Iron, cold iron, is the 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 concentration of elements affects 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, and has its concentrations in plant tissues finely regulated to avoid deficiency and excess stresses. It has been known that varying concentrations of Fe affect other components of the ionome, while variation in other elements's concentration also perturb iron homeostasis. Recently, molecular mechanisms that regulate the crosstalk of Fe homeostasis with that of zinc (Zn), copper (Cu), phosphorus (P) and nitrogen (N) 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 focused mainly on 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.