soil health and quality (Chandini et al. 2019). Thus present scenario insists consideration of rhizospheric microbiota, which is also considered as an extension of the
plant genome as it can influence plant fitness by transforming root architecture,
nutrient acquisition, or biotic and abiotic stress resistance (Almario et al. 2017).
Nitrogen (N 2 ), although abundant in the earth’s atmosphere, is a limiting nutrient
and its availability often limits ecosystem productivity (Perchlik and Tegeder 2017).
Unreactive, bio-unavailable N 2 in the atmosphere can be reduced to ammonium
(NH 4 ) by only a few bacteria and Archaea, which possess the molybdenum (Mo)dependent nitrogenase enzyme (Boyd et al. 2015). Legume plants fix a substantial
proportion of the biosphere’s available N 2 by developing mutualistic relation with
N 2 -fixing rhizobia that reside in root nodules (Rogers and Oldroyd 2014; Geddes
et al. 2019). However, the rising demand for food and fodder has made chemical
fertilization, an obligatory exercise adopted in today’s agriculture (Perchlik and
Tegeder 2017). Chemical synthesis of N fertilizers by means of the Haber–Bosch
process burns 3% of the world’s natural gas and adds the same percentage to global
carbon emissions. The energy-intensive manufacture of nitrogen fertilizers from
fossil fuels, runoff or leaching of applied chemical fertilizers, loss of fixed N 2 in
the form of nitrous oxides (potent greenhouse gases) and as soluble nitrates into
aquatic systems (causing eutrophication) is expected to devour approximately 2% of
global energy by 2050 (Vicente and Dean 2017). Thus for a sustainable environment
and economy, agricultural reliance on chemical fertilization needs to be scaled
down. Moreover, the requirement of modern agriculture far outstrips both
Legume-Rhizobium symbiotic N 2 fixation and industrially-produced nitrogenous
fertilizers (Heuer et al. 2017; Perchlik and Tegeder 2017). Collectively these aspects
have enlightened a renewed focus towards developing approaches to trim down the
global reliance on N fertilizers. The development of plants with enhanced N use
efficiency would aid in reducing fertilizer applications, lower energy costs and
greenhouse gas emissions associated with its synthesis, and help mitigate the
consequences of N loss into soil and water sources (Jez et al. 2016).
Since the preponderance of global calories is in cereals; hence the concept of
engineering them to reconstitute N 2 -fixing capability is a long-standing vision (Allen
et al. 2017; Vicente and Dean 2017). This perception has been an eye-opener since
the 1970s when nitrogenase was transferred from Klebsiella pneumoniae to
Escherichia coli (Geddes et al. 2015). This section highlights how SynBio can use
biological N 2 fixation as a blueprint for developing symbiotic N 2 fixation in cereals
and agriculturally important eudicots. Three approaches have been proposed to
achieve a common objective of enabling cereal crops with N 2 fixing ability (Rogers
and Oldroyd 2014; Geddes et al. 2015) (Fig. 15.3).
15.3.1 Engineering Perception of N 2 -Fixing Bacteria in Cereals
Four coordinated genetic programmes would provide a useful framework for structuring N 2 symbiosis in cereals: a) Nod factors recognition; b) bacterial infection; c)
root nodule organogenesis and d) instituting an appropriate atmosphere for
15 Plant Synthetic Biology: A Paradigm Shift Targeting Stress Mitigation,. . .
445
plant genome as it can influence plant fitness by transforming root architecture,
nutrient acquisition, or biotic and abiotic stress resistance (Almario et al. 2017).
Nitrogen (N 2 ), although abundant in the earth’s atmosphere, is a limiting nutrient
and its availability often limits ecosystem productivity (Perchlik and Tegeder 2017).
Unreactive, bio-unavailable N 2 in the atmosphere can be reduced to ammonium
(NH 4 ) by only a few bacteria and Archaea, which possess the molybdenum (Mo)dependent nitrogenase enzyme (Boyd et al. 2015). Legume plants fix a substantial
proportion of the biosphere’s available N 2 by developing mutualistic relation with
N 2 -fixing rhizobia that reside in root nodules (Rogers and Oldroyd 2014; Geddes
et al. 2019). However, the rising demand for food and fodder has made chemical
fertilization, an obligatory exercise adopted in today’s agriculture (Perchlik and
Tegeder 2017). Chemical synthesis of N fertilizers by means of the Haber–Bosch
process burns 3% of the world’s natural gas and adds the same percentage to global
carbon emissions. The energy-intensive manufacture of nitrogen fertilizers from
fossil fuels, runoff or leaching of applied chemical fertilizers, loss of fixed N 2 in
the form of nitrous oxides (potent greenhouse gases) and as soluble nitrates into
aquatic systems (causing eutrophication) is expected to devour approximately 2% of
global energy by 2050 (Vicente and Dean 2017). Thus for a sustainable environment
and economy, agricultural reliance on chemical fertilization needs to be scaled
down. Moreover, the requirement of modern agriculture far outstrips both
Legume-Rhizobium symbiotic N 2 fixation and industrially-produced nitrogenous
fertilizers (Heuer et al. 2017; Perchlik and Tegeder 2017). Collectively these aspects
have enlightened a renewed focus towards developing approaches to trim down the
global reliance on N fertilizers. The development of plants with enhanced N use
efficiency would aid in reducing fertilizer applications, lower energy costs and
greenhouse gas emissions associated with its synthesis, and help mitigate the
consequences of N loss into soil and water sources (Jez et al. 2016).
Since the preponderance of global calories is in cereals; hence the concept of
engineering them to reconstitute N 2 -fixing capability is a long-standing vision (Allen
et al. 2017; Vicente and Dean 2017). This perception has been an eye-opener since
the 1970s when nitrogenase was transferred from Klebsiella pneumoniae to
Escherichia coli (Geddes et al. 2015). This section highlights how SynBio can use
biological N 2 fixation as a blueprint for developing symbiotic N 2 fixation in cereals
and agriculturally important eudicots. Three approaches have been proposed to
achieve a common objective of enabling cereal crops with N 2 fixing ability (Rogers
and Oldroyd 2014; Geddes et al. 2015) (Fig. 15.3).
15.3.1 Engineering Perception of N 2 -Fixing Bacteria in Cereals
Four coordinated genetic programmes would provide a useful framework for structuring N 2 symbiosis in cereals: a) Nod factors recognition; b) bacterial infection; c)
root nodule organogenesis and d) instituting an appropriate atmosphere for
15 Plant Synthetic Biology: A Paradigm Shift Targeting Stress Mitigation,. . .
445