it wasn't until 1934 that gane reported that plants synthesize ethylene. in 1935, crocker proposed that ethylene was the plant hormone responsible for fruit ripening as well as senescence of vegetative tissues. the yang cycle ethylene is produced from essentially all parts of higher plants, including leaves, stems, roots, flowers, fruits, tubers, and seeds. ethylene production is regulated by a variety of developmental and environmental factors. during the life of the plant, ethylene production is induced during certain stages of growth such as germination, ripening of fruits, abscission of leaves, and senescence of flowers. ethylene production can also be induced by a variety of external aspects such as mechanical wounding, environmental stresses, and certain chemicals including auxin and other regulators. ethylene is biosynthesized from the amino acid methionine to s-adenosyl-l-methionine (sam, also called adomet) by the enzyme met adenosyltransferase. sam is then converted to 1-aminocyclopropane-1-carboxylic acid (acc) by the enzyme acc synthase (acs). the activity of acs determines the rate of ethylene production, therefore regulation of this enzyme is key for the ethylene biosynthesis. the final step requires oxygen and involves the action of the enzyme acc-oxidase (aco), formerly known as the ethylene forming enzyme (efe). ethylene biosynthesis can be induced by endogenous or exogenous ethylene. acc synthesis increases with high levels of auxins, especially indole acetic acid (iaa) and cytokinins. ethylene is perceived by a family of five transmembrane protein dimers such as the etr1 protein in arabidopsis. the gene encoding an ethylene receptor has been cloned in arabidopsis thaliana and then in tomato. ethylene receptors are encoded by multiple genes in the arabidopsis and tomato genomes. mutations in any of the gene family, which comprises five receptors in arabidopsis and at least six in tomato, can lead to insensitivity to ethylene. dna sequences for ethylene receptors have also been identified in many other plant species and an ethylene binding protein has even been identified in cyanobacteria. environmental cues such as flooding, drought, chilling, wounding, and pathogen attack can induce ethylene formation in plants. in flooding, roots suffer from lack of oxygen, or anoxia, which leads to the synthesis of 1-aminocyclopropane-1-carboxylic acid (acc). acc is transported upwards in the plant and then oxidized in leaves. the ethylene produced causes nastic movements (epinasty) of the leaves, perhaps helping the plant to lose water. ethylene in plant induces such responses: seedling triple response, thickening and shortening of hypocotyl with pronounced apical hook. in pollination, when the pollen reaches the stigma, the precursor of the ethene, acc, is secreted to the petal, the acc releases ethylene with acc oxidase. stimulates leaf and flower senescence stimulates senescence of mature xylem cells in preparation for plant use induces leaf abscission induces seed germination induces root hair growth — increasing the efficiency of water and mineral absorption through rhizosheath formation induces the growth of adventitious roots during flooding stimulates survival under low-oxygen conditions (hypoxia) in submerged plant tissues stimulates epinasty — leaf petiole grows out, leaf hangs down and curls into itself stimulates fruit ripening induces a climacteric rise in respiration in some fruit which causes a release of additional ethylene. affects gravitropism inhibits root growth in response to soil compaction, shade and flooding stimulates nutational bending inhibits stem growth and stimulates stem and cell broadening and lateral branch growth outside of seedling stage (see hyponastic response) interference with auxin transport (with high auxin concentrations) inhibits shoot growth and stomatal closing except in some water plants or habitually flooded ones such as some rice varieties, where the opposite occurs (conserving co2 and o2) induces flowering in pineapples inhibits short day induced flower initiation in pharbitus nil and chrysanthemum morifolium small amounts of endogenous ethylene are also produced in mammals, including humans, due to lipid peroxidation. some of endogenous ethylene is then oxidized to ethylene oxide, which is able to alkylate dna and proteins, including hemoglobin (forming a specific adduct with its n-terminal valine, n-hydroxyethyl-valine). endogenous ethylene oxide, just as like environmental (exogenous) one, can alkylate guanine in dna, forming an adduct 7-(2-hydroxyethyl)-guanine, and this poses an intrinsic carcinogenic risk. it is also mutagenic.