Virus Resistance in Rice

The genus Oryza of the Family Gramineae comprises 18 tropical and subtropical species, of which two species are cultivated as rice: O. sativa and O. glaberrima. Rice provides the staple food of 60% of mankind, and is cultivated in all tropical and subtropical countries. Oryza sativa is thought to have been domesticated in China before 6500 BC, in India between 2000 and 1500 BC and even earlier than 5000 BC in northeast India (Chauvet, 2004). Molecular markers strongly suggested that the asian rice has been domesticated twice independently to give rise to the so-called japonica and indica groups of varieties similar to subspecies in China and South-India, respectively (Second, 1982). A third domestication took place in West Africa probably around 1500 BC from the wild relative O. brevigulata (syn. O. barthii) to give the african cultivated rice species O. glaberrima which is isolated from O. sativa by reproductive barriers (Oka, 1958; Second, 1982). Rice is a natural host for 20 viruses and an experimental host for 17. About 16 viruses may seriously affect rice yield (Lapierre and Signoret, 2004). The distribution of each virus is generally restricted to only one of the continents in which rice is grown. Host plant resistance has been reported for several viruses. Intensive breeding programs have been carried out to obtain resistance to the main virus diseases: rice tungro viruses and Rice stripe virus in Asia, Rice hoja blanca virus in South-America, and Rice yellow mottle virus in Africa.