The International Wheat Genome Sequencing Consortium (IWGSC) to which India is a partner published today in the international journal Science a draft sequence of the bread wheat genome. Ten years back wheat was considered as toughest crop to decode because of the genome being 17000 million bases and each chromosome is represented three times with very high similarity to each other. Technological advances made in recent years and development of specialized wheat lines in 1950s made it possible to isolate individual chromosomes of wheat for sequencing. The chromosome-based draft sequence provides new insight into the structure, organization, and evolution of the large, complex genome of the world’s most widely grown cereal crop. Country’s three leading institutes, Punjab Agricultural University, Ludhiana, National Research Centre on Plant Biotechnology, New Delhi, and South Campus Delhi University, New Delhi, with financial support from Department of Biotechnology (DBT), Government of India, were entrusted with the responsibility of decoding one chromosome, designated as 2A, which is about 900 million bases in size and is about one third the size of the human genome or 2.5 times the size of rice genome.
The genetic blueprint is an invaluable resource to plant science researchers and breeders. For the first time, they have at their disposal a set of tools enabling them to rapidly locate specific genes on individual wheat chromosomes throughout the genome, said Dr Vijay Raghavan, Secretary, DBT. This genomics resource has made thousands of markers available to wheat researchers which will facilitate mapping and cloning of genes of agronomic importance in much lesser time and cheaper cost than was available earlier. Dr Swapan Datta, Deputy Director General (Crop Sciences) at Indian Council of Agricultural Research said decoding wheat genome will facilitate our understanding of gene function which will enable develop new genetic gains of wheat. Taking forward with molecular breeding and genetic engineering we would be able to develop climate smart wheat (drought/terminal heat tolerance) with higher yield. 22,000 wheat germplasm has been evaluated by NBPGR/ICAR. We would be able to use our germplasm better now with this science- discovery.
The draft sequence is a major landmark towards obtaining a complete reference sequence of the hexaploid bread wheat genome, the ultimate goal of the IWGSC. In the same issue of Science, another article presents the first reference sequence for the largest chromosome, 3B. This establishes a proof of concept and a template for sequencing the remaining chromosomes. “With the draft gene sequence for each of the bread wheat chromosomes and the first reference sequence of chromosome 3B, we have achieved a milestone in our roadmap,” said Catherine Feuillet, IWGSC Co-chair. As of today, researchers in the IWGSC estimate that the full genome sequence will be available within three years, said Dr Kuldeep Singh, Project Coordinator in India.
With a chromosome-based full sequence in hand, plant breeders will have high quality tools at their disposal to accelerate breeding programs and to identify how genes control complex traits such as yield, grain quality, disease, pest resistance, or abiotic stress tolerance and for mobilizing genes of interest from wild species said Dr B S Dhillon, Vice Chancellor Punjab Agricultural University. They will be able to produce a new generation of wheat varieties with higher yields and improved sustainability to meet the demands of a growing world population in a changing environment. Prof Akhilesh Tyagi, Director, NIPGR stated that the achievement reflects the strength of plant genomics research in India in global context. It is time to launch major efforts, by utilising resources generated, to create varieties which tolerate heat, utilise less water and show resistance to major disease by gene modification and genetic enhancement.
Wheat is a major dietary component for many countrues across the world. Grown on more land than any other crop, more than 215 million hectares of wheat are harvested annually to generate a world production of almost 700 million tons, making it the third most produced cereal after maize and rice. It is the leading source of vegetable protein in human food, having higher protein content than either maize or rice. The wheat plant is highly versatile due to its ability to grow in a wide range of environments. Also, wheat grain can easily be stored and can be converted readily into flour for making numerous varieties of high quality edible food. The goal of the IWGSC is to make a high quality genome sequence of bread wheat publicly available, in order to lay a foundation for basic research that will enable breeders to develop improved varieties.
Summary of the Publication
In nature, bread wheat evolved after cross hybridization of three different but closely related species (see figure). This evolutionary feature made it to accumulate largest content of DNA among all the food crops, making wheat genetics more complex than other food crops. Ten years back wheat was considered one of the toughest crops to decode due to its huge genome size of 17000 million base pairs, and presence of three sets of highly similar chromosomes in the genome and a very large proportion of repetitive DNA (ranging from 80-90%). Technological advances in DNA sequencing made in recent years and availability of specialized wheat lines, each having one chromosome arm added in duplicate to the normal chromosomes complement, developed during 1950s in wheat variety ‘Chinese Spring’ made it possible to isolate individual chromosomes for sequencing. The chromosome-based draft sequence provides new insight into the structure, organization, and evolution of the large, complex genome of the world’s most widely grown cereal crop. The decoding of wheat genome has helped in identifying more than 125,000 genes and almost 50 per cent of these assigned these to the individual wheat chromosomes. This has opened gates for understanding the biological function of each of these genes. The current draft sequence has generated thousands of DNA markers which could be used as ‘tag’ for identification of any gene in wheat and its subsequent transfer to any variety through cross hybridization much precisely and rapidly.
The blue print assembles only about 60 per cent of the genome and we expect to assemble complete genome in next three years. Completion of the wheat genome reference sequence is essential to accelerate breeding and the rapid identification of genes underlying complex traits such as yield, disease and pest resistance, or abiotic stress tolerance. The genome sequence will aid breeders by enabling new strains to be developed, including new “stacked” trait combinations, and by allowing the development of accelerated and smarter breeding schemes. This in turn will decrease the time from discovery to commercialisation of new varieties for farmers. The genome reference sequence will act as a benchmark point for understanding the differences between varieties that are associated with different traits, and provide breeders and plant scientists with a molecular toolkit for marker-based selection, high throughput screening, and the association of traits with specific genes and proteins. It holds the key to the production of a new generation of wheat varieties that will enable higher yields and improved sustainability of wheat production systems.
For each gene in wheat there are three copies, one from each of the three genomes, and in most cases only one copy expresses. How the plant decides which copy of the gene to express is most intriguing question in wheat biology. Availability of chromosome based sequence has open new horizons for researching this issue.
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