different frequencies of gametes to be identified with absolute certainty, and (v) the
genome constitution of some polyploids are unclear, making their inheritance pattern
difficult to determine (Kole 2010).
Modern hybrids of sugarcane are results of crosses between varieties and/or
clones, making the combination of chromosomes unique among each offspring,
due to random classification of chromosomes. Genetic and biotechnological breeding approaches in sugarcane germplasm can play a key role in improving this
biomass in the biofuel production. Variations in the phenotypes can be found in
the biomass yield, fiber content, and sugar composition (Hoang et al. 2015).
To produce bioethanol from sugarcane, transcription factors (TFs) that regulate
monolignol biosynthesis in lignin pathway have been studied, since the understanding of this metabolic pathway reduces and modifies the lignin content, which is
essential to address the biomass reclassification problem. The lignin biosynthesis
pathways are complex and involve at least 10 enzymes and 28 associated unigenes
identified in sugarcane. There are some key genes in lignin pathway that encode
terminal enzymes such as caffeic acid O-methyltransferase (COMT) and cinnamyl
alcohol dehydrogenase (CAD) (Hoang et al. 2015).
Lignin synthesis is like metabolism of phenylpropanoids in plants, containing a
series of enzymes common to other processes (phenylalanine ammonia-lyase (PAL),
cinnamate-4-hydroxylase (C4H), and COMT) and specific enzymes such as
cinnamoyl-CoA reductase (CCR) and cinnamyl alcohol dehydrogenase (CAD),
among
others
like
ferulate-5-hydroxylase
(F5H),
caffeoyl
CoA
O-methyltransferase (CCoAOMT), and hydroxycinnamate CoA ligase (4CL). Studies have shown that maize and sorghum mutants that were deficient in CAD and/or
COMT showed an easy digestibility due to the reduced lignin content (Ramos et al.
2001). Therefore, understanding enzymes and mechanisms involved in lignin biosynthesis is fundamental in genetic improvement of biomass to produce biofuels.
Downregulation of enzymes (COMT or CAD) in ending steps of the monolignol
biosynthetic pathway has little or no negative effect on plant growth. A moderate
reduction in lignin content (3.9% to 8.4%) can significantly reduce recalcitrance of
sugarcane biomass without affecting plant growth under indoor controlled environmental
conditions. Genetic studies demonstrate that downregulation of sugarcane COMT gene by
67% to 97% reduced lignin content by 3.9% to 13.7%, respectively. On reflection, the
syringyl/guaiacyl (S/G) ratio in lignin was reduced from 1.47 in wild type to values
between 1.27 and 0.79, and fermentable glucose increased 29% without pretreatment and
up to 34% with pretreatment, both after enzymatic hydrolysis (Jung et al. 2012).
The suppression of COMT on lignin biosynthesis has shown that transgenic lines
showed a significant reduction of S monomers and similar amount of G monomers in
relation to wild type. The S/G ratio was also lower in the transgenic lines, which in
turn had improvements in enzymatic digestibility. However, a microscopic analysis
demonstrated that vascular bundle tissues and sclerenchyma fiber cells were intact in
varieties with reduced lignin concentration. A histochemical analysis revealed the
reduction of S lignin units in sclerenchyma fiber cells (Fig. 2.2). However, the
transgenic lines showed thinner stems and lower biomass production in relation to
wild plants (Jung et al. 2012).
2 Biofuels Generation Based on Technical Process and Biomass Quality
49
genome constitution of some polyploids are unclear, making their inheritance pattern
difficult to determine (Kole 2010).
Modern hybrids of sugarcane are results of crosses between varieties and/or
clones, making the combination of chromosomes unique among each offspring,
due to random classification of chromosomes. Genetic and biotechnological breeding approaches in sugarcane germplasm can play a key role in improving this
biomass in the biofuel production. Variations in the phenotypes can be found in
the biomass yield, fiber content, and sugar composition (Hoang et al. 2015).
To produce bioethanol from sugarcane, transcription factors (TFs) that regulate
monolignol biosynthesis in lignin pathway have been studied, since the understanding of this metabolic pathway reduces and modifies the lignin content, which is
essential to address the biomass reclassification problem. The lignin biosynthesis
pathways are complex and involve at least 10 enzymes and 28 associated unigenes
identified in sugarcane. There are some key genes in lignin pathway that encode
terminal enzymes such as caffeic acid O-methyltransferase (COMT) and cinnamyl
alcohol dehydrogenase (CAD) (Hoang et al. 2015).
Lignin synthesis is like metabolism of phenylpropanoids in plants, containing a
series of enzymes common to other processes (phenylalanine ammonia-lyase (PAL),
cinnamate-4-hydroxylase (C4H), and COMT) and specific enzymes such as
cinnamoyl-CoA reductase (CCR) and cinnamyl alcohol dehydrogenase (CAD),
among
others
like
ferulate-5-hydroxylase
(F5H),
caffeoyl
CoA
O-methyltransferase (CCoAOMT), and hydroxycinnamate CoA ligase (4CL). Studies have shown that maize and sorghum mutants that were deficient in CAD and/or
COMT showed an easy digestibility due to the reduced lignin content (Ramos et al.
2001). Therefore, understanding enzymes and mechanisms involved in lignin biosynthesis is fundamental in genetic improvement of biomass to produce biofuels.
Downregulation of enzymes (COMT or CAD) in ending steps of the monolignol
biosynthetic pathway has little or no negative effect on plant growth. A moderate
reduction in lignin content (3.9% to 8.4%) can significantly reduce recalcitrance of
sugarcane biomass without affecting plant growth under indoor controlled environmental
conditions. Genetic studies demonstrate that downregulation of sugarcane COMT gene by
67% to 97% reduced lignin content by 3.9% to 13.7%, respectively. On reflection, the
syringyl/guaiacyl (S/G) ratio in lignin was reduced from 1.47 in wild type to values
between 1.27 and 0.79, and fermentable glucose increased 29% without pretreatment and
up to 34% with pretreatment, both after enzymatic hydrolysis (Jung et al. 2012).
The suppression of COMT on lignin biosynthesis has shown that transgenic lines
showed a significant reduction of S monomers and similar amount of G monomers in
relation to wild type. The S/G ratio was also lower in the transgenic lines, which in
turn had improvements in enzymatic digestibility. However, a microscopic analysis
demonstrated that vascular bundle tissues and sclerenchyma fiber cells were intact in
varieties with reduced lignin concentration. A histochemical analysis revealed the
reduction of S lignin units in sclerenchyma fiber cells (Fig. 2.2). However, the
transgenic lines showed thinner stems and lower biomass production in relation to
wild plants (Jung et al. 2012).
2 Biofuels Generation Based on Technical Process and Biomass Quality
49