Author(s):
In maize, zeins are the main protein components of seed stores. To alleviate protein deficiency, protein content can be increased to as high as 18% by increasing the prolamine (zein) fraction inmaize endosperm but unfortunately it consequently led to lysine and tryptophan deficiency. Zeins are the prolamins of maize grain which are soluble in an alcohol having one major class (α-zeins) and three minor classes (β, γ, and δ). These four types constitute about 50- 70% of maize endosperm and are essentially rich in glutamine, leucine and proline and poor in lysine and tryptophan causing malnutrition. The opaque-2 (o2)-a natural recessive mutation in maize led to nearly double the lysine and tryptophan content in endosperm due to a decrease in the synthesis of zein proteins and increase in the other seed protein bound lysine and tryptophan. The proportion of lysine and tryptophan in the total portion of protein were found to be almost double in QPM materials (4.1% and1%, respectively) than in non-QPM (2.7% and 0.6%, respectively). QPM also showed a corresponding increase in tryptophan content, which doubles the biological value of ordinary maize protein. Breeding for improved protein quality in maize commence in the mid-1960s with the discovery of mutants, such as opaque-2 (o2 However, successful utilization of these mutants is not achieved due to some adverse pleiotropic effects. So, researchers use two genetic systems Exploiting double-mutant combinations and Simultaneous use of o2gene and the genetic modifiers of the o2 locus. However, there was certain drawback like double mutant combination were not always vitreous and yield was severely affected due to the sum total of independent negative effects of two mutation. Combiningopaque-2 allele with its desirable genetic modifiers made it possible to breed QPM genotypes having hard kernel with high lysine and tryptophan content. Since, opaque-2 is a recessive mutation and endosperm specific, conventional backcross breeding alone is inefficient for the nutritional enrichment of maize. However, use of opaque-2 gene specific markers provided excellent opportunities for conversion of elite normal inbreds to homozygous o2/o2 forms through marker assisted selection (MAS)
