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Plant genetic resources Plant genetic resources It is estimated that nowadays only 30 crops provide 95 percent of human food energy needs and just four of them – rice, wheat, maize and potatoes – provide more than 60 percent. Given the significance of a relatively small number of crops for global food security, it is of pivotal importance to conserve the diversity within these major crops. While the number of plant species that supply most of the world’s energy and protein is relatively small, the diversity within such species is often immense. For example, the number of distinct varieties of the rice species Oryza sativa, is estimated at more than 100 000. It is this diversity within species that allows for the cultivation of crops across different regions and in different situations such as weather and soil conditions. Plant genetic diversity may also provide valuable traits needed for meeting challenges of the future, such as adapting our crops to changing climatic conditions or outbreaks of disease. A variety of Turkish wheat, collected and stored in 1948 was ignored until the 1980s when it was found to carry genes resistant to many disease-causing fungi. Plant breeders now use those genes to breed wheat varieties that are resistant to a range of diseases. Wild botanical relatives of our food crops – often found on the periphery of cultivated lands – may contain genes that allow them to survive under stressful conditions. These genes can add important traits to their cultivated relatives, such as robustness or frost resistance. Plant genetic diversity is threatened by “genetic erosion”, a term coined by scientists for the loss of individual genes and of combinations of genes, such as those found in locally adapted landraces. The main cause of genetic erosion, according to FAO’s State of the World’s Plant Genetic Resources for Food and Agriculture, is the replacement of local varieties by modern varieties. As old varieties in farmers’ fields are replaced by newer ones, genetic erosion frequently occurs because the genes found in the farmers’ varieties are not all contained in the modern variety. In addition, the sheer number of varieties is often reduced when commercial varieties are introduced into traditional farming systems. Other causes of genetic erosion include the emergence of new pests, weeds and diseases, environmental degradation, urbanization and land clearing through deforestation and bush fires. The International Treaty on Plant Genetic Resources for Food and Agriculture,popularly known as the International Seed Treaty, is a comprehensive international agreement in harmony with Convention on Biological Diversity, which aims at guaranteeing food security through the conservation, exchange and sustainable use of the world's plant genetic resources for food and agriculture, as well as the fair and equitable benefit sharing arising from its use. It also recognises farmers' rights: to freely access genetic resources, unrestricted by intellectual property rights; to be involved in relevant policy discussions and decision making; and to use, save, sell and exchange seeds, subject to national laws. The treaty was negotiated by the Food and Agriculture Organization (FAO) Commission on Genetic Resources for Food and Agriculture (CGRFA)and since 2006 has its own Governing Body under the aegis of the FAO. Gene banks Gene banks help preserve genetic material, be it plant or animal. In plants, this could be by freezing cuts from the plant, or stocking the seeds. In plants, it is possible to unfreeze the material and propagate it, however, in animals, a living female is required for artificial insemination. While it is often difficult to utilize frozen animal sperm and eggs, there are many examples of it being done successfully. In an effort to conserve agricultural biodiversity, gene banks are used to store and conserve the plant genetic resources of major crop plants and their crop wild relatives. There are many gene banks all over the world, with the Svalbard Global Seed Vault being probably the most famous one. Germplasm Conservation, Dissemination, and Evaluation The dynamic conservation of genetic resources—as a complementary approach to conserving germplasm in genebanks—aims to promote their adaptation to the environment through their continuous cultivation under biotic and abiotic selection pressures in various agroecological conditions. Through this project, scientists are identifying the opportunities for dynamic conservation of rice genetic resources based on farmer-managed systems by improving their understanding of how farmers manage rice diversity and the genetic consequences of their actions. Socioeconomic and genetic studies are being conducted in several agroecological and socioeconomic environments in India, Philippines, and Vietnam. The ultimate goal of this project is to define strategies for conservationists and breeders to promote the adoption and maintenance of rice diversity by farmers. Germplasm Conservation Conservation refers to protection of genetic diversity of crop plants from genetic erosion. There are two important methods of germpalsm conservation or preservation. i) In-situ conservation and ex situ conservation. These are described below. i) In - situ conservation: Conservation of germplasm under natural conditions is referred to as in situ conservation. This is achieved by protecting the area from – human interference, such an area is often called natural park, biosphere reserve or gene sanctuary. NBPGR, New Delhi, established gene sanctuaries in Meghalaya for citrus, north Eastern regions for musa, citrus, oryza and saccharum. Gene sanctuaries offer the following advantage. Merits: In this method of conservation, the wild species and the compete natural or seminatural ecosystems are preserved together. Demerits: Each protected area will cover only very small portion of total diversity of a crop species, hence several areas will have to be conserved for a single species. The management of such areas also poses several problems. This is a costly method of germplasm conservation. ii) Ex - situ conservation: It refers to preservation of germplasm in gene banks. This is the most practical method of germplasm conservation. This method has following advantages. It is possible to preserve entire genetic diversity of a crop species at one place. Handling of germplasm is also easy. This is a cheap method of germplams conservation. Ex - situ conservation: 5 types 1) Seed banks: Germplam is stored as seeds of various genotypes. Seed conservation is quite easy, relatively safe and needs minimum space. Seeds are classified, on the basis of their storability into two major groups. 2. Plant Bank: ( Field or plant bank )is an orchard or a field in which accessions of fruit trees or vegetatively propagated crops are grown and maintained. 3. Shoot tip banks: Germplasm is conserved as slow growth cultures of shoot-tips and node segments. 4. Cell and organ banks: A germplasm collection based on cryopreserved (at – 196OC in liquid nitrogen) embryogenic cell cultures, somatic/ zygotic embryos they be called cell and organ bank. 5. DNA banks: In these banks, DNA segments from the genomes of germplasm accessions are maintained and conserved Germplam evaluation Evaluation refers to screening of gemplasms in respect of morphological, genetical, economic, biochemical, physiological, pathological and entomological attributes. Evaluation of germplasm is essential from following angles. To identify gene sources for resistance to biotic and abiotic stresses, earliness, dwarfness, productivity and quality characters. To classify the germplasm into various groups To get a clear pictures about the significance of individual germplasm line. The evaluation of germplasm is done in three different places viz., (1) in the field (2) in green house a) 3) in the laboratory. List of important International Institutes conserving germplasm Name IRRI CIMMYT Institute International Rice Research Institute, Los Banos, Philippines Centre International de-Mejoramients de maize Trigo, El Baton, Mexico Activity Tropical rice Rice collection: 42,000 Maize and wheat (Triticale, barely, sorghum) Maize collection – 8000 CIAT Center International de-agricultural Tropical Palmira, Columbia Cassava and beans, (also maize and rice) in collobaration with CIMMYT and IRRI IITA International Institute of Tropical Agriculture, Ibadan, Nigeria. Grain legumes, roots, and tubers, farming systems. CIP ICRISAT Centre International de-papa-Lima. Peru International Crops Research Institute, for Semi-Arid Tropics, Hyderabad, India WARDA West African Rice Development Association, Regional Cooperative Rice Research in Collaboration with IITA Monrovia, Liberia and IRRI IPGRI AVRDC International Plant Genetic Research Institute, Rome Italy The Asian Vegetable Research and Development Centre, Taiwan Potatoes Sorghum, Groundnut, Cumbu, Bengalgram, Redgram. Genetic conservation. Tomato, Onion, Peppers Chinese cabbage. MODE OF REPRODUCTION Knowledge of the mode of reproduction and pollination is essential for a plant breeder, because these aspects help in deciding the breeding procedures to be used for the genetic improvement of a crop species. Choice of breeding procedure depends on the mode of reproduction and pollination of a crop species. Reproduction refers to the process by which living organisms give rise to the offspring of similar kind (species). In crop plants, the mode of reproduction is of two types: viz. 1) sexual reproduction and 2) asexual reproduction I. Sexual reproduction Multiplication of plants through embryos which have developed by fusion of male and female gametes is known as sexual reproduction. All the seed propagating species belong to this group. Sporogenesis Production of microspores and megaspores is known as sporogenesis. In anthers, microspores are formed through microsporogensis and in ovules, the megaspores are formed through megasporogenesis. Microsporogenesis The sporophytic cells in the pollen sacs of anther which undergo meiotic division to form haploid i.e., microspores are called microspore (MMC) or pollen mother cell (PMC) and the process is called microsporogenesis. Each PMC produce four microspores and each microspore after thickening of the wall transforms into pollen grain. Megasporogenesis A single sporophytic cell inside the ovule, which undergo meiotic division to form haploid megaspore, is called megaspore mother cell (MMC) and the process is called megasporogenesis. Each MMC produces four megaspores out of which three degenerate resulting in a single functional megaspore. Gametogenesis The production of male and female gametes in the microspores and megaspores is known as gametogenesis. Microgametogenesis This is nothing but the production of male gametes or sperm. On maturation of the pollen, the microspore nucleus divides mitotically to produce a generative and a vegetative or tube nucleus. The pollen is generally released in this binucleate stage. The reach of pollen over the stigma is called pollination. After the pollination, the pollen germinates. The pollen tube enters the stigma and travels down the style. The generative nucleus at this phase undergoes another mitotic division to produce two male gametes or sperm nuclei. The pollen along with the pollen tube possessing a pair of sperm nuclei is called microgametophyte. The pollen tube enters the embryo sac through micropyle and discharges the two sperm nuclei. Megagametogenesis The nucleus of the functional megaspore undergoes three mitotic divisions to produce eight or more nuclei. The exact number of nuclei and their arrangement varies from one species to another. The megaspore nucleus divides thrice to produce eight nuclei. Three of these nuclei move to one pole and produce a central egg cell and two synergid cells on either side. Another three nuclei migrate to the opposite pole to develop into three antipodal cells. The two nuclei remaining in the center, the polar nuclei, fuse to form the secondary nucleus. The megaspore thus develops into a mature female gametophyte called megagametophyte or embryo sac. The development of embryo sac from a megaspore is known as megagametogeneis. The embryo sac generally contains one egg cell, two synergids with the apparent function of guiding the sperm nucleus towards the egg cell and three antipodals which forms the prothalamus cells and one diploid secondary nucleus. Fertilization: The fusion of one of the two sperms with the egg cell producing a diploid zygote is known as fertilization. The fusion of the remaining sperm with the secondary nucleus leading to the formation of a triploid primary endosperm nucleus is termed as triple fusion. The primary endosperm nucleus after several mitotic divisions develops into mature endosperm, which nourishes the developing embryo. II. Asexual reproduction Multiplication of plants without the fusion of male and female gametes is known as asexual reproduction. Asexual reproduction can occur either by vegetative plant parts or by vegetative embryos which develop without sexual fusion (apomixis). Thus asexual reproduction is of two types: viz. a) vegetative reproduction and b) apomixis. Vegetative reproduction refers to multiplication of plants by means of various vegetative plant parts. Vegetative reproduction is again of two types: viz. i) natural vegetative reproduction and ii) artificial vegetative reproduction. Natural vegetative reproduction In nature, multiplication of certain plants occurs by underground stems, sub aerial stems, roots and bulbils. In some crop species, underground stems (a modified group of stems) give rise to new plants. Underground stems are of four types: viz. rhizome, tuber, corm and bulb. Artificial vegetative reproduction Multiplication of plants by vegetative parts through artificial method is known as artificial vegetative reproduction. Such reproduction occurs by cuttings of stem and roots, and by layering and grafting. Examples of such reproduction are given below: Stem cuttings: Sugarcane (Saccharum sp.) grapes (Vitis vinifera), roses, etc. Root cuttings: Sweet potato, citrus, lemon, etc. Layering and grafting are used in fruit and ornamental crops. Apomixis Apomixis refers to the development of seed without sexual fusion (fertilization). In apomixis embryo develops without fertilization. Thus apomixis is an asexual means of reproduction. Apomixis is found in many crop species. Reproduction in some species occurs only by apomixis. This apomixis is termed as obligate apomixis. But in some species sexual reproduction also occurs in addition to apomixis. Such apomixis is known as facultative apomixis. There are four types of apomixis: viz. 1. Parthenogenesis. Parthenogenesis refers to development of embryo from the egg cell without fertilization. 2. Apogamy. The origin of embryo from either synergids or antipodal cells of the embryosac is called as apogamy. 3. Apospory. In apospory, first diploid cell of ovule lying outside the embryosac develops into another embryosac without reduction. The embryo then develops directly from the diploid egg cell without fertilization. 4. Adventive embryony. The development of embryo directly from the diploid cells of ovule lying outside the embryosac belonging to either nucellus or integuments is referred to as adventive embryony. Pollination Transfer of pollen grains from the stamens, the flower parts that produce them, to the ovule-bearing organs or to the ovules (seed precursors) themselves. As a prerequisite for fertilization, pollination is essential to the production of fruit and seed crops and plays an important part in programs designed to improve plants by breeding. Types: self-pollination and cross-pollination Agents of pollen dispersal Beetles and flies Bees Wasps Butterflies and moths Wind Birds Self-pollination is a form of pollination that can occur when a flower has both stamen and a carpel(pistil) in which the cultivar or species is self fertile and the stamens and the sticky stigma of the carpel contact each other in order to accomplish pollination. The mechanism is seen most often in some legumes such as peanuts. In another legume, Soybeans, the flowers open and remain receptive to insect cross pollination during the day; if this is not accomplished, the flowers self pollinate as they are closing. Other plants that can self pollinate are many kinds of orchids, peas, sunflowers, tridax,etc. Self pollination, or more generally self pollenizing, limits the variety of progeny and may depress plant vigor. However, self pollenizing can be advantageous, allowing plants to spread beyond the range of suitable pollinators or produce offspring in areas where pollinator populations have been greatly reduced or are naturally variable. mating systems Mating system: the mode of transmission of genes from one generation to the next through sexual reproduction (e.g. maternal selfing rate) Selfing rate (s): the proportion of seeds that are self fertilized Outcrossing rate (t=1-s): the proportion of seeds that are outcrossed Inbreeding depression: the reduction in viability and fertility of inbred offspring compared with outbred offspring. Qualitative & quantitative traits The phenotypic traits of the different organisms may be of two kinds, viz., qualitative and quantitative. The qualitative traits are the classical Mendelian traits of kinds such as form (e.g., round or wrinkle seeds of pea); structure (e.g., horned or hornless condition in cattles); pigments (e.g., black or white coat of guinea pigs); and antigens and antibodies (e.g., blood group types of man) and so on. We have already discussed in previous chapters that each qualitative trait may be under genetic control of two or many alleles of a single gene with little or no environmental modifications to obscure the gene effects. The quantitative traits, however, are economically important measurable phenotypic traits of degree such as height, weight, skin pigmentation, susceptibility to pathological diseases or intelligence in man; amount of flowers, fruits, seeds, milk, meat or egg produced by plants or animals, etc. The quantitative traits are also called metric traits. They do not show clear cut differences between individuals and forms a spectrum of phenotypes which blend imperceptively from one type to another to cause continuous variations. In contrast to qualitative traits, the quantitative traits may be modified variously by the environmental conditions and are usually governed by many factors or genes (perhaps 10 or I00 or more), each contributing such a small amount of phenotype that their individual effects cannot be detected by Mendelian methods but by only statistical methods. Qualitative genetics Quantitative genetics It deals with the inheritance of traits of kind, viz., It deals with the inheritance of traits of form, structure, colour, etc. degree, viz., heights of length, weight, number, etc. Discrete phenotypic classes occur which display A spectrum of phenotypic classes occur discontinuous variations. which contain continuous variations. Each qualitative trait is governed by two Each quantitative trait is governed by or many alleles of a single gene. many non-allelic genes or polygenes. The phenotypic expression of a gene is Environmental conditions effect the not influenced by environment. phenotypic expression of polygenes variously. It concerns with individual matings and their It concerns with a population of organisms progeny. consisting of all possible kinds of matings. In it analysis is made by counts and ratios. In it analysis is made by statistical methods.