15.4 Green Sensors
Timely detection and anticipation of abiotic and biotic stresses in plants either during
pre- or post-harvesting stage can combat the challenge of sufficient food availability
throughout the world. Critical research is essential for the development of sensors to
timely monitor plant ecology and health (Kumar and Arora 2020). Green sensors are
genetically-encoded sensors, may be a promoter or a protein that can act in response
to external stimuli, and generate an output, characteristically gene expression (Shetty
et al. 2003). ‘Smart plant’ are the ones programmed with a synthetic signal transduction pathway with a modular receptor to act in response to a wide variety of
environmental factors (Pouvreau et al. 2018). The variety of exogenous stimuli
accessible to green sensors (commonly called as biosensors) is far-reaching, for
example, biosensors are reported for carbohydrates, coenzyme B12, amino acids,
organic acids, heavy metals, light, pathogens and plant hormones (Goold et al.
2018). There are two categories of biosensors, i.e. cell-free and whole cell-based
systems. In cell-free systems, purified enzymes, proteins and antibodies act as probes
to detect exogenous signal molecules quickly and specifically. However this
approach is expensive as the expression and refinement must be optimized, and
the active agents have to be immobilized (Mehta et al. 2016). Conversely, whole
cell-based biosensors are economical to cultivate and can be used in manifold assays,
i.e. depending on target analytes, biosensors can be modulated through genetic
manipulation (Hynninen and Virta 2010). For constructing cell-based biosensors,
standardized bioparts are recognized from genome databases. Such bioparts are
engineered to synthetically improve sensing modules such as transcription factors
and promoters, by increasing exogenous inputs into chassis cells or by bringing in
artificial genetic circuits or logic gates. On the other hand, cross-reactive bioparts
should be removed from the genome of chassis cells (Kim et al. 2018). The
specificity and sensitivity of constructed biosensors to exogenous analytes should
be analysed by a range of experimental methods such as measurement of total
bioluminescence and fluorescence of engineered chassis.
In order to assess the toxicity and mutagenicity of polluted soils, a whole plantbased sensing system act as cost-effective and real-time plant sentinels, or
‘phytosensors’ (Mazarei et al. 2008; Liu et al. 2013a, b). Some good examples of
sentinels include synthetic circuits for phytosensing of explosives (Antunes et al.
2011) or bacterial pathogens, nematodes or elicitors in transgenic tobacco and
Arabidopsis (Liu et al. 2013a, b, 2014). A histidine kinase (HK) based signalling
system, evolutionarily conserved between plants and bacteria, can be designed and
rapidly tested by SynBio tools in bacteria to be consequently implanted in plants
with little modifications such as codon optimization and proper targeting. Antunes
et al. (2011) assembled an absolute synthetic signal transduction pathway in
Arabidopsis root cells that connects input from computationally redesigned periplasmic binding proteins (PBPs) like ribose-binding protein (RBP) to a visual response
(de-greening). PBPs on the apoplast were engineered to sense and bind small
extracellular ligands, such as the explosive 2,4,6-trinitrotoluene (TNT) and develop
an affinity for the extracellular domain of a chemotactic protein, Trg. This complex
15 Plant Synthetic Biology: A Paradigm Shift Targeting Stress Mitigation,. . .
451
Timely detection and anticipation of abiotic and biotic stresses in plants either during
pre- or post-harvesting stage can combat the challenge of sufficient food availability
throughout the world. Critical research is essential for the development of sensors to
timely monitor plant ecology and health (Kumar and Arora 2020). Green sensors are
genetically-encoded sensors, may be a promoter or a protein that can act in response
to external stimuli, and generate an output, characteristically gene expression (Shetty
et al. 2003). ‘Smart plant’ are the ones programmed with a synthetic signal transduction pathway with a modular receptor to act in response to a wide variety of
environmental factors (Pouvreau et al. 2018). The variety of exogenous stimuli
accessible to green sensors (commonly called as biosensors) is far-reaching, for
example, biosensors are reported for carbohydrates, coenzyme B12, amino acids,
organic acids, heavy metals, light, pathogens and plant hormones (Goold et al.
2018). There are two categories of biosensors, i.e. cell-free and whole cell-based
systems. In cell-free systems, purified enzymes, proteins and antibodies act as probes
to detect exogenous signal molecules quickly and specifically. However this
approach is expensive as the expression and refinement must be optimized, and
the active agents have to be immobilized (Mehta et al. 2016). Conversely, whole
cell-based biosensors are economical to cultivate and can be used in manifold assays,
i.e. depending on target analytes, biosensors can be modulated through genetic
manipulation (Hynninen and Virta 2010). For constructing cell-based biosensors,
standardized bioparts are recognized from genome databases. Such bioparts are
engineered to synthetically improve sensing modules such as transcription factors
and promoters, by increasing exogenous inputs into chassis cells or by bringing in
artificial genetic circuits or logic gates. On the other hand, cross-reactive bioparts
should be removed from the genome of chassis cells (Kim et al. 2018). The
specificity and sensitivity of constructed biosensors to exogenous analytes should
be analysed by a range of experimental methods such as measurement of total
bioluminescence and fluorescence of engineered chassis.
In order to assess the toxicity and mutagenicity of polluted soils, a whole plantbased sensing system act as cost-effective and real-time plant sentinels, or
‘phytosensors’ (Mazarei et al. 2008; Liu et al. 2013a, b). Some good examples of
sentinels include synthetic circuits for phytosensing of explosives (Antunes et al.
2011) or bacterial pathogens, nematodes or elicitors in transgenic tobacco and
Arabidopsis (Liu et al. 2013a, b, 2014). A histidine kinase (HK) based signalling
system, evolutionarily conserved between plants and bacteria, can be designed and
rapidly tested by SynBio tools in bacteria to be consequently implanted in plants
with little modifications such as codon optimization and proper targeting. Antunes
et al. (2011) assembled an absolute synthetic signal transduction pathway in
Arabidopsis root cells that connects input from computationally redesigned periplasmic binding proteins (PBPs) like ribose-binding protein (RBP) to a visual response
(de-greening). PBPs on the apoplast were engineered to sense and bind small
extracellular ligands, such as the explosive 2,4,6-trinitrotoluene (TNT) and develop
an affinity for the extracellular domain of a chemotactic protein, Trg. This complex
15 Plant Synthetic Biology: A Paradigm Shift Targeting Stress Mitigation,. . .
451