Rice is the world's most commonly used cereal food, feeding half of humanity. However, rice production will have to double within 20 years from now to meet the needs of a growing population.
Two species are used for cultivation, one Asian and the other African. The Asian species gives much stronger agronomic performances, but the African one is more rustic, more resistant to pathogens, more tolerant to drought and soil salinity.
With the aim of transferring these properties to Asian rice, IRD scientists and their research partners1 are seeking to overcome the sterility between the two species2. They used genome sequencing to compare the structure of a portion of chromosome, identified as the factor behind the reproductive barrier. These investigations, the first results of which were published recently in the journal PLoS One, have led to the definition of genetic markers allowing more rapid development of fertile lineages of improved Asian rice.
Rice is the third cereal in the world after wheat and maize, but the leader as far as human nutrition is concerned. It is the staple food for half of humanity. The 155 million hectares of rice fields in the world, mainly in the tropical regions, produce about 660 million tone of rice pear year. But for it to remain the cornerstone of food security, production will have to double by 2030.
Africa and Asia the cradles of rice
The rice which is cultivated belongs to two distinct species. One, originating from Asia, has been grown for 7 000 years and is currently produced all over the world. The other is restricted to West Africa, where its wild ancestor was domesticated.
Although Asian rice, with specific name Oryza sativa, gives greatly superior agronomic performances, the very basis of all the world's commercialized production, the African one, Oryza glaberrima, holds genetic potential which is still poorly known and scarcely tapped to improve cultivated varieties. This rustic species has different qualities, indeed highly significant ones, generally missing from O. sativa. They possess strong resistance to the rice yellow mottle virus which causes substantial losses in O. sativa harvests in Africa, to attack by several species of nematodes, small parasitic worms living in the soil, and are more tolerant to certain harsh environmental conditions such as water stress or soil salinity.
Sterility of crosses
To transfer these valuable properties to Asian rice and further improve its yields, IRD scientists and their research partners1 wish to create interspecific bridges. The two species are intersterile2: they originate from the same wild Asian parent species, part of whose population appears to have become isolated in Africa about one million years ago, as a result of the aridification of the Sahel. This led to the emergence of a reproductive barrier, one of the central mechanisms of evolution which determines the generation of new species.
This means that although crossing O. sativa and O. glaberrima can produce a new plant, this will be sterile: the male gametes (in pollen grains) are not viable. Conversely, the female gametes contained in the flower can turn out to be fertile in some rare individual plants. Such plants have been the source for example of rice varieties such as NERICA3 in the 2000s, but only after a long laborious series of selections and crossing procedures.
Gene for sterility identified
Why are some female gametes of hybrids fertile, giving descendants which are also fertile while others do not? In a previous study, the scientists identified the portion of chromosome responsible for this sterility, region S1, using fine mapping of the genome, conducted in partnership with the International Center for Tropical Agriculture (CIAT) at Cali in Colombia. The same gene was found to be involved in both the male and female sterility.
The latest work, published in the journal PLoS One, compared the structure of the S1 region in the two rice species. The geneticists observed that few genetic changes had occurred in the course of the evolution of O. sativa and O. glaberrima. Although they have very different morpho-physiological traits that stem from domestication, their wild cousins became distinct only very recently in the scale of evolution: their speciation is at the most traced back to 600 000 or 700 000 years Before Present. The research team succeeded in identifying the genetic markers which could help to overcome the reproductive barrier.
These investigations will save scientists from the long laborious work of selection and hence accelerate the development of fertile lineages of improved Asian rice. In the long term, varieties better adapted to African conditions could be cultivated, where the Asian species makes up more than 90% of the rice-growing areas.
1. This research was conducted in partnership with the CIAT (International Center for Tropical Agriculture) at Cali, Colombia.
2. A species is a group of individual organisms that can reproduce between them and give rise to fertile descendants. Two distinct species cannot reproduce together.
3. Distributed by Africa Rice Centre (AfricaRice) at Cotonou in Benin.
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