Authors: Varun Kumar Badaya, Amit Dadheech and Meenakshi Dhoot
Department of Plant Breeding and Genetics, Rajasthan College of Agriculture, Maharana Pratap University of Agriculture and Technology, Udaipur-313 001, Rajasthan, India.
Email: varunbadaya67@gmail.com
Introduction
Maize is a major cereal crop for both livestock feed and human nutrition, worldwide. With its high content of carbohydrates, fats, proteins, some of the important vitamins and minerals, maize acquired a well-deserved reputation as a ‘poor man’s nutricerea. Several million people, particularly in the developing countries, derive their protein and calorie requirements from maize. The maize grain accounts for about 15 to 56% of the total daily calories in diets of people in about 25 developing countries, particularly in Africa and Latin America (FAO, 1992).
Cereal proteins, however, have poor nutritional value for monogastric animals, including humans, because of reduced content of essential amino acids such as lysine, tryptophan and threonine. Cereal proteins contain on an average about 2% lysine, which is less than one-half of the concentration recommended for human nutrition by the Food and Agriculture Organization (FAO) of the United Nations (WHO, 1985). Therefore, healthy diets for humans and other monogastric animals must include alternate sources of lysine and tryptophan. From the human nutrition view point, lysine is the most important limiting amino acid in the maize endosperm protein followed by tryptophan.
What is QPM?
Quality Protein Maize (QPM) is a cultivar that possesses almost double quantity of lysine and tryptophan than the normal maize variety.
Storage proteins in Maize
The maize grain largely consists of endosperm that is rich in starch (71%). Both the embryo and endosperm contain proteins but the germ proteins are superior in quality as well as quantity. Zeins are a class of alcohol soluble proteins that are specific to endosperm of maize (Prassana et al. 2001). These zeins consist of albumins, globulins, glutelins and prolamins and constitute about 50-60% of maize proteins. The prolamins are rich in proline and amide nitrogen derived from glutamine. All prolamins are alcohol soluble (Shewry and Halford, 2002). The prolamins of maize grain are called zeins and consist of one major class (α-zeins) and three minor classes (β, γ and δ). The zein fraction α is rich in cystein while β and γ fractions are rich in methionine. 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. Other proteins such as globulins (3%), glutelins (34%) and albumins (3%) are collectively called non-zeins. The zein fraction in normal maize normally contains smaller amounts of lysine and tryptophan while the non-zein protein fraction is balanced and rich in lysine and tryptophan (Vasal, 2000).
Fractions of Zein protein & its contents
| Protein | kind | Molecular Weight (kD) | Protein rich | Protein deficit |
| Zein (alcohol soluble) | a | 22 & 19 | Glutamin, Proline, Leucine, | Lysine & Tryptophan |
| b | 14 | Methionine, Cysteine | Lysine & Tryptophan | |
| g | 58, 27 & 16 | Cysteine | Lysine & Tryptophan | |
| d | 10 | methionine | Lysine & Tryptophan |
Breeding Strategies
In 1920, a naturally occurring maize mutant was identified in Connecticut maize fields in USA that had soft and opaque grains and was named as opaque-2 (o-2). In 1960, Nelson and Mertz worked with the mutant lines at Connecticut Experiment station to identify maize lines with improved protein profile (Krivanek et al. 2007). In 1961, researchers at Purdue University observed that mutant lines that were homozygous for o-2 allele had significantly higher lysine (almost double) in endosperm compared to normal maize. These discoveries aroused great enthusiasm and hope among researchers towards genetic manipulation of protein quality in maize and resulted in discovery of various other mutant types that had altered amino acid composition. These include the floury-2 (fl-2), Mucronate (Mc) and Defective endosperm B30 (DE B30). The opaque mutants are recessive (o1, o2, o5, o9-11, o13, o17), the floury mutation is semi-dormant (fl-1, fl-2 and fl-3) where as Mucronate and defective endosperm are dominant mutations. The opaque mutations affect the regulatory network whereas floury, Mucronate and Defective endosperm B30 affects the storage proteins (Gibbon and Larkin, 2005).
The improved protein quality of such mutants was apparently due to increase in proportion on non-zein fraction that is rich in lysine and tryptophan and repression of zein synthesis. Each of the zein polypeptide is a product of differential structural gene (Zp). These zp genes are simply inherited and are members of a large group of genes (upto 150). Epistatic interactions have also been reported among various regulatory mutants. Thus, o-2 and o-7 are epistatic over fl-2, whereas o-2 and Mc have synergistic effect. The reduction in levels of zeins by various mutant allele is accomplished either by reduction in levels of various zein sub-units, rate of accumulation of zeins, increase in methionine content and effect on timing and pattern of storage protein accumulation.
The only mutant gene conditioning protein quality in maize is o-2 which encodes a defective basic-domain-leucine-zipper transcription factor and has been mapped to short arm of chromosome-7. In its dominant form, o-2 regulates the expression of 22 KDa α-zeins. The increased level of lysine in o-2 mutants is due to higher levels of an elongation factor of protein synthesis (eEF1A). This factor though itself being rich in lysine (10%) but accounts for only 2% of lysine in the endosperm. Therefore, it is evident that the higher expression of eEF1A is accompanied by a number of transcription factors. QTL mapping studies have revealed linkage between eEF1A and genes encoding zein storage proteins (Villegas et al. 2004).
The recent studies using RNA interference based silencing of 22 and 19 KDa RNAi lines more profoundly caused opaque phenotype as compared to 19 KDa components. This is probably due to RNA interference induced down-regulation of 22 KDa α-zeins (Segal et al. 2003) and 19 KDa α-zeins (Huang et al. 2005) resulted in higher lysine content (upto 16-20% more lysine) much below the o-2 mutants. Double stranded RNA (ds RNA) had been used as a refined approach to simultaneously down regulate both 22 KDa and 19 KDa α-zeins resulting in increase in lysine from 2.83 to 5.62% and tryptophan from 0.69 to 1.22%.
Achievements through QPM
In India, QPM research was initiated under AICMIP in 1966 and resulted in three o-2 composites namely Shakti, Rattan and Protina. Later on using endosperm modification system, mo-2 composite “Shakti-1” was released in 1998. Later on under NATP, two hybrids “Shaktiman-1” and “Shaktiman-2” were also released using CIMMYT inbreds as parental lines. In China, a number of high yielding QPM hybrids are under cultivation covering an area of about 1000 hectares. It is expected that by 2020, about 30% of maize area in China will be under QPM cultivars (Gill, 2008).
References:
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About Author / Additional Info:
I am currently pursuing Ph.D in Plant Breeding and Genetics from RCA, MPUA&T, Udaipur, Rajasthan (INDIA)