Genomics and Applied Biology 2019, Vol.10, No.2, 10-19
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provides 86 calories/100 gm kernels. Green ears of maize are used on large scale for roasting, and consumed as
food at dough stage. The mixed feed manufacturing industry is the largest industrial user of shelled grain. About
three-fourth of the mixed feed industries output is used for manufacturing poultry and dairy feeds. Maize grains
serve as raw material for manufacture of starch, syrup, dextrose, maltodextrin, lactic acid, ethyl and butyl alcohol,
acetone and whiskey. Besides, variety of food products e.g., corn meal, corn flakes, cake, cookies, sweeteners and
porridge are prepared from maize grains. General public including tribals also consume normal maize as food
mixture and green cobs. Improper amino acid balance caused malnutrition. For this purpose the lysine is added as
inorganic source for preparation of poultry and cattle feed.
2 Seed Proteins in Maize
Endosperm occupies major portion (82%) of the maize grain and it is surrounded by thin pericarp (6%); while the
rest constitutes the embryo (12%) (Watson, 1987). Endosperm serves as the major source of seed storage proteins.
The grains contain around 9% protein, intermediate between rice and wheat. Therefore, maize seed protein turns
to be a frontier area research for many years. In maize, zein (60%) followed by glutelin (34%) are the major
fractions of seed storage proteins (Leite et al., 1999), while albumin and globulin occur in traces (3% each).
Average Indian adults need 52 gm protein per day against the available quantity of 26-30 gm in daily diet. There is
a scope to increase the protein content as high as 18% by increasing the prolamine (zein) fraction in maize
endosperm (Dudley and Lambert, 1969), but unfortunately it consequently led to lysine deficiency.
A number of multigene families are meant for zein proteins (Lending and Larkins, 1989) which form protein
bodies on the rough endoplasmic reticulum (Larkins et al., 1993). Each zein polypeptide is synthesized by
differential structural gene (Zp). These zp genes follow simple inheritance and serve as members of a large group
of genes (upto 150). Zeins are rich in glutamine (21-26%), leucine (20%), proline (10%) and alanine (10%), but
deficient in important essential amino acids e.g., lysine and tryptophan leading to protein malnutrition. In maize
grains, α-zeins are the major prolamin fraction, while β-15kD, γ-16-27kD and σ-10kD zeins (Coleman and
Larkins, 1999; Leite et al., 1999) occur in traces. α and σ-zeins form the protein body core which is surrounded by
β and γ-zeins (Esen and Stetler, 1992; Lending et al., 1992). α-zeins are in fact, constituted of two major
sub-classes e.g., 19kD and 22kD zeins. Polymorphism of these zein fractions can be detected by SDS-PAGE of
total seed protein.
3 Development of the Magic Quality Protein Maize (QPM)-the Story
Nutritional deficiency of maize was documented prior to 1960s. There was a dearth need to alter amino acid
composition. Altering amino acid composition become difficult by conventional breeding technique. To address
this problem, quest for improved protein content was first attempted. But, this turns to be futile, as protein content
maintains inverse relationship with lysine and tryptophan content in seed. In 1963, Purdue university researchers
isolated
opaque 2
(
o
2
) and
floury 2
(
fl
2
) natural mutants (Mertz et al., 1964; Nelson et al., 1965) with altered
amino acid profile and composition of corn endosperm protein which resulted in two-fold tryptophan content
compared to normal maize.
Later, the pleiotropic effects of
opaque-2
mutation were known that resulted poor consumer preference in terms of
unpleasant taste, chalky, lighter and soft endosperm. This led to damaged kernel while harvesting, increased
susceptibility to pest and diseases. The mutants showed reduced cob weight and lower yield due to reduced dry
matter accumulation. These undesirable features seem to be the major hindrances in genetic improvement for
higher productivity with enriched nutritional quality. Subsequently, numerous modifiers togetherly contributing
kernel vitreousness came to limelight. However, the mode of expression and inheritance pattern of the
o2
modifiers are still unclear. Later, in mid 1980s, Surinder K. Vasal and Evangelina Villegas of CIMMYT used
modified backcrossing and recurrent selection to improve kernel hardness and grain yield by combining the
opaque 2
and its complex genetic modifier systems. This resulted in a special category of biofortified maize
named as “Quality Protein Maize” (QPM) for which Vasal and Villegas were conferred the world food prize for
the year 2000 on world food day (16 October). Thereafter, serious efforts were undertaken for development of
