Grains, including cereal and grain legumes, have been the most important suppliers of energy and protein, and the main food source for mankind for centuries. Grains have been considered as the most prolific food source since the emergence of sedentism. The dried grains are not perishable and thus can be easily stored for a long time. Another advantage of cereals and legumes is that the calories, protein and other nutrients are concentrated and packaged in relatively small volumes.
Cereals including rice, wheat and maize by far have provided most of the calories, protein, B-vitamins and minerals. They are a staple for more than 6.5 billion inhabitants around the world, and also the main ingredients for animal feeds (Serna-Saldivar, 2010). Cereals are rich in starch, B-vitamins except B12 (Serna-Saldivar 2003), but are low in the protein quality which is commonly determined by the digestibility rate and the content and balance of essential amino acid. Cereals contain about 10% protein on average with the digestibility rate ranging from 80% to 90%. Cereal protein is rich in essential Sulphur-containing amino acids (SCAA), such as methionine, cystine and cysteine as well as in tryptophan, but lack lysine, an important essential amino acid for protein synthesis and proper growth. Legumes can complement in terms of the constitution of amino acids because they are rich in lysine but low in SCAA. Therefore, protein quality can be improved when legumes and cereals are combined in diets containing less or no meat. A ratio of 65:35 of cereals to legumes in grams of dry grains was proposed for the nutritional purpose in a balanced diet (Maphosa and Jideani, 2017).
Legumes are the second most important food source after cereals and are used as an alternative source of protein. They have higher protein content than most of the plant-based food. The protein content in legumes is about twice the content of cereals. Particularly, 20%-45% of the grain legume species are rich in essential amino acid lysine. Legumes also provide abundant dietary fibre, unsaturated fats, complex carbohydrates, vitamins and minerals. Furthermore, legumes have become increasingly attractive to health-conscious consumers because of the beneficial bioactive compounds they possess, most of which have antioxidant activities with health attributes such as antiatherogenic, anticarcinogenic and hypoglycemic properties.
The selection of cereals and legumes as the most important suppliers of energy and protein might have been determined by their genomic characters. All cereals and many legumes are annual plants and are more adapted to various environmental and climatic conditions. As a result, cereals and legumes have been grown in most of the ecosystems around the world. Particularly for cereals, more than 70% of the farm land was planted with cereal crops. They have been found in all continents and can be planted in tropical or subtropical areas that are characterized by high temperature, as well as semidesert areas.
Plants can be divided into three groups according to the photosynthesis pathway, C3 plants, C4 plants and CAM (Crassulacean acid metabolism) plants. Cereals have both C3 and C4 plants while legumes have only C3 plants. The C3 plants form three-carbon compounds in Calvin-Benson metabolic cycle and the C4 plants form four-carbon compounds in an acidic Crassulacean metabolic pathway. The C4 plants (maize, sorghum and millets) are more efficient in CO2 assimilation and water utilization. These characters give C4 plants the capability to cope with high light intensity and tolerance to the high temperature and low oxygen. The C4 crops could be key for the future because of the limited fresh water resources and the ongoing increasing of CO2 level in the atmosphere, which will undoubtedly lead to the increase of temperature.
Legumes are considered to be the most important supplier of plant protein, as an alternative to meat. The high content of protein in legumes might be associated with their symbiotic relationship with nitrogen-fixing bacteria, which converts unusable nitrogen gas into ammonium, which is further incorporated into proteins in plants.
It is estimated that by year 2050, the population on the planet will be at least 9.7 billion (FAO, 2018). To meet the demand of food of such a huge population, the annual production of cereals will need to be increased to about 3 billion tonnes from 2.1 billion today. Given the limitations in land and water resources, it is likely that the additional amounts of food needed in the coming decades will have to be produced mainly through yield increases, rather than through major expansions in cultivated areas. Genetic improvement of the crop species only accounts for 50 to 60 percent of the increase in yield, the rest of which is attributed to application of agro-chemicals and agricultural management (Oerke, 2006; Serna-Saldivar, 2010). It is now recognized that the gains in food production were often accompanied by negative effects on agricultural resources, including land degradation, salinization of irrigated areas, over-extraction of groundwater, the build-up of pesticide and herbicide resistance and the erosion of biodiversity for food and agriculture.
The genetic improvement of crops highly depends on the availability of genomic resources and technology innovations. Both Poceae and Leguminosae are large families, with more than 10,000 and 18,000 species, respectively. However, by far only a very limited number of these species have been researched and bred for improved food production. Underutilized crops are also known as orphan crops or neglected crops, most of which are superior in nutrients and flavors. Some of the underutilized legumes yield more than common legumes. Many underutilized legumes are known to have significant amount of bioactive compounds beneficial for human health and can be exploited in functional medicine. Most of these underutilized crops are cereals and legumes that are planted by small holders in remote regions in developing countries, the direct utilization of these crops can complement the yield of grains and help in mitigating the malnutrition problem in these regions. More importantly, the integration of the genomic resources of these underutilized crops into common cereals and crops can potentially help develop new varieties with higher yield and higher content of proteins as well as other essential nutrients.
Cereals and legumes not only provide energy and protein to humankind, but also provide possible solutions to future agriculture which is severely challenged by the increasing populations and changing climates. The clade of cereals have both C3 and C4 species, providing a good framework for scientists to study the evolution and formation of C4 pathway which could be more beneficial in coping with the changing climate in the future, such as higher temperature, higher light density and higher content of CO2 in the atmosphere. The high content of protein in legumes might be attributed to their symbiosis with nitrogen-fixing bacteria. The study of legumes could help uncover the evolution of this astonishing trait. The collection and study of diversified germplasms of cereal and legumes aided by modern advanced technologies would help accelerate the development of future crops with all these desirable traits incorporated.
Global grain genomic research program (G3RP) aims at developing future environment-friendly crops with higher yield and nutrients by collecting and profiling all the genomic resources of cereals and legumes around the globe. To achieve this goal, G3RP is divided into several parts, 1) global diversity of grains, 2) genomic innovation of desirable traits, 3) crop development through the utilization of neglected genomic resources.
1. FAO.2017. The future of food and agriculture – Trends and challenges. Rome.2
2. FAO. 2018. How to feed to the world in 2050. Rome.3
3. Oerke, E.C. 2006. Centenary review. Crop losses to pests. J. Agri. Sci 144:31-43.4
4. Sergio O. Serna-Saldivar. 2010. Cereal grains: properties, processing and nutritional attribute.5
5. Serna-Saldivar, S.O. 2003. Cereals: dietary importance. Pages 1027-1033 in Encyclopedia of Food Sciences and Nutrition. Second edition. B. Caballero, L. Trugo, and P. Finglas (eds.). Academic Press, London.