Luminescence is a useful trait to have, both for our use of lights and also in the study of animals and plants. And this is nothing new in science. The application of green fluorescent protein (GFP) has become extremely commonplace in biological research, as it allows for controlled expression and confirmation of other genetic changes being promoted properly. But it has its limitations. The most basic of which is that the light is often not all that bright and requires a high energy stream of particles in the UV range in order for the fluorescence to be expressed. Additionally, when using GFP in plants, the expression is often focused onto particular tissues or at certain times of development, meaning it is highly specialized, but also constrained.
A New Source
If true bioluminescence in the dark is required, the main go to for that has long been luciferin. This is the general compound that causes that biological phenomenon, but it has seen only partial application. That’s because injection or incorporation through substrates of luciferin is complicated and difficult and too much of the compound is inherently toxic. Other method attempts using bacterial bioluminescence genetics have seen similar failures and, even when successful, found that the organisms produced far too little light.
But there is an entire kingdom of life with a fair number of light producers that has been largely ignored up to this point, the fungi. Thanks to a recent genetic pathway breakdown, a greater scientific understanding of the caffeic acid pathway has been understood and its connection to naturally producing luciferin at sustainable non-harmful levels. This pathway is shared by plants and fungi and is used in the latter for their own form of luminescence. For plants, this pathway is an intermediate in a much broader system that creates lignin and many other compounds, often those used in creating the formidable toughness of the plant cell wall. While animals are unable to use this pathway directly, only two enzymes are missing to complete the biosynthesis system required, so it is within the realm of scientific possibility to create in them as well.
The Potential of Caffeic Acid
A research team consisting of members from the Institute of Bio-organic Chemistry of the Russian Academy of Sciences, MRC London Institute of Medical Sciences, and the Institute of Science and Technology Austria came together to try out this fungal novelty using the model organism plant tobacco, Nicotiana tabacum, and Agrobacterium as the gene transfer medium. Four separate genes from the fungus Neonothopanus nambi, first genetically described just last year, were used in order to incorporate this autoluminescent capability.
The insertion method was done numerous times to result in the gene cassette being incorporated into different parts of the plant genome, so the best outcome out of all those tested could be selected. The scientists ended up with fifteen independent lines of tobacco that generally had no trait differences from wild-type, other than a 12% height increase, suggesting that there is no detrimental effect from increased expression of the caffeic acid pathway, but that there may also be an effect on growth and development.
The light produced by the plants was visible to the naked eye of the researchers in the dark and this luminescence occurred at all stages of development and in all tissues. The brightness did vary, however, with there being a vivid green light made from younger, more actively developing cellular tissues. An additional test was conducted by infusing in luciferin, hispidin, and caffeic acid individually to see what impact they had on light emissions. The first two saw a sudden spike in brightness immediately after injection, whereas the caffeic acid saw a less bright emission that occurred over a period of time.
Transition zones of growing shoots and leaves were among the brightest of all, including the tips of roots. But the most glowing of all the tissues were the flowering parts (see the downloadable video here). Additionally, damaged leaves showed a bright line of color along the cut edge, showing the new cell growth to seal the wound. The research team were overall able to show that using these alternative fungal genes creates a form of luminescence that is an order of magnitude brighter than general previous bioluminescence techniques and also without buildup of toxic products in the plant that harms its growth.
A Luminescent Future
They hope to use these new glowing tobacco plants to better study how dynamic growth occurs in plants and where cell development is focused at all parts of the plant life cycle. They can also be planted directly into soil without the need of outside injections or involvement like with prior luciferin attempts. There is also the potential for eventual transference of these genes into larger plants such as trees and have them be a form of natural lumination within urban areas. Though that is, for now, just a pipe dream, and there is much more that needs to be studied before we can get to that point.
But this accomplishment has proven that autoluminescence is a feasible trait for use in plants and potentially animals without the damaging side effects and cost of earlier methods.
Photo CCs: Omphalotus subilludens (Murrill) H.E. Bigelow 814082 from Wikimedia Commons