Iron, cold iron, is master of them all: iron crosstalk with zinc, copper, phosphorus, and nitrogen homeostasis

The ionome is defined as the inorganic composition of an organism. In plants, the ionome has been shown to be integrated, as the concentrations of different elements affect one another, with complex regulatory mechanisms to keep nutrients, trace and toxic elements balanced. Iron (Fe) is an essential micronutrient that is necessary for photosynthesis, mitochondrial respiration, and redox metabolism; its concentration in plant tissues is finely regulated to avoid deficiency and excess stress. It has been known that varying concentrations of Fe affect other components of the ionome, while variations in the concentrations of other elements also perturb Fe homeostasis. Recently, molecular mechanisms that regulate the crosstalk of Fe homeostasis with that of zinc, copper, phosphorus, and nitrogen have been uncovered. Here we review these regulatory circuits, demonstrating that the ionome should be balanced and that micronutrients are important for nutrient use efficiency and to avoid nutrient deficiency as well as excess. We focus mainly on the model plant Arabidopsis thaliana and rice, for which mechanistic models have been proposed. Our review will help to integrate models to understand how plants balance the ionome.