turns on intracellular PhoR, the HK cognate of PhoB, which translocates to the
nucleus and activates transcription of an output gene under the control of a synthetic
plant promoter. When such plants are exposed to TNT, the de-greening circuit
hinders new chlorophyll synthesis rather upregulate the expression of chlorophyll
degradation genes and produces a visual response that is even distantly detectable
and quantifiable (Antunes et al. 2011). Morey et al. (2011) engineered a partial
synthetic pathway in which a synthetic promoter (PlantPho) is turned on using a
plant-adapted PhoB (PhoB-VP64) and the endogenous HK-based cytokinin signalling pathway. Periplasmic binding protein (PBP) computationally redesigned to bind
TNT, interacts with Trg which fuses with HK, PhoR allowing signal transduction
resulting in transcriptional activation of an output gene in the nucleus (Morey et al.
2011). Peng et al. (2014) developed a set of ‘Fukusensor’ transgenic phytosensors to
examine the loss-of-function scheme using a range of gamma radiations. They
harboured a marker gene (gfp gene) encoding a fluorescent protein in agenetic
background of defective DNA repair system and hypothesised that these mutant
plants would have relatively higher probability of accruing mutations in the marker
gene relative to engineered plants in a nonmutant background. Their study indicated
that latter plants could serve in an appropriate sensor system to report the genetic
effects to organisms in response to radionuclide contamination. Modular G-protein
coupled receptor (GPCR) system developed using chimeric BRET-biosensors (Bioluminescence Resonance Energy Transfer) is a broad-spectrum biosensor used by
Dacres et al. (2011) to sense the femtomolar quantity of diacetyl using an odorant
receptor protein from Caenorhabditis elegans. This approach in sentinel plants
would offer agricultural farming systems a better on-site judgement making aptitude,
thus dropping down the application of chemical fertilizers (Goold et al. 2018;
Wurtzel et al. 2019).
Auxin-dependent formation of a co-receptor complex between TIR1/AFB (F-box
proteins, constituents of an SCF E3 ubiquitin-ligase complex) and Aux/IAA (family
of negative regulators of the auxin response) results in ubiquitylation and proteolysis
of Aux/IAA, thereby relieving the suppression of auxin-responsive genes (Tan et al.
2007). In order to investigate time and space-resolved quantitative monitoring of
auxin dynamics, two types of biosensors have been synthetically designed one
relying on the ubiquitylation of fluorescent proteins fused to Aux/IAA and the
other on the co-receptor complex directly like chemi-luminescent ratiometric auxin
sensor (Wend et al. 2013). Green fluorescent protein (GFP) synthetic sensor has been
constructed in Arabidopsis and Zea mays to scrutinize known cytokinin function and
to also reveal a few novel functions. Cytokinin binding commences a multistep
phosphorelay signalling cascade that eventually phosphorylates nuclear transcription activators [i.e. the type-B nuclear ARABIDOPSIS RESPONSE REGULATORs
(ARRs)]. Type-B ARRs recognises concatemer of 24 repeats of the consensus
sequence which drives GFP expression patterns (Zürcher et al. 2013). A red lightregulated synthetic switch has been designed by utilising the operator site of
Arabidopsis phytochrome interacting factor 6 (PIF6) upstream of a minimal promoter. Red light illumination switch phytochrome B (PhyB) into the active far-red
form and consequently encourage heterodimerization with its PIF6 and turns on the
452
P. Singla
nucleus and activates transcription of an output gene under the control of a synthetic
plant promoter. When such plants are exposed to TNT, the de-greening circuit
hinders new chlorophyll synthesis rather upregulate the expression of chlorophyll
degradation genes and produces a visual response that is even distantly detectable
and quantifiable (Antunes et al. 2011). Morey et al. (2011) engineered a partial
synthetic pathway in which a synthetic promoter (PlantPho) is turned on using a
plant-adapted PhoB (PhoB-VP64) and the endogenous HK-based cytokinin signalling pathway. Periplasmic binding protein (PBP) computationally redesigned to bind
TNT, interacts with Trg which fuses with HK, PhoR allowing signal transduction
resulting in transcriptional activation of an output gene in the nucleus (Morey et al.
2011). Peng et al. (2014) developed a set of ‘Fukusensor’ transgenic phytosensors to
examine the loss-of-function scheme using a range of gamma radiations. They
harboured a marker gene (gfp gene) encoding a fluorescent protein in agenetic
background of defective DNA repair system and hypothesised that these mutant
plants would have relatively higher probability of accruing mutations in the marker
gene relative to engineered plants in a nonmutant background. Their study indicated
that latter plants could serve in an appropriate sensor system to report the genetic
effects to organisms in response to radionuclide contamination. Modular G-protein
coupled receptor (GPCR) system developed using chimeric BRET-biosensors (Bioluminescence Resonance Energy Transfer) is a broad-spectrum biosensor used by
Dacres et al. (2011) to sense the femtomolar quantity of diacetyl using an odorant
receptor protein from Caenorhabditis elegans. This approach in sentinel plants
would offer agricultural farming systems a better on-site judgement making aptitude,
thus dropping down the application of chemical fertilizers (Goold et al. 2018;
Wurtzel et al. 2019).
Auxin-dependent formation of a co-receptor complex between TIR1/AFB (F-box
proteins, constituents of an SCF E3 ubiquitin-ligase complex) and Aux/IAA (family
of negative regulators of the auxin response) results in ubiquitylation and proteolysis
of Aux/IAA, thereby relieving the suppression of auxin-responsive genes (Tan et al.
2007). In order to investigate time and space-resolved quantitative monitoring of
auxin dynamics, two types of biosensors have been synthetically designed one
relying on the ubiquitylation of fluorescent proteins fused to Aux/IAA and the
other on the co-receptor complex directly like chemi-luminescent ratiometric auxin
sensor (Wend et al. 2013). Green fluorescent protein (GFP) synthetic sensor has been
constructed in Arabidopsis and Zea mays to scrutinize known cytokinin function and
to also reveal a few novel functions. Cytokinin binding commences a multistep
phosphorelay signalling cascade that eventually phosphorylates nuclear transcription activators [i.e. the type-B nuclear ARABIDOPSIS RESPONSE REGULATORs
(ARRs)]. Type-B ARRs recognises concatemer of 24 repeats of the consensus
sequence which drives GFP expression patterns (Zürcher et al. 2013). A red lightregulated synthetic switch has been designed by utilising the operator site of
Arabidopsis phytochrome interacting factor 6 (PIF6) upstream of a minimal promoter. Red light illumination switch phytochrome B (PhyB) into the active far-red
form and consequently encourage heterodimerization with its PIF6 and turns on the
452
P. Singla