The Surprising Presence of 7-Dehydrocholesterol in the Plant Kingdom
For decades, 7-dehydrocholesterol (7-DHC), or provitamin D3, was understood as a compound primarily found in animals, serving as the precursor for vitamin D3 production in the skin upon UVB light exposure. This perspective led to the common belief that dietary vitamin D3 must come exclusively from animal products. However, advanced analytical methods have revealed that plants also produce 7-DHC, albeit in very low concentrations in most species. These trace amounts can be converted to vitamin D3 when exposed to ultraviolet light, although the yield is often negligible in commercial crops. The discovery that some plant families naturally accumulate higher levels has sparked significant interest among scientists seeking alternative, sustainable sources of vitamin D3.
Solanaceae: Natural Accumulators of 7-DHC
Certain plant families, most prominently the Solanaceae (nightshades), naturally produce and accumulate higher levels of cholesterol and its precursor, 7-DHC. This family includes economically important crops like tomatoes, potatoes, and eggplants. In these plants, cholesterol serves as a precursor for other bioactive compounds, such as defensive steroidal glycoalkaloids, rather than for widespread vitamin D synthesis. This inherent biochemical pathway makes them prime candidates for biofortification efforts.
Evidence from research shows that:
- Certain species like Solanum glaucophyllum (waxy-leaf nightshade) are known to accumulate significant amounts of vitamin D3 and its metabolites, causing calcium intoxication in grazing animals.
- Studies on plant oils, including wheat germ and avocado oils, have detected trace amounts of 7-DHC that can be converted to vitamin D3 with UVB exposure.
- A comprehensive review of vitamin D in plants confirmed the presence of 7-DHC and vitamin D3 in species like Solanum lycopersicum and Capsicum annuum, particularly after UVB irradiation.
Comparison of Plant vs. Animal Sterol Biosynthesis
While plants and animals both produce sterols, their biosynthetic pathways and primary sterol products differ significantly. This distinction explains the low natural levels of 7-DHC and cholesterol in most plant species.
| Feature | Animal Sterol Biosynthesis | Plant Sterol Biosynthesis |
|---|---|---|
| Primary Precursor | Lanosterol | Cycloartenol |
| Key Intermediates | 7-Dehydrocholesterol, Lathosterol | Episterol, 5-dehydroepisterol |
| Dominant End Product | Cholesterol | Phytosterols (sitosterol, campesterol) |
| 7-DHC Levels | Abundant in skin and other tissues | Generally low, with higher levels in certain families |
| Pathway Fate | Leads to cholesterol, steroid hormones, bile acids, and vitamin D3 | Primarily leads to phytosterols, brassinosteroids, and other metabolites |
Engineering Tomatoes to Produce 7-Dehydrocholesterol
Groundbreaking research at the John Innes Centre demonstrated the potential of biofortification by genetically modifying tomato plants to accumulate high levels of 7-DHC. Researchers used CRISPR-Cas9 gene editing to knock out a specific enzyme, sterol-Δ7 reductase 2 (Sl7-DR2), which is responsible for converting 7-DHC into cholesterol in the final step of the pathway.
This modification achieved several key results:
- Substantial accumulation: The engineered tomato leaves and fruit contained significantly higher levels of 7-DHC compared to non-edited plants.
- Successful conversion: When exposed to UVB light, the stored 7-DHC in the fruit and leaves was successfully converted into vitamin D3.
- No negative impact: The gene-edited plants showed no negative effects on their growth, development, or overall yield.
The Process of Vitamin D3 Conversion in Plants
The conversion of 7-DHC to vitamin D3 is a photochemical process, similar to how it occurs in human skin. When 7-DHC absorbs UVB light, its molecular structure undergoes a cleavage of the B-ring, forming an intermediate known as pre-vitamin D3. This unstable intermediate then undergoes a temperature-dependent thermal isomerization to form the stable, biologically active vitamin D3. In the gene-edited tomatoes, this process could be artificially triggered by exposing the sliced fruits or leaves to UVB light. This innovative approach offers a sustainable and cost-effective method for producing plant-based vitamin D3 supplements or fortifying food sources.
Conclusion
While the answer to "Do plants contain 7-dehydrocholesterol?" is a definitive yes, the naturally occurring levels are typically very low, making most plants a poor dietary source of vitamin D3. The field of metabolic engineering, however, is rapidly changing this reality. By precisely targeting and modifying the sterol biosynthesis pathway in crops like tomatoes, scientists have demonstrated a clear path towards developing sustainable, plant-based sources of vitamin D3. This not only offers a new solution to combat widespread vitamin D deficiency but also highlights the incredible potential of genetic technology to improve human nutrition through agricultural innovation. For those following a plant-based diet, this research could pave the way for a new generation of fortified foods. Read more about the gene-edited tomatoes on the John Innes Centre website: Gene-edited tomatoes could be a new source of vitamin D.
