This Primer describes the history of rice domestication, the importance of wild relatives of rice for crop improvement, and the domestication of wild species of rice not previously planted by farmers—a new approach called neodomestication.
Rice is a staple food crop for more than one-third of the global population, of which 90% live at or near the poverty line. Thus, rice genetic improvement is important for global food security and is critical for enhancing socioeconomic benefits and reducing the environmental impacts of agriculture. In continued efforts to address the long-standing problem of food security and sustainable agriculture, scientists are utilizing genes from diverse varieties of rice to improve the resilience of rice to pests, diseases and environmental stress.
This Primer describes the history of rice domestication, the importance of wild relatives of rice for crop improvement, and the domestication of wild species of rice not previously planted by farmers—a new approach called neodomestication.
Asian cultivated rice (Oryza sativa) is a semi-aquatic annual grass that can be grown across a wide breadth of agricultural ecosystems—from deeply flooded land to dry, hilly slopes. Historically, O. sativa has been divided into two major subspecies or varietal groups: japonica (or geng, “sticky” rice found in temperate East Asia, upland areas of Southeast Asia, and high elevations in South Asia), and indica (or xian, “non-sticky” lowland rice grown throughout tropical Asia). In addition to Asian rice, the genus Oryza contains one additional domesticated species—African rice (Oryza glaberrima)
In addition to Asian and African rice, the genus Oryza contains 25 globally distributed wild species, 16 of which contain two sets of chromosomes (i.e. their genomes are diploid), and 9 of which contain four copies of each chromosome (i.e. their genomes are tetraploid).
The domesticated and wild species are classified into 11 distinct and cytologically defined “genome types”. These genome types differ 3.6 fold in size and span 15 million years of evolutionary history.
Why is the diversity found in the genus Oryza important and how can it be harnessed to improve cultivated rice? In the field of genetics, variation within and among individuals of a population or species is defined as genetic diversity. Genetic diversity determines the range of inherited traits (phenotypic variation) among individuals, and serves as a mechanism for populations to adapt to changing environmental conditions.
With greater genetic diversity, it becomes more likely that some individuals in a population will possess key alleles (specific versions of genes) that are suited for adaptation to a specific environment. The domestication process — through the fixation of beneficial alleles giving superior characteristics and progressive removal of undesired ones — has inevitably reduced the genetic diversity of cultivated crops.
In contrast, the wild relatives of rice contain a vast reservoir of traits important for ecological adaptation that can be used to enhance the genetic diversity of cultivated rice. This type of variation is particularly important with regards to traits critical to sustainable agriculture, such as resistance to biotic stress (e.g., insect pests, fungal and bacterial diseases), tolerance to environmental stress (e.g., extreme temperature, floods, drought, salinity), and enhanced nutritional quality.
Rice “landraces” are local traditional varieties within a species that have been domesticated and adapted to their natural and cultural environments, and harbor higher genetic diversity than many cultivated species. Thus, landraces can also be harnessed as an additional source of diversity, in addition to the wild relatives, for crop improvement.
Access to natural genetic diversity (both physically in seed banks, and virtually in data banks) for both cultivated and wild Oryza species is essential for continued advancement in crop improvement. The T.T. Chang Genetic Resources Center (TTCGRC) is a well-known and internationally accessible germplasm bank, located in the Philippines at the International Rice Research Institute . The TTCGRC stores and maintains over 130,000 accessions (i.e. distinct and uniquely identifiable plant material representing a cultivar, breeding line or population) of Oryza that includes genetic stocks (plants or populations crossed and/or selected for genetic studies), landraces and wild relatives.
An increasing number of these genetic resources have been sequenced, thanks to the rapid advancement and decreasing costs of sequencing technologies. This genomic information is crucial to define genes and their functions, and to compare genomes from different individuals and species to assess important biological questions, such as tolerance to stress.
The importance of seed banks, and the landraces and wild species stored therein, is reflected by the numerous agronomically important genes that have been introgressed into wild species. Introgression is the process of incorporating novel genes from a donor plant (landrace or wild species) into a domesticated species.
Introgression is achieved when hybrid progeny derived from a cross-pollination between a domesticated and a wild species is back-crossed for several generations to the domesticated parent. Key examples of genes introduced through breeding include genes that confer resistance or tolerance to both biotic (Bph and Xa21) and abiotic (Sub1A) stresses.
Read the study:
Fornasiero A, Wing R, and Ronald P (2022) Rice domestication. Current Biology
Volume 32, Issue 1.
