Autoluminescence is the production of luminescence autonomously such as in bioluminescent organisms. Among organisms that are engineered to emit light, autoluminescent organisms are those that do not require the addition of substrate in order to do so. Autoluminescence also refers to the ultra-weak photon emission that occurs in the absence of external stimuli and arising from metabolic reactions in living systems (see section on biological autoluminescence).
Types of organisms that have bioluminescent members include bacteria, dinoflagellates, fungi, jellyfish, squid, insects, fish and sharks. Bioluminescence is due to energy released from a chemical reaction where a luciferase enzyme causes the substrate luciferin to be oxidized which releases energy. There are many unique biochemical families of luciferins and luciferases. Luciferins are small molecules the emit light when oxidized and many luciferins have been derived by evolution from unrelated biochemical pathways. Bioluminescence pathway genes have been used to genetically engineer assays, screening and bioimaging tools used in biology, chemistry and medicine.
There are no plants found in nature known to display autogenic bioluminescence. Plants that are engineered to express luciferases still require the exogenous application of the substrate molecules, luciferins, which can be toxic and have uneven distribution and result in low light emission levels. Autoluminescent plants that do not need the addition of exogenous substrate were generated in transplastomic tobacco by Alexander Krichevsky and his team at the State University of New York in 2010. The tobacco were engineered to contain the full bacterial lux operon, or set of genes that control the bacterial light-emission enzymatic system. In bacteria, the luciferin substrate is derived from flavin mononucleotide which is derived from riboflavin which is present in both plants and bacteria. Prokaryotic organisms share evolutionary origins with plant chloroplasts. The bacterial bioluminescence genes were transferred into chloroplasts which were introduced into the tobacco producing transplastomic tobacco that produces luciferase and luciferins.
In 2019, the fungal luminescence biochemical pathway was used to produce autoluminescence in transgenic tobacco plants by two research groups. Three of the fungal genes were able to recycle luciferin from caffeic acid which is a ubiquitous plant metabolite and a fourth fungal gene encodes the luciferase. The reaction produces enough green light to see a glow with the naked eyes and record it with only a few seconds of camera exposure time. The research team based in Russia and the UK generated transgenic tobacco plants that overexpress all four genes. The transgenic glowing tabaco have been reported to produce over a billion photons per minute. Research was led by Karen Sarkisyan and Ilia Yampolsky in collaboration with Moscow biotech company Planta and was published in 2020 in Nature Biotechnology.
Researchers based at University of Minnesota in the US used the fungal genes to build an autoluminescent quantitative reporter to study gene expression. Potential applications include studying spatiotemporal expression of genes and the engineering of biosensors.
Researchers at Planta and Arjun Khakhar of the University of Minnesota group have shown that the generation of other glowing plants such as periwinkle, petunia and rose are feasible. Since light production in these plants is connected to the metabolism of the plant, it serves as a metabolic indicator and reveal the physiological status of the plants and their responses to the environment. In the presence of a ripe banana skin which emits ethylene the plants glow more brightly. Light Bio plans to partner with Planta to commercialize the technology.
Distinct from bioluminescence phenomena exhibited by organisms such as fireflies, jellyfish and luminescent bacteria which involve luciferases and luciferins, the term autoluminescence or biological autoluminescence is also used for the spontaneous photon emission at extremely low intensity, invisible to the naked eye that is emitted from living organisms that occurs without external excitation or administrations of chemiluminescence agents. This type of autoluminescence, also termed ultra-weak photon emission or biophoton emission is thought to occur through the generation of metastable excited states from oxidative metabolic reactions. The intensity of bioluminescence is approximately three orders of magnitude higher than that of spontaneous photon emission. The ultra-weak photon emission type of autoluminescence has potential applications in non-invasive imaging in biological research. Autoluminescence is different from autofluorescence which the emission of fluorescent light due to stimulated light reactions with endogenous fluorophores, molecules that absorb light, reach an excited state and then emit light.
A brief review of bioluminescent systems (2019)
Aubin Fleiss, Karen S. Sarkisyan
March 8, 2019