Moving up, down, and everywhere: signaling of micronutrients in plants.

To cope with the variable availability of micronutrients, plants have evolved a complex set of physiological and developmental processes, which are under tight control of short-range and long-range signaling pathways. These signals act at the cellular and whole-plant scale to coordinate micronutrient homeostasis at the systemic and local level. Recently, several molecular components of the local and long-distance regulatory circuits as well as their putative positions in the signaling cascade have been identified. Since among the micronutrients comparatively most is known on the signaling of Fe, this review sets a focus on Fe, for which the regulatory pathway most likely involves signaling compounds such as nitric oxide and hormones (e.g. ethylene and cytokinin) that act upstream of a set of transcription factors.