Iron uptake mechanisms in plants: Functions of the FRO family of ferric reductases

Iron is essential for plants and plays critical roles in important processes such as photosynthesis and respiration. While our understanding of molecular mechanisms involved in iron uptake from the soil is relatively well developed, information regarding the mechanisms that serve to move iron into subcellular compartments, like mitochondria, chloroplasts and vacuoles, is just beginning to emerge. This review summarizes iron uptake from the soil and recent progress in understanding iron transport mechanisms occurring at organellar membranes with a focus on members of the FRO family of ferric chelate reductases. FROs reduce ferric iron chelates to form soluble ferrous iron. The founding member of the family, Arabidopsis FRO2, reduces iron at the root surface so that it may be transported across the plasma membrane. More recent studies in Arabidopsis suggest that FROs may function at organellar membranes. In particular, FRO7 plays an essential role in iron delivery to chloroplasts, while two other FROs (FRO3 and FRO8) localize to the mitochondria and might therefore contribute to mitochondrial iron homeostasis.