Plant Genetic Resources and Their Utilization in Crop Improvement
Authors: Vikas Gupta, Satish Kumar, Chandra Nath Mishra and Raj Pal Meena
Indian Institute of Wheat and Barley Research, Karnal, Haryana
The plant genetic resources (PGR) significantly contributed towards achieving the global objectives of food security, poverty alleviation and sustainable development. The worth of genetic resources in developing superior crop varieties is well recognized. The utilization of Norin 10 gene in wheat and Dee Geo Woo Gen in rice (sources for reducing plant height) have revolutionized the productivity of these crops globally. Wheat productivity increased by 137% and of rice by 93% in last 40 years due to the improved cultivars, coupled with good agronomic management (Upadhyaya and Gowda 2006). According to the international convention for biodiversity, genetic resources are living material that includes genes of present and potential value for humans. Plant genetic resources include all our agricultural crops and even some of their wild relatives because they too often have valuable traits.
Plant genetic resources include, i.e. landraces, varieties (obsolete and in use), genetic stocks, wild progenitors and breeding lines. Generally, varieties and breeding lines have contributed more than the others to date, and this will continue in the future. Wild relatives and genetic stocks, however, are expected to contribute much more in the next 20 years than the past and at present. The plant breeding contributes to diminution of the intraspecific diversity due to the development of genetically homogeneous cultivars and promotion of few widely adapted varieties. The lack of inter- and intraspecific genetic variability among cultivated crops can lead to epidemics of pests and diseases (genetic vulnerability).Most common examples of the past includes mainly; the Phytophthora infestans infestation of potato (Solanum tuberosum) in Western Europe in 1845/1846, the susceptibility of T-cytoplasm maize in the USA to Bipolaris maydis in 1970 and the Fusarium graminearum epidemic in wheat and barley in the western USA (1994â€"1996) (Haussmann et al., 2004).
Activities in germplasm conservation:
The main activities involved in conservation are:-
i. Collection: It involves collecting germplasm via explorations and collections from the sites of maximum diversity (centres of origin & centres of diversity) and introduction of germplasm from institutes and individuals concerned with germplasm conservation (IRRI-Rice, CIMMYT- Wheat & Maize).
ii. Conservation: Germplasm has to be maintained in a state so that there is a mimimum risk of losing. Many plant species and varieties are becoming extinct and many others are threatened and endangered. To reverse these genetic erosion, conservation of genetic diversity is a fundamental concern in conservation and evolutionary biology, as genetic variation is the raw material for evolutionary change within populations. Conservation is the process that actively retains the diversity of the gene pool with a view of actual or potential utilization. Different national and international institutes are involved in conservation of genetic resources for different crops.There are two ways of conserving germplasm:
a. Ex-situ conservation: the conservation of components of biological diversity outside their natural habitat. This includes field gene banks, tissue culture, green house, cryopreservation, seed gene banks and DNA banks etc.
b. In-situ conservation: the conservation of plant genetic resources in their ecosystems and natural habitats. These include genetic reserve conservation and on-farm conservation. Both of these involve the maintenance of genetic diversity in the locations where it is encountered but the first primarily deal with wild species in natural habitats/ecosystems and the latter with domesticated species in traditional farming.
iii. Characterization: Involves the assessment of germplasm collections for their distinctive traits for potential use in plant breeding. Germplasm evaluation involves the characterization for various traits viz., agronomic characteristics, abiotic (drought, salinity, wet conditions), biotic stress (pest and diseases). It also involves assigning the catalogue number to each accession and subsequently multiplication and storage.
iv. Utilization of Plant Genetic Resources: The use of PGR in crop improvement could be facilitated by systematic evaluation and documentation of the acquired data. Of particular importance are: information on valuable traits, e.g. resistances and specific quality traits;. reliable information on genotype environment interactions and specific adaptation;. information on general and specific combining abilities and af?liation to heterotic pools (if hybrid breeding is relevant); . on-farm evaluation to gain information about farmerâ€™s perception; . user-friendly information and documentation systems (standard formats); creation and evaluation of core collections; and International co-operation.
There are three ways of using PGR in plant breeding (Simmonds, 1993; Cooper et al., 2001): introgression involves the transfer of one or few genes or gene complexes (chromosome segments) from the PGR into breeding materials; . incorporation (also named genetic enhancement or base broadening) describes the development of new, genetically broad, adapted populations with large variation and acceptable performance level; prebreeding refers to more basic research activities with the goal of facilitating use of dif?cult materials. Nonetheless, the three categories cannot be clearly separated from each other as described below:
a) Introgression: It refers to the transfer of small chromosomal regions of one species or plant into the other through recombination. With the objective to improve highly heritable traits that are governed by one or few major genes. Generally, backcross method is used to introgress the desirable traits like resistances or restorer genes from wild relatives into breeding materials. Breeders generally look for traits that are not available in their breeding lines and the accessions which are relevant to particular trait are used in backcrossing programme for introgressing the desired traits into the high yielding backgrounds. For searching traits the first choice of breeders is primary gene pool followed by secondary and tertiary depending upon the ease in transferring the traits. According to Vavilovâ€™s law of â€˜homologous seriesâ€™, it is expected that an allele will be found in a cultivated form if genetic variation for it exists among wild relatives.
b) Incorporation: The varieties which are in cultivation are having very narrow genetic base thus making them vulnerable for epidemics. To broaden the genetic base, genetic variability is enhanced through incorporation of useful diversity from germplasm is an important component. With recent advances in biotechnology, traits of imporatance can be transferred from all genepools (Oceanic gene pool). In the initial stages the selection will be practised for highly heritable adaptation traits is made followed by yield traits in later stages of selection. The breeding method depends upon the mode of reproduction whether self or cross pollinated crop species and type of gene action governed by the trait.
c) Wide hybridization: It refers to the crossing of cultivated species with their wild species for transfer of few desirable traits inorder to broaden the genetic base. Genes from distantly related species can be introgressed in the cultivated lines through recombination of the homologous chromosomes, and undesirable gene linkages can be mostly broken by repeated backcrossing to cultivated lines. Moreover, the chromosome recombination enables a simultaneous gene transfer from different chromosomes, as well as introgression of traits with polygenic inheritance, in which the genes are distributed on different chromosome segments. The main objective of wide hybridizationis to provide breeders with genetic resources that are easy to use, i.e. different traits in acceptable genetic backgrounds.
d) PGR in Marker based research: Molecular markers and genome research has achieved several strides in the last 20 years. Molecular markers have been used in studying genetic diversity in almost every field crop. Molecular markers are very stringent in diversity studies due to their high power of resolution. Molecular markers are now a days used in identification, characterization and tagging of genes of economic inertest. In rice, bacterial blight resistance genes Xa21, Xa23, Xa38(t) Xa27, Xa29 and Xa32(t) have been identified and transferred from related wild species, O. longistaminata O. rufipogon, O. nivara O.minuta, O. officinalis and O. australiensis respectively. Some of the genes have been transferred into cultivated crop species and varieties have also been released using Marker Assisted Selection based breeding approach e.g., Xa 21 and Xa38(Oryza nivara). In addition to this genes for grassy stunt virus, blast resistance, brown plant hopper, cytoplasmic male sterility, sheath blight and tungro virus have been transferred from wild species into the cultivated lines.In wheat, genes like Lr9 (Aegilopes umbellulata), Lr21(Ae. taushii), Lr25(Secale cereale),Sr2(Triticum turgidium), Sr36 (T. timopheevi), Sr38 (Ae. ventricosa),Yr15 (T. dicoccoides) and Pm8 (Secale cereale) have been transferred from wild species for resistance to leaf rust, stem rust, stripe rust and powdery mildew resistance.
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About Author / Additional Info:
I am working as a Scientist in Indian Institute of Wheat and Barley Karnal, Haryana under ICAR, New Delhi.