**1. Introduction**

Though biofuels coevolved with cars, they received less importance in the past, due to abundant availability of fossil fuels at an economical price. Today, the world is experiencing

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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higher risks to energy security as well as its efficient utilization. These include disruptions to the supply of imported fossil fuels, limited availability of fossil fuels, and energy price spikes. Carbon dioxide emissions leading to global warming have further worsened the situation. In this context, biofuels are regaining interest, being able to present an attractive alternative to petroleum products since these are known to be biodegradable as well as renewable resources.

leaf length-width area, flag leaf area index, fresh and dry biomass index, brix value, and days

Genetic Improvement of Sorghum for Biomass Traits Using Genomics Approaches

http://dx.doi.org/10.5772/intechopen.73010

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Significant variability for genotype, general combining ability (GCA), and specific combining ability (SCA) for different components has been observed in sorghum [5, 6]. Genetic variability for biomass-related traits in sorghum has been reported by many scientists [7–9]. Hawkins [10] proposed an ideotype of high yielding and high biomass sorghum being tall, lodging

Plant height in sorghum is controlled by four independently inherited *Dw* (Dwarf) genes, viz., *Dw1*, *Dw2*, *Dw3*, and *Dw4* [11]. It is determined by the interplay of the internode length and the number of nodes it produces before flowering. The *Dw* genes have partial dominance for tallness, and their effects are additive in nature. Dwarfing genes have been isolated, and dwarf forage hybrids have been developed by incorporating *Dw2* gene into forage seed and pollen parents, leading to 11% increase in leafiness but 30% decrease in forage yield [12]. High heritability has also been reported for plant height [13], number of leaves/plant [14], brix value [15], and leaf area [16] in sorghum. All these traits are under genetic control and

Lignin is present within plant secondary cell wall. It not only gives rigidity and support to plant cell wall but also enhances water conductance and acts as a protective barrier against microbes [17]. Since lignin lowers the yield of fermentable sugars from cell walls, its higher concentration will negatively affect the morphogenic and industrial potential of lignocellulosic biomass [18]. Higher lignin is also associated with poor forage quality of sorghum owing to reduced access to proteins and other nutrients in the cell wall matrix. Hence, pretreatment of lignocellulosic biomass is essential to degrade lignin material in the cell wall. Researchers are trying to modify biomass composition of sorghum by targeting the genes that encode

Brown midrib (*bmr*) mutants arose from novel mutations in phenylpropanoid pathway leading to low lignin concentrations. Sorghum *bmr* mutants developed by chemical mutagenesis were characterized by low lignin [20]. Several allelic *bmr* genes, namely, *bmr 12*, *18*, *26*, and *6*, have been introgressed and characterized in sorghum. Most of the sorghum *bmr* mutants exhibited higher yield of fermentable sugars. A decreased caffeic acid O-methyltransferase activity was reported during evaluation of allelic genes *bmr12* and *bmr18*. Similarly, a low cin-

In sorghum, the stay-green is a recessive trait causing retention of green leaf area at maturity (GLAM). This character may be functional or cosmetic, indicating continued leaf photosynthesis capacity during grain filling or discontinued photosynthesis from leaf greenness, respectively. High yield potential of sorghum in water scarce environment is governed by the functional staygreen. The pleotropic stay-green leads to arrest protein decline in aging leaves [22]. Sorghum stay-green types have been developed worldwide by conventional breeding. Exploiting staygreen trait in breeding programs may result in genetic enhancement of sorghum yield, industrial value, and biotic and abiotic resistance. Stay-green alleles in sorghum individually enhance grain yield under limited water availability via modification in plant architecture and water

resistant and moderately photoperiod sensitive for maximum vegetation.

to maturity [4].

improved in early generations.

enzymes of the monolignol biosynthetic pathway [19].

namyl alcohol dehydrogenase activity was linked to *bmr6* [21].

While primary biofuels have utility in electricity generation, secondary biofuels generated by biomass processing are used in motorization and several industries. There are first-, second-, and third-generation categories of secondary biofuels depending on the type of raw material used and the processing technology applied. Feedstocks of starch and sugar are being utilized in the production of ethanol. Corn, wheat, and milo are starch-based feedstocks, whereas sugarcane and sugar beet are sugar-based feedstocks. Sugar-based feedstocks contain simple sugars, which can be readily extracted and fermented. Sorghum (Poaceae) serves as both sugar- and biomass-based feedstocks. The crop grows well with minimal input requirements on marginal areas. Sorghum varieties possess diverse phenotypic traits to suit their usage as food, feed, energy, and sugar production. Energy sorghum has high lignocellulosic biomass that can be converted into biofuels. An economic comparison shows that cost per ton of sorghum biomass is lesser than other potential biomass crops including switchgrass [1].

High yield and stress tolerance are two main characteristics attracting the scientists and researchers to sorghum as a promising source of biomass. Efforts for genetic improvement of biomass sorghum directly reduce the overall cost of biomass-to-ethanol conversion, mainly affected by lignin content and its composition. Plant cell walls constituting most of the biomass are mainly composed of cellulose, hemicelluloses, and lignin. Lignocellulosic biofuels are produced via three processes: pretreatment, hydrolysis, and fermentation. The enhanced yield of monomeric carbohydrates leads to cheap and efficient biofuel production. Progress in sorghum feedstock genomics research is a key to enhanced bioenergy production. It relies on the integrated use of breeding and biotechnology. The tremendous source of genetic variability in sorghum world collections has made a significant contribution to sorghum improvement in many countries. Sorghum has a diverse germplasm and a relatively small diploid genome of 760–810 Mbp [2] making it well suited for genomics approaches.
