were found between 6.96 and 20.75%, while cellulose and hemicellulose degradation ranged from 2.08% to 37.87% and from 14.71% to 52.50%, respectively. After
fermentation, biohydrogen production ranged from 14.59 to 40.66 mL per gram of
Miscanthus biomass. Improvement of varieties in their composition and cell wall
structure with high rates of degradability can significantly increase the sustainable
production of bioenergy (Zhao et al. 2014).
Switchgrass (Panicum virgatum) was genetically engineered and demonstrated to
be possible to increase cellulosic ethanol production by up to 2.6-fold
(by overexpressing the PvMYB4 transcription factor). This strategy decreased
carbon deposition into phenolic fermentation inhibitors and lignin and maintains
the propitiation of potentially fermentable soluble sugars. Overexpression of the
switchgrass R2-R3 MYB transcription factor PvMYB4 restricts genes from biosynthetic lignin pathway and directly affects saccharification without acid pretreatment,
directly affecting recalcitrance. This variety provides new germplasm for development of cultivars as a raw material to produce biomass for biofuels (Shen et al.
2013).
Hybrid poplars (genus Populus) are among the fastest growing temperate climate
trees in the world and are also considered one of the most promising raw materials
for biofuels and other value-added products. The estimated nominal yield (including
the moisture content at harvest) of hybrid poplar species grown in North America is
between 14 MgÁha
-1
Áyear
-1 and can be compared to switchgrass (14 MgÁha
-1
Áyear
-1
)
and higher than wheat straw and corn stover. The species and hybrids of the genus
Populus have cellulose contents between 42 and 49% (higher than that of switchgrass), hemicelluloses of 16 to 23% and lignin of 21 to 29%. Studies performed on
poplar clones demonstrated that a small decrease in S/G ratio resulted in a statistical
increase in releasing of xylose after hydrolysis with dilute sulfuric acid. Suppression
of the COMT gene in poplar does not result in a reduction in lignin content, but S/G
ratios were decreased. Sequencing of poplar genome paved ways for development of
new cultivars and clones optimized to produce biofuels (Sannigrahi and Ragauskas
2010).
Basically, the cell wall of plants consists of cellulose, non-cellulosic
polysaccharides, and lignin. Manipulations of molecular and structural levels of
wall components can improve performances of plants in biofuel production. This
can be done through genetic improvement or through methods such as screening of
natural variation and random mutagenesis. Altering genes encoding enzymes
responsible for lignin biosynthesis or manipulating genes involved in cell wall
polysaccharide biosynthesis may facilitate their deconstruction and improve mass/
energy balances during biofuel production (Burton and Fincher 2014). Plant crops
for biofuels that hold high yields and efficiency in conversion allow crops in smaller
areas, minimizing competition for food crops and conserving biodiversity. In this
way, development of genetically modified plants is key to biofuel production,
improving the efficiency and viability of sustainable production processes (Furtado
et al. 2014).
52
F. L. Shimizu et al.
fermentation, biohydrogen production ranged from 14.59 to 40.66 mL per gram of
Miscanthus biomass. Improvement of varieties in their composition and cell wall
structure with high rates of degradability can significantly increase the sustainable
production of bioenergy (Zhao et al. 2014).
Switchgrass (Panicum virgatum) was genetically engineered and demonstrated to
be possible to increase cellulosic ethanol production by up to 2.6-fold
(by overexpressing the PvMYB4 transcription factor). This strategy decreased
carbon deposition into phenolic fermentation inhibitors and lignin and maintains
the propitiation of potentially fermentable soluble sugars. Overexpression of the
switchgrass R2-R3 MYB transcription factor PvMYB4 restricts genes from biosynthetic lignin pathway and directly affects saccharification without acid pretreatment,
directly affecting recalcitrance. This variety provides new germplasm for development of cultivars as a raw material to produce biomass for biofuels (Shen et al.
2013).
Hybrid poplars (genus Populus) are among the fastest growing temperate climate
trees in the world and are also considered one of the most promising raw materials
for biofuels and other value-added products. The estimated nominal yield (including
the moisture content at harvest) of hybrid poplar species grown in North America is
between 14 MgÁha
-1
Áyear
-1 and can be compared to switchgrass (14 MgÁha
-1
Áyear
-1
)
and higher than wheat straw and corn stover. The species and hybrids of the genus
Populus have cellulose contents between 42 and 49% (higher than that of switchgrass), hemicelluloses of 16 to 23% and lignin of 21 to 29%. Studies performed on
poplar clones demonstrated that a small decrease in S/G ratio resulted in a statistical
increase in releasing of xylose after hydrolysis with dilute sulfuric acid. Suppression
of the COMT gene in poplar does not result in a reduction in lignin content, but S/G
ratios were decreased. Sequencing of poplar genome paved ways for development of
new cultivars and clones optimized to produce biofuels (Sannigrahi and Ragauskas
2010).
Basically, the cell wall of plants consists of cellulose, non-cellulosic
polysaccharides, and lignin. Manipulations of molecular and structural levels of
wall components can improve performances of plants in biofuel production. This
can be done through genetic improvement or through methods such as screening of
natural variation and random mutagenesis. Altering genes encoding enzymes
responsible for lignin biosynthesis or manipulating genes involved in cell wall
polysaccharide biosynthesis may facilitate their deconstruction and improve mass/
energy balances during biofuel production (Burton and Fincher 2014). Plant crops
for biofuels that hold high yields and efficiency in conversion allow crops in smaller
areas, minimizing competition for food crops and conserving biodiversity. In this
way, development of genetically modified plants is key to biofuel production,
improving the efficiency and viability of sustainable production processes (Furtado
et al. 2014).
52
F. L. Shimizu et al.