growth-promoting microbe can be harmonized by engineering plants to fabricate a
synthetic transkingdom signal to reroute bacteria towards roots. In analogy to natural
transkingdom signalling flanked by legumes and rhizobia, rhizopine transkingdom
signalling can direct synthetic symbioses to deliver N to cereals (Geddes et al. 2019).
They established synthetic rhizopine scyllo-inosamine 1 (SIA)-mediated
transkingdom signalling from transgenic Medicago truncatula and barley plants to
bacteria in their rhizospheres. Another strategy is to transfer the N 2 -fixing system
from N 2 -fixing bacteria to non-N 2 -fixing bacteria in the vicinity of cereal crop.
Transfer of large N 2 fixation arrangement from Pseudomonas stutzeri to the aerobic
associative bacterium Pseudomonas protegens Pf-5, resulted in constitutive nif
expression and imparted the ability to nurture micro-aerobically using N 2 as a lone
N source. In N-limited surroundings, inoculation of Arabidopsis, alfalfa, tall fescue
and wheat with transgenic P. protegens imparted substantial growth improvement in
contrast to the near-isogenic wild type (Setten et al. 2013). Recently, Ryu et al.
(2020) successfully engineered inducible nitrogenase activity in two cereal
endophytes (Azorhizobium caulinodans ORS571 and Rhizobium sp. IRBG74) and
an epiphyte Pseudomonas protegens Pf-5. As constant nitrogenase production can
confer an energy burden in plants, they placed nitrogenase expression under the
direction of agriculturally significant signals such as root exudates, biocontrol agents
and phytohormones. Interestingly, Brown et al. (2016) described synthetic
nanoparticles—cadmium sulphide nanocrystal/nitrogenase molybdenum-iron protein hybrid—as light-driven catalyst (instead of adenosine triphosphate hydrolysis)
to fix nitrogen. Though it needs a thorough investigation, it is an intriguing future
direction that can compete with the Haber–Bosch process (Jez et al. 2016). There are
several challenges in designing symbiotic plant–microbe interactions in cereal crops.
However, advanced information of the determinant and needs of biological N 2
fixation has converged with the development in SynBio technologies to make
synthetic plant–microbe N 2 fixing associations a tractable endeavour (Mus et al.
2016). Thus collectively these approaches can carry the vision of self-sustainable N 2
fixing cereal crops nearer to certainty (Geddes et al. 2015).
Designing synthetic consortia comprising a microbial community fabricated by
mixing selected strains using bottom-up combinations is an appropriate approach to
synchronously improve the exploitation of nutrients in plants (Vorholt et al. 2017).
Crop rhizosphere, when inoculated with this consortium, can inform about a range of
aspects of plant–microbe exchanges that can be decoded into the agricultural
appliance. Synthetic communities, with lesser intricacy as compared to native
microbiota, can be comprehensively studied for its components under controlled
conditions, for example, microbes can even be added, eliminated or substituted at the
strain level and via in situ gene expression or silencing, particular functions can be
explicitly added or removed. Further, by varying plant host genotypes for the
unchanged microbial community, the potential of novel plant factors affecting the
microbiota can be determined. For example, a synthetic community of seven strains
was constructed to analyse the variation in phyllosphere communities as a function
of Arabidopsis thaliana genotypes, where four out of nine A. Thaliana accessions
tested anchoraged different community as compared to reference accession Col-0
15 Plant Synthetic Biology: A Paradigm Shift Targeting Stress Mitigation,. . .
449
synthetic transkingdom signal to reroute bacteria towards roots. In analogy to natural
transkingdom signalling flanked by legumes and rhizobia, rhizopine transkingdom
signalling can direct synthetic symbioses to deliver N to cereals (Geddes et al. 2019).
They established synthetic rhizopine scyllo-inosamine 1 (SIA)-mediated
transkingdom signalling from transgenic Medicago truncatula and barley plants to
bacteria in their rhizospheres. Another strategy is to transfer the N 2 -fixing system
from N 2 -fixing bacteria to non-N 2 -fixing bacteria in the vicinity of cereal crop.
Transfer of large N 2 fixation arrangement from Pseudomonas stutzeri to the aerobic
associative bacterium Pseudomonas protegens Pf-5, resulted in constitutive nif
expression and imparted the ability to nurture micro-aerobically using N 2 as a lone
N source. In N-limited surroundings, inoculation of Arabidopsis, alfalfa, tall fescue
and wheat with transgenic P. protegens imparted substantial growth improvement in
contrast to the near-isogenic wild type (Setten et al. 2013). Recently, Ryu et al.
(2020) successfully engineered inducible nitrogenase activity in two cereal
endophytes (Azorhizobium caulinodans ORS571 and Rhizobium sp. IRBG74) and
an epiphyte Pseudomonas protegens Pf-5. As constant nitrogenase production can
confer an energy burden in plants, they placed nitrogenase expression under the
direction of agriculturally significant signals such as root exudates, biocontrol agents
and phytohormones. Interestingly, Brown et al. (2016) described synthetic
nanoparticles—cadmium sulphide nanocrystal/nitrogenase molybdenum-iron protein hybrid—as light-driven catalyst (instead of adenosine triphosphate hydrolysis)
to fix nitrogen. Though it needs a thorough investigation, it is an intriguing future
direction that can compete with the Haber–Bosch process (Jez et al. 2016). There are
several challenges in designing symbiotic plant–microbe interactions in cereal crops.
However, advanced information of the determinant and needs of biological N 2
fixation has converged with the development in SynBio technologies to make
synthetic plant–microbe N 2 fixing associations a tractable endeavour (Mus et al.
2016). Thus collectively these approaches can carry the vision of self-sustainable N 2
fixing cereal crops nearer to certainty (Geddes et al. 2015).
Designing synthetic consortia comprising a microbial community fabricated by
mixing selected strains using bottom-up combinations is an appropriate approach to
synchronously improve the exploitation of nutrients in plants (Vorholt et al. 2017).
Crop rhizosphere, when inoculated with this consortium, can inform about a range of
aspects of plant–microbe exchanges that can be decoded into the agricultural
appliance. Synthetic communities, with lesser intricacy as compared to native
microbiota, can be comprehensively studied for its components under controlled
conditions, for example, microbes can even be added, eliminated or substituted at the
strain level and via in situ gene expression or silencing, particular functions can be
explicitly added or removed. Further, by varying plant host genotypes for the
unchanged microbial community, the potential of novel plant factors affecting the
microbiota can be determined. For example, a synthetic community of seven strains
was constructed to analyse the variation in phyllosphere communities as a function
of Arabidopsis thaliana genotypes, where four out of nine A. Thaliana accessions
tested anchoraged different community as compared to reference accession Col-0
15 Plant Synthetic Biology: A Paradigm Shift Targeting Stress Mitigation,. . .
449