Improving the Content of Essential Amino Acids in Crop Plants: Goals and Opportunities

Abstract

The inability of humans and many farm animals to synthesize certain amino acids has long triggered tremendous interest in increasing the levels of these so-called essential amino acids in crop plants. Knowledge obtained from basic genetic and genetic engineering research has also been successfully used to enrich the content of some of these essential amino acids in crop plants. Among the essential amino acids, Lys, Trp, and Met have received the most attention because they are most limiting in cereals (particularly Lys and Trp) and legumes crops (particularly Met), which represent the major sources of human food and animal feed worldwide. Enriching crop plants in essential amino acids has both economical and humanitarian interest. In developed countries, the interest is mostly for the livestock feeding industry because farm animals generally provide sufficient amount of essential amino acids for human diets. In developing countries, where plants directly account for the majority of the food, the interest is both humanitarian and economical. So far, the success of genetic approaches has been mostly restricted to maize (Zea mays) by generating quality protein maize (QPM) cultivars, which are enriched in Lys and to some extent Trp in their seeds. However, genetic approaches have resulted in relatively limited success in other crop species. This is mostly due to limited availability of genetic resources for plant breeding, and the fact that genetic traits for high contents of Lys, Trp, or Met are generally associated with abnormal plant growth because these traits do not operate in a seed-specific manner. In contrast, results from genetic engineering research appear to be more promising, particularly because this approach allows seed-specific expression of specific traits of interest, using seed-specific promoters. In fact, one high-Lys maize cultivar, LY038, developed by genetic engineering, represents the first genetically modified (GM) crop with high nutritional value to be approved for commercial use in a number of countries. The potential to increase the contents of Trp and Met in a seed-specific manner have already been proven successful in basic research studies. Another advantage of genetically engineered traits is that they can be transformed into multiple plant species and genotypes and function synergistically with many other agronomically important traits. These genetic engineering approaches were generally aimed at tailor-made improvements of essential amino acid metabolic pathways and expressing native and genetically engineered proteins enriched in essential amino acid contents. However, improvements of metabolic pathways by genetic engineering also requires a detailed understanding of how these pathways interact with regulatory networks that fine tune plant development. These are now beginning to be elucidated by modern systems biology approaches, including transcriptomics, proteomics, and metabolomics. Due to space limitation, we focus this review only on approaches that have been extensively studied and proven suitable to improve the nutritional quality of food and feed. We also only cover research associated with nutritional improvements for monogastric mammals, namely, human and certain farm animals, particularly poultry and swine. These approaches are not as important for ruminant livestock, such as beef, because these animals require the presence of the essential amino acids in proteins that are resistant to rumen proteolysis. Approaches suitable for ruminant animals are discussed in the following reviews (Galili et al., 2002; Amir and Tabe, 2006). Maize is one of the most important cereal crops, providing between 50% and 70% of the dietary protein for humans, depending on geographical distribution. It is also one of the major crops used for feeding farm animals, particularly poultry and swine. Since maize seeds are very low in Lys, a major effort was initiated at the mid 20th century to identify high-Lys corn varieties by genetic approaches. These efforts resulted in the discovery of the high-Lys opaque2 mutant (Mertz et al., 1964; Mertz, 1997), which contains low levels of the Lys-poor seed storage proteins (called zeins) and a compensatory increase in Lys- and Trp-rich, non-zein, seed proteins as well as free Lys and Trp, compared to normal maize. The opaque2 mutant seeds, together with minerals and vitamins, were proven to significantly increase growth in rat feeding trials (Mertz et al., 1964), and they were found to have 90% the value of milk protein when fed to Guatemalan children (Bressani, 1966). A diet based solely on opaque2 flour was further shown to cure children who suffered from the protein deficiency disease kwashiorkor (Harpstead, 1971). Success with opaque2 maize stimulated extensive research to identify similar mutants in other cereals, including barley (Hordeum vulgare; Munck et al., 1970; Doll et al., 1974), and sorghum (Sorghum bicolor; Singh and Axtell, 1973). Despit