hand fulfils many important functions in plants but on the other can cause severe cell damage as a consequence of the formation of reactive hydroxyl radicals. Uptake of iron, its concentrations within particular tissues, and its subcellular distribution is therefore subject to careful control. In addition, the low bioavailability of iron in most soils necessitates the mobilization of sparingly soluble iron compounds. This has led to the evolution of concerted responses that assist in maintaining an adequate supply of iron for plant roots. These responses comprise morphological changes, such as the development of extra root hairs, formation of rhizodermal transfer cells, and induction of cluster roots, as well as induction of genes coding for enzymes involved in the mobilization and uptake of nutrients. Investigations at the protein, mRNA, and structural level showed that both systemic responses, involving transmission of long distance signals, and external nutritional signals, inducing localized responses, are involved in the complex control of iron homeostasis. A number of components have been identified at the molecular level, but the interplay between these components and the signal transduction cascades leading to an iron status within an adequate range are largely unknown. This review summarizes the available data that explain how these processes are coordinated to maintain a continuous and acceptable Fe supply despite changing environmental conditions.