Understanding the Biogenesis of Ethylene in Plants

Ethylene is a simple gaseous molecule – just two carbon atoms and four hydrogen atoms – yet it plays an outsized role in plant life. From triggering fruit ripening to orchestrating leaf drop and responding to environmental stress, ethylene is one of the most versatile plant hormones. But how do plants actually produce this critical gas? The answer lies in a well-defined biochemical pathway known as ethylene biogenesis (or biosynthesis), which starts with the amino acid methionine and proceeds through a series of enzymatic conversions. Understanding this pathway is essential for anyone studying post-harvest management, crop physiology, or food science.

The ethylene biosynthesis pathway: a quick overview 🔗

The ethylene biosynthesis pathway in higher plants is often described as remarkably simple – it involves just two dedicated enzymatic reactions. Despite this apparent simplicity, the regulation behind these reactions is layered and sophisticated. The pathway follows this sequence: Methionine → S-adenosylmethionine (SAM) → 1-aminocyclopropane-1-carboxylic acid (ACC) → Ethylene. Each step is catalysed by specific enzymes and regulated by both internal developmental signals and external environmental cues.

Step 1: Methionine to S-adenosylmethionine (SAM) 🔗

The journey begins with methionine, a sulfur-containing amino acid found in all plant cells. Methionine is converted to S-adenosylmethionine (SAM) by the enzyme SAM synthetase, and this reaction requires one molecule of ATP. This is not an exclusive step for ethylene production – SAM is also the primary methyl group donor for a wide range of cellular processes, including DNA methylation, protein modification, and polyamine biosynthesis. In fact, approximately 80% of cellular methionine is channelled into SAM production, making it one of the most metabolically active intermediates in plant cells.

Because SAM feeds into multiple pathways, its availability can influence how much ethylene a plant ultimately produces. However, studies on ripening tomato fruit have shown that different SAM-dependent pathways – including ethylene synthesis and polyamine production – can operate simultaneously thanks to an expanded SAM pool during ripening.

Step 2: SAM to ACC – the rate-limiting step 🔗

The first reaction unique to ethylene biosynthesis is the conversion of SAM to 1-aminocyclopropane-1-carboxylic acid (ACC). This step is catalysed by the enzyme ACC synthase (ACS), which belongs to the family of pyridoxal-5′-phosphate (PLP)-dependent enzymes. PLP (the active form of vitamin B6) serves as a cofactor for this reaction.

Along with ACC, this reaction also generates a by-product called 5′-methylthioadenosine (MTA). MTA does not go to waste – it is recycled back to methionine through a series of reactions collectively known as the Yang cycle (also called the methionine salvage pathway). This recycling mechanism is critical because it ensures that methionine levels remain stable even when ethylene is being produced at high rates, such as during fruit ripening. Without the Yang cycle, the limited methionine pool in plant cells would be quickly depleted.

Why is this step considered rate-limiting? 🔗

Under most physiological conditions, the conversion of SAM to ACC by ACC synthase is the slowest step in the entire pathway – making it the rate-limiting reaction. The rate of ethylene production in a tissue is closely tied to its endogenous ACC levels. When ACC levels are high (such as in wounded or ripening tissues), ethylene output increases correspondingly. This is why ACS has been a primary target for researchers aiming to control ethylene production in crops.

Step 3: ACC to ethylene – the final oxidation 🔗

In the final step, ACC is converted to ethylene by the enzyme ACC oxidase (ACO). This reaction is an oxidative process that requires molecular oxygen, along with iron (Fe²⁺) and ascorbate as cofactors. In addition to ethylene, this reaction produces CO₂ and hydrogen cyanide (HCN). The HCN generated is quickly detoxified by the enzyme β-cyanoalanine synthase, which converts it to β-cyanoalanine, preventing toxicity.

For many years, ACO was considered a secondary player relative to ACS. However, growing evidence now shows that ACO can also act as the rate-limiting enzyme in certain conditions – for instance, during specific stages of flower senescence or in certain fruit tissues. This has shifted attention toward understanding the regulation of ACO in greater detail.

The oxygen requirement 🔗

An important point about the final step is its strict dependence on oxygen. Under anaerobic (low-oxygen) conditions, ACC accumulates in plant tissues because it cannot be converted to ethylene. This has practical significance: controlled atmosphere storage of fruits, where oxygen levels are deliberately reduced, leverages this principle to slow down ethylene production and extend shelf life.

The Yang cycle: recycling methionine 🔗

The Yang cycle (named after Shang Fa Yang, who established key elements of this pathway using apple fruit tissue) is the methionine salvage pathway that operates alongside ethylene biosynthesis. When ACS converts SAM to ACC, the by-product MTA is generated. MTA is then converted back to methionine through several intermediate steps involving enzymes like MTA nucleosidase and MTR kinase.

This recycling loop has a special feature: it conserves the methylthio group of methionine. This means the sulfur atom is never lost during ethylene production. The carbon atoms that end up as ethylene (C-3 and C-4 of methionine) are replenished from the ribose portion of ATP. This elegant recycling system allows plants to sustain continuous ethylene production without depleting their methionine reserves – a crucial adaptation during high-ethylene events like climacteric fruit ripening.

ACC conjugation: an alternative fate for ACC 🔗

Not all ACC in plant cells is converted to ethylene. Some of it is diverted into conjugated forms, which serve as a mechanism to regulate the free ACC pool. The three main conjugates are malonyl-ACC (MACC), the most abundant form, produced by the enzyme ACC-N-malonyl transferase; γ-glutamyl-ACC (GACC); and jasmonyl-ACC (JA-ACC).

Conjugation of ACC effectively reduces the amount of precursor available for ethylene production. This is one of the ways plants fine-tune ethylene levels in different tissues and developmental stages. Interestingly, recent research has also shown that ACC itself may function as a signalling molecule independent of ethylene, playing roles in cell wall metabolism, guard cell development, and pollen tube attraction. This dual function makes ACC homeostasis even more important.

Regulation of ethylene biosynthesis 🔗

The ethylene biosynthesis pathway may be short, but its regulation occurs at multiple levels – transcriptional, post-transcriptional, and post-translational.

Transcriptional regulation 🔗

Both ACS and ACO are encoded by multigene families. In tomato, for example, there are multiple ACS and ACO genes that are expressed at different times during fruit development. This means the plant can activate specific gene family members in response to particular signals – such as wounding, pathogen attack, flooding, or developmental cues like ripening.

Post-translational regulation of ACS 🔗

ACS protein stability is tightly controlled through phosphorylation and ubiquitin-mediated degradation. Phosphorylation by kinases can stabilise ACS proteins, increasing their half-life and thereby boosting ethylene production. Conversely, dephosphorylation can target ACS for degradation via the 26S proteasome pathway. This layered post-translational control allows plants to rapidly adjust ethylene output in response to changing conditions.

Environmental and hormonal signals 🔗

A range of factors stimulate ethylene biosynthesis. Wounding, flooding, drought, pathogen infection, and exposure to auxin are all known to upregulate ACS expression and activity. At low concentrations, ethylene promotes normal growth and development. However, elevated ethylene – commonly produced under stress – can trigger adverse effects such as premature leaf senescence, flower wilting, and accelerated fruit softening.

Inhibitors of ethylene biosynthesis 🔗

Understanding how to block ethylene production has enormous practical value, especially in post-harvest management of fruits, vegetables, and cut flowers.

Aminoethoxyvinylglycine (AVG) 🔗

AVG is one of the best-known inhibitors of ethylene biosynthesis. It works by irreversibly blocking ACC synthase, the enzyme responsible for converting SAM to ACC. AVG binds to the PLP cofactor within the enzyme’s active site, effectively shutting down its catalytic activity. Commercially, AVG is marketed as ReTain® and is widely used by apple growers in the United States to delay fruit ripening, reduce pre-harvest fruit drop, and maintain flesh firmness. However, it is worth noting that AVG is not entirely specific to ACC synthase – it can also inhibit other PLP-dependent enzymes, including tryptophan aminotransferase, which is involved in auxin biosynthesis.

Aminooxyacetic acid (AOA) 🔗

AOA is another inhibitor that targets ACC synthase. It works by forming a complex with the PLP cofactor, thereby competing with SAM for access to the enzyme’s catalytic site. While effective in laboratory settings, AOA is less commonly used in commercial applications compared to AVG.

Cobalt ions (Co²⁺) and other ACO inhibitors 🔗

Cobalt ions inhibit the activity of ACC oxidase, the enzyme catalysing the final step. Since ACO requires Fe²⁺ as a cofactor, cobalt can displace iron at the active site and prevent the conversion of ACC to ethylene. Similarly, researchers have recently identified pyrazinamide (PZA), a drug used to treat tuberculosis, as a novel ACO inhibitor. PZA is converted inside plant cells to pyrazinecarboxylic acid (POA), which binds at the active site of ACO and blocks its interaction with natural substrates.

1-Methylcyclopropene (1-MCP) 🔗

While not an inhibitor of ethylene biosynthesis per se, 1-MCP deserves mention because it is the most widely used commercial tool for managing ethylene effects. It works by binding to ethylene receptors, blocking the plant’s ability to perceive ethylene and thereby preventing downstream responses like ripening and senescence. 1-MCP is sold under various brand names and is extensively used in the storage and transport of fruits, vegetables, and ornamental plants.

Why ethylene biogenesis matters in agriculture and food science 🔗

Ethylene’s influence on plant life is vast. It regulates seed germination, root hair formation, stem elongation, leaf and flower senescence, fruit ripening, and abscission. In climacteric fruits such as tomatoes, bananas, apples, and mangoes, ethylene triggers the coordinated changes in colour, texture, aroma, and nutritional content that define ripening. Even in non-climacteric fruits like strawberries and grapes, ethylene plays a regulatory role, though at lower concentrations.

From a food science perspective, controlling ethylene biosynthesis is directly linked to reducing post-harvest losses. Globally, a significant proportion of harvested fruits and vegetables is lost due to over-ripening and decay – much of it driven by uncontrolled ethylene accumulation. Tools like AVG, 1-MCP, and controlled atmosphere storage are all based on the understanding of ethylene biogenesis discussed above.

In the context of environmental stress – whether drought, flooding, salinity, or extreme heat – ethylene production typically increases, activating defence responses but also potentially accelerating damaging processes like premature senescence. Understanding the biosynthetic pathway allows researchers and breeders to develop crop varieties with optimised ethylene responses, improving both yield stability and post-harvest quality.

Key takeaways 🔗

The biogenesis of ethylene in plants follows a concise but tightly regulated route: methionine is activated to SAM, SAM is converted to ACC by ACC synthase, and ACC is oxidised to ethylene by ACC oxidase. The Yang cycle ensures methionine is recycled to sustain production. The pathway is regulated at every level – from gene expression to enzyme stability – and is responsive to developmental programmes, hormonal signals, and environmental stress. Inhibitors like AVG and cobalt ions provide practical tools for managing ethylene in agriculture and post-harvest handling.

What do you think? Given that ethylene biosynthesis can be influenced at multiple points – from SAM availability to ACC conjugation to ACO inhibition – which intervention point do you think holds the most promise for improving shelf life of perishable crops? And how might climate change, with its associated increase in abiotic stresses, alter the way we approach ethylene management in the future?

References

1. https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2019.00695/full
2. https://pmc.ncbi.nlm.nih.gov/articles/PMC151252/
3. https://en.wikipedia.org/wiki/1-Aminocyclopropane-1-carboxylate_synthase
4. https://nph.onlinelibrary.wiley.com/doi/10.1111/nph.16873
5. https://pmc.ncbi.nlm.nih.gov/articles/PMC9460115/
6. https://link.springer.com/chapter/10.1007/978-94-017-9484-8_1
7. https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2017.00475/full
8. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/aminoethoxyvinylglycine
9. https://www.nature.com/articles/ncomms15758
10. https://pmc.ncbi.nlm.nih.gov/articles/PMC5378820/
11. https://www.mdpi.com/2218-273X/14/1/90