Ethylene is a plant hormone. Unlike other plant hormones that are typically liquid compounds, ethylene is a simple hydrocarbon gas, C2H4. This gaseous nature allows it to diffuse rapidly through plant tissues and the surrounding air, acting as a potent signal even at trace concentrations. Ethylene is produced by nearly all parts of a plant, including stems, roots, leaves, and flowers, throughout its life cycle. Its primary function is to coordinate various developmental processes and environmental responses, often triggering a shift from growth to maturation or defense.
The Role in Fruit Ripening and Quality
Ethylene is most famous as the primary trigger for fruit maturation, particularly in climacteric fruits such as bananas, avocados, and apples. In these fruits, a surge in ethylene production is accompanied by a dramatic increase in respiration, known as the climacteric peak. This burst initiates a cascade of biochemical changes that define the fruit’s edible quality.
The hormone directly regulates the expression of genes responsible for these ripening processes. Enzymes like polygalacturonase and pectin methylesterase are activated, which begin to break down the cell wall components, leading to the characteristic softening of the fruit flesh. Furthermore, ethylene promotes the degradation of chlorophyll, replacing the green color with pigments like carotenoids or anthocyanins.
Climacteric fruits exhibit autocatalytic ethylene production, meaning a small initial amount stimulates the fruit to produce much more. This positive feedback loop explains why placing one ripe apple with unripe ones causes the entire batch to ripen quickly. Non-climacteric fruits, including grapes, citrus, and strawberries, do not exhibit this response and must be fully ripened before harvest, as their maturation is less dependent on ethylene.
Developmental Control and Growth Inhibition
Beyond maturation, ethylene acts as a morphogen, shaping the architecture of young plants, especially in response to dark or subterranean conditions. This is classically demonstrated by the “triple response” observed in dark-grown seedlings exposed to the gas, which aids their emergence from soil or debris.
The first change is the inhibition of stem (hypocotyl) elongation, resulting in a shorter shoot. The second is a noticeable thickening of the stem, caused by a change in the direction of cell expansion from longitudinal to radial. These two changes make the seedling sturdier and better able to push through compacted soil.
The third component is the exaggeration of the apical hook, a protective bend that shields the delicate growing tip and embryonic leaves from mechanical damage. Ethylene also influences root development, generally inhibiting primary root growth but promoting the formation of root hairs. This increased surface area enhances the plant’s capacity for water and nutrient absorption, which is beneficial for a struggling seedling.
Ethylene’s Function in Stress and Aging
Ethylene serves a protective function as a stress signal, with production increasing significantly when a plant encounters environmental threats. Exposure to flooding, for example, rapidly triggers ethylene synthesis in submerged tissues. Because the gas cannot diffuse out of the waterlogged tissue, it accumulates and induces responses like the rapid elongation of stems to help leaves reach the surface for air.
Ethylene also plays a central part in senescence (programmed aging) and abscission (the shedding of organs). As a leaf or flower ages, or following wounding, ethylene levels rise, signaling the breakdown of cellular components. This process facilitates the recycling of valuable nutrients, such as nitrogen and phosphorus, back into the main plant body before the organ is shed.
Abscission occurs at a specialized layer of cells, the abscission zone, found at the base of the petiole or fruit stalk. Ethylene promotes the activation of cell wall-degrading enzymes within this zone, chemically weakening the connection until the organ detaches. This programmed shedding allows the plant to conserve resources or disperse mature seeds.
Commercial Manipulation of Ethylene
The effects of ethylene on plant life cycles have made its manipulation a standard practice in commercial agriculture and floriculture. To promote or synchronize ripening, a synthetic chemical called ethephon is widely used. Ethephon is absorbed by the plant and slowly breaks down inside the cells to release active ethylene gas. This application is commonly used to ensure uniform ripening of crops like tomatoes and coffee or to synchronize flowering for easier harvesting.
Conversely, effort is dedicated to inhibiting ethylene action to extend the shelf life of produce and cut flowers. Two primary anti-ethylene agents are used: 1-methylcyclopropene (1-MCP) and silver thiosulfate (STS). 1-MCP is a gaseous compound that competitively binds to the plant’s ethylene receptors, effectively blocking the natural hormone from initiating its signaling cascade. This treatment is highly effective and widely used on apples, pears, and bananas to prevent over-ripening during storage and transport.
Silver thiosulfate (STS) is another inhibitor containing silver ions, which interfere with the ethylene receptor proteins, preventing hormone binding. While effective, STS has environmental concerns due to its heavy metal content, leading to a greater focus on 1-MCP as the preferred commercial inhibitor. These chemical interventions allow growers and distributors to precisely control the timing of ripening and senescence, minimizing waste and maximizing product quality for consumers.