Ethylene and Fruit Ripening

Ethylene (C₂H₄, ethene) is unique among the classical plant hormones: it is a simple, small, hydrophobic gas rather than a solid organic molecule dissolved in water. This physical property has enormous biological consequences. Because ethylene is a gas, it moves through intercellular air spaces and diffuses out of the plant into the surrounding atmosphere, meaning its "signal" can travel between organs and even between different fruits or plants sharing a closed space, such as a storage room or a paper bag. Ethylene is synthesized by essentially all plant tissues and acts at extraordinarily low concentrations, often in the range of 0.1 to 1 part per million (ppm), making it among the most potent hormones known by mass. Two processes anchor this article. First, ethylene biosynthesis proceeds through a short, well-characterized pathway from the amino acid methionine, via S-adenosylmethionine (SAM) and 1-aminocyclopropane-1-carboxylic acid (ACC), with ACC synthase (ACS) as the committed and rate-limiting enzymatic step. Second, ethylene action is most dramatically visible in climacteric fruit ripening, a coordinated developmental program in which a burst of respiration (the "climacteric rise") coincides with a spike in ethylene production, triggering coordinated softening, sugar accumulation, pigment change, and aroma volatile synthesis. Not all fruits behave this way; non-climacteric fruits ripen without this respiratory and ethylene burst and are far less responsive to exogenous ethylene, a distinction whose molecular basis (differences in ripening-control transcription factors, not merely ethylene levels) is still an active area of research. Ethylene signaling connects tightly to other topics in Plant Hormones: An Overview, since ethylene rarely acts alone: its effects are frequently modulated by auxin (which stimulates ethylene synthesis), and the balance of ethylene with auxin and abscisic acid shapes processes as diverse as fruit abscission, leaf senescence, and the seedling's emergence strategy from soil.

1. Biosynthesis — the Yang cycle. Ethylene is built from the amino acid methionine in three enzymatic steps that constitute what is called the Yang cycle (named for Shang Fa Yang, who elucidated it): 1. Methionine + ATP → S-adenosylmethionine (SAM), catalyzed by SAM synthetase. 2. SAM → 1-aminocyclopropane-1-carboxylic acid (ACC) + 5′-methylthioadenosine (MTA), catalyzed by ACC synthase (ACS). This is the committed, rate-limiting step of the pathway, and ACS is encoded by a multigene family whose members are induced by wounding, auxin, and ripening signals themselves. 3. ACC → ethylene + CO₂ + HCN, catalyzed by ACC oxidase (ACO), an enzyme requiring O₂ and ascorbate as cosubstrate. The MTA byproduct is salvaged back to methionine (the "methionine salvage pathway" or Yang cycle proper), meaning the plant does not need to expend a whole methionine molecule's worth of carbon and nitrogen each time it makes ethylene — only the methyl-thio group is recycled while the ring-forming carbons of ACC become the ethylene itself. Because ACS is transcriptionally inducible and rapidly turned over by ubiquitin-mediated proteolysis, cells can switch ethylene production on and off (or up and down) within hours, giving ethylene physiology its characteristic rapid, pulse-like kinetics.

2. Perception — receptors that repress in the "off" state. Ethylene is perceived by a small family of receptors located, unusually, in the endoplasmic reticulum membrane rather than the plasma membrane: ETR1, ETR2, ERS1, ERS2, and EIN4 in Arabidopsis, with a homolog called NR (Never-ripe) especially well studied in tomato. Each receptor has a hydrophobic N-terminal domain that binds ethylene directly, using a copper cofactor delivered by the copper transporter RAN1 as an essential part of the binding pocket. The crucial and counterintuitive logic of this system is that the unbound receptor is the active signaling state, and it actively represses the ripening/ethylene response. Unbound receptors activate a Raf-like kinase, CTR1 (Constitutive Triple Response 1), which phosphorylates and inhibits the downstream membrane protein EIN2. When ethylene binds the receptor, the receptor (and CTR1) is switched off, releasing EIN2 from inhibition. Ethylene signaling is therefore a double-negative cascade: hormone turns off a repressor of a repressor, ultimately turning genes ON.

3. Signal relay to the nucleus. Unphosphorylated, active EIN2 is cleaved, and its C-terminal fragment translocates to the nucleus (and also to cytoplasmic P-bodies to help degrade repressor mRNAs). In the nucleus, EIN2 signaling stabilizes the transcription factor EIN3 (and related EILs) by blocking their degradation via the ubiquitin-proteasome system (through F-box proteins EBF1/EBF2). Stabilized EIN3 binds promoters of primary ethylene-response genes, including a cascade of ERF (Ethylene Response Factor) transcription factors, which in turn activate the large downstream battery of ripening genes: cell-wall hydrolases (polygalacturonase, pectin methylesterase, expansins) that soften tissue, enzymes of carotenoid and anthocyanin synthesis that change color, enzymes producing volatile esters and terpenoids that create aroma, and further copies of ACS and ACO themselves.

4. Autocatalysis — why climacteric ripening is a switch, not a dial. This last point is the key to climacteric physiology. Ethylene signaling upregulates ACS and ACO gene expression, which increases ethylene production, which further activates the receptors and signaling pathway, which further upregulates ACS/ACO — a positive feedback loop called System 2 ethylene production (autocatalytic ethylene), distinguished from the low, developmentally programmed, auxin-regulated System 1 ethylene production present throughout normal vegetative growth and in non-climacteric tissues. Once a climacteric fruit crosses a threshold and enters System 2, ripening becomes effectively self-sustaining and difficult to reverse, explaining why climacteric fruit ripening looks like an all-or-none developmental switch rather than a graded response to hormone dose.

5. The triple response — the classic seedling bioassay. Long before molecular biology, ethylene's action was diagnosed using a simple, robust morphological syndrome in dark-grown (etiolated) dicot seedlings exposed to ethylene or its precursors:

• Radial swelling / inhibition of stem elongation — cells expand laterally more than they elongate vertically, thickening the stem.
• Exaggeration of the apical hook — the curved region just behind the shoot tip curls more tightly, thought to help protect the delicate shoot apex as it pushes through soil.
• Horizontal (diagravitropic) growth — the seedling grows sideways rather than straight up.

This "triple response" remains the standard whole-organism bioassay used to screen ethylene mutants (constitutive triple response, ctr, mutants show the phenotype even without ethylene; ethylene-insensitive, ein, mutants fail to show it even with high ethylene).

Neljubow (1901) — discovery of the triple response. Dimitry Neljubow, working in St. Petersburg, noticed that dark-grown pea seedlings in his laboratory showed abnormal, bent, thickened growth compared with seedlings grown outdoors. He correctly deduced that the culprit was illuminating gas (coal gas, used for lab lighting at the time) leaking into the growth chamber, and he identified ethylene as the specific active component of that gas mixture. This established, well before auxin was chemically characterized, that a simple gas could act as a potent, specific regulator of plant growth — the founding observation of the entire field and the origin of the "triple response" bioassay described above.

Denny and Crocker (early 20th century) and Gane (1934) — ethylene as a natural plant product. For decades after Neljubow, ethylene's effects were studied using external gas sources (illuminating gas, or fruits known to emit "ripening gas"), leaving open the question of whether plants themselves make ethylene or merely respond to it. Richard Gane demonstrated chemically in 1934 that ripening apples themselves release ethylene, proving that ethylene is an endogenous plant product and not merely an environmental contaminant — a necessary step before ethylene could be accepted as a true hormone.

Burg and Burg (1960s) — climacteric physiology quantified. Stanley Burg and colleagues used gas chromatography to precisely measure the coincidence of the respiratory climacteric with the ethylene peak in ripening fruit, and showed that treating pre-climacteric fruit with exogenous ethylene could trigger premature, synchronized ripening, while removing ethylene (by ventilation, or later by hypobaric/low-pressure storage) delayed it. This causally linked ethylene concentration to the timing of the ripening program, rather than ethylene being merely a byproduct of ripening.

Silver ion and 1-MCP inhibitor experiments — proving necessity. Silver ions (Ag⁺, applied as silver thiosulfate) block ethylene action by interfering with receptor function without blocking ethylene synthesis; aminoethoxyvinylglycine (AVG) and aminooxyacetic acid (AOA) block ACS activity and thus block ethylene synthesis itself. Fruits and flowers treated with these inhibitors fail to ripen or senesce on schedule even though they are otherwise developmentally mature, demonstrating that ethylene action (not just its correlation with ripening) is causally required. The compound 1-methylcyclopropene (1-MCP), developed later, binds ethylene receptors essentially irreversibly and is now used commercially (marketed as SmartFresh) to keep apples and other climacteric fruit in storage for many months by preventing ethylene perception.

Tomato mutants — dissecting the pathway genetically. Three classic tomato mutants provided decisive, independent genetic proof of different pathway components:

• Never-ripe (Nr) carries a dominant, defective allele of the ethylene receptor gene ETR-like NR; fruit remain firm and green far longer than normal because the receptor cannot properly relay the "ethylene present" signal, despite the plant still producing ethylene normally. This showed perception, not just production, is essential.
• ripening inhibitor (rin) and non-ripening (nor) carry mutations in MADS-box and NAC-domain transcription factors, respectively, that act upstream of and alongside ethylene signaling to control ripening onset; rin fruit barely ripen even when treated with exogenous ethylene, showing that ethylene signaling alone is not sufficient without these transcriptional "master regulators" also being intact — an important refinement showing ethylene is necessary but its downstream transcriptional context also matters.

Together, inhibitor pharmacology (loss of function chemically) and mutant genetics (loss of function heritably) converged on the same conclusion from two independent directions: ethylene production and perception are both required for normal climacteric ripening, and interrupting either step, chemically or genetically, blocks or delays it.

Example 1 — Diagnosing a storage problem using inhibitor logic. A postharvest physiologist has two batches of pre-climacteric avocados. Batch A is treated with AVG (an ACS inhibitor) before harvest; Batch B is treated with 1-MCP (a receptor blocker) after harvest. Both batches are then exposed to 10 ppm exogenous ethylene gas in a sealed room.

• Batch A avocados do ripen normally, because although their own ethylene synthesis is blocked, exogenous ethylene supplies the missing hormone directly to intact, functional receptors.
• Batch B avocados fail to ripen on schedule even with the same 10 ppm exogenous ethylene, because 1-MCP has occupied and disabled the receptors themselves, so no signal can be perceived regardless of how much ethylene gas is present.

The logic: a synthesis inhibitor can be "rescued" by adding hormone back exogenously, but a perception inhibitor cannot, because the lesion is downstream of the hormone itself. This distinction is exactly how researchers experimentally separate biosynthesis mutants/inhibitors from perception mutants/inhibitors, and it is the same reasoning tested in the Never-ripe mutant (which also fails to be rescued by exogenous ethylene, confirming NR acts in perception, not synthesis).

Example 2 — Quantifying the climacteric respiratory burst. Suppose a banana's CO₂ output is monitored during ripening at 20 °C in a sealed chamber of fixed volume. Baseline (pre-climacteric) respiration is measured at 15 mg CO₂ kg⁻¹ h⁻¹, and ethylene evolution is negligible (< 0.05 μL kg⁻¹ h⁻¹). Over the next 48 hours, ethylene evolution rises sharply to a peak of 30 μL kg⁻¹ h⁻¹, and CO₂ output rises in parallel to a peak of 150 mg kg⁻¹ h⁻¹ before declining as the fruit fully ripens and senesces. Two calculations follow from this: 1. Fold-increase in respiration: 150 / 15 = a 10-fold climacteric rise in respiration rate, the defining signature of climacteric physiology (in real bananas the climacteric rise is typically 2- to 5-fold; a 10-fold rise here is used to make the arithmetic vivid, but the qualitative pattern — a sharp, transient peak — is realistic). 2. Timing logic: because the ethylene rise precedes or is coincident with the CO₂ rise, and because applying exogenous ethylene to a pre-climacteric banana advances the timing of both curves without changing their shape, this is the kind of dose-and-timing evidence (in the tradition of Burg and Burg) that supports ethylene as the trigger of the respiratory burst rather than a downstream consequence of increased respiration. A student should note the logical trap here: correlation of two rising curves is not enough on its own; the causal claim rests additionally on the intervention experiments (exogenous ethylene advances ripening; inhibitors delay it) described in the evidence section.

Ethylene research does not have a single founding figure the way Mendel or Morgan anchor genetics, but if one person must be singled out for turning ethylene from a curiosity into a rigorously measured physiological signal, it is Stanley P. Burg. Working at the University of Miami in the 1950s–60s, often alongside his father Ellen Burg, Stanley Burg pioneered gas chromatography methods sensitive enough to measure the vanishingly small quantities of ethylene (parts per billion to parts per million) produced by ripening fruit, and he used these methods to establish, quantitatively, the tight temporal coupling between the ethylene peak and the respiratory climacteric. Burg also developed hypobaric (low-pressure) storage technology, which removes ethylene and other volatiles from around stored produce by continuously ventilating at reduced atmospheric pressure, directly translating his mechanistic insight into a commercial postharvest storage method still influential in the shipping and long-term storage of fruit today.

Postharvest storage and food security. Controlled-atmosphere storage rooms for apples reduce O₂, elevate CO₂, and use 1-MCP treatment to suppress ethylene perception, extending storage life from weeks to many months and dramatically reducing food waste and stabilizing year-round fruit supply.

Deliberate ripening ("gassing") of produce. Bananas, tomatoes, and other climacteric fruit are commonly picked green (for shipping durability) and then exposed to controlled ethylene gas in ripening rooms shortly before retail sale, giving distributors precise control over when fruit becomes ripe on the shelf.

Genetic engineering and breeding. The rin mutation, despite blocking full ripening, is deliberately bred into commercial tomato varieties (heterozygous "long shelf-life" hybrids) to slow softening during transport while preserving acceptable color and flavor development, a direct commercial application of the classic ripening-mutant genetics.

Cut flower and ornamental industries. Because ethylene triggers senescence and floral wilting, silver thiosulfate (an ethylene action inhibitor) and 1-MCP are used commercially to extend vase life of cut flowers such as carnations, which are notoriously ethylene-sensitive.

Agriculture and stress physiology. Ethylene-mediated aerenchyma formation and the "escape" or "quiescence" strategies of flood-tolerant crops (well studied in deepwater and submergence-tolerant rice varieties) are being incorporated into flood-resilient crop breeding programs, directly relevant to climate adaptation in agriculture.

Diagnostics and quality control. Portable ethylene sensors are now used in shipping containers and warehouses to monitor and manage ethylene accumulation in real time, preventing accidental premature ripening or spoilage of mixed produce loads, since ethylene-sensitive commodities (like lettuce) must be kept segregated from high-ethylene emitters (like ripening apples or melons).

Ask the tutor: How do auxin and ethylene signaling interact during fruit abscission and the formation of the abscission zone?