Glowing plants are here! Maybe they will be your next desk lamp?

In the fantasy world of literary works, glowing plants can be seen everywhere. Fantasy forests, street lamp flowers, fluorescent seeds floating in the air... These wonderful plants are beautiful and inspiring. Have you ever thought that plants may also glow in the real world?

Stills from the movie Avatar

Advanced experience with bioluminescence biology

Bioluminescence is actually quite common in nature. Currently, there are about 30 independent bioluminescent systems known, and various luminous species include bacteria, algae, fungi and invertebrates.

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Bioluminescent organisms are very common, so some scientists began to wonder: Can plants also glow?

It is important to note that you may have seen "luminous mushrooms" before, but "mushrooms" are not plants, but fungi.

Initially, scientists created luminous plants so that they could use fluorescent genes as reporter genes under laboratory conditions, thereby identifying whether exogenous genes have been successfully introduced into the plant by detecting the luminescence, or judging the growth status or gene expression of the experimental plants based on the intensity of the luminescence.

When studying luminous plants, we can first learn from the advanced experience of luminous organisms.

Bioluminescence relies on luciferase to catalyze its substrate luciferin to produce chemiluminescence. As early as the 1980s, scientists introduced firefly luciferase into plant cells or plants for expression. When the substrate luciferin and the energy substance adenosine triphosphate (ATP) are added through culture medium or irrigation, plant tissues will emit firefly-like light.

However, these luminous plants require external sources to provide substrates and energy, and have limitations such as weak luminescence, short duration, and difficulty in observation with the naked eye. Even the brightness effect of "reading at night with fireflies" cannot be achieved.

In addition to the above methods, scientists can also obtain fluorescent plants by transferring jellyfish fluorescent protein or improved fluorescent protein into plant tissues for expression through genetic engineering in the laboratory. However, these plants must be stimulated by ultraviolet or blue light to emit short-term fluorescence, and they need to be detected by instruments. They are not truly plants with self-luminous ability.

Tobacco leaves with firefly luciferase (left) and citrus leaves with fluorescent protein (right). Image source: References [4] and [5]

It's bright! Here come plants that can emit light

The experience gained from fireflies and jellyfish cannot be fully applied to plants, so scientists began to learn from luminous fungi, and this time they succeeded.

The plant luminescence system that can truly be seen by the naked eye made an important breakthrough in 2020. Scientists from the United States and Russia used the fungal bioluminescence pathway (FBP) present in luminescent fungi to transform and establish a bioluminescence system that can work in plants.

In the FBP system, caffeic acid is first converted into the intermediate product Hispidin, and then further converted into luciferin through enzyme catalysis. Finally, luciferin is oxidized under the catalysis of luciferase and releases light energy.

Catharanthus roseus and Rosa fruticosa transiently expressing FBP. Image source: Reference [1]

Caffeic acid, which is the raw material for the conversion of luciferin, is crucial to this system. Caffeic acid is a common molecule in plants and a key intermediate product of lignin and other important plant metabolites. Therefore, it is feasible to introduce the FBP system into plants and integrate the caffeic acid cycle metabolic pathway of fungal luminescence into the metabolic process of plants, thereby constructing plants that can produce self-luminescence without adding any chemicals.

In addition, the green light produced by the caffeic acid cycle does not overlap with the spectrum absorbed by colored plants to a large extent, so the light produced by the FBP pathway will not lose much brightness due to absorption by the plant itself.

When the self-luminous plants breathe, the absorbed oxygen can promote the interaction between luciferase and luciferin to produce an oxidation reaction, at which time the plants will release energy in the form of light. If the released light energy is strong enough, the transformed plants can produce self-luminescence visible to the naked eye while growing alive.

Self-luminous tobacco plant that has been introduced into the FBP system. Image source: Reference [3]

Each generation is brighter than the previous one

Plants may provide lighting in the future

In May 2023, Du Hao's team at Zhejiang University further improved the system on this basis.

In their study, they found that the content of caffeic acid, the precursor of luciferin biosynthesis, and the intermediate product galactoside is the limiting factor of plant luminescence intensity. Through identification and screening, the research team obtained two catalytic enzyme genes from Brassica napus and Aspergillus nidulans, respectively.

By introducing these two genes into the FBP system, the catalytic enzymes produced by them can efficiently promote the synthesis and accumulation of large amounts of caffeic acid and lactobacillus in plants, thereby significantly increasing the content of luciferin and successfully enhancing the luminescence intensity of self-luminous plants.

This plant self-luminous system optimized by metabolic engineering has increased the brightness of the original light by more than five times, and can continuously and stably emit light visible to the human eye. Even the leaves separated from the body can continue to glow for three days.

When multiple flowering plants are placed together, the light they produce can illuminate a dark environment, bright enough for people to clearly see larger text nearby.

Enhanced FBP Plants

After further research, the team found that the lack of sucrose supply would lead to a significant decrease in the biosynthesis of caffeic acid and lactone in enhanced self-luminous plants, indicating that the sugar produced by photosynthesis in self-luminous plants is essential for the synthesis of luciferin.

Comparison with the self-luminescence intensity of ordinary FBP plants. Image source: Reference [6]

These findings provide an explanation for the mechanism by which bioluminescent plants fix carbon dioxide in the air through photosynthesis during the day, converting solar energy into sugars and other organic matter, and then releasing light energy through catabolism at night.

The luminescence effect of enhanced FBP tobacco during the flowering period. Image source: Reference [6]

This research result has deeply analyzed and verified the mechanism of plant self-luminescence, and provides an important direction for further designing and optimizing the luminescence system. If the luminescence intensity is further improved on the basis of the existing results, and strong luminescent plant varieties are created, these plants will no longer be limited to laboratory scientific research and testing purposes, but are also expected to be used in environmental lighting and other fields.

Conclusion

With the continuous in-depth research, the brightness of luminous plants is getting higher and higher. In the future, plants that can really illuminate may appear. Just imagine that planting a few luminous trees can illuminate the road, and holding a branch or planting a luminous flower can be used as a lighting lamp. How wonderful it feels. At that time, we can directly convert and utilize bioenergy, which can not only save electricity, but also effectively reduce carbon emissions, playing a dual role of energy conservation and environmental protection and beautification.

Let us look forward to the real-life “Wonderland Garden” and “Magic Forest” wonders created by scientists.

References

[1] Khakhar A, et al. Elife, 2020, 9:e52786.

[2] Krichevsky A, et al. PLoS One, 2010, 5(11):e15461.

[3] Mitiouchkina T, et al. Nat Biotechnol, 2020, 38(8): 944-946.

[4] Ow DW, et al. Science, 1986, 234(4778): 856-859.

[5] Wu H, et al. Crop Sci, 2015, 55:2786–2797.

[6] Zheng P, et al. Plant Biotechnol J, 2023, 21: 1671–1681.

Planning and production

Produced by Science Popularization China

Author: Li Yin, School of Life Sciences, Sun Yat-sen University

Producer丨China Science Expo

Editors: Lin Lin, Jin Yufen (Intern)

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