Ethylene is the central hormone regulating climacteric fruit ripening. It is synthesized from L-methionine through the formation of S-adenosyl-L-methionine (SAM) and 1-aminocyclopropane-1-carboxylic acid (ACC) via ACC synthase (ACS), followed by conversion of ACC to ethylene by ACC oxidase (ACO). The ethylene biosynthesis pathway is shown in Figure 1, while the ethylene signaling pathway is illustrated in Figure 2.
This process is regulated at multiple levels: auxin and abscisic acid (ABA) enhance ACS/ACO gene expression, whereas gibberellin generally inhibits it. Notably, ethylene exhibits autocatalytic regulation, leading to the characteristic ethylene burst in climacteric fruits such as tomato, apple, and banana.
Ethylene is perceived by ETR/ERS receptors on the endoplasmic reticulum membrane and transduced via CTR1 to EIN2, which subsequently activates transcription factors EIN3/EIL and ERF. This signaling cascade regulates genes associated with ripening traits, including color change, softening, sugar accumulation, and aroma production.
Other hormones closely interact with ethylene: auxin activates ARF5 to enhance ACS/ACO expression; ABA promotes ethylene biosynthesis; while gibberellin delays ripening. Environmental factors such as high light, temperature, and mechanical injury also stimulate ethylene production.
Various technologies have been applied to control ripening, including genetic modification, chemical treatments (ethylene, 1-MCP), sensing technologies, and -omics approaches. However, knowledge gaps remain, particularly regarding the independent signaling role of ACC and the detailed regulation of ACS/ACO isoforms.
Ethylene Biosynthesis and Regulation
Ethylene biosynthesis involves three main steps:
- L-methionine → SAM (via SAM synthetase)
- SAM → ACC (via ACS; rate-limiting step)
- ACC → ethylene (via ACO, requiring oxygen and Fe²⁺)
Because ACS is the rate-limiting enzyme, regulation primarily targets its expression and activity.
At the transcriptional level, auxin and ABA upregulate ACS/ACO expression, while gibberellin downregulates it. Ethylene also stimulates its own production once a threshold is reached.
At the post-transcriptional level, ACS isoforms are regulated by phosphorylation: MPK3/MPK6 kinases stabilize ACS proteins (e.g., ACS2, ACS6), whereas PP2A-mediated dephosphorylation promotes their degradation. This allows flexible control of ethylene production under varying conditions.
Molecular Mechanism Linking Ethylene to Fruit Ripening
In the absence of ethylene, ETR/ERS receptors activate CTR1, which keeps EIN2 inactive. Upon ethylene binding, CTR1 is inhibited, allowing cleavage of EIN2 and translocation of its C-terminal fragment into the nucleus, where it activates EIN3/EIL and downstream ERF transcription factors.
ERFs directly regulate genes involved in ripening processes, including:
- Pigment biosynthesis (color change)
- Cell wall degradation (softening)
- Starch-to-sugar conversion
- Aroma compound production
However, some ripening processes can still occur, albeit more slowly, when ethylene is inhibited, indicating that ethylene is the primary trigger but not the sole regulator.
Hormonal Interactions and Environmental Factors
- Auxin: activates ARF5 to enhance ACS/ACO expression and ethylene production; may delay some ripening traits at later stages.
- ABA: increases prior to or during ripening, promoting sugar accumulation and ethylene biosynthesis.
- Gibberellin: suppresses ACS/ACO expression and delays ripening.
- Jasmonate: stimulates ethylene production at the pre-climacteric stage, enhancing color and aroma.
Environmental factors such as strong light, high temperature, mechanical damage, and stress conditions (drought, cold, hypoxia) can increase ethylene production and accelerate ripening.
Research Methods and Applications
- Molecular genetics: CRISPR/Cas9 and RNAi are used to modify ACS/ACO genes to extend shelf life.
- Chemical treatments: ethylene or ethephon induces ripening; 1-MCP inhibits ethylene perception to delay ripening.
- Measurement techniques: gas chromatography (GC), ethylene sensors, and ACC quantification via HPLC or GC-MS.
- -Omics approaches: transcriptomics, proteomics, and metabolomics provide comprehensive insights into ripening mechanisms.
Representative Studies and Applications
- Tomato: transcription factors RIN and NOR interact with ethylene; ACS suppression delays ripening.
- Apple: ACS1 plays a key role in ethylene production; 1-MCP and cold storage extend shelf life.
- Banana: ACO gene editing reduces ethylene production and prolongs storage.
- Melon: reduced ethylene lowers aroma formation but does not completely inhibit other metabolic processes.
- Non-climacteric fruits: ripening is mainly regulated by ABA and sugar accumulation, with minimal ethylene involvement.
Knowledge Gaps and Future Directions
Key areas for further research include:
- The independent signaling role of ACC
- Isoform-specific regulation of ACS/ACO
- Interactions between ethylene and other hormones
In practice, developing low-ethylene-producing cultivars and advanced sensing technologies will improve postharvest management and fruit quality on a global scale.
REFERENCES
Alonso-Salinas, R., López-Miranda, S., José Pérez-López, A., Acosta-Motos, J.R. (2024). Strategies to Delay Ethylene-Mediated Ripening in Climacteric Fruits: Implications for Shelf Life Extension and Postharvest Quality. Horticulturae 2024, 10(8), 840; https://doi.org/10.3390/horticulturae10080840
Liu, M., Wang, C., Ji, H., Sun, M., Liu, T., Wang, J., Cao, H., Zhu, Q. (2024). Ethylene biosynthesis and signal transduction during ripening and softening in non-climacteric fruits: an overview (2024). Front. Plant Sci., Sec. Crop and Product Physiology, Volume 15 - 2024 | https://doi.org/10.3389/fpls.2024.1368692.
Thao Minh Viet Nguyen, Dinh Thi Tran, Clara I Mata, Bram Van de Poel, Bart M Nicolaï, Maarten LATM Hertog, (2025). Gene expression driving ethylene biosynthesis and signaling pathways in ripening tomato fruit; a kinetic modelling approach. Journal of Experimental Botany, eraf055.
Thao Minh Viet Nguyen, Dinh Thi Tran, Bram Van De Poel, Maarten L. A. T. M. Hertog, Bart Nicolai (2024). The impact of growing season on the ethylene biosynthesis and signaling pathways of a heat tolerant tomato during off-vine postharvest ripening. Postharvest Biology and Technology, Vol.207,112637.
Tipu, M.M.H., Sherif S.M. (2024). Ethylene and its crosstalk with hormonal pathways in fruit ripening: mechanisms, modulation, and commercial exploitation. Front Plant Sci.,15:1475496. doi: 10.3389/fpls.2024.1475496.
Assoc. Prof. Tran Thi Dinh
Faculty of Food Science and Technology