The role of ethylene and other signals in the regulation of fe deficiency responses by Dicot plants

Abstract

Iron (Fe) is abundant but its availability for plants is low specially on calcareous soils. To facilitate its acquisition, dicot (Strategy I) plants switch on several morphological and physiological changes in their roots, known as Fe responses. Once plants acquire enough Fe, the responses are switched off. Their regulation is not totally known but since the 1990s different results have supported a role for the plant hormone ethylene in such a process. Most of those results have been based on morphological and physiological studies and have been previously reviewed. Although the role of ethylene in the regulation of morphological Fe responses has been generally accepted, its role in the regulation of physiological Fe responses has been more controversial. In this review, we discuss the most recent results supporting a role for ethylene in the regulation of physiological Fe responses, most of them based on transcriptomic, proteomic, molecular and genetic analyses. In addition, we review results suggesting a role, either as activators or suppressors of physiological Fe responses, of other hormones and non-hormonal substances, such as auxin, nitric oxide, glutathione, and phloem Fe. As conclusion, we propose a Working Model that integrates both positive and negative signals in the regulation of physiological Fe responses. As positive signals, ethylene and nitric oxide would act at the end of the signalling cascade leading to the activation of physiological Fe responses while phloem Fe could play an important role as repressor.