John Timmer

Collage of images of glowing green plant parts.
Enlarge / An image of some of the plant parts imaged under their own glow.

Wander into the wrong section of a home improvement store and you’ll discover entire aisles dedicated to a product category called “landscape lighting.” Apparently, many people find it aesthetically pleasing to have systems to illuminate trees and flowers after the Sun goes down. If a group of Russian scientists has its way, however, the trees may ultimately be able to illuminate themselves.

A not-necessarily healthy glow

Plenty of living things glow in the dark. Some of these, like jellyfish and corals, rely on fluorescent proteins, which absorb energy at one wavelength and then re-emit it later at a visible one. Others, like fireflies, have enzymes that convert chemical energy to photons and so aren’t limited by the need for a source of photons to power their glow. Typically, these latter organisms have a specific chemical—generically called a “luciferin”—that provides the energy. An enzyme called a “luciferase” (again, a generic term) cleaves a chemical bond in the luciferin and releases its energy as a photon.

(The use of these generic terms can be confusing, as the terms are interchangeable but the molecules are not. If you try to feed the luciferin from bacteria to the firefly luciferase, nothing will happen. It also means that the list of molecules in the “luciferin” category is regularly expanding as biologist categorize more systems.)

Obviously, if you wanted to make a glow-in-the-dark plant (and honestly, who wouldn’t?), you’d choose a luciferin/luciferase system rather than a fluorescent protein. Otherwise, you’d have to set up landscape lighting anyway in order to feed the fluorescent protein a steady stream of photons to make sure it glows. But choosing the luciferin/luciferase system means that you have to either supply your plant with the right chemical or, ideally, get the plant to make the chemical itself.

This has been tried and… well, the results were not optimal. Dumping firefly luciferin on plants is extremely expensive and can be toxic. Getting plants to produce a luciferin themselves is tough, since most of the metabolic pathways used to provide starting material for the chemical aren’t present in plants. While there have been some attempts to engineer the pathways that make luciferins into plants, the results have been disappointing.

Bio-engineering-luminescent

The new work relies on the discovery of a new pathway in fungi that feeds on rotting wood. As you might imagine, a fungus that feeds on wood has to have some metabolic pathways that overlap with those of the plants that provide its food source. In this case, the fungus produces its luciferin through a biochemical pathway called the caffeic acid cycle. Conveniently, caffeic acid is an intermediate on the pathway that plants use to make lignin, a key component of plant cell walls and a major contributor to the robustness of wood.

It turns out that only three enzymes were needed to convert caffeic acid to a luciferin. Add in the need for a luciferase to get the glow and the researchers were looking at a total of four genes. They engineered them into a single stretch of DNA and inserted that into the cells of a tobacco plant. While tobacco may seem like an odd choice, it has been intensively researched and is often used for experiments like this.

The first bit of good news is that the production of an extra chemical made no difference to the plants. The growth, flowering, and germination time of the engineered plants was indistinguishable from those of controls. The only apparent difference was that the engineered plants are slightly taller (12 percent).

The second is that it worked. The fungal system produces a green glow, and most plant tissue is transparent at green wavelengths. The light in tissues like leaves and flowers was bright enough that it could be captured with consumer cameras instead of needing high-end imaging equipment. The researchers tested adding different luciferin precursors to the leaves and found that the conversion of caffeic acid was the big limitation on the light production, suggesting possible ways of improving the pathway.

Incidental science

The researchers don’t go into their motivations for doing this, although it’s clear there’s an element of “wouldn’t it be neat if… ” to the whole enterprise. Still, as it turns out, these glowing plants could be scientifically valuable. The researchers find that the glow is brightest in areas that have high levels of metabolic activity, including actively growing leaves and sites of injury. It can trace the location of veins within leaves, and the researchers see a wave of activity in old leaves as they’re being replaced, possibly because the plant is pulling nutrients back out of those leaves.

Finally, the researchers found they could massively boost the glow of the plants by exposing them to ripe banana skins or other sources of plant hormones. So maybe once your yard is fully planted with glowing plants, you’ll need to peel some bananas before fully enjoying it at night.

And you know it’s coming, because several authors disclosed that they work at a company called Planta that has filed for patents on this system.

Nature Biotechnology, 2020. DOI: 10.1038/s41587-020-0500-9  (About DOIs).



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