Under Fe deficiency, Strategy I (non-graminaceous) plants up-regulate the expression of many Fe acquisition genes and develop morphological changes in their roots. The regulation of these responses is not completely known, but since the 1980's different results suggest a role for auxin, ethylene and, more recently, nitric oxide. The up-regulation of the Fe acquisition genes does not depend solely on these hormones, that would act as activators, but also on some other signals, probably phloem Fe, that would act as an inhibitor. It is not known which of the hormones considered is the last activator of the Fe acquisition genes, but some results suggest that auxin acts upstream of ethylene and NO and that, perhaps, ethylene is the last activator.