Chapter 11 - Ethylene

Abstract

Ethylene, a simple organic molecule, has important roles in the ripening of many fruits, in the induction of senescence in leaves and flowers, and in the abscission of leaf petioles and flower peduncles. It also maintains the apical hook during the germination of dicot seeds and acts to trigger defense responses under stressful situations, such as flooding, extremes of temperature, wounding, and pest and pathogen attack. Ethylene is synthesized in flowering plants from methionine via a cyclic pathway, which preserves the methylthio group of methionine while using ATP. The enzymes—1-aminocyclopropane-1-carboxylic acid (ACC) synthase (ACS) and ACC oxidase (ACO)—are encoded by multigene families in all plants investigated, and different members of these gene families are expressed differentially in different tissues and organs in response to developmental cues, as well as environmental factors and hormones, such as auxins, cytokinins, and ethylene itself. Ethylene also acts to inhibit its own production. While synthesis mutants of ethylene are not found, several mutants in Arabidopsis overproduce ethylene; one of them does so because the regulatory control over one of the ACS genes is lost. Inhibitors of ethylene production inhibit the activity of either of the two enzymes, ACC synthase or ACC oxidase. Transgenic silencing of ACC synthase or ACC oxidase genes has been accomplished, and used to delay ripening of fruits.