Fruit Ripening and Ethylene

Why fruit ripening and ethylene matter

• Ethylene is the smallest hormone in biology. A molecule of just two carbons and four hydrogens, ethylene diffuses freely through air and through the intercellular spaces of tissue. Because it is a gas, it is the only plant hormone that can signal cell-to-cell, fruit-to-fruit, and even plant-to-plant without any transport protein — a feature no peptide or steroid hormone can match.
• It defines how food is shipped worldwide. Bananas — the world's most exported fruit, roughly 20 million tonnes traded annually — are picked hard and green in the tropics, shipped at 13–14 °C to suppress ethylene, and then deliberately gassed with 100–150 ppm ethylene in ripening rooms at the destination. The entire global banana trade is an applied lesson in climacteric physiology.
• Controlled atmosphere storage feeds us out of season. An apple bought in June was very likely harvested the previous September and held for months in sealed rooms at ~1–3% oxygen and near 0 °C. Low O₂ starves ACC oxidase of the oxygen it needs to make ethylene, so the climacteric is postponed almost indefinitely.
• Food waste is an ethylene problem. Roughly a third of fruit and vegetables are lost between farm and plate, much of it to premature ethylene-driven ripening and rot. Ethylene scrubbers (potassium permanganate sachets), receptor blockers (1-MCP), and cold chains all exist to manage this single gas.
• The Flavr Savr tomato was the first commercial GM food. Approved by the U.S. FDA in 1994, it carried an antisense construct that suppressed polygalacturonase — a cell-wall enzyme downstream of ethylene — to slow softening. It failed commercially, but it proved that dissecting the ripening pathway could be turned into a product.
• Ethylene is more than ripening. The same hormone drives leaf and flower senescence, abscission (the drop of leaves and fruit), the triple response of dark-grown seedlings, aerenchyma formation in flooded roots, and defense against pathogens. Ripening is just its most delicious job.

How ethylene ripens a fruit, step by step

The story begins with an amino acid. Ethylene is synthesized from methionine through what is now called the Yang cycle, worked out by Shang Fa Yang and colleagues in the late 1970s and early 1980s. Methionine is first activated to S-adenosyl-L-methionine (SAM) by SAM synthetase. Then the pivotal enzyme ACC synthase (ACS) converts SAM into 1-aminocyclopropane-1-carboxylic acid (ACC), releasing 5′-methylthioadenosine, which is recycled back to methionine so the plant does not exhaust its sulfur. Finally, ACC oxidase (ACO) uses molecular oxygen, Fe²⁺, and ascorbate to convert ACC into ethylene, carbon dioxide, and cyanide (detoxified by β-cyanoalanine synthase). The ACS step is the rate-limiting valve on the whole pathway — control ACS and you control ethylene.

In an unripe climacteric fruit, ethylene is made only in tiny amounts by what Shang Fa Yang termed System 1 — basal, self-inhibiting synthesis. The transition to ripening flips this to System 2, in which ethylene induces the expression of the very ACS and ACO genes that make it. This positive feedback is the autocatalytic burst: a small trigger — the fruit reaching horticultural maturity, a wound, or an outside puff of ethylene — pushes the tissue past a threshold, and ethylene production climbs exponentially. It is the biochemical definition of "ripe."

Ethylene is perceived not by making anything happen, but by switching a brake off. Copper-binding receptors of the ETR1 family (ETR1, ETR2, ERS1, ERS2, EIN4 in the model plant Arabidopsis) sit in the membrane of the endoplasmic reticulum. In clean air these receptors are active and, through the Raf-like kinase CTR1, keep the master regulator EIN2 phosphorylated and silent — so the ripening genes stay off. When ethylene binds (via a copper cofactor delivered by the transporter RAN1), the receptors and CTR1 shut down. EIN2 is de-repressed; its cleaved C-terminal fragment travels to the nucleus, where it stabilizes the transcription factors EIN3 and EIL1. These switch on a cascade of ERF (Ethylene Response Factor) genes. Ethylene signaling is therefore a double negative: the hormone works by inhibiting an inhibitor.

The downstream ERF program remodels the fruit on three fronts at once. Softening comes from cell-wall enzymes — polygalacturonase (PG) and pectate lyase dissolve pectin, pectin methylesterase de-esterifies it, and expansins loosen the cellulose–hemicellulose network — turning a firm tomato into a soft one. Color changes as chlorophyllase and the PAO pathway strip green chlorophyll while carotenoid biosynthesis (lycopene in tomato, β-carotene in mango) and anthocyanins are switched on. Sweetness and aroma arise as amylases hydrolyze stored starch into glucose, fructose, and sucrose, organic acids are consumed, and alcohol acyltransferases build the volatile esters that give a ripe banana or apple its smell. In a banana this starch-to-sugar swing is dramatic: starch falls from ~20–25% of fresh weight to under 1%, while sugars climb to ~15–20%.

Climacteric vs non-climacteric fruit

Feature	Climacteric fruit	Non-climacteric fruit
Examples	Apple, banana, tomato, avocado, pear, mango, peach, kiwi, melon	Strawberry, grape, citrus, pineapple, cherry, watermelon
Respiration during ripening	Sharp climacteric spike (CO2 several-fold)	No spike — flat or gradual decline
Ethylene pattern	Autocatalytic burst (System 2)	No burst; low basal ethylene (System 1)
Ripens off the plant?	Yes — can be picked mature-green	Largely no — must ripen on the plant
Response to applied ethylene	Triggers full ripening	Modest, does not self-amplify
Storage strategy	Delay the climacteric (cold, low O2, 1-MCP)	Slow senescence and decay; harvest ripe
Classic trade example	Green bananas shipped and gassed on arrival	Strawberries picked ripe and rushed to market

The "one bad apple" and how growers weaponize it

The proverb is a physiology lesson. A bruised, overripe, or fungus-infected apple releases a flood of ethylene. Because ethylene is autocatalytic in climacteric fruit, that gas pushes neighboring apples past their own ripening threshold, triggering their ACS and ACO genes and their own bursts — and the signal marches through the crate fruit-to-fruit until the whole barrel is overripe. Ethylene diffuses at parts-per-billion sensitivity, so it does not take much.

The reverse of the proverb is a kitchen trick. Seal an unripe avocado, peach, or pear in a paper bag with a ripe banana or apple; the trapped ethylene from the ripe fruit accelerates the climacteric of the hard one, and it softens in a day or two. Growers do this at industrial scale in ripening rooms, dosing tightly stacked pallets of green bananas or tomatoes with 100–150 ppm ethylene to synchronize a whole shipment. The proverb and the trick are the same molecule read in two directions.

Delaying ripening: controlled atmosphere and 1-MCP

If ethylene is the accelerator, commercial storage is the science of pressing the brake. Controlled atmosphere (CA) storage, pioneered by Franklin Kidd and Cyril West in Britain in the 1920s–1930s, holds fruit in sealed rooms at roughly 1–3% oxygen (versus 21% in air), 1–5% carbon dioxide, and temperatures near 0 °C. Low oxygen starves ACC oxidase, which requires O₂ as a substrate, so the fruit physically cannot complete the last step of ethylene synthesis; high CO₂ antagonizes ethylene action; and cold slows every enzyme. This is why an apple in the supermarket can be a year old and still crisp.

The modern complement is 1-methylcyclopropene (1-MCP), marketed as SmartFresh, discovered by Edward Sisler and Sylvia Blankenship and commercialized around 1999–2002. 1-MCP is a small cyclic molecule that binds the ethylene receptors essentially irreversibly, occupying the copper-containing binding pocket. Treated fruit becomes deaf to ethylene — it cannot perceive even the gas it makes itself — and ripening stalls until new receptors are synthesized. On the synthesis side, aminoethoxyvinylglycine (AVG), sold as ReTain, inhibits ACC synthase to keep apples on the tree longer without dropping. Together, CA rooms, 1-MCP, AVG, and ethylene scrubbers form a layered defense against a single two-carbon gas.

Common misconceptions

• "Ethylene makes fruit ripen by activating ripening genes." The opposite in mechanism: the receptors are negative regulators. In air they actively keep ripening off through CTR1 and EIN2. Ethylene binding switches the receptors off, releasing the brake. Loss-of-function receptor mutants (like the Arabidopsis triple and quadruple receptor mutants) are constitutively ethylene-responsive; the hormone works by de-repression.
• "Non-climacteric fruit ignore ethylene." They still produce and respond to ethylene — citrus degreening is routinely done with ethylene gas, and it regulates aspects of strawberry and grape ripening — but they lack the autocatalytic System-2 burst and the respiratory climacteric, so ripening does not self-amplify and does not continue meaningfully after harvest.
• "Refrigerating everything is best." Cold suppresses ethylene, but many tropical climacteric fruit suffer chilling injury below ~10–13 °C: bananas blacken, and tomatoes lose flavor and never ripen properly. This is why bananas ride at 13–14 °C, not in the fridge, and why a supermarket tomato refrigerated at home turns mealy.
• "Ethylene is the only ripening hormone." Ethylene dominates climacteric ripening, but abscisic acid (ABA) is a key driver in many non-climacteric fruit like grape and strawberry, and auxin, gibberellins, and sugars all modulate the timing. Ripening is a hormonal chorus with ethylene as the loudest voice in climacteric fruit.
• "Ripening equals rotting." Ripening is a programmed, coordinated developmental phase that maximizes seed-dispersal appeal — sweet, soft, colored, fragrant. Senescence and microbial rot are the uncontrolled decay that follows if the fruit is not eaten. Ethylene drives both, but they are distinct stages.
• "Ethylene is a modern additive." Ethylene is endogenous — the plant makes it. The ancient practice of gashing figs or burning incense to ripen fruit (recorded in Egypt and China) unknowingly exposed fruit to combustion-derived ethylene millennia before its chemistry was understood.

A famous history: from smoky streetlamps to a Nobel-adjacent pathway

• Dimitry Neljubow (1901). A Russian graduate student noticed that pea seedlings grown in his lab near leaking illuminating-gas pipes grew stunted, thickened, and horizontal — the "triple response." By systematically testing the components of coal gas, Neljubow identified ethylene as the active agent, at astonishingly low concentrations. It was the first demonstration that a simple gas could act as a plant growth regulator.
• Nineteenth-century citrus and streetlamps. Growers had long noticed that oranges and lemons stored in sheds heated by kerosene or gas stoves degreened faster. The "kerosene stove" ripening rooms of Florida and California worked because incomplete combustion released ethylene — folk practice that pre-dated the mechanism by decades.
• Richard Gane (1934). Working in England, Gane provided the definitive proof that ripening fruit themselves produce ethylene — collecting and chemically identifying the gas emitted by apples. This closed the loop: ethylene was not just something fruit responded to; it was something fruit made to signal ripening.
• Shang Fa Yang and the biosynthetic pathway (1979–1984). Yang's laboratory at UC Davis mapped methionine → SAM → ACC → ethylene, identified ACC as the key intermediate, established ACC synthase as rate-limiting, and described the methionine-recycling Yang cycle. This work made ethylene biosynthesis one of the best-understood hormone pathways in plants.
• Chang, Kwok, Bleecker, and Meyerowitz (1993). Using ethylene-insensitive Arabidopsis mutants isolated via the triple-response screen, this team cloned ETR1, the first ethylene receptor — a two-component histidine-kinase-like protein anchored in the ER. That discovery, followed by cloning of CTR1, EIN2, and EIN3, built the modern double-negative signaling model of how a gas turns into gene expression.