Ethylene is a gas produced by the petrochemical industry for plastic production and by plants as a hormone affecting plant processes. It speeds up fruit ripening, inhibits flower production, promotes seed germination, and affects seedling development. Biosynthesis of ethylene in plants occurs in response to stress. Methionine is the starting point for biosynthesis, and ACC synthase produces more ethylene in response to stress. Auxin herbicides stimulate ethylene production, which may play a role in plant death. Ethylene’s multiple roles make it an active research area.
Ethylene is a compound of carbon and hydrogen with the chemical formula C2H4. It is a colorless gas with a sweet smell that is produced on a large scale by the petrochemical industry for use primarily in the production of plastics. Ethylene is also produced by plants and acts as a hormone that affects crucial plant processes in several ways. It is unusual for such a small molecule to be active as a hormone. The biosynthesis of ethylene in plants occurs in response to various stresses, including pest and disease attacks, drought, and tissue damage.
The effects of ethylene on plants are many and varied. Its best-known effect is to speed up the ripening of some fruits, such as apples, bananas and tomatoes, but not citrus fruits. It had been known since at least ancient Egyptian times that some fruits could ripen more quickly by bruising; often it is enough to bruise or cut a fruit to hasten the ripening of a large number stored in the same container. Ethylene was not identified as the cause of this response until 1901, and it was not until the late 20th century that the details of the biosynthetic process of ethylene in plant tissues were revealed.
Ethylene inhibits flower production in most plants, but promotes seed germination and can affect seedling development in an interesting way known as the “triple response.” Seedlings grown in the dark and exposed to ethylene show characteristic thickening and shortening of the stem and increased curvature of the apical hook, a structure that protects the growth center at the tip of the stem. Ethylene also promotes the destruction of chlorophyll, the production of pigments called anthocyanins, associated with autumn colors, and the aging and falling leaves. Since the compound is a gas and, like most hormones, is effective at very low concentrations, it can diffuse easily through plant tissue and therefore the production of this compound by one plant can affect nearby others. Ethylene from industrial sources and automobile engines can also affect plants.
The starting point for the biosynthesis of ethylene in plants is methionine, an essential amino acid produced in chloroplasts. This reacts with adenosine triphosphate (ATP) to produce S-adenosyl-L-methionine (SAM), also known as S-AdoMet, catalyzed by an enzyme called SAM synthetase. A further reaction converts SAM to 1-amino-cyclopropan-1-carboxylic acid (ACC), catalyzed by the enzyme ACC synthase. Finally, ACC reacts with oxygen to produce ethylene, hydrogen cyanide and carbon dioxide, catalyzed by the enzyme ACC oxidase. Hydrocyanic acid is converted into a harmless compound by another enzyme, so the biosynthesis of ethylene does not release toxic chemicals.
ACC synthase is produced by plants in response to stress, resulting in the production of more ACC and consequently more ethylene. Stress can take the form of an attack by insect pests or plant diseases, or it can be due to environmental factors such as drought, cold or floods. Harmful chemicals can also cause stress, leading to the production of ethylene.
The plant hormone auxin, when present in large quantities, stimulates the production of ethylene. Auxin herbicides, such as 2,4-dichlorophenoxyacetic acid (2,4-D), mimic the action of this hormone, causing many plants to produce ethylene. While the exact mechanism of action of these herbicides is unclear, it appears that excessive ethylene production may play a role in plant death in susceptible species.
The scope of ethylene biosynthesis in plants is, as of 2011, an active research area. Given this hormone’s wide range of effects, it is likely to have multiple roles. In the case of seedlings, it appears to be produced in response to soil resistance to the developing seedling and to trigger growth responses that help protect the growing center. There is also evidence that it may play a role in disease resistance; experimental studies suggest that plants lacking response to ethylene are more susceptible to some diseases.
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