Synthetic routine of luciferin: footstone of auto-glowing plants
Here we define “plantern” (a chimera of “plant” and “lantern” in both semantics and biology) as transgenic plant that is capable to produce visible light need neither external illumination nor exogenous substances, and the bioluminescence is bright enough to be observed with naked eyes. Feasibility of designing “planterns” as streetlights have been evaluated according to photosynthetic efficiency and luminous energy output. To produce as much light as tranditional streetlights, glowing plants have to output more than 1000 lumens by the transformed firefly bioluminescent system. It was estimated that a fast-grow tree glowing with the firefly luciferase-luciferin pairs only have to divert approximately 0.3 % of the stored energy to generate bright-enough bioluminesence during the nighttime (Reeve et al., 2014). In addition to producing light, planterns are also leveraged for whole-plant imaging and studying spatiotemporal expression of genes during the entire life-cycle, for limitations on delivering exogenous luciferin to the whole plant and tissue would no longer be a bottleneck (Reuter et al., 2020).
Rebuilding luciferin biosynthetic routine is the priority of generating “planterns”, which could be achieved by introducing all genes associated with luciferin biosynthesis (Krichevsky et al., 2010) or integrating luciferin biosynthetic routine into native plant metabolic pathway (Khakhar et al., 2020; Mitiouchkina et al., 2019). In the latter situation, the complicated biosynthesis pathway could be simplified. The unclarified intermediate steps would even be skipped if products of the unknown enzymatic reaction have been found in plant metabolome. Once the luciferin biosynthetic pathway in plant cells is reconstrcuted, common plant genetic engineering technique would be used to make the transgenic glowing plants (Figure 1 ).
Besides those well-known bioluminescent systems that have been reconstructed in plant metabolome, new bioluminescent systems were also discovered recently. More alternatives of intracellular illuminants would be available once unveiling the mysteries of newly-found bioluminescent systems (Kaskova et al., 2016). Some of these systems might be suitable for plant cells or whose synthetic pathways could be reconstituted in plant metabolic routine. Recently, a novel yellow bioluminescent system from annelid worms in the genus Tomopteris , kinds of pelagic polychaetes, was separated and characterized (Glagoleva et al., 2020). The worm luciferin turned out to be aloeemodin, which had been also discovered in plants, such as aloe and senna(Heidemann et al., 1996). Arachnocampa luminosa , a New Zealand luminescent glowworm, can generate blue-green light. The glowworm luciferase shares 31% sequence identity with firefly luciferase, but catalyzing an entirely new luciferin whose biosynthetic pathway begins from xanthurenic acid and tyrosine (Watkins et al., 2018). Dinoflagellate, notorious protist responsible for causing the surface of the ocean to sparkle at night, produce bluish-green gimmer under mechanical disturbance (Valiadi & Iglesias-Rodriguez, 2013). The dinoflagellate luciferin is believed as derivative of chlorophylla because of their similar molecular structures (Topalov & Kishi, 2001). Genes associated with reaction converting chlorophylla into dinoflagellate luciferin could be introduced into parts of chloroplasts when synthetic routine of dinoflagellate luciferin is clarified.