Bioluminescence in plants: beyond as the biomarkers
During past decades, tremendous efforts have been invested for the
development of optical non-invasive biomarkers in plant cells. The first
bioluminescent system that was introduced into plants is firefly
luciferase-luciferin system (Ow et al., 1986). Firefly luciferase can be
expressed independently or fused with proteins of interst in plant
cells, which provides a useful tool to study protein expression and
localization, and protein-protein interactions (PPIs) (Guan et al.,
2013; Sanchez et al., 2018). To detect PPIs, firefly luciferase is
splited as N- and C-terminal halves and fused with two candidates of
interactional proteins (Chen et al. 2008). However, firefly luciferin
could not be biosynthesized in plant cells yet, so that it have to be
supplied exogenously, which causes low cost-efficency.
To break those bottlenecks, several strategies were established,
including using FPs as alternative biomarkers and searching novel
luciferase-luciferin bioluminescent systems that can be completely
biosynthesized in plants. In 1995, the green fluorescent protein (GFP)
gene from Aequorea victoria was successfully expressed inArabidopsis , which brings novel revoluion in plant research (Hu
& Cheng, 1995). Since then, FPs as tags are widely used in plants to
monitor protein localization in vivo , recognize specific plant
cells and tissues, obtain cell-specific transcriptomes and record plant
cell fate selection (Ckurshumova et al., 2011). Spectral characteristics
of FPs could be altered to reduce autofluorescence background from plant
tissues. Various FPs covering most fluorescent ranges (i.e. yellow,
blue, cyan, green, and red) have been adapted for plant cells (Blatt and
Grefen, 2014). However, there are still problems remained when using
FPs, including cellular damage caused by excited laser, signal weakening
by plant tissues and migrating from plasmodesmata due to its smaller
molecular weight (Brunkard & Zambryski 2017).
Another alternative strategy that developing novel bioluminescent
systems was emerging. After elucidation of the luciferase-luciferin
pairs original from bacteria and fungi, two distinguished optical
reporters have been developed from these systems and now accessible in
plant science. These systems have been described in their entirety and
could be biosynthesized in plant cells. However, due to cytotoxicity and
low photon yield of bacterial bioluminescent reactions in eukaryotes
(Kotlobay et al., 2018), since autoluminescent system was firstly
reported in Nicotiana tabacum , only few bacterial
luciferase-luciferin pairs were reported in other transgenic plants (Cui
et al., 2014; Krichevsky et al., 2010). Fungal bioluminescent system is
an unique genetically encodable eukaryote luciferase-luciferin system,
which was introduced into plant cells recently (Khakhar et al., 2020;
Mitiouchkina et al., 2020). Less cytotoxicity and high luminescent
intensity make fungal bioluminescent system as one of the most promising
optical molecular tools (Reuter et al., 2020).
Nowdays, protein-directed revolution and synthetic biology boosted the
development for novel bioluminescent systems. Luciferase engineering
could be used to optimize the natural bioluminescent systems, such as
increasing bioluminescent intensity and stability. Rational designing
novel bioluminescent systems provides two representative cases, Nano
Luciferase (NanoLuc) (England et al., 2016) and Nano-lantern (Doerr,
2013). NanoLuc is a small subunit (19 kDa) from shrimp Oplophorous
gracilirotris luciferase (Hall et al., 2012). Split NanoLuc was
developed to screen PPIs, which avoids imposing steric hindrance caused
by huge firefly luciferase halves (Wang et al., 2020). Nano-lantern is
an artificially designed luminescent protein fused with enhancedRenilla reniformis luciferase and yellow FP Venus. ATP
production from chloroplast could be observed by Nano-lantern (Saito et
al., 2012). Today, fluorescence and bioluminescence is no longer a new
tool for molecular and celluar biology, but becoming a novel light
sources, with which glowing plants have been obtained.