Molecular Interventions for Enhancing the Protein Quality of Maize

Abstract

Maize, one of the three most popular cereal crops of the world, globally contributes 15 % of the protein and 20 % of the calories derived from food crops in the world’s diet. However, cereals do not provide a nutritionally balanced source of protein. For nutritional security, it is necessary to adopt a genetic enhancement strategy in which essential amino acids are either incorporated or increased in grain protein to alleviate hunger, increase income, and improve livelihood. Quality protein maize (QPM) is having high nutritive value of endosperm protein with opaque2 (o2) mutation leading to 60–100 % increased content of lysine and tryptophan. The lysine value of o2 maize is 2.5–4.0 g/100 g of endosperm protein, which is more than twice that of the normal maize (1.3 g lysine/100 g protein). International Maize and Wheat Improvement Center (CIMMYT), Mexico, played a significant role in the development of QPM maize. The breeding of QPM involves three genetic systems: (i) the recessive mutant allele of the o2 gene, (ii) the endosperm hardness modifier genes, and (iii) the amino acid modifier genes influencing free amino acid content in the endosperm. Due to recessive nature of the o2 gene, complex action of modifier genes, and presence of amino acid enhancer genes, the use of DNA marker-assisted selection (MAS) accelerated the selection efficiency and expedited the development of new QPM cultivars. Using a combination of MAS and phenotypic selection techniques, a single cross, short duration Vivek QPM 9 hybrid was developed and released in 2008 by Vivekananda Parvatiya Krishi Anusandhan Sansthan, Almora, India. Alternatively, manipulating the plant lysine metabolic pathway provides possible enzyme targets for genetic engineering to increase free lysine content in corn grain. Furthermore, RNA interference (RNAi) has been used to specifically suppress α-zein production in transgenic corn, resulting in a doubling of the lysine content of corn grain. QPM is likely to gain wider acceptance if QTLs for kernel modification, and enhancers for amino acids are fine mapped to develop markers to follow MAS for vitreous kernels and high levels of lysine. However, the major constraints in adoption of QPM hybrids are contamination with normal maize pollen in field, resulting in erosion of the trait in farmer-saved seed system. It is essential to give training on good seed production practices to the local communities and development of linkage between the seed producers, farmers, and the industry for sustainable higher nutritional benefits of QPM in the long term.