Advances in Crop Science and Technology
Open Access

Antinutrients; Breeding methods; Nutritional quality; Sorghum quality

Introduction

Sorghum (Sorghum bicolor) is an important food crop that is consumed by millions of people as a staple food source in arid and semi-arid areas of the developing world, especially Africa. In the developed world, including Australia and the United States, sorghum is primarily produced for livestock feed and industrial uses such as bioethanol production. However, due to the increasing body of evidence and awareness of its beneficial health-promoting properties (Girad and Awika, 2018), there is growing demand for and use of sorghum in the food markets of the developed world (Awika, 2017). It was consumed mostly in northern China, India, and southern Russia, where about 85% of the crops are consumed directly as human food (Dicko et al., 2006) [1,2].

Sorghum is a self-pollinated, diploid (2n=2x =20) crop species and belongs to the Poaceae family with a genome size of 730 Mb (Paterson et al., 2009). It is an indigenous crop to Ethiopia, beginning from 4000- 3000 and domesticated to the surrounding countries as Ethiopia is the origin of its domestication (Getahun et al., 2014). It is an economically, socially, and culturally important crop grown over a wide range of ecological habitats in the country, in the range of 400-3000 masl (Teshome et al., 2007). it is also widely grown in the arid and semi-arid tropics because of its unique adaptation to harsh and drought-prone environments where other crops can least survive and food insecurity is rampant (Adugna, 2007). The sorghum crop is grown in almost all regions of Ethiopia and used as a staple food crop on which the lives of millions of poor Ethiopians depend on it [3,4].

The crop is a C4 species with high photosynthetic capacity and inherent high biomass yield potential. Its high levels of tolerance to drought and high temperature, and adaptation to inherent low soil fertility make the crop progressively more relevant for food security because of climate change. In Ethiopia, sorghum is the third most important staple cereal crop after teff and maize (Spielman et al., 2012), and it is widely used as a food, feed, fiber, and bioenergy crop [5]. Consumption of sorghum by urban dwellers is partly explained by the availability and the market price of teff. This means that sorghum serves as a substitute for making enjera when the price of teff picks high (Adugna, 2014). The yield and quality of sorghum produced worldwide are influenced by a wide array of biotic and abiotic constraints. Grain sorghum quality is determined by several factors, such as visual quality, nutritional quality (including whole grain, protein, and starch digestibility; nutrient bioavailability), anti-nutritional factors such as tannins, processing characteristics, cooking quality, and consumer acceptability (Hulse et al., 1980). Nutritional aspects such as vitamins, and mineral elements such as iron, and zinc received little attention, but efforts are being made to improve them in various crops including sorghum. Among other nutritional needs, proteins are essential to people and animal nutrition [6]. Therefore, the present research aimed to profile the nutritional value, anti-nutritional value, and mineral contents of sorghum varieties and different breeding approaches to improve sorghum quality (Sleper and Poehlman, 2006) [7].

Possessing the sorghum nutritional diversity would have a direct impact on the improvement of sorghum for quality breeding and for food product development to reduce the prevalence of food insecurity, malnutrition, and mineral deficiency, especially for low-income communities. Besides the milling quality of sorghum is determined mainly by kernel shape, density, hardness, structure, presence of a pigmented testa, pericarp thickness, and color (Legesse, 2018). The goal of sorghum improvement is mainly for direct enhancement of yield potential and stability However, in the present market economy,the product quality of sorghum has become increasingly important to satisfying the end users [8].

Therefore, breeding for improved quality, consideration is given to the physical and chemical characteristics of the product harvested that affect its nutritional value, processing, and utilization. For example, our ability to improve the nutritional quality of sorghum grain protein by classical plant breeding is limited by the low level of variation in the gene pool available for crossing. Exploration of the available genetic variation in landraces and improved cultivars for chemical and physical grain attributes and their association with end-uses, such as Injera quality, would require the screening of germplasm for quality characteristics before subsequent inclusion in breeding programs. Therefore, the objective of this term paper is

• To assess the available methods that could be applied in sorghum breeding programs for quality traits

• To review the components of quality descriptor parameter traits in sorghum

Literature Review

Importance of sorghum grain quality traits

There are several components to grain quality, such as visual quality; nutritional quality, including digestibility and big availability of nutrients; anti-nutritional factors; milling characteristics; cooking quality; consumer acceptability; and storage stability. Grain quality improvement in cereal crops continues to be an important area of research as cereals represent the largest constituent of global food supplies (Gilland, 2002) [9]. Grain quality deserves as important a place in the evaluation of new varieties as high yield and yield stability. Grain produced in a farmer’s field passes through several transformations before it is consumed in the form of food. Grain sorghum, like other cereal crops, can also be utilized to create additional end-use products. Sorghum grain has been used to make baking flours, pop sorghum, alcoholic beverages pet foods, and packaging materials (Zhu, 2014) [10].

Grain quality is a nebulous term that means different things to different people. Grain quality largely depends on the grain type and its end use. It includes a range of properties that can be defined in moisture content of kernel size, sanitary fungi and mycotoxin count, intrinsic fat content, protein content, hardness, and starch content quality characteristics [11]. The quality properties of grain are affected by its genetic traits; the growing period, timing of harvest, grain harvesting and handling equipment, drying system, storage management practices, and transportation procedures, and various products can require different grain characteristics and thus can alter crop ideotype. Identifying genes influencing sorghum grain composition would help manipulate grain texture and quality to accommodate existing end-use markets and promote new product development (Bean et al. 2016) [12].

In addition, understanding the chemical and genetic components underlying the gross energy content of sorghum would enable breeders to increase the overall feed efficiency when the grain is grown for livestock feed through selective breeding and trait introgression. Cereal grain quality traits including the five evaluated in this study have been previously measured with high predictability using NIRS (Kays and Barton, 2002). The marketability of sorghum could also expand by tailoring its grain quality for various end-users, including flour mills, animal producers, and ethanol companies (Bean et al., 2016) [13,14].

Genetic control of nutritional traits in sorghum

Knowledge of the nature of genetic control of both qualitative and quantitative characters of agronomic importance is fundamental in the systematic and rapid improvement of these traits (House 1985). Excellent reviews on the genetics of various traits found in sorghum can be controlled (Rooney, 2000). The genetics of traits important to sorghum production are presented in this term paper. A better understanding of the genetic basis of grain size, including underlying molecular mechanisms, will provide new targets for improving yield and grain quality in cereal breeding [15].

Source of quality traits in sorghum

Sorghum quality traits vary among different types of sorghum and their cultivated environments. Genetic improvement of grain quality can help sorghum to adapt to varying demands for enduse products. Cultivated Variety Cultivated varieties are the most preferred source for quality or any other trait since they are the easiest to utilize in breeding programs [16]. Only when a trait is not present in the cultivated varieties of the concerned crop, a breeder should look elsewhere for the trait. In general, most quality traits are available in the current or old varieties. The cultivated sorghum had its origin in Africa near present-day Sudan and Ethiopia. It includes five basic races, viz., bicolor, guinea, caudatum, kafir and dura (Harlan and De Wet, (1972). The sorghum cultivars have the potential to produce more grain per unit area (Seetharama et al., 2003) [17].

A germplasm line of quality traits not available in cultivated varieties may be found in a germplasm line if an extensive search were made. For example, high lysine (near to 3% of the total protein) lines of sorghum, viz., ISI 1167 and ISI 1758 were identified from Ethiopian collections. Many quality traits have been contributed by spontaneous/induced mutants. This is particularly true for protein and oil quality traits. There are also examples of the isolation of desirable mutants from mutant lines for quality traits [18]. For example, the P-721 opaque mutant of sorghum has an opaque endosperm, which is not liked by consumers. A vitreous (horny/hard) endosperm DES-induced mutant was isolated from P-271 opaque line; this mutant has high lysine content as well. There is a large genetic variation within sorghum that is suitable for sustainable biofortification (Ashok-Kumar et al., 2013) [19].

However, to ensure success in research and development, a multidisciplinary approach is necessary for screening and selection of breeding lines with higher concentrations of essential nutrients (Paiva et al., 2017; Vasconcelos et al., 2017). The grain iron and zinc concentrations in sorghum are under polygenic control and show continuous variation. While grain zinc concentration is controlled by additive genes, both additive and non-additive genes play a major role in controlling grain iron concentration (Ashok Kumar et al., 2013) [20].

Description of some parameters of sorghum quality Protein quality

Sorghum proteins, also known as kafirins, belong to a group of grain proteins called prolamins. Sorghum kafirins have higher hydrophobicity compared with maize zein protein, rice proteins, and wheat prolamins (Beltan et al., 2006). There are three major subclasses of sorghum kafirin proteins based on molecular weight: α, β, and γ-kafirins. The relative abundance of α, β, and γ-kafirin subclasses are 80, 13, and 7%, respectively (Winn et al., 2009). Kafirin protein bodies are organized into small globular protein bodies in such a way that the more protease-resistant and hydrophobic β and γ-kafirins enclose the more digestible and less hydrophobic α-kafirin component (Winn et al., 2009). Thus, sorghum proteins have 40–60% lower digestibility compared with proteins from other cereal grains (Duodu et al., 2003), and digestibility decreases further during cooking due to the formation of disulfide cross-linking of the kafirin proteins (Aboubacar et al., 2001) [22,23].

Gluten quality

Gluten is a protein that can be extracted from flour when starch and other minor components of the flour are removed by washing it out with running water. The resulting gluten contains approximately 65% water (Moreno and Sousa, 2014). On a dry matter basis, gluten contains 75– 86% protein, the remainder being carbohydrates and lipids, which are held strongly within the gluten-protein matrix (Gallagher and Arendt, 2004). Gluten has some characteristics (extensibility, resistance to stretch, mixing tolerance, and gas-holding ability) that favor its use in many food products (Gallagher et al., 2004). Sorghum grain is glutenfree, high in resistant starch, and is a rich source of nutrients, and most importantly, contains a diverse range of bioactive Phenolic compounds (Dykes & Rooney, 2007) [24,25].

Sorghum contains more abundant and diverse Phenolic compounds compared to other major cereal crops; it contains nearly all classes of Phenolic compounds, with simple Phenolic acids, flavonoids, and tannins being the dominant groups (Shen et al., 2018). Recently, the motivation for the consumption of gluten foodstuffs has led to a rising demand for its availability. This has led to an exponential increase in the market for gluten-free foods and beverages which has now continued to grow even faster than anticipated [26].

Grain and flour color

The color of sorghum grain was affecting the color of the resulting food, which has implications for consumer preference. Thus, the color of sorghum grains and flour is an important quality parameter, which is taken into consideration for the selection of grain for specific food uses. For sorghum, white or light-colored types are generally preferred for porridge making (Belton and Taylor, 2004), while red-colored sorghums are generally preferred for brewing traditional beer (Taylor and Duodu, 2017). For industrial production of lager beer, white sorghums are generally selected. Ultraviolet-visible spectrophotometer using absorbance or transmittance at a specific wavelength (Feillet et al., 2000) or visual inspection can also be used to determine color (Taylor and Dewar, 2001) [27].

Dark brown pigment

Some sorghum cultivars have pigmented sub-coat (testa) located between the pericarp and the endosperm (Earp et al., 2004). The pigmented testa contains tannins proanthocyanidins. Tannins protect the grain against insects, birds, and fungal attack but condensed tannins are associated with nutritional disadvantages and reduced food quality (Waniska and Rooney, 2000) [28].

Grain hardness

Grain hardness or endosperm texture (grain strength) is an important grain quality attribute that plays a role in the processing of cereal grains and the end-use quality of cereal grain products such as bread and snack foods (Bettge and Morris 2000). Grain hardness also plays a role in plant defense against molds and possibly from insect attacks (Chandrashekar and Mazhar 1999). For sorghum, grain hardness has been linked to several specific end-use quality traits [29]. Cagampang and Kirleis (1984) reported that sorghum cooking quality parameters such as adhesion, cooked grain texture, alkali gel stiffness, and amylograph viscosities were significantly related to grain hardness. Rooney et al. (1986) reported that sorghum grain hardness was the most important component related to porridge quality. Grain hardness was also related to the production of high-quality couscous granules from sorghum and within a given grain lot, large sorghum kernels were harder than small kernels and related to higher-quality grain (Lee et al., 2002). Large sorghum kernels with corneous endosperm are usually preferred for human consumption and are associated with desirable physical and chemical quality parameters such as high protein concentration, low ash, and high milling yields, high water absorbance, bright white color, and large particle size (Lee et al., 2002). Small kernel sorghum that is more likely to be harder and more difficult to mill is not popular in the grain market (Wills and Ali, 1983) [30].

Nutritional content of sorghum quality

The nutritional quality of sorghum is poor compared to other cereals, mainly due to the predominance of storage proteins, i.e., prolamins (Kajirins), which are known to be extremely low in the essential amino acid lysine, rich in leucine, and have low protein digestibility (which is lowered when the grain is cooked) and People who depend on sorghum in their diet often develop pellagra mainly on account of a high leucine and isoleucine ratio (Magnavaca et al., 1993) [31].

Chemical composition

In developing countries, especially in Africa, over 78% of the sorghum produced is used for food, with about 14% for animal feeding and 7% for other uses (Betay et al., 2017). Extensive studies on the composition of sorghum have indicated that the grain is a good source of energy, carbohydrates, polyunsaturated fatty acids (PUFAs), minerals, vitamins, and some essential amino acids (Awika et al., 2016; Pontieri et al 2016; Taylor et al., 2018). Chemical quality parameters such as crude protein (CP), fat, P, starch, and total digestible nutrients (TDN) directly influence sorghum’s nutritional value. Starch content in sorghum kernels affects the consistency of thick porridge, cooked couscous firmness, and rollability (Beta et al., 2001) [32,33].

Carbohydrates

The majority of the carbohydrates in sorghum are starch, while soluble sugar, pentose, cellulose, and hemicellulose are low. Regular endosperm sorghum types contain 23 to 30% amylase, but waxy varieties contain less than 5% amylase. Sorghum is a good source of fiber, mainly the insoluble (86.2%) fiber. As indicated by previous authors Awika et al. (2017), starch, including dietary fiber derived from cellulosic cell wall carbohydrates, is a major component of sorghum, constituting about 75% of the grain. The presence of non-starch polysaccharides (NSPs) in sorghum grains could be suggestive of their potential ability to improve bowel function and lower cholesterol levels (Topping, 1991) [34].

Sorghum starch is composed of 70–80% amyl pectin and 20- 30% amylase. Waxy sorghums contain starch with 100% amyl pectin; their properties and uses are similar to those of waxy maize. Sorghum endosperm requires a little longer cooking compared to maize endosperm particles. Increased grain weight amounts to a yield increase when sorghum is grown on a large scale, and grain weight has positive relationships with milling and nutritional qualities. Between 62-71% of whole grain dry matter (Rhodes et al., 2016) [35].

Proteins

Protein content and composition vary due to genotype, and water availability, temperature, soil fertility, and environmental conditions during grain development. The protein content of sorghum is usually 11-13% but sometimes higher values are reported (David, 1995) [36]. Prolamins (kafirins) constitute the major protein fractions in sorghum, followed by glutelins. Sorghum components, especially its protein is less digestible than other cereals for humans and monogastric animals, because of its anti-nutritional factors such as tannins and phytic acid. Removal of these undesirable components is essential to improve the nutritional quality of sorghum and effectively utilize its potential as human food or animal feed (Soetan et al., 2009; Kumar et al., 2010). Interaction between tannins and sorghum proteins reduces both protein and starch digestibility. Proteins rich in proline bind more sorghum tannins than other proteins. The low digestibility of sorghum proteins is presumably due to the high protein cross-linking. Goodquality proteins are those that are readily digestible and contain the essential amino acids in quantities that correspond to human requirements (Zhao et al., 2011) [37].

Lipids

The crude fat content of sorghum averages about 3%, which is higher than that of wheat and rice. Fatty acid composition is similar to that of corn oil, with high concentrations of linoleic (49%), oleic (31%), and palmitic acids (14%). Like maize, the energy content of sorghum is high. Sorghum grain contains about 1.5 ppm of total carotenoids. Apart from maize and durum wheat, sorghum is the only cereal that contains a significant amount of β-carotene, the provitamin of vitamin A, which is important in human physiology [38].

Vitamins and Minerals

Available studies have also indicated that sorghum contains minerals and vitamins, both of which constitute part of the essential nutrients required by humans to perform the functions necessary to sustain life. Sorghum contains fairly high levels of potassium (K) (900- 6957.67 mg/kg), and phosphorus (P) (1498-3787.25 mg/kg), minerals known to assist with muscle movement, keeping the nervous system healthy and building strong bones and teeth. Minerals are located in the pericarp, aleurone layer, and germ. The refined sorghum products lose part of these important nutrients, as in all other refined cereal fractions. Vital vitamins reported in sorghum also include the B vitamins (0.1-19.9 mg/100 g), and vitamin E (1.38 mg/100 g). Trace amounts (maximum of 0.01 mg/100 g) of β-carotene (a vitamin and precursor of vitamin A) have also been reported by Khalil et al. (1984), an indication that sorghum cannot be considered a good source of β-carotene and vitamin A nutrient deficiencies [39,40].

Soluble sugars

Sorghum at maturity the average soluble sugar content ranges from 0.8 to 4.2% with sucrose being 75% of the sugars (Jambunathan et al., 1980). Mature caryopsis contains 2.2 to 3.8% soluble sugars, 0.9 to 2.5% free-reducing sugars, and 1.3 to 1.4% non-reducing sugars (Bhatia et al., 1972). Glucose ranges from 0.6 to 1.8% and fructose from 0.3 to 0.7%. High lysine and sugary cultivars contain more soluble sugars than normal sorghums Subramanian et al. (1992) reported that the highlysine sorghum lines IS11167 and IS11758 from Ethiopia comprised the highest percentages of total soluble sugars (5.2 and 4.4%, respectively). During germination, sugars accumulate in the endosperm after the second day with maximum concentration occurring after eight days (Waniska and Rooney, 2000) [41].

Anti-nutritional contents of sorghum

Tannins (Condensed Polyphenols)

Tannin is a large polyphenolic polymer and is known to bind proteins, limiting their digestibility, but is also an excellent antioxidant. Tannin is widespread throughout the plant kingdom, with diverse biological and biochemical functions, such as protection against predation from herbivorous animals and pathogenic attack from bacteria and fungi (Xie et al., 2003). High-tannin sorghums are desirable in making food products due to their palatability (Awika et al., 2004). Good quality bread containing tannin sorghum bran has high antioxidant and dietary fiber levels, with a natural dark brown color and excellent whole-grain flavor (Gordon, 2001) [42].

In addition, healthy bread mixes containing tannin sorghum bran, barley flour, and flax seed have been developed (Rudiger, 2003). Kiprotich et al. (2014) recommended that sorghum grains suitable for malting and brewing should not have tannin levels greater than 18.13 mg/100 ml since high tannin levels pose a challenge during the brewing process. Tannins inhibit the activity of alpha-amylase (Alonso et al., 2000), and this lowers the hydrolysis of starch which is essential for brewing [43].

Phytic acid

Sorghum contains phytic acid which binds minerals and reduces their availability to the consumer. Its phytic acid levels are similar to those reported for wheat, barley, and maize, but lower than that of soybeans and other oilseeds. Since sorghum grain is usually low in mineral content (with phytin and mineral contents equivalent to maize), and the presence of phytic acid likely renders its low mineral content unavailable, supplementation with other mineral sources is necessary where sorghum is a major component of the diet. As with tannin content, phytic acid content (and mineral content) may be reduced by abrasive decortication of the grain to remove the pericarp and aleurone layers (Waniska and Rooney, 2000) [44].

Cyanogenic glycosides

Cyanogenic glycosides are mainly present in germinating seeds, sprouts, and the leaves of immature sorghum plants. Traore et al. (2004) showed that malted red sorghum that had been dried contained on average 320 ppm cyanogens.

The most abundant of cyanogens is dhurrin, which may comprise three to four percent of the leaves of germinating seeds (Waniska and Rooney, 2000). Stressors such as drought, frost, heavy insect infestation, or overgrazing can result in increased levels of these compounds, which, along with tannins, are part of the plant’s defense mechanisms. The use of potassium nitrate fertilizer was also shown to increase cyanogen production in sorghum (Busk and Moller, 2002). Processing of germinating seeds for feed may result in the release of cyanide. It is generally recommended not to graze animals on young plants or cut them for green chop until they are at least 18 to 51 cm tall (Undersander and Lane, 2001) [45].

Different breeding approaches to improve sorghum quality traits

Breeding methods in sorghum improvement at various centers have taken into account the geographical mandate of individual research organizations, their materials and manpower resources, and short-term and long-term goals concerning increasing productivity and genetic diversification (Rai et al., 1999). Breeding methods used in sorghum quality improvement are those developed for self-pollinated crops such as pure line selection, pedigree breeding, backcross breeding, population improvement, and hybrid breeding. Different breeding methods are used for developing and selecting high-quality sorghum. The major methods are conventional breeding methods, molecular breeding, mutation breeding, etc [46].

Conventional breeding for improvement sorghum quality

The conventional breeding methods involved in the choice of parents for crosses require a prioritization of the goals to be achieved by breeding, and the collection and characterization of genetic sources carrying favorable alleles for the target traits. Exotic germplasm is a rich source of genes for sorghum breeding, with crosses of exotic x exotic parents generally the most rewarding compared with local x local crosses (House et al. 1997). The most common breeding concept employed to introgression important traits is backcrossing, and it is most effective for simply inheriting qualitative traits seed color, seed quality, nutritional quality, etc. Globally most sorghum released were derived from the pedigree breeding program rather than from the population improvement programs. It is therefore evident that the targeted gene pool approach is appropriate for a program that aims at a broad geographic mandate (Reddy et al., 2004) [47].

Selection criteria of sorghum parents for quality traits

Parents for crosses are selected based on their yield, quality traits, disease resistance, and agronomic performance. The initial selection of superior parental lines is typically based on their additive genetic values estimated from testcross performance trials. In a more recent study by Hunt et al. (2018) genomic models were applied for prediction of testcross yield performance in the context of individual trial analysis. Early-stage breeding of hybrid sorghum involves the development of elite inbred lines, which will be subsequently used as parents of commercial hybrids [48].

Landrace selection for sorghum quality development

A collection of diverse germplasm is the foundation for the genetic improvement of crop plants. Most crops grown worldwide are the direct result of selection from wild cultivated and a landrace represents the equilibrium between heterogeneous and heterozygous genotypes within a population of a crop that is maintained by continuous multiplication under a given set of climatic, soil, and husbandry conditions [49].

Earliest landrace introductions that contributed to grain sorghum development included Black hull Kafir, Feterita, Giant Milo, Hegari, Pink Kafir, White and Brown Durra, and White and Red Kafir (Smith and Frederiksen, 2000). Landraces contribute 84% of the total collection compared with wild species, which contribute only 1 %. The germplasm maintained at ICRISAT is classified into five races -bicolor, guinea, caudate, kafir, and durra - and their derivatives, predominantly represented by the three basic races: durra (21.8%), caudate (20.9%), and guinea (13.4%). The intermediate races durra-caudatum (12.1 %), guinea-caudatum (9.5%), and durra-bicolor (6.6%) are common. India, Uganda, and Zimbabwe have all 5 basic and 10 intermediate races (Gopal et al., 2002).

Screening quality sorghum from introduced germplasm materials

Plant genetic resources play an important role in generating new crop varieties with high-yield potential quality and resistance to biotic and abiotic stresses (Sajid et al., 2008). Sorghum landraces have a wide genetic diversity rich in traits useful in crop improvement (Mutegi et al., 2010). Characterization and evaluation of germplasm are the prerequisites for the utilization of the available diversity in the sorghum quality improvement program. Hence, the accessions were characterized to assess the variability and identify the promising accessions for different traits. Plant genetic resources play an important role in generating new crop varieties with high yield potential and resistance to biotic and abiotic stresses (Sajid et al., 2008). Characterization of germplasm is important for the sustainable conservation and increased use of crop genetic resources (Sergio and Gianni, 2005) [50].

Molecular breeding for improvement of sorghum quality traits

Recently, biotechnology has provided ways of improving crops with special reference to sorghum. This is through the use of various techniques such as molecular markers, gene identification, and cloning, in vitro protocols for efficient plant regeneration, genetic engineering, and gene transfer technology to introduce desirable traits into sorghum genomes and genomics, and germplasm databases (O’Kennedy et al., 2006). Sorghum genome mapping based on DNA markers began in the early 1990s, and since then, many sorghum linkage maps have been constructed (Subudhi and Nguyen 2000).

Genetic improvement has been made easily accessible through the use of easily assayable molecular DNA genetics of DNA markers that enable accurate identification of genotype without the confounding effect of environment, thereby increasing heritability. Marker-assisted selection (MAS) in sorghum reduces the length of time required for introgression of characters unlike with the use of the pedigree breeding method. Also, the selection of progenies based on genetic values derived from molecular marker data substantially increases the rate of genetic gain, particularly if the number of cycles of evaluation or generations can be reduced (Meuwissen et al., 2001).

The DNA-based methods are independent of environmental factors and give rise to a high number of polymorphisms. Various molecular markers that have been used in sorghum breeding including randomized amplified polymorphic DNA (RAPD) (Prakash et al., 2006), amplified fragment length polymorphism (AFLP) (Wu et al., 2006), and simple sequence repeat (SSR) (Manzelli et al., 2006), single nucleotide polymorphism (SNP) (Zeng et al., 2011), microarrays (Buchanan et al., 2005), and diversity array technology (DArT) (Mace et al., 2009). The latter technique has been reported recently in several studies [51].

These markers were mainly used for genetic diversity, cultivar identification, gene mapping and discovery, and gene pyramiding. For instance, the β-, γ-, and δ-kafirin genes were sequenced from 35 sorghum genotypes to investigate the allelic diversity of seed storage proteins (Laidlaw et al., 2010). Marker-assisted selection (MAS) is becoming an increasingly useful tool to breeders, and it is our goal to add them to the toolbox by providing a genetic analysis of sorghum protein digestibility. Winn et al. (2009) mapped QTLS governing protein digestibility in sorghum. The results uncovered that two major QTLs on chromosome 1 are associated with protein digestibility one QTL (locus 1 from the high digestibility/ high lysine content parent) unfavorably affects digestibility and one QTL (locus 2 from the HD parent) only 20 cM away favorably affects digestibility [52].

Grain quality attributes like kernel flouriness, kernel friability, kernel hardiness, amylose content (%), protein content, and lipid content (%); fodder quality traits like stay-green; and juicy mid-rib have also been mapped (Xu et al., 2000; Haussmann et al., 2002). Depending on their relative effects and position, many QTL could be used as targets for marker-assisted selection and provide an opportunity for accelerating breeding programs (Subudhi and Nguyen 2000). The selectable marker bar gene is isolated from Streptomyces hygroscopcus and codes for phosphinothricin acetyl transferase (PAT) proteins of 183 amino acids and shows 85 % DNA sequence homology with another marker gene pat isolated from S. viridochromogenes. Phosphinothricin inhibits glutamine synthetase (GS) irreversibly, resulting in inhibition of amino acid biosynthesis. Almost all the transformation studies conducted earlier in sorghum used the bar gene as a selectable marker gene except for (Battraw and Hall, 1991)

Mutation breeding to improve sorghum quality traits

A mutation is a sudden heritable change in the DNA in a living cell, not caused by genetic segregation or genetic recombination (van, 1998). Insertion mutagenesis is a useful method for producing mutants to investigate gene function because those mutations are tagged with DNA fragments of known sequence. Mutagens are induced in many ways; physical (X-ray, gamma-ray etc) and chemical mutagenesis are used. Sorghum grain is loaded with starch and is relatively poor in protein and lipids. Tadesse and Jacob, (2004) introduced the dhdps-raec1 mutated gene, which encodes an insensitive form of dihydropicolinate syntheses, the key regulatory enzyme of the lysine pathway. Their acceptance is limited due to many problems associated with their opaque kernel, reduced grain yield, slow drying in the field, increased susceptibility to molds and insects, and the tendency of the seed to crack when mechanically harvested (Ejeta and Axtell 1990). Similarly, the chemically induced high-lysine strain p-721 Q has a soft kernel and floury endosperm with reduced yielding ability (Axtell 2001). Two mutants were identified in sorghum, the hl gene in an Ethiopian line (Singh and Axtell, 1973), and the P721 opaque gene which was induced with the chemical mutagen diethyl sulfate (Axtell et al., 1979). Ejeta and Axtell, (1990) were able to select modified endosperm of p-721 opaque (high-lysine mutant) with vitreous kernels similar to normal types. However, vitreous phenotypes have been detected in an advanced generation (F6) of breeding [53].

However, crosses between p-721 Q (high protein digestibility) and other elite lines resulted in improved yields (Axtell et al., 1979). The recent identification of a sorghum line, p-851171 (a derivative of p-721 Q), with protein digestibility levels surpassing that of maize raises the hopes of improving protein digestibility in sorghum (aria et al. 2000). However, to date, no studies have examined the association between protein digestibility and lysine concentration in crosses involving p-851171 (Axtell, 2001). Studies show that the lines contain ‘‘low prolamin’’ in which the proportion of kafirin is reduced by 50% with compensatory increases in other more lysine-rich proteins and free amino acids. The lysine content was enhanced by 40-60% [54].

The challenge of protein improvement is the association with deleterious effects on seed weight and yield. Oria et al. (2000) reported the identification of a novel line with high protein digestibility from a cross involving the high lysine P721 opaque mutant. Sorghum lines from the African Centre for Crop Improvement, and breeding lines from other sources were mutagenesis with gamma irradiation and cyclotron to improve agronomic and nutritional traits (Brauteseth, 2009). The author used the FOSS near-infrared spectroscopy (NIR) for the analysis of nutritional quality traits such as protein, and starch among others. Significant differences and deviations were observed among the populations. Sorghum mutant lines with high digestible (HD) protein have been developed (Weaver et al., 1998), and QTLs underlying the HD trait have been mapped to chromosome 1 (Winn et al., 2009).

Genetic engineering / biofortification

Africa Biofortified Sorghum (ABS) project developed improved sorghum lines through the process of genetic engineering techniques (ABSP, 2009). Sorghum was genetically transformed with the use of agrobacterium to improve nutritional quality (Zhao et al., 2000). Sorghum can be genetically transformed by using agrobacterium to improve nutritional quality due to its low lysine content, a high-lysine gene HT12 can be inserted into the sorghum gene using agrobacterium vector together with a herbicide-resistant gene bar (Alina et al., 2017). This way can increase 40-60% of lysine (Zhao et al., 2003). The benefit of improved transgenic lysine variety is to eradicate malnutrition because sorghum is low in lysine and a high-lysine gene HT12 was inserted into the sorghum gene using an agrobacterium vector together with herbicide-resistant gene bar. Ultimately, increased levels (40-60%) of lysine were observed in hemizygous sorghum grains (Zhao et al., 2003) [56,57].

Da Silva et al. (2011) transformed sorghum lines to suppress the synthesis of various kafirin sub-classes (alpha, gamma, and deltakafirins) or backcrossed into transgenic lines with improved protein quality. The transgenic lines had high protein digestibility, improved amino acid score, and protein digestibility corrected amino acid score in contrast to untransformed sorghum lines. Vendemiatti et al. (2008) reported that a commercial sorghum line, Massa 03, and nine ICRISAT high lysine enriched sorghum lines from India, were evaluated for storage protein content and amino acid composition [58-64].

Transgenic approach for sorghum quality improvement

Unlike conventional plant breeding, in transgenesis, only the cloned genes of agronomic importance are introduced into the plants without linkage drag from the donor. This approach has the potential to serve as an effective means of removing certain specific defects of an otherwise well-adapted cultivar, which is difficult using conventional breeding approaches (Chahal and Gosal, 2002). Successful transformation of sorghum was first achieved in the early 1990s, with highly advanced transformation technology now available that is capable of transforming at least 150 kbp of foreign DNA into the sorghum genome (Seetharama et al., 2003) [65,66].

Transformation of sorghum protoplasts

Explants for transformation and plant regeneration Protoplast, suspension cell cultures, immature embryos, immature inflorescences and shoots tips from germinating seedlings are used as explant material to introduce various transgenic into the sorghum genome. The first report of direct DNA uptake into protoplasts of sorghum by electroporation was carried out by Ou-Lee et al. (1986) and later by Battraw and Hall (1991). Hagio et al. (1991) reported stable transformation from suspension cell cultures of Sorghum vulgare through micro projectile bombardment [67]. Though these above investigators reported expression of the integrated foreign gene in detectable amounts, attempts to regenerate them in to whole plants were not attempted. Bombardment of cell suspension cultures directly has advantage as it eliminates the need for preparing protoplasts and reduces the formation of chimeras which are often seen when embryos are bombarded. Shoot tips from germinating seedlings are also widely used explants in sorghum transformation (Tadesse et al., 2003) [68].

Electroporation of protoplasts

The first attempt to transform sorghum was done by using the electroporation method of gene transfer into sorghum protoplasts where transient expression of cat gene that coded for chloramphenicol acetyltransferase (cat) was reported by (Ou-Lee et al., 1986). Battraw and Hall (1991) electroporated protoplasts isolated from embryogenic suspension cultures to introduce nptII along with the uidA reporter gene [69,70]. They studied, transient reporter gene expression using different factors such as linearization of the plasmid and the effect of electroporating with two different gene constructs. They obtained about 77 different Kanamycin-resistant calli. However, plant regeneration could not be achieved. This method is outdated with the development of superior and less cumbersome transformation methods like particle bombardment and Agrobacterium-mediated transformation [71,72].

Particle bombardment

Microprojectile bombardment is the most widely deployed method for genotype-independent sorghum transformation of the various methods for DNA delivery. Initially, Hagio et al. (1991) reported transient expression of reporter genes in sorghum suspension cells by particle bombardment. During the first decade of sorghum transformation studies, there are only a few reports of successful recovery and analysis of transgenic sorghum plants (Casas et al., 1997; Zhu et al., 1998). So far, the PDS-1000/He gene delivery device of Biorad laboratories, Richmond, California has been the most common and successful device used for direct gene transfer. Particle bombardment is an efficient method of genetic transformation of cereals; where biological molecules are driven at high velocity into the target tissue [73]. It offers advantages such as the introduction of multiple genes, simplicity of introducing transgenic, and transformation in those plants where agro-infection is difficult. Production of transgenic plants by particle bombardment can be divided into two processes: (i) introduction of DNA into cells with minimum tissue damage and (ii) Regeneration from transformed cells. Sorghum grain is loaded with starch and is relatively poor in protein and lipids. Tadesse and Jacobs (2004) to improve the amino acid content (lysine) of sorghum grain tried to modify the regulation of the lysine branch of the aspartate metabolic pathway. The deregulation process involves the introduction of a mutated dhdps-rl gene encoding a feedback-insensitive dihydropicolinate synthetase enzyme leading to the accumulation of more amount of lysine [74-78].

Agro-bacterium mediated transformation

Zhao et al. (2000) achieved success in the production of transgenic sorghum plants with an average transformation frequency of 2.1% after co‐cultivation of immature embryos with Agrobacterium carrying a super‐binary vector. It was found that the source of the embryos had a very significant impact on the transformation efficiency, with field‐grown embryos producing a higher transformation frequency than glasshouse‐grown embryos. Recently, work on Agrobacteriummediated co-transformation and regeneration from immature embryo callus, Lu et al. (2009) reported transgenic recovery of sorghum plants harboring a modified tRNAlys (from Arabidopsis thaliana) and sorghum lys1 tRNA synthase elements (TC2 or SKRS) for improving lysine content in sorghum seeds. The SKRS fragment is under the control of the CZ19 B1 element of maize 19KDa zein protein. Though they could successfully generate transgenic sorghum plants, the expression of the lysine gene or amino acid content in sorghum seed is not reported [79,80].

Summary and Conclusion

Sorghum (Sorghum bicolor) is an important food crop that is consumed by millions of people as a staple food source in arid and semi-arid areas of the developing world. Nutritionally, sorghum has a key advantage over other cereal grains in that it has a higher proportion of RS and SDS compared with many staple cereals, improving satiety and glycemic control. Sorghum is a gluten-free cereal used as whole grain and it is a source of energy, protein, vitamins, minerals, and nutraceuticals such as antioxidant Phenolic and cholesterol-lowering waxes. In addition, sorghum grain is utilized for human diets and also used for livestock feed, fuel, construction materials, and an increasing number of industrial products. This diversity of uses increases the complexity of breeding for improved quality. Grain quality varies among different types of sorghum and their cultivated environments.

On the other hand, sorghum use in human food is limited, largely by the nature of its proteins. The protein bodies in sorghum endosperm (kafirins) are relatively hydrophobic, which restricts the interaction of the endosperm with water and, thus, the ability of starch to swell and form a gel network. The effects of gene action on sorghum for yield and protein quality are essential for researchers and plant breeders for their breeding programs aiming to develop hybrids and for further genetic improvement. Information on nutritional diversity assessment is also important for plant breeders, curators, millers, dieticians, and nutritionists for handling and conservation of the sorghum genetic material. The new genetically improved sorghum varieties with enhanced endosperm functionality, developed from mutants, show a lot of promise for unlocking the food use potential of sorghum.

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