Plant 178

Oryza sativa L. (Poaceae)



Rice feeds more people than any other plant on the planet. For at least 9,000 years people have depended on rice, have modified it to their lives and in turn been modified by the demands of rice. Different types of rice are adapted to the multiplicity of environments ranging from the tropics to the temperate latitudes of northern Japan, from the Cambodian lowlands to the Himalayas and from the deep waters of Bangladesh to the dry uplands of Nepal. Consequently, there is tremendous variation in the characteristics of rice.

Rice is an annual grass of two main types. The indica type has non-sticky, long grains and is adapted to warm, temperate climates. The japonica type has sticky, short grains and is adapted to damp, tropical climates. Over thousands of years, people have participated in vast communal engineering feats to terrace steep hillsides, dam fields and channel water for paddy fields. Although not essential for rice cultivation, flooding reduces weeding and encourages the fertilization of fields by the aquatic fern Azolla.

Trying to understand the origins of domesticated rice is a challenge. Available evidence suggests the indica and japonica types diverged well before rice was domesticated; today they are so different they cannot interbreed. However, both types do interbreed with rice's wild ancestor, Oryza rufipogon, a perennial species distributed across Asia. The scientific debate continues about whether domestication happened once or on multiple occasions.

Rice is nutritionally incomplete. Cleaned rice grains, for example, are deficient in vitamins such as A and B1, and minerals such as iron. Vitamin A deficiency has enormous public health consequences; between 250,000 and 500,000 vitamin A-deficient children go blind each year, and half die within twelve months. Strategies for alleviating vitamin A deficiency have met with varying degrees of success but one proposal has been the development of Golden Rice. Golden Rice has high levels of a vitamin A precursor, which was achieved by introducing appropriate genes from other species, such as daffodil and maize, into rice. Golden Rice is not the complete solution to vitamin A deficiency worldwide, but Golden Rice varieties are being used in some current rice breeding programmes.

Another international programme that is using biotechnological approaches for the development of high-yielding rice varieties aimed at smallholder farmers is the C4 Rice Project. Changing the photosynthetic metabolism of rice is predicted to have considerable yield benefits including increased photosynthetic, nitrogen-use and water-use efficiencies.

Further reading

Beyer P 2010. Golden rice and 'golden' crops for human nutrition. New Biotechnology 27: 478-481.

Sweeney M and McCouch S 2007. The complex history of the domestication of rice. Annals of Botany 100: 951-957.

Wang P et al. 2016. Finding the genes to build C4 rice. Current Opinion in Plant Biology 31: 44-50.

Stephen Harris