Significance of nutritional improvement of rice endosperm
Together with wheat and maize, rice is one of the most important food crops for humans and the main staple food for half of the world population. It provides around 21% of per capita energy and 15% of protein to global human populations (IRRI, http://irri.org/about-rice/rice-facts/rice-basics). Around 3 billion people, mostly in Asia, depend on rice for 35–59% of their caloric intake (Meng et al., 2005) and in many developing countries the dietary contribution of rice is substantially higher. For example, in Cambodia, Myanmar, Bangladesh and Vietnam, rice provides over 70% of dietary energy (Kennedy et al., 2003). This contribution of rice is a success of breeders who over thousands of years made rice productive, high yielding, and an affordable calorie source. However, progress towards improving the micronutrient content of rice has been limited, partially because it was the not the focus of rice breeding until recently.
Starch accounts for approximately 90% of total dry matter in rice seeds, followed by a fraction of storage proteins. Furthermore, rice grains are polished since the oil-rich aleurone layer turns the seed rancid upon storage and therefore makes rice unsuitable for consumption. The essential micronutrients which are almost exclusively stored in the husk, aleurone and embryo of rice, are removed during this process. Consequently, polished rice grains contain only small amount of key micronutrients or lack them entirely (Kennedy et al., 2003 and Lucca et al., 2006). Importantly, many rice consumers are among the world's poorest whose diet is largely restricted to rice because it is filling and the most accessible and affordable food. Considering the above facts and the severity of widespread micronutrient malnutrition, improving rice grain (specifically endosperm) nutritional quality would have a significant impact on global human health. This calls for new, high-yielding and high-quality rice varieties containing higher levels of bioavailable vitamins, minerals and essential amino acids for nutrition.