Lectures

RICE GENOME - A GLOBAL HERITAGE

By Professor Akhilesh K. Tyagi

Rice is one of the most important crops for mankind. It feeds nearly half the world's population and accounts for more than 50% of their daily calorie intake. Although, in the past 30 years, world rice production has doubled due to the introduction of new high yielding varieties and improved cultivation practices, it is still insufficient to cope up with the ever﷓increasing global demands. It is expected that the demand for rice in the world would increase at about 1 % per annum from 2005 to 2025, implying that the current average yield of 3.9 tons/hectare has to be raised considerably in order to meet the growing needs. This is not an easy task in view of the fact that the land available for cultivation is decreasing due to continuous urbanization and inappropriate land use. It necessitates the development of high yielding varieties and also minimizing yield loss due to disease and abiotic stresses (such as drought and salinization). Development of plants for traits like yield or resistance to various stresses (biotic and abiotic) requires a thorough understanding of the cellular and functional aspects of the plant, which is dictated by its genetic make up. Thus, it is essential to identify all the genes and understand their function as well as networking. This idea invoked the interest of researchers who made a contribution in setting the pace for studies in rice genomics.

Beyond its importance as the world's premier crop, rice is also an excellent model plant for genomics. Among cereals, rice has the smallest genome with an estimated size of 430 megabase pairs (Mbp) as compared to the significantly 'large genome sizes of sorghum, maize, barley, and wheat (about 750, 3000, 5000, and 16000 Mbp, respectively). This also gives it a relatively higher gene density with the number of genes estimated to about 50,000. Other factors that aid in the use of rice as a model plant species include the fact that it can be transformed by exogenous DNA allowing geneticists to complement mutations or to confer dominant phenotypes to verify gene function, and it has a vast germplasm of cultivated and wild species serving as source of useful alleles.

With the widespread studies being performed on rice, a number of genetic markers have been mapped aiding in the development of a comprehensive genetic and physical map of rice. Such information has also been used for extensive comparative mapping studies, which have established that the gene order is significantly conserved between rice chromosomes and other cultivated cereals, thus suggesting that rice could provide a road map for the characterization of larger genomes like that of maize, barley and wheat. Furthermore, the release of the genome sequence from two subspecies of Oryza sativa over a short span of time has brought rice to the forefront of all genomic studies. One of the projects aimed at generating map﷓based high quality sequence of rice genome was the International Rice Genome Sequencing Project (1RGSP) that was started in 1998. The IRGSP involves scientists of ten countries, vz. Brazil, China, France, India, Japan, Korea, Taiwan, Thailand, UK and USA. The group has completed the sequence of rice genome with production of about 400 million base sequence. Considering that one page of the rice genome book contains 1000 bases, the book contains 400000 pages out of which a chapter of about 12000 pages is written by scientists working at University of Delhi South Campus and Indian Agricultural Research Institute. Analysis of gene contents has uncovered about 50,000 genes, half of them with unknown function. Thus, the challenge for future would be to identify useful alleles of genes as well as to define functions of novel genes for crop improvement. To elaborate on the function of genes, both classical and, more importantly, reverse genetics studies are required. These include technologies like tagging, transgenics, microarray, SAGE, complementation and proteomics. Our group has already identified novel genes involved in reproductive development or stress response. This should pave the way for crop improvement. (Summary by Prof Tyagi)

(Professor Tyagi is coordinating at present the Indian Initiative for Rice Genome Sequencing (IIRGS) and Centre for Plant Molecular Biology (CPMB) at the Department of Plant Molecular Biology of the University of Delhi South Campus. He is actively involved in different aspects of rice molecular biology. Under his leadership of the work in the South Campus and at Indian Agricultural Research Institute, IIRGS has produced ~16 Mb of high quality rice genome sequence from a region of rice chromosome 11 assigned to India as part of International Consortium to sequence rice genome. Besides, he is also involved in identification of important novel genes in rice, especially those involved in abiotic stress. Apart from studies in rice, he is contributing to the development of edible vaccine against cholera in tomato and whole genome high throughput sequencing of a microbial genome, Mycobacterium w. )