Genetics and Crop Improvement:
Genetics is the science of heredity and variation which deals with resemblances and differences among individuals related by descent”.
Heredity is the transfer of traits from parents to offspring’s/progeny.
Heredity means the similarities progeny shows to its parents and develops the progeny in the parental image.
Deviation from the heredity/similarity is called variation. Variation in various characteristics of the members of the same race arises due to the change in gene structure or change in the environment.
Gene & Gene Function:
A gene is a unit of inheritance which by interaction with the other genes and environment controls the development of a character.
The genetic material present in the chromosomes is DNA (Deoxyribonucleic acid). Gene is represented by a given sequence of the nucleotides of which the DNA is composed.
Nucleotides: Adenine, Guanine, Cytosine and Thymine.
Arrangement in DNA: Adenine-Thymine : Guanine- Cytosine.
The Gene influences the mechanism of protein synthesis, and thus controls the metabolic activities inside the plants.
The genes occur in alternative kinds, called alleles which result in the form of contrasting forms of the characters they determine. Genes that express themselves in the exclusion of their alleles are called dominant. The contrasting form of a gene, which is not expressed in the presence of the dominant, is referred to as the recessive.
Plants with like pair genes on chromosome are called homozygous plants. (AA)
Plants with different gene pair on chromosome are called heterozygous plants. (AB).
Genes sometime change in nature so that they produce a different type of character, the new form being reproduced in different/succeeding generations. Such a change in a gene is called mutation.
A mutation represents a change in the chromosome number or structure and may occur spontaneously or be inclined artificially by different methods e.g., irradiation and chemicals.
Genetic recombination is simply a recombination of genetic material through independent assortment following hybridization.
Applications of Genetics in Crop Management:
- To maintain or increase yields by selecting verities for:
- Pest/ Disease resistance.
- Drought resistance.
- Increased response to fertilizers.
- Tolerance to adverse soil conditions.
- Adaptation to shorter seasons, longer seasons, heavy grazing, or frequent cutting.
2. To increase the value of the yield by selecting verities with such traits as
- Increased oil content
- Improved storage qualities.
- Improved milling and baking qualities.
- Increased nutritional value such as higher levels of proteins.
3. To reduce production costs by selecting verities that:
- Can be mechanically harvested, reducing labour requirement.
- Require fewer chemical protectants or fertilizers.
- Can be used with minimum tillage systems, reducing cost by conserving fuel or labour operations.
Methods of Plant Breeding:
There general methods of crop improvement are commonly used either alone or in combination for improvement of cultivars.
Introduction is the least expensive and least time consuming method of improving crop plants. A large number of varities, lines, population or strains are brought from other localities or countries given accession number and planted in observation or screening nurseries. The samples that are very late, poor in growth, susceptible to disease or low yielding are discarded. The samples which show promise are selected and cultivated along with standard cultivars as a control. The samples that out yield control cultivars are tested again in comparison with standard cultivars. Samples that show higher yields are tested at more than one location. The best sample, based on two three years of yields tests at more than one location are selected, named and approved for commercial cultivation.
Wheat variety Dirk was a successful introduction in NWFP from Australia. IR-8 from International Rice Research Institute (IRRI).
Pak-81. Variety of wheat is an introduction from (CIMMVT) in Mexico.
Selection is the sorting out of best variety of plants from mixed populations or intercrossing populations. The selection procedure depends upon the type of reproduction and mode of pollination. With self-pollinated crops, pure line selection and mass selection are used. With cross pollinated crops, mass selection and progeny selection are used. In asexually propagated plants, clonal selection is used.
Selection is needed to identify
- Superior verities, hybrids, populations or strains for direct use.
- Parents for hybridization programs.
- Superior strains in asexually propagated plants.
Crossing is done to combine in one cultivar the traits found in two or more cultivars, experimental lives or populations/generations.
The handling of material after the hybridization depends upon the mode of reproduction and pollination.
- In self-pollinated crops, a large number of genetically different plants can be grown side by side with natural reproduction. After crossing two cultivars, the material segregates, and homozygosity is reached in a few generations.
- In cross-pollinated species, individual plants are heterozygous, and self-pollination or inbreeding leads to loss of vigor. Due to cross pollinations, which cause cages in gene frequencies, plant materials in which cross-pollination is the rule cannot be grown side by side without artificial control of pollination.
Biotechnology & Genetic Engineering For crop Improvement:
Biotechnology is the integrated use of principles of biology and engineering with special regards for the chemistry and physics of micro-organisms.
Manipulation of genetic material of an organism to produce desired results is termed as Genetic Engineering. Plant genetic engineering has the potential to transfer a beneficial gene from a distantly related wild species into an economical species. Genetic engineering techniques make the use of cellular transformation or gene splicing through recombinant gene technology.
Principles of Genetic Engineering:
The genetic information coded within a gene DNA is dictated by the sequence of the base pairs. The basic tools of recombinant DNA technology are different enzymes that can be used to cut and paste the piece of DNA together. A large number of site-specific enzymes have been identified; these are like molecular scissors that will cut a DNA strand when they encounter a specific sequence.
The basis of recombinant DNA technology is the ability to isolate DNA fragments containing one or more genes and replicate them in a different host, such as bacteria. For example, the human insulin gene transferred to bacteria produces a human insulin called humilin. Herbicide resistant genes have been transferred to certain plant species.
Enzymes called “restriction endonucleases” permit DNA to be cleaved at desired points and into fragments of desired sizes that will hybridize i.e., attach to other complementary DNA fragments. The plasmids are modified to accept alien DNA which is incorporated by genetic recombination and then labeled with biochemical markers to allow them to be identified after replication in the bacteria. The replication and subsequent selection of these bacteria containing the recombined DNA is called gene cloning.
Genetic Engineering & Crop Improvement Prospects:
Tolerance to Herbicides:
The production of plants resistant to herbicides incurs crop safety against herbicide damages. Two general approaches have been taken in engineering herbicide tolerance,
- Altering the level and sensitivity of the target enzyme for the herbicide.
- Incorporating a gene that will detoxify the herbicide.
2. Insect Resistance:
Insect resistance in transgenic plants have been achieved through the insect control protein genes of Bacillus Thuringiensis (B.t) which produces an insect control protein lethal to selected insect pests and unharmful for productive insects. Most strains of B.t. are toxic to moth & butterfly larvae, & fly larvae.
3. Disease Resistance:
Significant resistance to tobacco mosaic virus (TMV) infection has been achieved by expressing only coat protein gene of TMV in transgenic plants. This approach has produced similar results in transgenic plants of tomato, tobacco, and potato against broad spectrum of plant viruses, including alfalfa virus, cucumber mosaic virus, potato virus X & potato virus Y.
Biological Nitrogen Fixation:
The symbiotic fixation is controlled by genes in both the bacteria and plant cells. Altering the expression of some of the genes involved in nitrogen fixation can enhance the amount of nitrogen being fixed both by the bacteria as well as plant.
Tree improvement and forestry plant breeding higher yields, resistance to disease and insect pests. Conventional method of selection and breeding are very slow. The problem is long life cycle of trees, from 20 to 50 years.
Tissue culture technique could reduce the time for selection and propagation of desirable traits. Selection of genetic variants through cell and tissue culture requires very little space. They offer new ways to increase productivity & make trees harder & resistant to pests & diseases.
Corn plants with a higher photosynthesis rate, in order to manufacture food more efficiently.
Salt tolerant barley & tomato plants. Black cherry trees that produce higher quality fruit.
Walnut trees resistant to “coding moth” & “black line” diseases.
In vitro culture technique. “the process of regenerating whole plants or plant products from ex-plants on nutrient media under aseptic conditions”.
The central concept of tissue culture is “titopotency”, i.e., every living cell has the genetic information needed to develop into a complete organism. It is possible to obtain entire plant from almost every part of the mother plant, i.e., small segments of roots, stems, flowers, embroyas, pollens, protoplasts etc.
The discovery of plant growth hormones (auxins & Cytokinins) enabled research to acheive this goal.
Techniques of Tissue Culture:
Isolation, culture, & fusion of protoplasts Possible.
Somatic hybridization achieved by fusing protoplasts of two incompatible species and then regenerating plants allows the transfer of favourable genes, not otherwise transfable.
2. Anther Culture:
Haploid plants are sterile.
Anther lobes contain pollen grains, Induction off anther lobes of fertile diploid plants into sterile haploid plants to make them fertile Anther Culture.
3. Embryo Culture:
Technique in which a premature baby is removed from the mother womb and incubated under controlled conditions.
In plants such premature babies exist when certain intragenic and interspecific crosses are made. Their further development is stopped due to incompatibility in plant tissues. The embryo culture can be used to rescue a hybrid which is unable to mature in vivo.
Application of Tissue Culture:
Different plant parts (even a single cell & tissue) can be used to regenerate whole new plants, under suitable culture media and environment.
- Mean of quick multiplication for making available a bulk supply of commercial forms to growers.
- Commercial propagation of horticultural trees like orchards at a speedy rate.
- Preserving germplasm to avoid extinction of precious genetic material.
- Preservation of tissue, especially meristems, valued where seed production is not easy, seed is viable for a limited period, is highly heterozygous and is unstable because of pathogens.
- Production of mutants or in screening for stress.
- Resistance to diseases e.g., sugarcane cultivars resistant to eyespot disease, downy mildew& sugarcane mosaic caused by viruses.
- All the plant production free of parasites by blocking their cycle of vegetative propagation.