Ethylene is a gaseous hydrocarbon with the molecular structure C₂H₄. It is commonly produced when hydrocarbons are exposed to oxidative stress, such as that found during lightning, volcanic eruptions, forest fires, and photochemical reactions on the ocean surface. Plants coopted ethylene biosynthesis during evolution to manage their response to oxidative stress from biotic and abiotic sources. Further exaptations of ethylene include modulation of plant life history events such as development, transformation, senescence, and death.

Due to a number of factors described below, humans may be subject to increasing ethylene exposure. The potential health consequences of ethylene exposure are not part of the public consciousness and warrant further exploration.

Ethylene In Nature

• Ethylene is produced when hydrocarbons are exposed to oxidative stress. Examples include photochemical reactions in various environments such as the atmosphere, ocean surface, and volcanoes. It is also produced in seawater by photochemical reactions of the dissolved organic carbon and is present in the growth environment of sponges.
• Ethylene is produced from fire and present in smoke during the oxidation of a hydrocarbon source. Ethylene is explosive at relatively low concentrations, suggesting that it may be a cause and effect of fire in a feed-forward manner. That is, ethylene is both a product and a substrate of hydrocarbon combustion.

Ethylene In Plant Life

• Ethylene serves as a simple gaseous hormone of plants, integrating external signals with internal processes and adjusting the plant’s phenotype to its environment.
• Triggers of ethylene biosynthesis in plants include biotic and abiotic stresses such as infection, wounding, dehydration, hypoxia, abscission, burning, or freezing. Oxidative stress is a potent trigger of ethylene production.
• In fruits, ethylene promotes its own further biosynthesis in a chain reaction. Such feed-forward synthesis is somewhat similar to fire’s production of ethylene, which induces further fire. In both cases, the formation of ethylene is oxygen-dependent and anaerobic conditions inhibit ethylene formation.
• Effects of ethylene include modulation of many developmental and aging aspects of the plant biology, including seed germination, root hair development, root nodulation, flower senescence, fruit ripening, abscission, senescence, and apoptosis.
• Zegzouti et al showed that ethylene controls the expression of many genes, suggesting plants co-opt ethylene for broader adaptive use.
• Wounded trees emit ethylene gas, which attracts insects that sense the gas as an indication of exposed tissue they can invade. It is as if such insects smell the fear or stress of plants.
• Ethylene gas is responsible for the “one bad apple spoils the whole bushel” phenomenon, suggesting that it signals stress within a group, not unlike the alarm call of animals.
• Ethylene released from the fire and smoke of forest fires induces an ethylene-mediated stress response in surrounding trees. These responses include flowering, senescence, ripening and abscission of fruits. It is as if a burning tree is sending alarm calls to the surrounding forest, and the neighboring plants respond by activating mechanisms that can increase the probability of seed dispersion before potential annihilation.

Ethylene In Human History

• The effect of ethylene has been harnessed for food cultivation since antiquity. In the Bible, Amos was described as a “gasher” of figs. Gashing was known to promote stress-mediated ripening of figs.
• In I779, Priestley referred to Ingenhousz as the first to generate ethylene.
• In 1924, Denny found that smoke from kerosene combustion in lanterns used to de-green citrus fruits contained ethylene as the active ingredient. This demonstrated that ethylene is a fruit-ripening agent that acts in very small amounts. These observations are in agreement with many similar historic reports such as those from China where incense was burnt in closed chambers to ripen pears.
• 107 million metric tons of ethylene were synthesized in 2005, making it the most produced organic compound in the world. Ethylene is used for thousands of applications, including oxidation for surfactants and detergents, halogenation in the PVC process, alkylation for packaging, oxo-reactions in making n-propyl alcohol, as an anesthetic agent, fruit ripening, and as a welding gas. Ethylene can be produced from natural gas and crude oil, which are the core of the hydrocarbon economy and the source of plastics.
• Ethylene is present in auto-emission.
• Ethylene is approved for spraying bananas but used for a multitude of other produce. In grocery stores, a fruit is sprayed with synthetic ethylene to “finish” the product — triggering the fruit stress response — so that it takes on a color and taste that appeal to the consumer.
• Ethylene in the US is currently regulated by the Environmental Protection Agency (EPA) and classified as a pesticide. Ironically, as discussed above, many insects are drawn to plant wounds by sensing ethylene.
• The US Center for Disease Control (CDC) lists ethylene as a potential human carcinogen. If ethylene were regulated by the US FDA, it might have been considered for the GRAS (Generally Recognized as Safe) designation. However, since the EPA regulates ethylene, it cannot receive that designation. Hence, there is only environmental regulation of one of the most synthesized chemicals on the planet that is widely consumed by humans and has little to no FDA oversight.

Ethylene Effects On Humans

• It is believed that ethylene is not produced in species outside the plant kingdom and some microorganisms.
• One fundamental question is whether plant ethylene production has a xenohormetic effect (cross-species hormonal effect) on animals that consume them. Ethylene modulates calcium regulation and other functions in animal cells. Ethylene has a myriad of effects on insects including shortened lifespan.
• Adrenaline and glucocorticoids, which are commonly assumed to be exclusively the stress hormones of animals, appear to have a function in plant life, too. Glucocorticoid receptors have been found in plants recently. Their function is now thought to be conserved across animals and plants. Furthermore, adrenaline biosynthesis has been observed in plants and its functions include response to stress and promotion of ethylene biosynthesis. It is intuitively appealing to speculate that stress pathways may be more conserved across the biome than we realize.
• Ethylene has been detected in the human exhaled air. It is found in gut microflora and can lead to the production of ethylene oxide, which is known to be a genotoxic human carcinogen.
• Ethylene has been detected emanating from human skin.
• Ethylene was introduced as a gaseous anesthesia in 1923 by Luckhardt and Carter. After many decades of experience, ethylene use declined as better agents emerged. From a risk perspective, ethylene use as an anesthetic has been linked to cardiac arrest, blood pressure fluctuations, water retention, nausea, hyperglycemia, and loss of appetite. Explosions have been reported.
• Nutritional effects of consumption on human healthcare are not clear.
• In the debate between corn and grass-feeding of livestock, the former is considered less healthy for the cows and the humans that consumed them. Is it possible that corn-fed cows are consuming highly processed corn with high concentrations of ethylene, whereas grass contains less ethylene?
• Humans metabolize ethylene to ethylene oxide, and ethylene oxide has been shown to have potential untoward health consequences.
• In the presence of heavy cigarette smoke, a flower in the room will wilt. High concentrations of ethylene present in cigarette smoke are thought to be responsible for this phenomenon. Smoking has long been known to increase the risk of stress-mediated human diseases such as hypertension and diabetes. Is it possible that ethylene from cigarette smoke is a contributor to human diseases through the stress pathway?

Implications

What are ethylene’s implications for human health? This is a critical question because so many aspects of modern plant food production, delivery, and preparation methods likely increase its ethylene content. Off-the-vine ripening, injuries during transport, ethylene spraying, disease, and cultivation in inappropriate environments or seasons can induce a stress response in plant flesh. Fermentation is an inducer of ethylene. Cutting a fruit or vegetable, which wounds the flesh and exposes it to oxidative stress, activates ethylene-mediated stress response including browning and sweetening. Food processing inflicts substantial wounds to plant flesh, triggering a massive stress response. Storage steps such as drying, salting, and freezing can slow oxidation, but also increase oxidative processes. Preparation techniques including heating, cooking, burning all add oxidative stress to plant-derived foods and trigger ethylene-mediated stress response in the remaining flesh. It is likely, then, those modern plant-derived foods consumed by humans are subject to extreme stress and contain high levels of ethylene and other mediators of a stress response. Despite these considerations, the degree to which humans may be consuming ethylene through consumption of “stressed food” remains uninvestigated.

Increasing evidence suggests that environmental stress experienced by a species can be sensed by another species that consumes the former, such that the latter can mount a preemptive, adaptive response to the stress. In the modern food system, however, the stress humans engender in the food supply chain may be coming back full circle to cause maladaptive stress response in humans who consume the stressed foods in a perverse version of “you are what you eat”. The stress that humans induce in plants during cultivation, production, transport, and preparation may elicit stress response in food, including the production of stress response molecules such as ethylene, saturated fats, and simple sugars.

Do these signaling molecules modulate stress responses in those who consume them? Does consumption of such stress-signaling molecules increase the risk of stress-mediated chronic diseases observed in humans including hypertension, diabetes, depression, heart disease, and obesity? We believe these questions warrant further exploration.

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Originally published in The Journal of the Palo Alto Institute in January 2012.