Establishment, Curation, And Utilization Of The World Rice Collection
USDA/ARS
The USDA-ARS World Rice Collection consists of over 19,000 accessions and 12 species of the genus Oryza are represented. The majority of rice that is grown for human consumption comes from just one of these species, Oryza sativa. The other species are described as wild relatives of rice and generally have undesirable traits like seed shattering and poor grain yield. However, research has shown that these wild relatives also harbor genes, that when crossed with O. sativa, can enhance yield, disease and insect resistance, and tolerance to some environmental stresses.
Rice is not indigenous to the USA, thus the original rice collection came from exploration trips and donations from countries around the world where rice is naturally found. The first accessions trace back to seed collection trips from over a century ago and the seed has been maintained ever since. Rice has been grown in the USA since pre-colonial days, however it was not until the development of mechanized planting and harvesting equipment that significant acreage became established in the mid-south and California, where it is still grown today. Seed of rice accessions from other countries was the basis for establishing the USA rice industry.
These genetic resources have proven valuable to the USA because they have been a source of novel traits and genes that have increased the productivity and the value of the USA rice crop. Here are some examples of how they have been used in public breeding programs.
The rice variety, Jojutla, grown in Mexico was found to have excellent parboiling quality. Parboiled rice is what is most commonly used in large restaurant chains but can also be found in the local grocery store.
To produce parboiled rice, the harvested paddy rice is cooked with the husk still on the grain. Once the grain is milled, it has a nice golden color because of having been cooked with the husk on. The reason why parboiled rice is desired by the restaurant and canned soup business is because it has uniform cooking quality and maintains its integrity and grain shape after cooking (i.e. does not get mushy).
The USA parboiling industry desired to have this superior parboiling quality in varieties that were grown in the USA. The variety Jojutla was intercrossed with several US varieties in the 1960’s and, after many years of testing for processing quality, the variety Newrex was released in 1980 that was used for many years by the US parboiling industry.
Another extremely important example of the value of the world rice collection was the identification and use of the semidwarfing gene. Most rice grown around the world prior to the 1960s was very tall and would commonly fall down (lodge) prior to harvest thus, reducing the grain yield. Farmers were limited in the amount of fertilizer that could be applied to the crop because this caused further lodging. The variety Taichung Native 1, from Taiwan, possesses a gene (sd-1, semidwarf) which dramatically reduces the height of the plant. This gene has been used around the world in rice breeding and resulted in the “green revolution” in rice. Farmers could apply fertilizer to their crop, dramatically increasing the grain yield without fear of the rice plants toppling to the ground before it could be harvested. In the USA, Taichung Native 1 was also used extensively in breeding to reduce the height of US varieties. One of the first widely grown semidwarf US varieties was Lemont, released in 1986. Today, most USA varieties possess thesd-1gene.
Diseases can also reduce grain production. Rice blast is one of the most damaging diseases to the crop and is a constant worldwide threat. The disease can be managed to some extent by applying fungicides, but these are expensive and the timing of application is critical for good disease control. The variety Tetep, from Vietnam, was found to have excellent resistance to rice blast disease. This variety was introduced into the USA in 1978 and used in rice breeding programs. It was later found that Tetep possesses thePi-tagene that conveys resistance to all but one of the known isolates of the blast fungus that occur in the USA. The first USA variety (Katy) havingPi-tawas released in 1989. Many other varieties have been subsequently released in the US that possess this same gene that traces back to Tetep. ∆
USDA/ARS