Essential and Detrimental — an Update on Intracellular Iron Trafficking and Homeostasis

Chloroplasts, mitochondria and vacuoles represent characteristic organelles of the plant cell, with a predominant function in cellular metabolism. Chloroplasts are the site of photosynthesis and therefore basic and essential for photoautotrophic growth of plants. Mitochondria produce energy during respiration and vacuoles act as internal waste and storage compartments. Moreover, chloroplasts and mitochondria are sites for the biosynthesis of various compounds of primary and secondary metabolism. For photosynthesis and energy generation, the internal membranes of chloroplasts and mitochondria are equipped with electron transport chains. To perform proper electron transfer and several biosynthetic functions, both organelles contain transition metals and here iron is by far the most abundant. Although iron is thus essential for plant growth and development, it becomes toxic when present in excess and/or in its free, ionic form. The harmful effect of the latter is caused by the generation of oxidative stress. As a consequence, iron transport and homeostasis have to be tightly controlled during plant growth and development. In addition to the corresponding transport and homeostasis proteins, the vacuole plays an important role as an intracellular iron storage and release compartment at certain developmental stages. In this review, we will summarize current knowledge on iron transport and homeostasis in chloroplasts, mitochondria and vacuoles. In addition, we aim to integrate the physiological impact of intracellular iron homeostasis on cellular and developmental processes.