Transgenic high-lysine rice – a realistic solution to malnutrition?

Lysine is considered an 'essential amino acid' required at sufficient levels to prevent malnutrition and serious diseases, particularly in developing countries. It is mostly obtained from various plant foods. In the current issue (pages 4285–4296) Yang and colleagues report on successful stimulation of lysine biosynthesis and suppression of its catabolism in transgenic rice plants without changing the plant phenotype. This approach led to the production of high-lysine rice plants, rendering them as nutritionally favourable crops. Lysine is considered the first limiting essential amino acid in cereal and legume crops – i.e. it is present in the smallest quantity (Galili et al., 1994). This restricted content significantly reduces the nutritional values of these crops to 50–70% compared with those containing more balanced levels (Galili and Amir, 2013). As an 'essential amino acid', not produced in the bodies of humans and farm animals, lysine must be obtained from other sources. It is quite extensively present in livestock-derived foods, such as meat, eggs and cheese. However, where diets rely on plant-derived foods – which is the case for huge populations living in poverty in developing countries – people suffer from insufficient lysine levels, leading to vulnerability to disease, decreased levels of blood proteins and retarded mental and physical development in young children (Galili and Amir, 2013). To prevent this, enriching the content of lysine in those crop plants which serve as the major sources of human foods and livestock feed in these countries is essential. Among the cereal crops, rice (Oryza sativa) is a stable source of calories and protein intake for approximately one-third of the world's population (Kusano et al., 2015). Lysine metabolism in plants has been studied for over 50 years, since the discovery of the maize high-lysine mutant opaque-2 (o2), which contains low levels of lysine-poor seed storage proteins (zeins) and consequently an increased lysine and tryptophan content compared with the wild type (Mertz et al., 1964). However, enrichment of lysine levels in crops using classical genetics and breeding approaches is difficult because: (i) lysine synthesis is highly negatively regulated by a feedback inhibition loop in which lysine feedback inhibits the activity of dihydrodipicolinate synthase (DHPS), the first enzyme of the lysine biosynthesis pathway, slowing down its synthesis (Box 1); and (ii) lysine is efficiently degraded by its catabolism into the tricarboxylic (TCA) cycle, a pathway initiated by the bi-functional enzyme LKR/SDH (Box 1), which exhibits both lysine-ketoglutarate reductase (LKR) …