The groundbreaking discovery of ACC oxidase in sugarcane and its implications for agriculture
Imagine an invisible gas, released in tiny quantities, that can dictate the fate of a plant—from the sweet ripening of fruit to the final breath of senescence. This regulatory molecule is ethylene, a simple plant hormone with complex effects. For sugarcane, a crop that provides nearly 86% of the world's sugar, understanding how ethylene operates could hold the key to improving cultivation and yield 5 .
At the heart of ethylene production lies a crucial enzyme: ACC oxidase (ACO), which catalyzes the final step in ethylene biosynthesis. The quest to isolate and understand the gene encoding this enzyme in sugarcane represents a fascinating journey into plant molecular biology.
Ethylene's role in plants extends far beyond triggering fruit ripening. This gaseous hormone regulates essential processes throughout a plant's life cycle:
Waking the embryo from dormancy
Building the foundation for nutrient uptake
Including sex differentiation in some species
The ethylene biosynthesis pathway is a two-step process beginning with S-adenosylmethionine (SAM) being converted to 1-aminocyclopropane-1-carboxylic acid (ACC) by ACC synthase (ACS). The final reaction involves ACC oxidase transforming ACC into ethylene, carbon dioxide, and cyanide 3 6 .
What makes ACC oxidase particularly fascinating is its requirement for specific co-factors: ferrous ions (Fe²⁺), ascorbic acid (vitamin C), and oxygen 8 . This places ACO in the family of Fe(II)-dependent oxygenases.
ACO typically exists as a multigene family in most plants, with different members expressed in response to various developmental cues and environmental stresses 6 . This complexity allows plants fine-tuned control over where and when ethylene is produced.
In 2003, Chinese researchers achieved a significant milestone: cloning and sequencing a fragment of the ACC oxidase gene from sugarcane (Saccharum L. Hybrid cv. ROC16) 1 . Their experimental approach demonstrated elegant scientific problem-solving.
The research team faced a challenge: how to identify an unknown gene amidst thousands of others in the sugarcane genome. Their solution was to look for conserved regions—stretches of the genetic code that remain similar across related species.
The experiment yielded a DNA fragment of approximately 940 base pairs. But was this truly part of a sugarcane ACC oxidase gene? The researchers turned to computational tools to verify their discovery 1 .
| Plant Species | Percentage Homology | Evolutionary Relationship |
|---|---|---|
| Rice | 88% | Monocot |
| Bamboo | 88% | Monocot |
| Banana | 74.6% | Monocot |
| Tomato | 70% | Dicot |
| Potato | 70% | Dicot |
| Melon | 68% | Dicot |
| Carnation | 68% | Dicot |
The data revealed a clear pattern: sugarcane ACO shared higher homology with other monocotyledonous plants (like rice and bamboo) than with dicotyledonous species 1 .
The cloning of sugarcane ACC oxidase followed a systematic approach that exemplifies standard practices in gene isolation and characterization:
Researchers created oligonucleotide primers corresponding to conserved amino acid regions in the ACO protein family
Using these primers, they amplified a 940 bp fragment from sugarcane DNA
The amplified fragment was sequenced and compared to known sequences in genomic databases
The evolutionary relationships between the sugarcane sequence and other ACO genes were determined
The confirmed sequence was submitted to GenBank with accession number AF442821 1
| Research Reagent | Function in Experiment |
|---|---|
| Degenerate Primers | Molecular probes designed to bind to conserved regions of target gene |
| DNA Template | Source of genetic material containing the gene of interest |
| PCR Components | Enzymes and nucleotides to amplify specific DNA fragments |
| BLAST Database | Computational tool for comparing DNA sequences across species |
| Phylogenetic Analysis Software | Programs to evolutionary relationships between genes |
The cloning of sugarcane ACC oxidase opened doors to deeper investigations into how this gene is regulated and functions within the plant.
Subsequent research in other plants, particularly Arabidopsis, has revealed that ACO gene expression is finely tuned by multiple signals 2 :
Both quality and duration affect ACO transcription
Including ethylene itself in a feedback loop
Such as drought and cold
Research has demonstrated that ACO isn't merely a housekeeping enzyme—it plays active, specific roles in plant development. Studies in Arabidopsis show that the aco1-1 mutant produces less ethylene in root tips and displays altered lateral root formation 2 .
Understanding the ACC oxidase gene has opened exciting possibilities for sugarcane improvement through biotechnology.
With the ACO gene in hand, researchers have attempted to modify ethylene production in sugarcane. In one study, scientists constructed plant expression vectors containing ACO antisense genes and successfully transformed sugarcane variety ROC22 7 .
The results were telling: transgenic plants grew more slowly and were more dwarfed compared to non-transgenic plants 7 . This demonstrates both the feasibility of genetically modifying ethylene production in sugarcane and the importance of balanced ethylene levels for normal growth.
| Component | Measurement Method | Key Features |
|---|---|---|
| Ethylene Gas | Gas Chromatography | Measures actual ethylene production in headspace |
| ACC (precursor) | Chemical Conversion to Ethylene | Indirect measurement using NaOCl/HgCl₂ |
| ACO Enzyme Activity | In vitro assay with co-factors | Measures enzyme performance directly |
| Gene Expression | RNA sequencing | Reveals when and where ACO genes are active |
Ethylene plays significant roles in plant stress responses, and ACO forms a key part of this system. Research has shown that ethylene biosynthesis in sugarcane can be induced by various environmental stresses 7 . Meanwhile, studies examining sugarcane responses to drought and cold have revealed complex regulatory networks that potentially interface with ethylene signaling pathways 5 9 .
The cloning of sugarcane ACC oxidase marked a beginning rather than an endpoint. Many questions remain to be explored:
As research continues, the initial discovery of the ACC oxidase gene sequence will serve as a foundation for developing sugarcane varieties with enhanced resilience and productivity—an important goal in the face of changing climate conditions and growing global demand for sugar and bioenergy.