Optimization of “planterns”
One of the problems is low luminescent intensity, which hinders the
population of planterns. Though some glowing plants generated a bright
emission of 1.44 × 1012 photons/s (Kwak et al. 2017),
many other auto-glowing plants produce a much dimmer luminescence of 1.3
× 106 photons/s (Krichevsky et al., 2010) and 1.6 ×
109 photons/s (Mitiouchkina et al., 2020), which is
only bright enough to see in entire darkness with naked eyes.
Enhancement of whole-plant glowing intensity could be considered as
follow. Bioluminescent intensity is determined by both characteristics
and accumulation of luciferase-luciferin pairs. Plant intracellular
conditions also interfere bioluminescent intensity. Once the
luminescence is produced, transparency window (TW) effects transmission
efficiency (Strack, 2019).TW can be measured and sketched by
spectrophotometer (Mitiouchkina et al., 2020). Because the TW is
effected by pigments, plant species, apparatus and growth status, the
bioluminescent spectrum should be altered according to TW.
As a result, the improvement of glowing intensity could be achieved from
two aspects, i.e., the optimization of bioluminescent systems and the
transgenic strategies. By engineering of luciferase, promotes
feasibility to enhance bioluminescent intensity, spectral
characteristics, pH and thermal stability by random mutagenesis,
site-specific mutagenesis and in scilicon design (Branchini et
al., 2019; Fujii et al., 2007; Modestova & Ugarova, 2016). Protein
fusion or spilt could also alter bioluminescent characteristics. A
chimeric firefly luciferase of North American firefly (Photinus
pyralis ) and Japanese firefly (Luciola italica ) luciferase
exhibited 1.4-fold enhanced bioluminescence quantum yield compared with
the wide-type firefly luciferases (Branchini et al., 2014). Nano-lantern
and NanoLuc are shown to improve luminescent intensity significantly,
though exogenously supplied luciferin is necessary. Similar nano-lantern
or NanoLuc might be developed based on the other bioluminescent systems
whose luciferin could be biosynthesized, e.g., fungal bioluminescent
system. Recently, as structural basis for the spectral difference was
gradually clarified, bioluminescent color could also be changed by
designing luciferin. It was reported that firefly luciferins with
elongated π-system, such as phenyl or phenol, exhibit an obvious
red-shift of emission spectrum (Yao et al., 2020). It was reported that
firefly luciferin analog with less conjugated π-system present
blue-shift in absorption and fluorescence (Pirrung et al., 2019). The
designed luminescent pairs provide more selections, and these luciferin
analogs might be able to be biosynthesized in plant cells. Given that
the potentially cytotoxicity and growth inhibition to higher plants
caused by bioluminescent systems, protecting groups were introduced into
luciferin is reported possible, which can decrease cytotoxicity and
enhance the accumulation (Yuan et al., 2017).
To optimize transgenic methodologies, substanial accumulation of
luciferase-luciferin pairs can be achieved via common genetic
engineering technique, such as codon optimization, inserting
translational enhancer and several tandemly ranging the gene cluster
(Sasaki et al., 2014). Engineering subcellular organelles (e.g.
chloroplast and vacuole) into specified “scintillon” is another
promising strategy. Increasing the accumulation of the whole luciferin
pathway can be achieved through the development of chloroplast
transformation technique (Kwak et al., 2019; Jin & Daniell, 2015).
Bacterial bioluminescent system has been stably expressed in chloroplast
(Krichevsky et al., 2010). Maternal inheritance of the chloroplast
genome prevents genes from escaping into the environment through pollen
grains (Yu et al., 2020), which may helps popularize planterns as
alternative light sources in future (Callaway, 2013). Meanwhile,
vacuoles could be employed to accumulate FPs or luciferase-luciferin
pairs. Proteins associatd to bioluminescence can be fused with sequence
specific vacuolar sorting signals (Viegas et al., 2017). The potential
strategy to enhance the suboptimized glowing plants would be helpful for
designing better glowing plants in future.