Quality protein maize (QPM): Genetic basis and breeding perspective

Abstract

Major fraction (60%) of seed storage protein in maize is zein which determines the quality of food and feed. Zeins comprise four subfamilies e.g., α , β, γ and δ zeins. Among these, α-zeins are the major prolamin subunits in maize. α-zeins are rich in glutamine, leucine and proline but, deficient in essential amino acids like 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. RNAi studies proved down regulation of 22kD zeins than the 19kD component as the biochemical basis of QPM phenotype. However, the opaque-2 mutation made the endosperm chalky and soft resulting damaged kernel while harvesting, poor germination, increased susceptibility to pest and diseases, inferior for food processing and in general reduced yield. Later, combining opaque-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, and biochemical analysis of lysine and tryptophan content is expensive; 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). In India, Vivek QPM-9: a hybrid of two QPM introgression lines is being widely used for commercial cultivation.

Breeding for Quality Protein Maize (QPM) Varieties: A Review

“…In 1963, researchers at Purdue University in USA discovered that opaque-2 (op2) created grain proteins in the endosperm nearly twice as those found in normal maize [29]. Endosperm of maize grain is the store house of storage proteins [13,31]. Homozygous maize varieties with two copies of the mutation (recessive o2 allele) have substantially higher lysine (69%) in grain endosperm compared to normal maize [29].…”

“…New inbred lines are developed from these segregating families [5]. The opaque-2 mutation is reported to be associated with numerous modifiers which together behave as a polygenic trait [13]. In general, QPM materials had 10-15% less grain yield with the kernel appearance being dull and chalky, slower drying, as well as the greater kernel rots and greater vulnerability to stored grain pests [2].…”

“…Quite a number of studies have been conducted on QPM regarding the improvement of the nutritional value and diseases resistance of QPM over the past years in breeding programs around the world [13,[29][30][31][32][33][34]. Increasing the protein content and investigating genetic variability among QPM genotypes and normal endosperm maize varieties have been the major research focus.…”

Incorporation ofopaque-2into ‘UMI 1200’, an elite maize inbred line, through marker-assisted backcross breeding

“…Zein is rich in glutamine (21-26%), leucine (20%), proline (10%) and alanine (10%), but deficient in important essential amino acids, for example, lysine (2.0% in the protein) and tryptophan (0.4% in the protein). The deficiency in these two amino acids essential for building proteins in humans leads to protein malnutrition [4]. To solve the problem of malnutrition, a potential solution is sought in the adoption of quality protein maize (QPM) varieties possessing balanced protein due to higher lysine and tryptophan.…”

Quality Protein Maize: An Alternative Food to Mitigate Protein Deficiency in Developing Countries

“…Zein is a class of prolamin proteins that are mainly present in maize. All the zein polypeptides are products of different structural genes [21]. Most of the prolamin genes have a promoter element called the endosperm or prolamin box.…”