Ethylene is a simple, gaseous hydrocarbon that functions as a plant hormone, influencing many aspects of growth and development. Its discovery traces back to the 19th century, when people observed that gas leaking from street lamps was causing nearby plants to shed their leaves prematurely. In 1901, Russian scientist Dimitry Neljubow identified ethylene as the specific component in the illuminating gas responsible for these effects. Later, in the 1930s, researchers confirmed that plants naturally produce their own ethylene, establishing it as an endogenous regulator.
The unique nature of ethylene as a gas allows it to easily diffuse through plant tissues and even travel to neighboring plants, coordinating responses on a broad scale. This mobility, combined with its activity at very low concentrations, makes it a powerful signaling molecule.
Ethylene’s Key Functions in Plant Biology
Ethylene is widely recognized for its role in the ripening of climacteric fruits, such as bananas, tomatoes, and avocados. In these fruits, a sharp increase in ethylene production triggers and coordinates the changes associated with ripening. This includes the conversion of starches to sugars, the softening of the fruit’s texture through cell wall degradation, and the development of characteristic colors and aromas that attract seed-dispersing animals.
The hormone also governs senescence, the programmed aging and death of plant parts, and abscission, the shedding of leaves, flowers, and fruit. As leaves or petals age, the plant increases ethylene production, which promotes the breakdown of chlorophyll and other cellular components. In abscission, ethylene stimulates the formation of a specialized layer of cells at the base of a leaf stalk or fruit stem, weakening the connection and allowing it to detach without harming the plant.
Plants produce ethylene as a rapid response to a variety of environmental challenges. When a plant is wounded or attacked by pathogens, a burst of ethylene can help initiate defense mechanisms. In situations of abiotic stress, such as flooding or drought, ethylene signaling can trigger adaptive changes. For instance, in waterlogged conditions, it can promote cell death to create air channels (aerenchyma) in roots, allowing them to survive low-oxygen environments.
Ethylene also regulates plant growth from the very beginning, helping to break seed dormancy and promote germination. In seedlings growing in the dark, ethylene mediates the “triple response”: inhibited stem elongation, radial swelling of the stem, and an exaggerated curve in the apical hook. This response protects the delicate shoot tip as it pushes through soil.
Understanding the Ethylene Signaling Pathway
Plants perceive ethylene through a counterintuitive molecular process. Ethylene receptors are located in the membrane of a cellular compartment called the endoplasmic reticulum. In the absence of ethylene, these receptors are active and function as a “brake,” sending a signal to repress ethylene-related responses within the cell.
When ethylene gas diffuses into the cell and binds to these receptors, it inactivates them, which releases the “brake” on the signaling pathway. By turning off the repressor, the presence of ethylene allows the downstream signaling components to become active. This is a form of negative regulation, where the signal works by inhibiting an inhibitor.
Once the receptors are inactivated, a protein called CTR1, which is normally kept active by the receptors, is also turned off. This inactivation allows another protein, EIN2, to become active. EIN2’s activation leads to the processing and movement of a portion of the protein to the cell’s nucleus.
Inside the nucleus, the activated portion of EIN2 stabilizes transcription factors. These factors are the master switches that directly control the plant’s response. They bind to specific DNA sequences in the promoter regions of ethylene-responsive genes, turning on the machinery needed to produce the proteins that carry out the physiological changes. This could include enzymes for fruit ripening, cell wall degradation for leaf fall, or proteins involved in defense against pathogens.
Applications of Ethylene Knowledge in Everyday Life
Commercially, ethylene gas is used to control the ripening of fruits after they have been harvested and shipped. Bananas, for example, are picked while green and hard, and then exposed to ethylene in special ripening rooms to ensure they reach the consumer at the perfect stage of ripeness and color.
Conversely, extending the shelf life of produce often involves inhibiting the effects of ethylene. Technologies have been developed to remove ethylene from storage and transport environments using scrubbers or filters. A compound known as 1-methylcyclopropene (1-MCP) is widely used to block ethylene perception. By binding to the ethylene receptors, 1-MCP prevents the hormone from initiating the ripening or aging process, keeping fruits, vegetables, and cut flowers fresh for longer periods.
In horticulture and floriculture, this knowledge is used to manage the longevity of ornamental plants and flowers. Preventing premature wilting, petal drop, and fading is a major goal for the floral industry. By treating flowers with ethylene inhibitors, their vase life can be significantly extended, ensuring they remain vibrant from the grower to the consumer.