**5. Efforts in enhancing QPM production**

### **5.1 Genetics of QPM**

QPM contains the mutation at opaque-2 loci, which changes the protein composition of the maize endosperm, resulting in increased concentrations of lysine and tryptophan [33]. The increase in concentration (60–100%) of these two essential amino acids increased the biological value of QPM (80%), when compared to normal maize (40–57%) [34]. The biological value of cow milk protein was about 90%, whereas QPM has about 80% value [35].

QTL mapping of o2 modifiers insights that it encodes that the 27-kDa - zein protein and it is observed on chromosome-7 long arm [36]. The function of the 27-kDa zein protein in the formation of vitreous endosperm was revealed when the protein quantity increased threefold in QPM compared with soft opaque-2 mutant [37]. An increase in the number of zein proteins and their compaction between starch grains is partially involved in endosperm modification in QPM [38]. The o2 modifier genes have complexity in inheritance [12]; it reveals that several other loci control the formation of a vitreous kernel in QPM. For identifying the other factors linked to the endosperm modification, [39] performed a proteomic study of the non-zein proteins, and it was observed that the quantity of a starch synthesis enzyme and the amylopectin branching structure are changed in QPM. It is supported that QPM starch expands more than normal maize. It reveals that suppression of the opaque endosperm in QPM was associated with the starch grain properties.

Maize protein quantity can be enhanced with the opaque-2 (o2) mutation, which increases the lysine and tryptophan levels by decreasing the synthesis of zeins. The QPM utilization mainly restricts due to chalky and soft texture kernels [3]. The quality protein maize was developed based on introgression of opaque-2 QTLs, called o2 modifiers which convert to hard and vitreous endosperm [40]. QPM development has significantly improved the status of nutrient-deficient people who suffer from malnutrition and protein energy deficiency in the developing countries [41].
