Genetic and Genomics Resources in 'Finger Millet- A Climate Resilient Nutri-Millet'
Authors: Dr. Amolkumar U Solanke and Dr. SV Amitha Mithra
NRC on Plant Biotechnology, LBS Building, IARI, Pusa Campus, New Delhi
Finger millet (Eleusine coracana (L.) Gaertn.), is a cereal crop serving as a staple food for millions of poor people living on the most marginal agricultural lands of Africa and Indian subcontinent. It is a self-pollinating allotetraploid (AABB) with chromosome number 2n=4x=36, and genome size of 1593 Mbp. It is well adapted to drought and rainfed agriculture in Africa and India. It has extraordinary ability to survive under the most exacting environmental stresses such as drought, low soil fertility, and high temperature. Reported from the Hindustani and African Centers of Diversity, finger millet is reported to tolerate alkalinity, salinity, both high and low pH, fungal and viral diseases, insect pests and drought (Duke, 1978). Because of its tolerance to difficult growing conditions, it can be grown in areas of India where other cereal crops, such as maize, sorghum or wheat, would not survive. Owing to its ability to grow under varied agro-climatic and resource poor conditions, it can be a valuable resource for understanding genomics of tolerance to various abiotic stresses and important genes and alleles from finger millet can be utilized for crop improvement of other major cereals.
Grains of finger millet are higher in protein, fat and mineral content than rice, maize and Sorghum. Protein content in finger millet grains ranges from 7 to 14 per cent with high methionine, and other essential amino acid content. Every 100 grams of finger millet seeds contain 12.6 mg iron, 410 mg calcium and 290 mg phosphorus. It also has low glycemic index and high fiber content. Consumption of finger millet prevents constipation and lowers cholesterol. All parts of the finger millet plant provide the raw materials for antiphlogistic medicines used to treat liver disease, measles, pleurisy, pneumonia, and smallpox. Because of these high nutritional and medicinal properties, it is called as nutri-millet., Moreover, finger millet seeds can be stored without damage for as long as 50 years and thus are highly valued as a reserve food in times of famine.
Genetic and Genomics resources in Finger Millet:
Owing to its nutritional value and climate resilient nature, the value of this crop as a rich resource of superior genes and alleles has now been recognized. Finger millet is believed to have been domesticated more than 5,000 years ago from the tetraploid E. coracana ssp. africana Cytogenetic analyses of the hybrids and chloroplast DNA restriction analysis of diploid and polyploid species have shown that E. indica as the A genome donor to E. coracana. Later on E. Floccifolia was confirmed as the B genome donar by genomic in situ hybridization (GISH) of E. coracana to various diploid species of the genus (Bisht and Mukai, 2001). The genome size (1C value) of finger millet has been estimated by flow cytometry to be about 1.8 pg (Mysore and Baird, 1997).
Initially breeding efforts in finger millet were very limited due to highly self-pollinating nature of the crop and its small flower size making hand-emasculation difficult. The development of hot water emasculation, combined with contact pollination, in the 1960s opened the way for the development of new crossbred varieties. However, all released varieties in Africa are germplasm selections. In India, the hybridization of Indian and African varieties has resulted in the production and release of high-yielding 'Indaf' types that have proven popular with farmers.
Although the finger millet germplasm pool remains largely uncharacterized, small-scale analysis of the nutritional value of seeds of wild and cultivated E. coracana lines have shown a wide variation in protein, calcium, and iron content (Vadivoo et al., 1998). An evaluation of around 2,000 finger millet lines from ICRISAT, Asia Center, has shown a considerable range in flowering time (54-120 days), plant height (45-165 cm), number of basal tillers (1-70), peduncle length (2-28 cm), inflorescence length (1-32 cm) and other morphological traits (Prasada Rao and Wet, 1997). Further Upadhyaya et al. (2006) developed a core subset of finger millet germplasm (622 accessions) based on origin and data on 14 quantitative traits from the entire global collection of 5940 accessions held in the genebank at ICRISAT, Patancheru, India. RAPD and microsatellite approach (Dida et al., 2008) have already demonstrated wider level of genetic diversity among Indian and Indo-African domesticated finger millet cultivars and their phylogenetic relationship with wild annual species of finger millet.
The first major milestone in genetic genomics was achieved with the creation of a molecular marker-based genetic linkage map of ragi A and B genomes from a F2 population derived from E. coracana subsp. coracana cv. Okhale-1 and its wild progenitor E. coracana subsp. africana acc. MD-20, using RFLP and AFLP markers (Dida et al., 2007). The constructed map spans 721 cM on the A genome and 787 cM on the B genome and cover 18 finger millet chromosomes partially.
Comparative genome analysis with rice, a model crop with rich genomic resources, revealed that six of the nine finger millet homoeologous groups corresponded to a single rice chromosome each. The remaining three finger millet groups were orthologous to two rice chromosomes with one rice chromosome being inserted into the centromeric region of a second rice chromosome to give the finger millet chromosomal configurations. Gene orders between rice and finger millet were also found to be highly conserved. A comparison of the organization of finger millet, Panicoideae and Pooideae genomes relative to rice allowed inferring putative ancestral chromosome configurations in the grasses (Srinivasachary et al., 2007).
With availability NGS and new technologies, researchers have generated a large number of ESTs and molecular markers in this crop. Many transcription factor genes like EcNAC1 and EcDREB1A have been characterized from finger millet for stress tolerance. Regeneration and Agrobacterium-mediated transformation protocols are also standardized by many researchers for use in gene characterization.
Although a large number of genes have been implicated to be involved in abiotic stress response, huge gaps still remain in our understanding. Therefore, structural genomics of such an important stress tolerant crop like finger millet would be useful for characterizing the novel genes and alleles of agricultural importance and validating their functions through comparative and functional genomics approaches. Besides, with the advent of new tools and techniques, it would be possible to isolate such useful genes from finger millet and transfer into various target species to enhance and sustain agricultural productivity.
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