Plant breeding techniques

Introduction

Green revolution is the need of the hour. In the era where a lot of destruction is seen in the ozone, greeneries play a vital role in stabilizing the damage occurring in the atmosphere. Apart from this, the green revolution is the key driver of enhancing food production through artificial hybridization of plants and genetic therapies. All these strategies can assist in fulfilling the needs of the world’s population suffering from hunger crisis. As already  India is in the top 10 list of malnutrition, and the majority of the African nations are also in the list which necessitates a quick and healthy remedy in the form of a green revolution. As always, most of our traditional farming methods lack the scientific ideology that yield better productivity. So, the plant breeding techniques help to increase the yield at a reasonable time and cost. Plant breeding is the scientific technique of producing genetically improved crops that help in generating desired plant types which are productive and safe for consumption. Apart from the yield, plant breeding assist in developing resistance to microbes, pests, drought and other adverse conditions. Although, traditional breeding was in practice since 9,000-11,000 years, genetically advanced technologies have witnessed their popularity from the past 2 decades. An ideal plant breeding technique must include hybridization (crossing) of pure lines, artificial selection of the breeds to produce plants with desirable characters of higher yield, nutrition and resistance to diseases. 

 

Table of Contents

1. Introduction

2. Steps used for developing new plant varieties

3. Steps involved in cross-hybridization of selected parents

4. Testing, Release and Commercialization of New Cultivars

 

5.  Plant Breeding for Disease Resistance

6. Concept of polyploidy in Crop Improvement (Polyploidy Breeding) 

    6.1 Advantages of polyploidy

7. Breeding by mutation

8. Genetic Engineering (Recombinant DNA Technology) used in plant culture

9. Plant Breeding for Developing Resistance against Insects and pests

10. Plant Breeding for Improved Food Quality

 

Steps used for developing new plant varieties

  1. Germplasm collection: A germplasm is the sum total of all the alleles of the genes present in a crop and it`s related species. 

  2. Cultivating improved varieties: The varieties that we chose must be unique and do not exist in a traditional cultivation method.

  3. Evaluation and Selection of Parents: It is the process of identifying plants with desirable combination of traits. For example, if the colour of a wheat germ is brownish then the germ can produce better offspring. In the next stage,  cross-Hybridization is carried out.

 

Steps involved in cross-hybridization of selected parents

We already learnt that hybridization helps to provide a great genetic variation among the resulting progeny. It also assists in bringing about heritable variations of 2 or more lines together. A line is called so when a group of individuals are related to a common descent with a similar genotype.  A hybridization may involve a single crossing between the b. Wheat variety C-306 was developed through multiple cross between C-591 (Reagent 1974 x Ch2-3) and hybrid of (P-19 x C-281).  Hybridization can also be intraparietal, intervarietal, interspecific and intergeneric. Common steps involved in hybridization are explained below

  1. Selection of Parents with Desired Characters depending upon what we exactly need for future progeny.

  2. Selfing: A self-breeding experiments among selected plants that helps to bring about homozygosis –purity of the desired traits.

  3. Emasculation: After completing the pollination, the anthers (male parts) must be removed out of a bisexual flower before they undergo maturation  and this is known as emasculation.

  4. Bagging: After the emasculation, flowers are immediately covered by a plastic cover or paper known as bagging. Bagging prevents unwanted pollen to spill out making them contact with emasculated flowers. Such a prevention of contact helps to prevent the contamination from foreign pollen grains.

  5. Tagging: After emasculation and bagging, the flowers must be marked with the details of events and the date and time on which they happened. This is essential to track the changes. 

  6. Artificial Pollination or Crossing: Controlled pollination by bringing selected pollen grains in contact with a stigma through human efforts is called artificial pollination.  Pollen grains are collected from the covered flowers of the ‘male’ parent in a clean and sterile paper or polythene bags or test tubes. The collected pollen grains can be stored for later use. When the stigma of the emasculated flower of ‘female’ parent matures, the covering of the bag is removed for a short while to bring in contact with the male part. By using a clean brush the stigma is dusted with pollen grains. After the pollination, the emasculated flower is covered again till the stigma remains receptive. Bags are discarded when fruits begin to develop. The seeds produced by the flowers of the female parent are known as the hybrid seeds  and such seeds are stored for testing or later use. 

  7. Selection and Testing of Superior Recombinants: This involves choosing the best of all the progeny produced from the hybrids. While selecting, it is important to make a careful choice of those plants that have the desired character combination. This will ensure the superior quality necessary for future pollination. The selected plants are subjected to self-pollination for several generations until they reach a state of uniformity (homozygosity). 

  8. Selection in Self-pollinated Crops: The degree of cross-pollination is less than 5% at once so, a repeated self-pollination of selected plants is carried out until the superior homozygous genotypes are obtained.  The self-pollinated progeny of homozy­gous plant should be of a pure line that have identical genotype. The wheat variety HUW 468 is a good example of pure line. 

  9. Selection in Cross-pollinated Crops: The cross-pollinated crops are heterozy­gous as the population resulting from this process contains plants of several different genotypes. Some of these genotypes are superior but many are inferior. Only the plants with superior genotype are selected and allowed to crossbreed so that the heterozygosity is also maintained. Selection can be continued until the successive genera­tions of cross-pollinated crops that yield good breeds.

 

Testing, Release and Commercialization of New Cultivars

The newly selected lines are evaluated for their yield and other agronomic traits of quality and disease resistance. The evaluation is done by growing these plants in the research field and recording their perfor­mance under ideal conditions (regular irrigation conditions). After the evaluation is done in the research field, the testing of the materials has to be done in the farmer’s fields, for at least 3 growing seasons at different locations of the country,( This will ensure the breeds are exposed to all the agro-climatic zones).In order to decide on to better breeds, the material is evaluated by comparing them to the best available crops in the market. Some of the  improved varieties that are developed by hybridization in India are:

  1. Wheat obtained by crossing Kalyan Sona, Sonalika.

  2. Rice obtained by crossing Jaya and Ratna,

  3. Sugarcane obtained by crossing Saccharum Barberi  and Sachharum officinarum, 

  4. Rapeseed mustard obtained by crossing Brassica and Pu/sa swarnim.

 Plant Breeding for Disease Resistance: Biotechnology and genetic engineering made it possible for the plants to develop having an ability to resist infections like fungal infection viral infection bacterial infection tomato and pests. Doing this so will not only improve the end but also generate stay healthy crop free from infections for the consumption by animals and human beings.  The resistance can be developed against some fungal diseases such as brown rust of wheat, red rot of sugarcane and late blight of potato. In addition, some of the bacterial infections like black rot of crucifers and viral diseases like tobacco mosaic, turnip mosaic, etc can also be controlled. Methodology followed in breeding for disease resistance is same as explained above under the heading Common steps involved in hybridization. Some of the examples of disease-resistant crop varieties developed by hybridization are Wheat, Cauliflower and Chilli.

Concept of polyploidy in Crop Improvement (Polyploidy Breeding): A polyploidy organisms are those that have more than two sets of chromosomes or genomes. Some of the polyploidy crops are wheat, bananas, cotton, potatoes, sugarcane and tobacco. Polyploidy is the result of the failure of chromosomes to separate at the time of anaphase ( non-disjunction). it can also happen due to the failure to form a spindle that assist in the exchange of nuclear material to opposite poles of cells. Polyploidy can be triploid (3n), tetraploid (4n), petaloid (5n) or even beyoned this. Polyploids with odd-numbered genomes such as triploids and pentapolis are sexually sterile because the odd chromosomes do not undergo synapsis. Therefore, the resulting polyploidy can be subjected to propagation by vegetative methods as seen in banana and Pineapple. 

Advantages of polyploidy:

  1. Faster growth makes the business of vegetables and fruits more profitable.

  2. Bigger and better size of fruits adding value to its weight and quantity.

  3. The number of fruits and vegetables per plant is relatively higher than the normal.

Breeding by mutation: Mutation is a dramatic, and heritable change of a trait or group of traits in a given species. Such an enrichment of traits are due to the addition, deletion, insertion, or frameshift happening within their chromosomes. Mutations occurring naturally are called spontaneous mutations. Spontaneous mutation can be germinal or somatic. Somatic mutations are integrated to produce crop improvement only in vegetatively propagated plants ( for example; seedless grape). Another class of mutation is the Induced Mutations by mutagens. Mutagens can be chemical agents such as ethyl methane sulphonate (EMS) and sodium azide. Physical mutagens are agents like X-rays, gamma-rays, ultraviolet rays. When the mutagens are being introduced they induce certain sensitive changes in the structure, numbers, and arrangement of DNA and chromosomes, which can produce mutations. It was first demonstrated by Muller in the year 1927 where he used X-rays on Drosophila. More than 200 varieties of plant breeds have been developed in India by using mutation breeding.

Genetic Engineering (Recombinant DNA Technology) used in plant culture: It is a process by which the genetic makeup of cells are being altered for a specific purpose. It is carried deliberately by an external method to transfer or replace the genes to create a recombinant DNA. It is done by cutting DNA molecules exactly at a specific site and make it into fine fragments that has desirable and useful genes. Such genes can be further inserted into a suitable carrier or vector and transfer them to the completely different cell of the plant so that the host cells will acquire useful characters like disease resistance.

Plant Breeding for Developing Resistance against Insects and pests: There are 4 mechanisms that regulate the insect resistance in plants, namely, non-preference, use of antibiotics, developing tolerance, and avoidance or escape from the scavenger (pest). Painter – a botanist has introduced the first 3 mechanisms in the year 1951 and the  4th one was added subsequently. A resistant variety is characterized by exhibiting 2 or more of these mechanisms. Since insects and pests account for 25% of the destruction of yield, it is imperative to create a breed that can withstand pests. Such breeding produce changes in the in morphological, biochemical or physi­ological characters of a plant that repel pests. For example, the development of hairy leaves in many plants help in developing resistance against insects. Another example is the growth of solid stems that are genetically developed as in the case of wheat has resulted in aversion against the stem sawfly.

Plant Breeding for Improved Food Quality: Objectives of plant breeding for food quality are:

  1. To maintain protein content and quality

  2. To regulate the oil content and quality

  3. To maintain the balanced composition of vitamin content and other micronutrient s like mineral content.

World’s hunger rate is on the rise that has led to new ideas on how to improve the grain production per hectare or per specific area.  Around 840 million people in the world do not have either adequate food or the nutritionally balanced food. 3 billion people suffer from protein, vitamins and micronutrient deficiencies as these people are either vegans or cannot afford to fish and meat. And other non-vegetarian foods. As a resolution, genetic engineers and plant breeding experts have undertaken a project to improve nutritional quality of the plants by certain techniques under the administration of Indian Agricultural Research Institute (IARI), New Delhi. IARI has successfully developed many vegetable crops rich in micronutrients like minerals and vitamins. Some of the examples are carrots with rich vitamin, healthy pumpkin, vitamin С enriched bitter gourd etc.

 

 



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