Rice is the seed of the monocot plant Oryza sativa. As a cereal grain, it is the most important staple food for a large part of the world’s human population, especially in East, South, Southeast Asia, the Middle East, Latin America, and the West Indies. It is the grain with the second-highest worldwide production, after maize (corn).Rice is the most important grain with regard to human nutrition and caloric intake, providing more than one fifth of the calories consumed worldwide by the human species.
Rice cultivation is well-suited to countries and regions with low labor costs and high rainfall, as it is labor-intensive to cultivate and requires ample water. Rice can be grown practically anywhere, even on a steep hill or mountain. Although its parent species are native to South Asia and certain parts of Africa, centuries of trade and exportation have made it commonplace in many cultures worldwide.
(The name wild rice is usually used for species of the grass genus Zizania, both wild and domesticated, although the term may also be used for primitive or uncultivated varieties of Oryza.)
The direct ancestor of rice (Oryza sativa L.) is believed to be AA genome wild relatives of rice in Asia. However, the AA genome wild relatives involve both annual and perennial forms. The distribution of the retrotransposon p-SINE1-r2, a short interspersed nuclear element (SINE) at the waxy locus was analyzed in diverse accessions of the AA genome wild relatives of rice (O. rufipogon sensu lato). Most annual wild rice accessions had this retrotransposon, while most perennial types lacked this element, contradicting results to the previous studies. O. sativa has dual origin that lead to indica-japonica differentiation. Results suggest the indica line of rice varieties evolved from the annual gene pool of AA genome and the japonica varieties from the perennial gene pool of AA genome wild rice.
In Asian countries
To date, archaeological evidence suggests that domesticated rice (Oryza sativa) was developed from the wild variety Oryza rufipogon, probably in the lower and middle Yangtse River Valley in China, before 10,000 years ago. Some preliminary data have been published suggesting dates as early as 12,000-14,000 years ago. According to the latest molecular studies, Oryza sativa was domesticated at least twice: O. sativa japonica, developed in south China (probably the Yangtse valley), and O. sativa indica, developed in eastern India or Indonesia. Both of these were domesticated from the original plant called Oryza rufipogon. Rice farming at this early date was dry land cultivation; rice paddies were not developed until about 2500 BC. Morphological studies of rice phytoliths from the Diaotonghuan archaeological site clearly show the transition from the collection of wild rice to the cultivation of domesticated rice. The large number of wild rice phytoliths at the Diaotonghuan level dating from 12,000-11,000 BP indicates that wild rice collection was part of the local means of subsistence. Changes in the morphology of Diaotonghuan phytoliths dating from 10,000-8,000 BP show that rice had by this time been domesticated. Soon afterwards the two major varieties of indica and Japonica rice were being grown in Central China. In the late 3rd millennium B.C. there was a rapid expansion of rice cultivation into mainland Southeast Asia and westwards across India and Pakistan.
The earliest remains of cultivated rice in India have been found in the north and west and date from around 2000 BC. Perennial wild rice still grow in Assam and Nepal. It seems to have appeared around 1400 BC in southern India after its domestication in the northern plains. It then spread to all the fertile alluvial plains watered by rivers. Cultivation and cooking methods are thought to have spread to the west rapidly and by medieval times, southern Europe saw the introduction of rice as a hearty grain. Domesticated is not the same as cultivated. Domesticated types usually have a mutation that keeps the seed attached to the stalk. These non-shattering genes make the domesticated plant dependent on people to spread the seeds, and would be automatically selected for in the normal course of cultivation. There are two views; that the two types were domesticated entirely independently, or that there was a single domestication event (selection of the non-shattering gene) which then found its way from japonica types to indica types, perhaps by crossing with wild varieties. Domestication might be a process driven by conscious and unconscious selection of adaptive gene blocks distributed over the genome. In the past decade, a wealth of data provided by molecular markers, together with phenotypic, ecological, and archaeological data, significantly increased our evolutionary understanding of the genus Oryza.
Today, the majority of all rice produced comes from China, India, Pakistan, Indonesia, Bangladesh, Vietnam, Thailand, Myanmar, Philippines, and Japan. Asian farmers still account for 92-percent of the world’s total rice production. Rice is grown in all parts of India, Northern and Central Pakistan. Basmati rice cultivated in the northern plains of the Punjab region is famous all over the world for its smell and quality.
The History of Rice in China
The best known evidence for early domestication is japonica. Rice phytoliths (some of which are identifiable to japonica) were identified in the sediment deposits of Diaotonghuan Cave, located near Poyang Lake in the middle Yangtse river valley radiocarbon dated about 10,000-9000 years before the present. Additional soil core testing of the lake sediments revealed rice phytoliths from rice of some sort present in the valley before 12,820 BP (although these were not necessarily domesticated). Shangshan, a Neolithic village in the lower Yangtse valley dated to about 10,000 BP contained ceramic sherds tempered with charred plants, including rice and containing fan-shaped phytoliths. By about 7,000 years ago, japonica is found throughout the Yangtse valley, including large amounts of rice kernels at such sites as TongZian Luojiajiao (7100 BP) and Hemuda (7000 BP).
Evidence from ancient times
Four grains of rice were recovered from the Yuchanyan site, a rock shelter in Dao County, Hunan Province in China. They seem to represent very early forms of domestication having characteristics of both japonica and sativa, and are said to be dated between 12,000 and 14,000 years ago, although there is no discussion of what exactly was dated in the very preliminary report. Rice has been one of the most important foods of Chinese for thousands of years. It is believed that Chinese started to eat rice about 5,000 years ago. It is also believed that rice cultivation was originated by Shen Nung, the Divine Farmer, around 2,737 BC.
In African countries
African rice has been cultivated for 3500 years. Between 1500 and 800 BC, Oryza glaberrima propagated from its original centre, the Niger River delta, and extended to Senegal. However, it never developed far from its original region. Its cultivation even declined in favour of the Asian species, possibly brought to the African continent by Arabs coming from the east coast between the 6th and 11th centuries CE.
A traditional food plant in Africa, its cultivation declined in colonial times, but its production has the potential to improve nutrition, boost food security, foster rural development and support sustainable land care. It helped Africa conquer its famine of 1203. a perennial and can produce a ratoon crop for up to 30 years.
In Middle East countries
According to Zohary and Hopf, O. sativa was introduced to the Middle East in Hellenistic and Parthian times, and was familiar to both Greek and Roman writers. They report that a large sample of rice grains was recovered from a grave at Susa in Iran (dated to the first century AD) at one end of the ancient world, while at the same time rice was grown in the Po valley in Italy. In Iraq rice was grown in some areas of southern Iraq. With the rise of Islam it moved north to Nisibin, the southern shores of the Caspian Sea and then beyond the Muslim world into the valley of Volga. In Palestine, rice came to be grown in the Jordan Valley. Rice is also grown in Yemen.
In European countries
The Moors brought Asiatic rice to the Iberian Peninsula in the tenth century. Records indicate it was grown in Valencia and Majorca. In Majorca, rice cultivation seems to have stopped after the Christian conquest, although historians are not certain. Muslims also brought rice to Sicily, where it was an important crop. After the middle of the 15th century, rice spread throughout Italy and then France, later propagating to all the continents during the age of European exploration.
In Caribbean countries
Rice is not native to the Americas but was introduced to the Caribbean and South America by European colonizers at an early date with Spanish colonizers introducing Asian rice to Mexico in the 1520s at Veracruz and the Portuguese and their African slaves introducing it at about the same time to Colonial Brazil. Recent studies suggest that African slaves played an active role in the establishment of rice in the New World and that African rice was an important crop from an early period.In either case, varieties of rice and bean dishes were a staple dish along the peoples of West Africa and they remained a staple among their descendants subjected to slavery in the Spanish New World colonies, Brazil and elsewhere in the Americas.
In United States
In 1694, rice arrived in South Carolina, probably originating from Madagascar. In the United States, colonial South Carolina and Georgia grew and amassed great wealth from the slave labor obtained from the Senegambia area of West Africa and from coastal Sierra Leone. At the port of Charleston, through which 40% of all American slave imports passed, slaves from this region of Africa brought the highest prices, in recognition of their prior knowledge of rice culture, which was put to use on the many rice plantations around Georgetown, Charleston, and Savannah. From the slaves, plantation owners learned how to dyke the marshes and periodically flood the fields. At first the rice was milled by hand with wooden paddles, and then winnowed in sweet grass baskets (the making of which was another skill brought by the slaves). The invention of the rice mill increased profitability of the crop, and the addition of water power for the mills in 1787 by millwright Jonathan Lucas was another step forward. Rice culture in the south-eastern U.S. became less profitable with the loss of slave labor after the American Civil War, and it finally died out just after the turn of the 20th century. Today, people can visit the only remaining rice plantation in South Carolina that still has the original winnowing barn and rice mill from the mid-1800s at the historic Mansfield Plantation in Georgetown, South Carolina. The predominant strain of rice in the Carolinas was from Africa and was known as “Carolina Gold.” The cultivar has been preserved and there are current attempts to reintroduce it as a commercially grown crop.
In the southern United States, rice has been grown in southern Arkansas, Louisiana, and east Texas since the mid 1800s. Many Cajun farmers grew rice in wet marshes and low lying prairies where they could also farm crayfish when the fields were flooded. In recent years rice production has risen in North America, especially in the Mississippi River Delta areas in the states of Arkansas and Mississippi.
Rice cultivation began in California during the California Gold Rush, when an estimated 40,000 Chinese laborers immigrated to the state and grew small amounts of the grain for their own consumption. However, commercial production began only in 1912 in the town of Richvale in Butte County. By 2006, California produced the second largest rice crop in the United States, after Arkansas, with production concentrated in six counties north of Sacramento. Unlike the Mississippi Delta region, California’s production is dominated by short- and medium-grain japonica varieties, including cultivars developed for the local climate such as Calrose, which makes up as much as eighty five percent of the state’s crop. References to wild rice in the Americas are to the unrelated Zizania palustris.
More than 100 varieties of rice are commercially produced primarily in six states (Arkansas, Texas, Louisiana, Mississippi, Missouri, and California) in the U.S. According to estimates for the 2006 crop year, rice production in the U.S. is valued at .88 billion, approximately half of which is expected to be exported. The U.S. provides about 12% of world rice trade. The majority of domestic utilization of U.S. rice is direct food use (58%), while 16 percent is used in each of processed foods and beer. The remaining 10 percent is found in pet food.
Although attempts to grow rice in the well-watered north of Australia have been made for many years, they have consistently failed because of inherent iron and manganese toxicities in the soils and destruction by pests. In the 1920s it was seen as a possible irrigation crop on soils within the Murray-Darling Basin that were too heavy for the cultivation of fruit and too infertile for wheat. Because irrigation water, despite the extremely low runoff of temperate Australia, was (and remains) very cheap, the growing of rice was taken up by agricultural groups over the following decades. Californian varieties of rice were found suitable for the climate in the Riverina, and the first mill opened at Leeton in 1951.
Even before this Australia’s rice production greatly exceeded local needs,and rice exports to Japan have become a major source of foreign currency. Above-average rainfall from the 1950s to the middle 1990s encouraged the expansion of the Riverina rice industry, but its prodigious water use in a practically waterless region began to attract the attention of environmental scientists. These became severely concerned with declining flow in the Snowy River and the lower Murray River.
Although rice growing in Australia is highly profitable due to the cheapness of land, several recent years of severe drought have led many to call for its elimination because of its effects on extremely fragile aquatic ecosystems. The Australian rice industry is somewhat opportunistic, with the area planted varying significantly from season to season depending on water allocations in the Murray and Murrumbidgee irrigation regions.
Rice production is one of the most important agricultural activities on the planet as more than half the people in the world eat rice at least once a day. Australia produces enough rice to feed almost 40 million people a meal a day for 365 days and Australian rice is exported to 72 countries. This is 1 percent of the world population per day.
Rice is the most important food crop and the staple food for 40 percent of the world population. More than 90 percent of rice is produced and consumed in Asia. It is grown under a wide range of agro-climatic conditions. There have been major advances in increasing rice production worldwide thanks to the large-scale adoption of modern high-yielding rice varieties and improved cultural practices. World rice production more than doubled from 257 million tonnes (Mt) in 1966 to 599 Mt in 2000. This was mainly achieved through the application of the principles of classical Mendelian genetics and conventional plant breeding methods. The current world population of 6.1 billion is expected to reach 8.0 billion by 2030 and rice production must increase by 50 percent in order to meet the growing demand. If this goal is to be met, it is necessary to use rice varieties with higher yield potential, durable resistance to diseases and insects and tolerance to abiotic stresses
The High Yielding Varieties are a group of crops created intentionally during the Green Revolution to increase global food production. Rice, like corn and wheat, was genetically manipulated to increase its yield. This project enabled labor markets in Asia to shift away from agriculture, and into industrial sectors. The first “Rice Car”, IR8 was produced in 1966 at the International Rice Research Institute which is based in the Philippines at the University of the Philippines’ Los Baños site. IR8 was created through a cross between an Indonesian variety named “Peta” and a Chinese variety named “Dee Geo Woo Gen.”
Scientists have identified and cloned many genes involved in the gibberellin signaling pathway, including GAI1 (Gibberellin Insensitive) and SLR1 (Slender Rice). Disruption of gibberellin signaling can lead to significantly reduced stem growth leading to a dwarf phenotype. Photosynthetic investment in the stem is reduced dramatically as the shorter plants are inherently more stable mechanically. Assimilates become redirected to grain production, amplifying in particular the effect of chemical fertilizers on commercial yield. In the presence of nitrogen fertilizers, and intensive crop management, these varieties increase their yield two to three times.
As the UN Millennium Development project seeks to spread global economic development to Africa, the “Green Revolution” is cited as the model for economic development. With the intent of replicating the successful Asian boom in agronomic productivity, groups like the Earth Institute are doing research on African agricultural systems, hoping to increase productivity. An important way this can happen is the production of “New Rices for Africa” (NERICA). These rice, selected to tolerate the low input and harsh growing conditions of African agriculture are produced by the African Rice Center, and billed as technology “from Africa, for Africa. Ongoing research in China to develop perennial rice could result in enhanced sustainability and food security.
German and Swiss researchers have engineered rice to produce Beta-carotene, with the intent that it might someday be used to treat vitamin A deficiency. Additional efforts are being made to improve the quantity and quality of other nutrients in golden rice.The addition of the carotene turns the rice a “gold” color, hence the name.
Expression of human proteins
Ventria Bioscience has genetically modified rice to express lactoferrin, lysozyme, and human serum albumin which are proteins usually found in breast milk. These proteins have antiviral, antibacterial, and antifungal effects.
Rice containing these added proteins can be used as a component in oral rehydration solutions which are used to treat diarrheal diseases, thereby shortening their duration and reducing recurrence. Such supplements may also help reverse anemia.
Recent advances in cellular and molecular biology and rice biotechnology have produced new tools to increase the efficiency of both phases.
Genetic variability for agronomic traits is the key component of breeding programmes for broadening the gene pool of both rice and other crops. A number of rice cultivars and elite breeding lines characterized by resistance to major diseases and pests, tolerance to abiotic stresses and improved quality characteristics have been developed through conventional plant breeding approaches. In such breeding programmes, it is mainly the genetic variability available in O. sativa germplasm which has been exploited. However, the genetic variability for many traits, such as tolerance to stem borer, tungro, sheath blight and salt stress, is limited in cultivated germplasm. Breeders therefore search for genetic variability in other gene pools involving wild relatives of Oryza and new techniques are applied for the creation and transfer of variability through somaclonal variation and genetic engineering.
Wide hybridization involving hybridization between rice and related wild species is adopted to broaden the gene pool of rice. The genus Oryza comprises 24 species representing AA, BB, CC, BBCC, CCDD, EE, FF, GG, HHJJ and HHKK genomes. Of these species, O. sativa (2n=24 AA) is cultivated worldwide, whereas O. glaberrima(2n=24 AA) is limited to certain areas in West Africa. Wild species are an important reservoir of useful genes for resistance to major diseases and pests, tolerance to abiotic stresses and cytoplasmic male sterility. The barrier most commonly encountered is lack of crossability resulting from chromosomal and genic differences. Biotechnology tools, such as embryo rescue and protoplast fusion, have been employed to overcome this difficulty, resulting in the production of several interspecific hybrids. More recently, molecular techniques have been employed for precise monitoring of alien gene introgression.
Gene transfer from wild species to rice
Hybrids between cultivated rice and AA genome wild species can be produced through normal procedures. Hybrids between rice and distantly related wild species, on the other hand, are usually difficult to produce; low cross ability and abortion of hybrid embryos are common features in such crosses. Hybrids have been produced through embryo rescue between elite breeding lines or varieties and several accessions of wild species representing BBCC, CC, CCDD, EE, FF, GG, HHJJ and HHKK genomes. A number of useful genes for resistance to brown plant hopper (BPH), white backed plant hopper (WBPH), bacterial blight (BB), blast and tungro disease have been transferred from wild species to rice. The first example of transfer of a useful gene from wild species is the introgression of a gene for grassy stunt virus resistance from O. nivara to cultivated rice.
Wild species with AA genome have been an important source of cytoplasmic male sterility (CMS). The development of a male sterile line with the cytoplasm of wild species (O. sativa L. f.spontanea) and the nuclear genome of rice. This cytoplasmic source has been designated as wild abortive (WA), i.e. a male sterile wild rice plant with abortive pollen. About 95 percent of the male sterile lines used in commercial rice hybrids grown in China and other countries have WA cytoplasm. A new CMS source from O. perennis was transferred into indica rice.
Tagging alien genes with molecular markers
Alien genes for resistance to BPH, BB and blast have been tagged with molecular markers. RFLP (restriction fragment length polymorphism) analysis of the introgression lines derived from O. sativa x O. officinalis showed introgression of the chromosome segments in 11 of the 12 chromosomes of O. officinalis. Most introgressed segments were detected by single RFLP markers and the flanking markers were negative for introgression.Co-segregation for BPH resistance and RG457 showed that the gene for resistance to BPH is linked to the molecular marker RG457.
Somaclonal variation refers to the variation arising through tissue culture in regenerated plants and their progenies. Somaclonal variation is reported in various plant species and occurs for a series of agronomic traits, such as disease resistance, plant height, tiller number and maturity, and for various biochemical traits. The technique consists of growing callus or cell suspension cultures for several cycles and regenerating plants from these long-term cultures. The regenerated plants and their progenies are evaluated in order to identify individuals with a new phenotype. Dama, a somacl it is resistant to Pyricularia and has good cooking qualities.
The introduction of alien genes from bacteria, viruses, fungi, animals and, of course, unrelated plants into crop species allows plant breeders to achieve breeding objectives which until just a decade ago were not considered possible. Several techniques are now available for the transformation of rice, e.g.: electroporation, polyethylene-glycol-induced uptake of DNA into protoplasts, microprojectile bombardment and, more recently, Agrobacterium-mediated transformation. Transgenic rices carrying agronomically important genes for resistance to stem borer and fungal pathogens, and tolerance to herbicide, have been produced in both japonica and indica rice. Several laboratories have produced transgenic rices – mainly through protoplast-mediated DNA transformation, but also via microprojectile bombardment. Transgenic plants were produced after co-bombarding embryogenic callus and cell suspensions with a mixture of 14 different pUC-based plasmids. Eighty-five percent of the R0 plants contained more than two of the target genes and 17 percent more than nine. Plants containing multiple transgenes had normal morphology and 63 percent of the plants set viable seeds.
Transgenic rice for modifying yield potential
Starch biosynthesis plays an important role in plant metabolism. Several enzymatic steps are involved in starch biosynthesis in plants. ADP-glucose pyrophosphorylase (ADPGPP) is a critical enzyme for regulating starch biosynthesis in plant tissues. Starch levels and dry matter accumulation were enhanced in potato tubers of plants transformed with glgC16 gene from E. coli encoding ADPGPP. The glgC16 gene has been introduced into rice and the yield potential of these lines will soon be evaluated.
The transfer of C4 traits into C3 rice is being explored with the aim of improving photosynthetic efficiency; however, it is difficult to incorporate genes for C4 traits into C3 plants through traditional plant breeding methods. Agrobacterium-mediated transformation was introduced and introduced from maize a gene for phosphoenolpyruvate carboxylase (PEPC), which catalyzes the initial fixation of atmospheric CO2 in C4 plants. Most transgenic rice plants showed high level expression of the maize gene; the activities of PEPC in the leaves of some transgenic plants were two to three times higher than those in maize, and the enzyme accounted for up to 12 percent of the total leaf soluble protein. The level of expression of the maize PEPC in transgenic rice plants correlated with the amount of transcript and the copy number of the inserted maize gene. The transgenic rice plants exhibited reduced O2 inhibition of photosynthesis and photosynthetic rates comparable to those of untransformed plants. These findings demonstrate a successful strategy for introducing the key biochemical component of the C4 pathway of photosynthesis into rice.
Transgenic rice for insect resistance
As early as 1987, genes coding for toxins from Bacillus thuringiensis (Bt) were transferred to tomato, tobacco and potato, where they provided protection against lepidopteran insects. A major target for Bt deployment in transgenic rice is the yellow stem borer, Scirpophaga incertulas. The pest is widespread in Asia and is the cause of potentially substantial crop losses. Improved rice cultivars are either susceptible to the insect or have only moderate levels of resistance; transgenic rice with Bt is therefore very promising for the control of yellow stem borer.
A truncated d-endotoxin gene, cry1A(b),was introduced into rice. Transgenic plants in the R2 generation expressing the cry1A(b)protein showed increased resistance to striped stem borer and leaf folder.
Transgenic rice for disease resistance
Sources of resistance to some diseases (blast and bacterial blight) have been identified within cultivated rice germplasm, and improved germplasm with resistance has been developed. However, sources of resistance to sheath blight are not available and there are only a few known donors for resistance to tungro disease (caused by two types of virus). A highly successful strategy, known as coat protein (CP) mediated protection, has been employed against certain viral diseases, such as tobacco mosaic virus, in tobacco and tomato. A CP gene for rice stripe virus was introduced into two japonica varieties by electroporation of the protoplasts. The resultant transgenic plants had high levels of CP (up to 0.5% of the total soluble proteins) and exhibited a significant level of resistance to virus infection. The resistance was inherited in the progenies.
.Sheath blight in rice is caused by the fungus, Rhizoctonia solani, which has a wide host range. About six chitinase genes have been identified in rice and are being manipulated to increase the level of resistance to fungal diseases.1.1 kilobase rice genomic DNA fragment containing a chitinase gene has been introduced through PEG-mediated transformation.
Transgenic rice for abiotic stress tolerance
A series of abiotic stresses, such as drought, excess water, mineral toxicities/deficiencies in soil and unfavourable temperature, affect rice productivity. Genetic engineering approaches hold great promise for the development of rice cultivars with higher levels of tolerance to abiotic stresses. codA gene was also introduced for choline oxidase from Arthrobacter globiformis. The codA gene was inherited into the second generation of transgenic rice and its expression was stably maintained at levels of the mRNA, the protein and enzyme activity. Levels of glycine betaine were estimated to be as high as 1 and 5 m mol/g of fresh leaves in two types of transgenic plants (Chl COD and Cyt COD plants) in which choline oxidase was targeted to the chloroplasts and cytosol, respectively. Further analysis of transgenic plants demonstrated their ability to synthesize betaine and confer enhanced tolerance to salt and the cold.
Transgenic rice for improved nutritional quality
Rice contains neither b-carotene (provitamin A) nor C40 carotenoid precursors in its endosperm. Rice in its milled form (as it is usually consumed) is therefore entirely without vitamin A and its carotenoid precursors. Millions of rice consumers who depend on rice for a large proportion of their calories suffer from vitamin A deficiency. Transgenic rice (Golden Rice) was produced with the provitamin A(b-carotene) biosynthetic pathway engineered into its endosperm. Agrobacterium-mediated transformation was applied to introduce three genes: phytoene synthase (psy), phytoene desaturase (crt1) and lycopene cyclase (lcy). HPLC (high performance liquid chromatography) analysis revealed the presence of b-carotene in transgenic seeds.
The entire coding sequence of the soybean ferritin gene was inserted into kita-ake, a rice cultivar via Agrobacterium-mediated transformation. The introduced ferritin gene was regulated by the rice seed storage protein glutelin promoter, GluB-1, and terminated by the Nos polyadenylation signal. Synthesis of soybean ferritin protein was confirmed in each of the transformed rice seeds by western blot analysis, and specific accumulation in endosperm was determined by immunological tissue printing. The iron content of T1 seeds was up to three times higher than in untransformed seeds.
Escalating effectiveness of variety
The anther culture technique has been refined greatly; it is now possible to produce haploids from the anther culture of many japonica and indica rices, although the frequency of plant regeneration is lower in indica varieties. Anther culture is important for the development of true breeding lines in the immediate generation from any segregating population, producing a shorter breeding cycle in new varieties. Selection efficiency in doubled haploid (DH) lines is higher, especially when dominance variation is significant. DH lines are also useful for developing mapping populations for molecular analysis. Seeds of such populations can be distributed to workers in other laboratories and populations can be grown repeatedly in many different environments, greatly facilitating the additional mapping of both DNA markers and genetic loci controlling traits of agronomic importance. A number of varieties and improved breeding lines have been developed through anther culture in China, the Republic of Korea, Japan and the United States. Most of the anther-culture-derived varieties are japonica. Indica rice are generally regarded as recalcitrant for anther culture.
Molecular marker technology
One of the most exciting developments in rice biotechnology is the advent of molecular markers. A series of molecular markers, e.g. random fragment length polymorphism (RFLP), random amplified polymorphic DNA (RAPD), amplified fragment length polymorphism (AFLP) and microsatellites, have become available.
These markers offer new opportunities for various studies in genetic and breeding research, particularly in the construction of saturated molecular maps, gene tagging, QTL mapping, marker-assisted breeding, gene pyramiding, physical mapping of genome, map-based gene cloning, alien introgression and DNA finger-printing of pathogen populations.
Probable consequences of enhancement of rice
Recent advances in cellular and molecular biology of rice offer new opportunities to enhance the efficiency of both the evolutionary and the evaluation phase of rice breeding. Biotechnology is becoming an important component of rice breeding. Anther culture has become an important technique for use by plant breeders to shorten the breeding cycle for the development of rice varieties, to fix recombinants and in overcoming sterility in distant crosses. Several rice varieties have been developed through anther culture. Doubled haploid populations are important for mapping genes governing agronomic traits including QTL.
Molecular markers have led to the tagging of numerous genes for tolerance to major biotic and abiotic stresses. MAS has become an important tool in rice breeding: for moving genes from one varietal background to another; for pyramiding genes; and for the development of durable pest-resistant cultivars. Fine mapping of QTL should provide a means to pyramid QTLs for tolerance to major abiotic stresses. Map-based cloning has made it possible to isolate useful genes governing important agronomic traits and the incorporation of these genes into elite rice cultivars through transformation. Advances in tissue culture and molecular marker technology have resulted in broadening of the gene pool of rice and have enhanced the efficiency of introgression of useful genes from wild species across crossability barriers. Advances in genetic engineering have facilitated the introduction of cloned novel genes into rice through transformation.
Transgenic rices with enhanced resistance to diseases and insects and improved nutritional quality have been produced and will have a great impact in terms of increasing rice production and improving the nutritional value of rice. Many rice cultivars have been developed through the application of biotechnology tools. Future food and nutritional security will depend upon the availability of rice cultivars with higher yield potential, durable resistance to diseases and insects, tolerance to abiotic stresses and higher levels of micronutrients in the grain.
Name: RANJEETA BACHASPATIMAYUM
Currently doing Phd in Genetics.
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