Glowing plants with bioluminescent systems
Up to now, three bioluminescent systems that origin from firefly,
bacteria and fungi have been reconstructed in plants. The firefly
luciferase-luciferin system is a kind of D‐luciferin‐dependent systems,
which is one of the most intensively studied bioluminescent systems
(Fleiss & Sarkisyan, 2019). In the presence of Mg2+,
oxygen and ATPs, reduced firefly luciferin can be catalyzed by firefly
luciferase, and generate yellow-green luminescence. To prolong glowing,
CoA could be used to deoxidize the oxidized luciferin (Gosset et al.,
2020). Ow. et al. (1986) introduced Photinus pyralis luciferase
gene into N. tabacum and Daucus carota for the first time.
Firefly luciferase was expressed during growing process of the
transgenic plant and accumulated in various plant organs, e.g., roots,
stems and leaves. When watering the tobacco with solution of firefly
luciferin and ATP, the substrate was transmitted to every tissues
through the vasculature and react with luciferase, leading to lightening
of the whole transgenic tobacco. Due to the technical difficulty to
synthesize firefly luciferin in plant cells, the firefly luciferins can
only be exogenously supplied. In 2017, a novel nano-technique named
pressurized bath infusion of nanoparticles (PBIN) was utilized to
introduce the whole firefly bioluminescent system. In PBIN, luciferin
and coenzyme A (CoA) were packaged into two different nano capsules, and
luciferase was linked onto the surface of silica-PEG nanocarriers. These
nanoparticles could reduce luciferin cytotoxicity and diminish chemical
denaturation of luciferase to some extent, causing a long light-emitting
duration of up to 8 hours. To transmit the whole firefly bioluminescent
system, the plant was immersed in solution containing the nanoparticles
in a closed pressured aqueous chamber, in which pressurization
delievered the nanoparticles into mesophyll cells through stomata. Once
these nanoparticle gathered in mescophyll cells, it glowed, and produced
the brightest glowing plants to date. The PBIN-based light-emitting
plants were also the only glowing plants which could be modulate “on”
and “off” states when CoA- and dehydroluciferin-nanoparticles were
provided, respectively, thus providing viability as alternative light
sources (Kwak et al., 2017).
The second bioluminescent system is discovered in three Gram-negative
motile rods luminous bacteria genera, that is Vibrio ,Photobacterium , and Xenorhabdus . The lux operons
(luxCDABEG ) are highly conserved through all luminous bacterial,
which encode the whole bacterial bioluminescent system (Close et al.,
2009). Gene luxA and luxB encode α and β subunits of
bacterial luciferase, respectively, and luxCDEG encodes
biosynthesis pathway of tetradecanal. Krichevsky et al. (2010)
transferred the whole Photobacterium leiognathi lux operon
into tobacco chloroplast and obtained the first auto-luminescent plant.
Although only very dim green luminescence was observed, this
auto-emission needs neither exogenous luciferin nor optical excitation.
However, the cytotoxicity of the bacterial luciferin, long-chain fatty
aldehyde, is ineluctable to the eukaryotic cells. It is reported that a
low concentration of bacterial luciferin n-decyl aldehyde can
cause toxity to many model eukaryotes, including mammalian and higher
plant cells (Hollis et al., 2001), which hindered application of
bacterial bioluminescent system as biomarkers or illuminants.
Recently, the third bioluminescent system originated from fungi was used
to generate auto-stronge-glowing plants.The biosynthetic pathway of
fungal luciferin is named caffeic acid cycle (Kotlobay et al., 2018). A
gene cluster encoded four key enzymes catalyze the caffeic acid cycle inNeonothopanus gardneri , including fungal luciferase (Luz),
hispidin-3-hydroxylase (H3H), hispidin synthase (Hisps), and
caffeylpyruvate hydrolase (CPH). Hisps is post-translationally activated
by another enzyme, NPGA (4’-phosphopantetheinyl transferase). Precursor
of fungal luciferin is hispidin, which can be convert to fungal
luciferin, termed 3-hydroxyhispidin. Then the luciferin is oxidized by
Luz to yield photons and oxy-luciferin, named caffelpyruvic acid,
followed by hydrolyzing the oxy-luciferin to pyruvic acid and caffic
acid. Caffic acid can be recycled to hispidin by Hips (Purtov et al.
2015;
Oba
et al. 2017). Fungal caffeic acid cycle shares three intermediary
metabolites with three major biosynthetic pathways of vascular plants,
including shikimate, lignins and flavonoid anthocyanins condensed
tannins. These common molecules make a bridge connecting the gap between
fungal caffeic acid cycle and the native biosynthetic pathway in plant
cells. As a result, it is reasonable to design a sophisticated fungal
luciferin synthetic pathway reconstituted into tobacco metabolic
routines (Mitiouchkina et al., 2019). Mitiouchkina et al. (2020) created
a luminescent N. tabacum through synthetic biology, which stably
displayed green luminescence. Gene of Luz , H3H ,Hisps , and CPH were transferred in the tobacco. The
auto-luminescent tobacco was brighter than other reported
auto-light-emitting plants. Coninsistantly, Khakhar et al. (2020)
described the transiently transformation and expression of fungal
bioluminescent system in both model and commercial plant species.NPGA gene was also transferred besides the four gene mentioned
above. Stronge luminescence was generated from cotyledons, leaves, and
petals of these genetically modified plants. The fungal bioluminescent
system produces brighter light and shows no cytotoxicity and growth
inhibition to plant cells, compared with that employed bacterial
luminescent system (Kotlobay et al., 2018). However, drawbacks are also
existed on the plant-expressed fungal bioluminescent system. For
example, low pH and temperature in plant cells reduced photon yield
(Reuter et al., 2020).