Ethylene involvement in the regulation of Fe-deficiency stress responses by Strategy I plants

Plants have developed different mechanisms for the acquisition of iron (Fe). Depending on the mechanisms, plants are classified into two groups: Strategy I and Strategy II. Strategy I plants include all higher plants except the Gramineae, while Strategy II plants comprise the Gramineae. When plants suffer from Fe-deficiency, they develop several morphological and physiological changes in their roots, known as Fe-deficiency stress responses, which disappear when the plants acquire enough Fe. In Strategy I plants, these changes include subapical swelling with abundant root hairs, transfer cells, acidification of the rhizosphere, enhancement of the capacity to reduce Fe³⁺ to Fe²⁺, enhancement of the capacity for Fe²⁺ uptake, release of flavins, and others. The regulation of these responses is not fully understood but in recent years there has been evidence suggesting the involvement of ethylene in this process. This review summarises different results that support a role for this hormone in the regulation of Fe-deficiency stress responses by Strategy I plants.