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Genetic engineering, an alternative approach to enhance nutritional values by transgenic has been considered to be the potential tool for the sustainable and an efficient strategy for increasing the nutritional quality traits in target area of plants (Lucca et al. 2001; Zimmermann and Hurrell 2002; Holm et al. 2002). Genetic engineering flavors transferring of desired genes precisely between interspecies/ intregeneric into the target area, there by higher capability for producing significant expression levels of nutritional elements in the edible part. In contrary, crossbreeding, leads to chances of trial and error to produce the desired phenotype. The malnutrition problem is further exaggerated by increasing world population which is likely to reach 8 billion by 2030. The increase in malnutrition (93%) will take place in the developing world, whose share of global population is projected to increase from 78% in 1995 to 83% in 2020 (Khush 2008; Khush et al. 2012). Numerous evidences are pileup available in public databases and literature information shows that, significant increase of bioavailable content in rice grains by transferring of biofortfication genes through biolistic and Agrobacterium-mediate transformation method (Fig.1) (Table 4).
Through the transgenic approaches, Goto et al. (1999) first observed three fold enhancement of Fe in the starchy endosperm of rice by transferring of ferritin gene of soybean. Similarly in 2001, Lucca et al. introduced ferritin gene from common bean into rice observed had double the concentration of Fe in seeds as compared to controls. Rice, lacking in producing β-carotene precursor of Vitamin A. Ye et al. (2000) developed golden rice that, yields 1.6-2.0 μg/g of β-carotene of dry rice which is very beneficial to retina (Vitamin A) to create visual pigment and ultimately leads decreasing of night blindness particularly in developing countries. It is profile by incorporating of major four genes phytoene synthase, phytoene desaturase, β-carotene desaturase, and lycopene β-cyclase into rice.
Vasconcelos et al. (2003) transferred soybean ferritin gene into two rice varieties as IR 68144 having 7 mg/g of Fe in milled grain and 17 mg/g of Fe in rough rice while commercial high-yielding variety IR64 has 2 mg/g of Fe in milled grain and 10 mg/g in rough rice. Through the biolistic method, they observed significant enrichment of Fe in transgenic T0 lines (Fr18) had a 34.7 fig/g in compare to control 15.7fig/g and also Zn enrichment in Fr19 line had a 55.5fig/g in compare to control 33.6fig/g in unpolished transgenic seed The increase amount of Fe concentration was more than double in compare control seeds. The results are corroborated with earlier studies of Goto et al. 1999, Lucca et al. 2001 and Zimmerman and Hurrel 2002. Similarly, Khalekuzzaman et al. (2006) observed increased in Fe content of 10.5 to 15.0 mg/kg in T1 brown seeds compared to control 9.70 mg/kg. In T2 generation, polished rice seeds have the Fe content of 4.9 to 11.0 mg/kg compared to control 3.3 to 3.8 mg/kg. Thus, the Fe content significantly increased more than 2-fold in transgenic lines in compare to non-transformed plants by an endosperm specific glutelin promoter into BRRI Dhan 29. Subsequently many researchers have attempted to increase Fe content in rice endosperm by over expressing genes involved in Fe up take from the soil and translocation from roots, shoot, flag leaf to grains, and by increaseing the efficiency of Fe storage proteins (Kobayashi and Nishizawa 2012; Lee et al. 2012; Bashir et al. 2013a; Masuda et al. 2013a; Slamet-Loedin et al. 2015). Among these studies, the associated increase in Fe and Zn content in rice grains was obtained by the over expression or activation of the Nicotianamine Synthase (NAS) genes or influenced with other transporters genes (Table 4).
Christou and Twyman (2004) reported almost 50 % of the world’s population is currently affected by malnutrition and majority of the cereal such as maize, wheat or rice, which are deficient in several essential nutrients as lysine and threonine (rice and wheat) and tryptophan, methionine and cysteine (legumes) (Zhu et al. 2007). The improvement of nutritional quality trait is prominent objective to develop novel rice varieties with increased essential amino acid content, protein content and biofortification traits to reducing the nutritional deficiency. Masuda et al. (2009) transferred NAS gene of Hordeum vulgare to rice cultivar Tsukinohikari and by expression analysis target traits were they observed significant enhancement of accumulated 2- to 3-fold higher iron and zinc in polished rice grains. Zheng et al. (2010) observed 5-fold iron accumulation in polished rice grain through the over expression of endosperm specific endogenous rice NAS gene. Through the higher expression of 3 rice NAS homologous proteins, (OsNAS1, OsNAS2, and OsNAS3), Johnson et al. (2011) observed 2-fold increase in Fe and Zn concentration in polished rice. Similarly, Lee et al. (2009) observed transferred of NAS gene (OsNAS3-D1), increases the expression level of Fe content (2.9-fold), Zn (2.2-fold), and Cu (1.7-fold) compared to WT grain at seedling stage. Soumitra et al. (2012) observed 7.8 fold increases of Fe content in a line 276- 1-2 and six lines showed a 4.1 to 4.5 fold increment over NT control by over expression of ferritin gene. Recently, over-expression of barley genes related to phytosiderophore synthesis resulted in enhanced Fe and Zn concentration in rice unpolished and polished seeds (Masuda et al. 2008; Masuda et al. 2009).
Masuda et al. (2013) described different transgenic approaches used to introduce multiple genes, including ferritin under the control of endosperm-specific promoters, NAS over expression, OsSUT1 promoter-driven OsYSL2 expression, and barley IDS3 genome fragment, and significantly increased 2-fold, 3-fold, 1.4-fold, and 6-fold of Fe concentration as in polished rice seeds.In similar way earlier reports of Lucca et al. (2002) observed 2.2-fold of Fe concentration was increased in brown seeds of transgenic japonica cv. Taipei 309, Vasconcelos et al. (2003) Showed 3.7-fold increase in polished seeds of indica cv. IR68144 and Paul et al. (2012) identified 2.1-fold increase in polished seeds of indica cv. Pusa Sugandhi II. Enhanced expression of the three genes from the rice nicotianamine synthase family (OsNAS genes) also facilitated increase in Fe and Zn concentration of rice grain (Lee et al. 2009; Zheng et al. 2010; Johnson et al. 2011; Lee et al. 2011). These results suggest that, targeting multiple genes would be more successful in crop improvement.
The ever increasing demand of rice production with higher quality drives to the identification of superior and novel rice cultivars. In 2012, the global population increased to 7 billion and it continuously rising. Tester and Langridge (2010) have been estimated that a 70% increase in rice production by 2050 will be needed to meet the predicted demand of the growing global population. Therefore, an urgent need to accelerate crop breeding programme and to implement new management strategies that together can achieve sustainable yield increases with better nutritional grain quality traits in rice.
To meet these challenges, plant breeders and biotechnologist together has to explore efficient breeding strategies that integrate genomic technologies by using available germplasm resources to a new revolution in the field of plant breeding to better understanding of genotype and its relationship with the phenotype, in particular for complex traits. In this new plant breeding era, genomics will be an essential aspects to develop more efficient nutritional rich rice cultivars (Perez-de-Castro et al. 2012), which is necessary to avoid human health problems concern to mineral nutrition
Genomics approaches are particularly useful when dealing with complex traits, as these traits usually have a multi-genic nature and an important environmental influence. Sequenced rice genome has provided new technologies and tools in functional genomics, transcriptomics and proteomics of important agronomic traits in rice. Now a day, trends in molecular biology technology fully updated and using the different molecular approaches as Whole Genome SNP Array (Xing et al.2013;), Genome-wide association mapping (Zhao et al. 2011; Huang et al. 2010), genomic-based genotyping platforms and re-sequencing (Gao et al. 2013; Han and Huang 2013), Genome-guided RNA-seq (Loraine et al. 2013, Szczesniak et al. 2013), Map-based cloning approach (Salvi and Tuberosa 2005; Price 2006), transcriptome profiling (Kathiresan et al. 2006; Venu et al. 2011), genomics approahes (Mochida and Shinozaki 2011, Plocik and Graveley 2013) Next generation sequencing (NGS) technologies (James et al. 2013, Miyao et al. 2012, Uchida et al. 2011) massively parallel signature sequencing (MPSS) and sequencing-by-synthesis (SBS) (Venu et al. 2011) etc., effective strategy to understand molecular mechanism and their relation between the genotypes and phenotypic traits.
Using the recent breakthrough in sequencing technologies helps to explore relationship between genotype and phenotype with greater resolution. As the cost of sequencing has decreased, breeders and biotechnologist have begun to utilize next generation sequencing (NGS) with increasing regularity to sequence large number of populations of plants, increasing the resolution of gene of interest and QTLs discovery and providing the source for identification and development of nutrition quality traits in rice at the whole-genome level. Zhao et al. (2011) based on genotyping with 44,100 SNP variants across 413 diverse accessions of O. sativa were phenotyped for 34 traits including grain quality. Venu et al. (2011) performed a deep transcriptional analysis for the milling yield and eating quality trait genes in rice through MPSS and SBS. Many of the transcription factors have been identified in Cypress (282), LaGrue (312), Ilpumbyeo (363), YR15965 (260), and Nipponbare (357) which are involved in key grain quality-related genes.
Nutritional grain quality traits are economically important sources for the reducing of malnutrition in particularly developing countries. Therefore, consumption of right food in the right quantity of the right time (i.e age) is necessary to avoid malnutrition. The demand of global food production will increase as world population will reach to 10.5 billion in 2050.by 70 %, including 3 billion tons of cereals, which means global rice and maize production must double (FAO 2009) for increasing of world population will reach to 10.5 billion in 2050. Therefore, improving the nutritional value of rice is an essential to reduce the global health particularly children’s and women of micronutrient deficiency.
Through the conventional breeding approach, improving grain nutrition in rice is difficult as the breeding process requires significant time for the several rounds of selection and testing at various sites over different years. With the current gain in knowledge on the morpho-physiological traits and molecular basis helps to understand grain nutritional quality traits and mineral accumulation in different rice germplasm and the DNA information of specific genes will be useful to develop molecular markers.
The increasing availability of more accurate genetic markers, advanced breeding tools could be used, for selecting the presence or absence of specific alleles of genes known to play a role in nutritional grain quality traits in rice. In particular, MAS greatly accelerates the efficiency and precision of quality rice breeding programs and it is unaffected by environmental factors. While, transgenic approaches aids to enrich grain nutrition to desired level (concentration) comparatively lesser period than other means. The recent development of genomic technologies will be helpful to improve the nutritional quality in rice when it goes hand in hand with crop improvement breeding programme.
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