Plant breeding techniques

Introduction

The world is bombarded with food shortage requiring scientific attention. This can be resolved by relying upon the scientific methods of plant breeding techniques. In the era of drastic destruction of ozone, green revolution plays a vital role in stabilizing the damage arising out of the pollution in our atmosphere. The green revolution is the key driver of enhancing food production through artificial hybridization of plants and animals.  All these strategies can assist in fulfilling the hunger gap preponderating across many nations including India.  India is in the top 10 list of countries suffering from malnutrition, which definitely needs a quick-fix in the form of a green revolution. Added to this, most of our traditional farming methods lack the scientific ideology necessary for better productivity hence our farmers need to master the modern high-yielding breeding techniques.

 

 

What is plant breeding

Plant breeding is the scientific technique of producing genetically improved crops to aid in generating desired plant types that can ascertain the safety and productivity of the yield. Apart from the yield, the technique also assists in developing resistance to microbes, pests, drought and other adverse conditions exist in the environment. Although traditional breeding was in practice since 9,000-11,000 years, genetically superior technologies have testified their popularity from the past 2 decades. A perfect plant breeding technique must embrace hybridization (crossing) of pure lines and the artificial selection of the breeds to produce higher yield, nutrition and resistance to diseases. 

 

 

Steps used for developing new plant varieties

There are 3 major steps while developing a new variety of plants. They are Germplasm collection, Cultivating improved varieties and the evaluation of varieties. Germplasm is the sum total of all the alleles of the genes present in a crop and it`s related species. Germplasm is collected from centres of diversity, specialized gene banks, plant gene sanctuaries, farmer's fields, markers and some seed companies. It is the primary material essential to drive crop improvement programmes. Germplasm may be indigenous when collected within the country or it can be exotic if it is collected from foreign countries. Cultivating improved varieties needs proper testing and guaranty of the breed quality. The varieties that we chose must be unique and does not exist in a traditional cultivation method. Evaluation and Selection of Parents is the process of recognising plants with a desirable combination of traits. Selection of parents is picking up seeds of only those plants for multiplication which have the desired traits. For example, if we are successful in identifying the colour of a wheat germ as brownish, and the brown coloured wheat germ can produce a better offspring, in the next phase, crossbreeding is carried in a phased manner.

 

 

Steps involved in cross-hybridization of selected parents

We already learnt that hybridization provides a great genetic variation among the resulting progeny. Furthermore, the process also assists in inducing the 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 parents or it can be done multiple times till we get the desired progeny. Wheat variety C-306 was developed through multiple crosses between C-591 (Reagent 1974 x Ch2-3) and hybrid of (P-19 x C-281).  Hybridization can be intraparietal, intervarietal, interspecific or intergeneric. Intervarietal hybridisation is a cross between the two varieties of the same species whereas interspecific hybridisation is a cross between the different species of the same genus. An intergeneric hybrid is a cross between plants in two different genera in the same family. They are closely related enough that pollination will produce a hybrid, though the seeds of this hybrid are usually sterile.

 

 

Steps of plant hybridization 

  • Selection of Parents: It is carried out by keeping in mind about the desired traits depending upon what we exactly need for future progeny.

  • Selfing: It is a self-breeding experiment between the 2 or more selected plants that assist to bring about homozygosis –purity of the desired traits. Selfing can be done twice or more and we need to wait until the purity of the progeny is attained. However, going for multiple crossing may influence negatively on the purity of the resulting offspring.  

  • Emasculation: Emasculation is the scientific process of extracting anthers from bisexual flowers without affecting the pistils and the technique is readily used in various plant hybridization techniques. It is done after completing the pollination before a flower undergoes maturation.  

  • Bagging: After the emasculation, flowers are immediately covered by a plastic cover or a thick paper and the process is called bagging. Bagging prevents unwanted pollen to spill out which can avoid contact between the normal flowers with emasculated flowers. Such a restriction of contact helps to prevent the contamination from foreign pollen grains.

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

  • Artificial Pollination or Crossing: It is the process of the controlled pollination by bringing the selected pollen grains in contact with stigma through human efforts.  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 called the hybrid seeds  and such seeds are stored for testing or later use. 

  • Selection and Testing of Superior Recombinants: This includes choosing the best of all the progeny produced from the hybrids. While selecting, it is important to make a careful choice of those plants with 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). 

  • Selection of the  Self-pollinated Crops: The range of cross-pollination at once hardly assures 5% of the results at once, therefore, repeated self-pollination of selected plants is carried out till the superior homozygous genotypes are obtained.  The self-pollinated progeny of homozy­gous plant should be of a pure line containing identical genotype. The wheat variety HUW 468 is a good example of pure line. 

  • Selection in Cross-pollinated Crops: The cross-pollinated crops are heterozy­gous as the population resulting from the process comprises plants of several different genotypes. Some of those genotypes are superior but many are inferior. Only the plants with superior genotype are chosen and approved to crossbreed so that the heterozygosity is maintained. Selection can be sustained until the successive genera­tions of cross-pollinated crops yield good breeds.

 

 

Testing, Release and Commercialization of New Cultivars

The newly selected lines are evaluated for their yield, quality and disease resistance and the evaluation is done by growing these plants in a laboratory setting and recording their perfor­mance under ideal conditions (regular irrigation conditions). After the evaluation in the research field, the testing of the materials has to be repeated 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  developed by hybridization in the past 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 an ability to resist infections like fungal, viral and bacterial infections. Doing this will not only improve the end results of the breeds but also produces healthy crops which are free from infections and are good 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. The methodology followed while breeding for disease resistance is the 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)

Polyploidy organisms are those that contain more than 2 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). Nevertheless, it can also happen due to the failure to form a spindle that assists in the exchange of nuclear material to opposite poles of cells. Polyploidy can be triploid (3n), tetraploid (4n), petaloid (5n) or even beyond this. Polyploids with odd-numbered genomes such as triploids and pentapolis are sexually sterile because the odd-numbered 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

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. The mutation is a dramatic, and heritable change of a trait or group of traits in a given species.It is performed by artificially inducing addition, deletion, insertion, or frameshift mutations within the chromosomes of a target breed. Mutations happening naturally are called spontaneous mutations while the artificially conducted ones are called induced mutations. The spontaneous mutation is again classified into germinal or somatic. Somatic mutations are useful to produce crop improvement only in vegetatively propagated plants ( for example; seedless grape) while the germinal mutation broadly focuses on the germ level manipulations. Mutagens are 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 introduced, they induce certain sensitive changes in the structure, numbers, and arrangement of DNA and chromosomes, which have the capacity to alter the genome causing one or more types of mutation including inducing addition, deletion, insertion, or frameshift.

 

 

Genetic Engineering (Recombinant DNA Technology) used in plant culture

It is a process by which the genetic makeup of cells is 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 have 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 regulating 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 the aforementioned mechanisms. Since insects and pests account for 25% of the destruction of yield, it is imperative to create a breed that can resist pests. Such breeding results in the morphological, biochemical or physi­ological changes of a plant so that pests are not entertained. For example, the development of hairy leaves in many plants has helped in amplifying 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, as a result, new ideas on how to improve the grain production per hectare or per specific area have been evolved.  Around 840 million people in the world do not have access to either adequate food or nutritionally balanced food. 3 billion people suffer from protein, vitamins and micronutrient deficiencies as these people are either vegans or cannot afford the costs of fish and meat. As a resolution, genetic engineers and plant breeding experts have initiated a project to improve the nutritional quality of the plants by several 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 vitamins, healthy pumpkin, vitamin С enriched bitter gourd etc.

 



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