hydroxypropionate–hydroxybutyrate cycle, iii) metabolically engineering carbon
products into molecules such as acetyl-CoA, which is an originator for a lot of
fuels (Chu and Majumdar 2012). Engineering pathways for novel biofuel molecules
has been employed widely in yeasts, for example, a monoterpene synthase from
sweet basil was introduced into S. cerevisiae resulted in a strain synthesizing and
secreting monoterpene geraniol into the medium, thus letting go the cost of extraction (Oswald et al. 2007; Scott et al. 2010). Starch, feedstock required for bioethanol
fabrication, can be produced from nutritionally less valuable cellulose and hemicellulose present in the bagasse of many crops. The implantation of polymer-degrading
enzymes in plants β-glucanase for hydrolysis of cellulose to glucose and β-Dxylanase, α-glucuronidase for hydrolysis of xylan into fermentable pentoses has
been effectively executed (Li et al. 2014). For example, US company Agrivida
produced Zea mays lines individually expressing endo-β-1,4-glucanase and
endo-β-1,4-xylanase, to amplify bioethanol production (Zhang et al. 2011). Shen
et al. (2012) engineered temperature resistant xylanase (with a bacterial self-splicing
intein) that prevents autohydrolysis during development but can be induced by heat
treatment post-harvest (Shen et al. 2012). Aquatic phototrophic organism, such as
cyanobacteria and green algae, is also the candidate organisms for producing biofuel
at large scale, as their complete biomass can be digested into methane or converted
into crude by thermochemical processes (reviewed by Wijffels et al. 2013). Since
utilising lignin rich hosts for biofuel production is challenging, SynBio offers
designing lignin which is more open to biotechnological applications. A ferulolylCoA:monolignol transferase (FMT from Angelica sinensis) was implanted in poplar
to synthesize ferulated monolignol subunits, which can then be incorporated into the
lignin polymer to construct so-called Zip-ligninTM (Wilkerson et al. 2014).
Engineered poplar trees had superior cell wall digestibility under mild alkaline
pre-treatment (Zhou et al. 2017). In gain-of-function approach, bacterial
3-dehydroshikimate dehydratase (QsuB), under the regulation of the plant
Cinnamate-4-Hydroxylase (C4H) promoter was targeted in plastids of Arabidopsis
to reduce the accessibility of an intermediate in the shikimate pathway (Eudes et al.
2015). This in turn reduced monolignol production for lignin synthesis in
non-essential plant parts. Since hexose sugars are preferred (over pentose and lignin)
as a carbon source, thus there is a need to optimize the biomass to match the
microbial preference. The cell wall content of the pectin galactan (composed of
the hexose galactose) was increased in A. thaliana by overexpressing Galactan
Synthase1 (GALS1), cytosolic UDP-Glucose/UDP-Galactose-4-Epimerase2
(UGE2) synthesizing UDP-galactose and a gene for UDP-galactose transporter
moving the substrate to Golgi lumen to be utilized by GALS1 (Aznar et al. 2018).
This study was extended by using 3-dehydroshikimate dehydratase (QsuB) strategy
via SynBio technology, where high-galactan lines were combined with cell-specific
low xylan and low lignin (Aznar et al. 2018). Production of lignocellulosic-based
biofuels is less preferred because sequestration of cell wall polysaccharides by lignin
polymers makes the sugar extraction process considerable complex, thus demanding
effective techniques for decreasing lignin content (Eudes et al. 2015; Shih et al.
2016). Techniques of RNAi genetic engineering in corn resulted in plants with less
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P. Singla
products into molecules such as acetyl-CoA, which is an originator for a lot of
fuels (Chu and Majumdar 2012). Engineering pathways for novel biofuel molecules
has been employed widely in yeasts, for example, a monoterpene synthase from
sweet basil was introduced into S. cerevisiae resulted in a strain synthesizing and
secreting monoterpene geraniol into the medium, thus letting go the cost of extraction (Oswald et al. 2007; Scott et al. 2010). Starch, feedstock required for bioethanol
fabrication, can be produced from nutritionally less valuable cellulose and hemicellulose present in the bagasse of many crops. The implantation of polymer-degrading
enzymes in plants β-glucanase for hydrolysis of cellulose to glucose and β-Dxylanase, α-glucuronidase for hydrolysis of xylan into fermentable pentoses has
been effectively executed (Li et al. 2014). For example, US company Agrivida
produced Zea mays lines individually expressing endo-β-1,4-glucanase and
endo-β-1,4-xylanase, to amplify bioethanol production (Zhang et al. 2011). Shen
et al. (2012) engineered temperature resistant xylanase (with a bacterial self-splicing
intein) that prevents autohydrolysis during development but can be induced by heat
treatment post-harvest (Shen et al. 2012). Aquatic phototrophic organism, such as
cyanobacteria and green algae, is also the candidate organisms for producing biofuel
at large scale, as their complete biomass can be digested into methane or converted
into crude by thermochemical processes (reviewed by Wijffels et al. 2013). Since
utilising lignin rich hosts for biofuel production is challenging, SynBio offers
designing lignin which is more open to biotechnological applications. A ferulolylCoA:monolignol transferase (FMT from Angelica sinensis) was implanted in poplar
to synthesize ferulated monolignol subunits, which can then be incorporated into the
lignin polymer to construct so-called Zip-ligninTM (Wilkerson et al. 2014).
Engineered poplar trees had superior cell wall digestibility under mild alkaline
pre-treatment (Zhou et al. 2017). In gain-of-function approach, bacterial
3-dehydroshikimate dehydratase (QsuB), under the regulation of the plant
Cinnamate-4-Hydroxylase (C4H) promoter was targeted in plastids of Arabidopsis
to reduce the accessibility of an intermediate in the shikimate pathway (Eudes et al.
2015). This in turn reduced monolignol production for lignin synthesis in
non-essential plant parts. Since hexose sugars are preferred (over pentose and lignin)
as a carbon source, thus there is a need to optimize the biomass to match the
microbial preference. The cell wall content of the pectin galactan (composed of
the hexose galactose) was increased in A. thaliana by overexpressing Galactan
Synthase1 (GALS1), cytosolic UDP-Glucose/UDP-Galactose-4-Epimerase2
(UGE2) synthesizing UDP-galactose and a gene for UDP-galactose transporter
moving the substrate to Golgi lumen to be utilized by GALS1 (Aznar et al. 2018).
This study was extended by using 3-dehydroshikimate dehydratase (QsuB) strategy
via SynBio technology, where high-galactan lines were combined with cell-specific
low xylan and low lignin (Aznar et al. 2018). Production of lignocellulosic-based
biofuels is less preferred because sequestration of cell wall polysaccharides by lignin
polymers makes the sugar extraction process considerable complex, thus demanding
effective techniques for decreasing lignin content (Eudes et al. 2015; Shih et al.
2016). Techniques of RNAi genetic engineering in corn resulted in plants with less
462
P. Singla