What is Plant Breeding? – Definition, Objectives, Types, Methods

The Biology Topics of ecology involve studying the relationships between living organisms and their environment.

Introduction and Basic Concepts of Plant Breeding – How Plant Breeding Works

Plant breeding is the science of improvement in the hereditary characteristics of economically important crop plants and the production of new crop varieties which are better than the original varieties in one or many features useful to humans.

The population of the world is increasing day by day at an alarming rate. Such population explosion is creating a huge problem of shortage of the minimum necessities of food, cloth, and shelter. To cope with this problem newer ways are constantly being worked upon. The techniques like plant breeding, tissue culture, genetic engineering, micropropagation, animal husbandry, animal breeding, etc., are proving to be of utter importance so as to provide the basic needs to the growing population. The techniques like plant breeding, animal husbandry, farm management, and pisciculture have been in use since ancient times, but the advancement of science has improvised and implemented newer ideas to give a greater and better yield.

Plant Breeding

Plant breeding is the art and science of changing the genetics of plants in order to produce desired characteristics. Plant breeding can be accomplished through many different techniques ranging from simply selecting plants with desirable characteristics for propagation, to more complex molecular techniques. Modern plant breeding is applied genetics, but its scientific basis is broader, covering molecular biology, cytology, systematics, physiology, pathology, entomology, chemistry, and statistics (biometrics). It has also developed its own technology.

Classical plant breeding uses deliberate interbreeding (crossing) of closely or distantly related individuals to produce new crop varieties or lines with desirable properties. Plants are crossbred to introduce traits/genes from one variety or line into a new genetic background. For example, a mildew-resistant pea may be crossed with a high-yielding but susceptible pea, the goal of the cross being to introduce mildew resistance without losing the high-yield characteristics. Progeny from the cross would then be crossed with the high-yielding parent to ensure that the progeny were most like the high-yielding parent, (backcrossing). The progeny from that cross would then be tested for yield and high-yielding mildew-resistant plants would be further developed. Plants may also be crossed with themselves to produce inbred varieties for breeding.

AIMS and Objectives of Plant Breeding

The aim of plant breeding is to develop a variety that combines as many of the desirable and beneficial characteristics of economic value as possible. These are

• Increase in Yield: Only high-yielding varieties are selected from the newly developed mutant or hybrid varieties. A higher yield of plant products like grain, fibre, fodder, oil, timber, fruit, etc. is achieved.
• Improved Quality: Quality regarding size, shape, colour, food value, taste, etc. in food grains, vegetables, and fruits is improved. The colour, size, vitamin and juice content in mango, higher sugar content in sugarcane and protein content in pulses, the fine fiber in cotton, etc., can be obtained by plant breeding.
• Improved Structure: Agronomic characteristics, such as plant height, tillering, branching, erecting or trailing habit, etc., are improved.
• Resistance to Disease and Insect Pests: Viruses, bacteria, fungi, and nematodes produce diseases in plants. The genotype of some varieties of plants provides them resistance to such pathogens and other pests or parasites. Such strains of crop plants are called resistant strains.
• Adaptability to New Regions and Climates: Drought-resistant varieties may be developed by changing the genotype of plants.
• Change in the duration of Maturity: Early maturity and early harvest save much labour and care.
• Elimination of Harmful and Toxic Substances: Plants that produce harmful metabolites causing harm to man and animals can be reduced.
• Development of Varieties for Saline Soils: Saline soils are generally less fertile. Hybrid plants may be obtained that can tolerate salinity. It will result in increased use of agricultural land.
• Sensitivity to Light and Temperature: It has been possible to obtain varieties of rice and wheat plants in which their light and temperature requirement is diminished. As a result, their cultivation is possible in a region with less available light and temperature.

Role of Plant Breeding in Organic Agriculture

Critics of organic agriculture claim that it is too low-yielding to be a viable alternative to conventional agriculture. However, a growing body of evidence suggests that poor performance is not an intrinsic property of organic production, but rather the result of growing poorly adapted varieties. It is estimated that over 95% of organic agriculture is based on conventionally adapted varieties, even though the production environments found in organic vs. conventional farming systems are vastly different due to their distinctive management practices. Most notably, organic farmers have fewer inputs available than conventional growers to control their production environments. Breeding varieties specifically adapted to the unique conditions of organic agriculture are critical for this sector to realize its full potential. This requires the selection of traits such as

• Water use efficiency
• Nutrient use efficiency (particularly nitrogen and phosphorus)
• Weed competitiveness
• Tolerance of mechanical weed control
• Pest/disease resistance
• Early maturity (as a mechanism for avoidance of particular stresses)
• Abiotic stress tolerance {i.e., drought, salinity, etc.)

Currently, few breeding programs are directed at organic agriculture and until recently those that did address this sector have generally relied on indirect selection (i.e., selection in conventional environments of traits considered important for organic agriculture). However, as the difference between organic and conventional environments is large, a given genotype may perform very differently in each environment due to an interaction between genes and the environment (see gene-environment interaction). If this interaction is severe enough, an important trait required for the organic environment may not be revealed in the conventional environment, which can result in the selection of poorly adapted individuals. To ensure that the most adapted varieties are identified, advocates of organic breeding now promote the use of direct selection (i.e., selection in the target environment), of many agronomic traits.

Methods of Plant Breeding

Plant breeding started at the beginning of the 20th century after the rediscovery of Mendel’s work. Today plant breeding has become a specialized technology based on genetics. In India, Plant breeding is actively carried out at Indian Agricultural Research Institute, Delhi; Central Potato Research Institute, Cuttack; Sugarcane Research Institute, Coimbatore and at a number of agricultural universities and research stations. Some points must be kept in mind before plant breeding works. These are habit, habitat of the concerned plant and their life cycle, floral structure, and mode of reproduction. The source of variability for selection and crossing should be kept in mind. Crop Improvement has the Following Aspects.

1. Plant Introduction

The process of introducing new plants from their growing place to a new locality with different climates is termed Plant Introduction. The adjustment of such plants to their new locality is called Acclimatization. The new crops or the new varieties may be introduced in the form of seeds or cuttings. In vegetatively propagated crops, the cuttings are imported and in sexually propagated crops, the seeds are imported. Introductions from other countries are valuable sources of germplasm. The introduction of the IR-8 variety of rice and dwarf Mexican varieties of wheat contributed much to the green revolution in India around 1970. Such introductions have given us many of our valuable crops like potatoes, tomatoes, cauliflower, soybeans, grapes, guava, etc.

Along with introduced materials, some unwanted and harmful weeds, insect pests and diseases also entered in our country. Argemone mexicana is an example of a weed that entered India with an introduction. Therefore, all the introductions are carefully examined for the presence of harmful materials like weeds, insects, plant pests, and other disease-causing organisms. This process is known as quarantine. Quarantine is the prohibition of the entry of plants or plant materials into the country having infectious diseases. There are two types of plant introduction

• Primary Introduction: When a selected new variety is properly developed in a new environmental condition and is economically cultivated without change in genotype, it is called primary introduction, e.g., the dwarf variety of wheat called Sonara-64 and the dwarf variety of rice called IR-8.
• Secondary Introduction: When a selected variety is hybridized with a local variety and the incorporation of new characters takes place, then it is used for agricultural purposes. This process is known as secondary introduction, e.g., Kalyansona or Sonalika (varieties of wheat).

Purpose of Introduction:

• Hybridization in collected plants for the production of new varieties.
• Collection of plants from different sites or conservation of their seeds, i.e., called germplasm conservation.
• To get a proper concept of the origin and evolution of collected plant material.
• To collect wood, fiber, and food from plants in different places.

The procedure of Plant Introduction:

• Procurement of Plants or Germplasm: Germplasm is mainly collected from NBPGR (National Bureau of Plant Genetic Resources) or from IBPGR (International Bureau of Plant Genetic Resources).
• Packing and Despatch: The reproductive propagules of plants, seed, tuber, root or their cutting or runner, etc., are sterilized and transported to different sites.
• Plant-Quarantine: Plant quarantine is a technique for ensuring disease-free and pest-free plants by isolating them during a period while performing tests for the presence of problems.
• Cataloguing: Catalogues will be made on the basis of the origin of plants. For example, In the case of exotic plants ‘EC’ type abbreviation or in the case of indigenous plants ‘IC’ type abbreviation, or in the case of indigenous wild plants ‘IW’ type abbreviation is added before their name. [Where EC = Exotic collection; IC = Indigenous collection and IW = Indigenous wild]
• Multiplication and Distribution: This is the final stage, where the multiplication and distribution are done in a nearby area of selected plants.

Merits of Plant Introduction:

• The new species variety is formed by the hybridization of selected plants.
• The proper modification of crop varieties is done by the introduction.
• High-yielding, disease-resistant plant varieties will be formed.
• Through this process, a new plant species is brought from a new place.

Demerits of Plant Introduction:

• During the introduction, several exotic weeds (e.g., Argemone, Lantana) entered the agricultural fields.
• Due to the collection of exotic plant species, several plant diseases like rust of coffee, spread over our country.

2. Selection

Selection is an important step in all breeding experiments which is being practiced by man since the early days of agriculture. The selection involves picking up the better ones out of the entire crop plants. The selected plants are separated from the inferior ones and are favoured by reproducing them under controlled conditions. The process involves picking up the high-yielding, healthy, disease-free crop plants with desired characteristics out of the entire crop plants for next time cultivation. Selection may be of two main types Natural selection and Artificial selection.

A. Natural Selection:
This type of selection is a normal rule in nature and has resulted in evolution. Charles Darwin and A.R. Wallace (1858) have explained natural selection and according to this concept of natural selection, the fittest can survive and the rest are wiped out. Natural selection has produced the cultivated crops and ecotypes in plants. Such ecotypes are the basis of present-day artificial selection and hybridization. By the process of natural selection, favorable traits are transmitted through generations.

B. Artificial Selection:
It means choosing certain superior individual plants for the purpose of having better crops from a mixed population in which individuals differ in character. There are three patterns of artificial selection Mass selection, Pure line selection, and Clonal selection.

(i) Mass Selection:
This is the most common and old method of selection. Selection is the breeding stock of those members of a population exhibiting desirable qualities or elimination of those showing undesirable qualities i.e., phenotypic selection. This selection procedure depends on the selection of plants on the basis of their phenotype and performance.

• The procedure is selecting a number of plants, heads of seeds phenotypically superior in desired traits from the field population.
• Harvesting and bulking their products together for sowing the next year’s crop.
• This process is repeated till the desired characters are obtained uniformly. It actually takes about eight years to produce a new variety.
• In the next year, a new variety of seeds are sowed.

Mass selection is a quick and easy method of crop improvement but it has the following drawbacks:

• It is not very effective in bringing about an increase in the yield which is dependent mostly on the environment.
• It is not possible to know whether the superior phe¬notype is due to a better genotype or due to favourable environment.
• Selected characters of the crop exhibit segregation because of natural cross-pollination.
• The selection is made from the female individuals and there is no check on the male plant which contributes equally to the progeny.

In spite of certain drawbacks, mass selection is the most common method of crop improvement among farmers. Improvement of many important plants, such as maize, watermelon, radish, grapes, apples, onion, pear, etc., are the result of mass selection.

Some Plant Varieties Obtained from Artificial Selection:

Plant	Varieties
1. Paddy	Co-4, Nu-3
2. Wheat	Kalyansona, NP-6, K-13
3. Mango	Neelam
4. Potato	Kufri red, Kufri safed
5. Tobacco	NP-28, T-59
6. Banana	Highgate, Bombay green

(ii) Pure Line Selection:
The pure line is the progeny of a single homozygous, self-pollinated plant. W.L. Johanssen (1903) first established the pure line theory.

• The single plants with desired traits or traits are selected out of the variable population in the field.
• Seeds from selected plants are sown in separate rows to produce a progeny by self-pollination.
• Desired plants are again selected from this progeny and self-pollinated. This is continued for several generations.
• The inferiors are eliminated from each generation. Wheat varieties like Kalyan-227 and PV-18 have been developed at Ludhiana by this type of selection.

Pure line selection is better than mass selection because the selected plants retain desirable characters for several generations. But the main drawback of this method is that it takes 7-12 years for raising the desired variety.

(iii) Clonal Selection:
It is practiced in vegetatively propagated plants like sugarcane, banana, potato, etc. In this method, the best plant-based on phenotypic characters is selected. It is then multiplied vegetatively and supplied to the farmers. A population of plants raised from a single vegetatively propagated plant is called a clone. This selection procedure is used for asexually propagated crops.

• In the first year, desired plants are selected from a population.
• Next year, the desired plants are propagated vegetatively i.e., cloned.
• In the Third year, from these cloned plants best varieties are selected and examined in different conditions such as environment, soil, etc., year after year. The suitable one is selected as a new variety.
• In the eighth year, this new cloned variety is supplied to the farmers.

The main advantages of this method are that a desired clone can be obtained within one year, and varieties are stable and easy to maintain. The main drawback of this type of selection is that this method is applicable only to vegetatively propagated crops and further it does not produce any new variation.

Differences between Mass Selection and Pure-line Selection:

Mass Selection	Pure Line Selection
1. This is a procedure for selecting a number of plants that are phenotypically superior in desired traits from the field population.	1. It is a procedure for selecting a single plant with the desired trait from the variable population.
2. It is not very effective in bringing about an increase in the yield.	2. It is better because the selected plants retain desirable characteristics for several generations.
3. Improvement in maize, grapes, and apple production is the result.	3. Wheat varieties have been developed by using this selection method.

3. Hybridization

Hybridization is the technique of introducing characters of two desirable species into a single offspring (hybrid) by means of artificial pollination to create new genetic combinations. In this process of cross-breeding, genetically different parents produce a hybrid and resulting polyploid offspring. Hybrids are first-generation (F1) crosses between genetically different parents. Hybrids differ in their vigour, growth, size, and yield. As a result of hybridization, hybrid varieties of cereals, oil seeds, pulses, sugar beet, onion, tomato, and fruits have been developed.

Depending upon the parents involved, hybridization may be of the following types:

• Intravarietal Hybridization: Cross between two different plants of the same variety.
• Intervarietal or Intraspecific Hybridization: Cross between two different varieties of the same species. Varieties of cotton, crossing between Gossypium arboreum and Gossypium hirsutum.
• Interspecific Hybridization: Cross between two different species of the same genus, e.g., a cross between two wheat plants of the same species.
• Intergeneric Hybridization: Cross between two different genera, e.g., Triticale (Triticum aestivum × Secale cereale).
• Introgressive Hybridization: By a back-crossing system, some desired genes (disease resistant) of one species may be transferred into the genotype of another species (disease susceptible).
• Transgressive Hybridization: Desirable genes from two different parents are introduced to form an off-spring plant with completely separate characteristic features.

Aims of Hybridization:

• To bring all possible good characters into a single variety.
• Creation of genetic variability by introducing various combinations of genes.
• Utilization of hybrid vigour plants.
• To create genetic variability.
• Improvement of one or more quantitative characters.

Technique of Hybridization

It involves the following steps.

(i) Selection and Isolation of Plants:
First, the plants to be used as male and female are selected. Among both the parents one should be well adapted to the area and another one should possess desired characteristics. These plants are induced to flower at the same time. If required the selected plants are undergone self-breeding to bring about homozygosity. In the case of cross-pollinated plants isolation must be done very carefully.

(ii) Emasculation:
The removal of anthers from the female parents before their dehiscence is called emasculation. When two parent plants have bisexual flowers, the stamens of the female plant are removed in the bud stage. This is to prevent cross-pollination. The process of emasculation differs in different plants. Some common methods are
(a) Forcep or Scissor Method:
It consists opening of the flower and removing the stamens which can be done easily by using a pair of forceps or scissors.

(b) Hot or Cold Water and Alcohol Emasculation:
In plants having small-sized flowers, such as rice (Oryza sativa), sorghum (Sorghum vulgare), and bajra (Pennisetum typhoides), etc., emasculation is done by dipping the panicles in hot water having a desired temperature for a definite period. Similarly, cold water or alcohol emasculation is carried out.

(c) Male-Sterility Method:
In some self-pollinated plants, such as barley, sorghum, onion, and bajra the emasculation operation may be eliminated by the use of male-sterile plants which have sterile anthers. Male sterility is created by the manipulation of such genes. Male sterility may also be induced by spraying some chemicals, such as 2, 4-D, Naphthalene acetic acid (NAA), Maleic hydrazide (MH), and tri-iodobenzoic acid, etc.

(iii) Bagging:
The flowers on female plants are kept in isolation by enclosing them in bags so that no foreign pollen may fall on the stigma. This process is called bagging. Paper or polythene bags or muslin clothing are used for this. The flowers on the male plants are also covered by the bags so that the pollen grains may not be contaminated by unwanted foreign pollen.

(iv) Crossing:
Bags are opened at the time when the act of pollination is to be performed. After that, the female cross-pollinated flower is again bagged. If the plants are dioecious, the male plants are completely eliminated from the vicinity of the female plants which are to be cross-pollinated by pollen of a desired species. Pollination is done by brushing or dusting collected pollens over the receptive stigma. In most of the crops the stigmas are receptive in the morning hours and so crossing at that time is most effective.

(v) Labelling:
The crossed flowers are properly tagged and labeled. The labeling is done either on the bag itself or on the labels specially designed for this purpose. The labeling should have the following information:

• Date of emasculation
• Date of crossing
• Details of parents, male and female

(vi) Harvesting of Crop and Collection of Hybrid Seed:
The bags are removed and the crossed heads of desirable characters are harvested and collected with their attached labels separately in envelopes. After complete drying, they are threshed individually and preserved as such. In the coming season, these seeds are sown separately to raise the F1 generation. The plants of the F1 generation are progenies of crossed seeds and therefore, are hybrids.

Post-hybridization Selection Procedure

The seeds of the F1 generation and subsequent generations are then collected by different selection methods. These methods are different for self and cross-pollinated crops. Here two methods adopted for self-pollinated crops are described as follows:

(i) Pedigree Method:
The individual plants are selected from the Fj population on the basis of desired characteristics. These plants are harvested and threshed separately, and sown in separate rows in the next year to raise F2 generation. The plants possessing desirable characteristics and found disease resistant are harvested and threshed separately, and the seeds of each plant are sown in a separate row in the next year to raise F3 generation. This process is continued upto F5 generation. At F6, due to successive self-pollination, most of the lines become homozygous and fairly uniform. The plants uniform in desired characteristics are harvested and bulked together to constitute a variety. These varieties are then tested and given field trials. The seeds of superior variety thus obtained are multiplied and distributed to farmers. The whole process takes about 10 to 13 years.

The pedigree method is well-suited to those crops where the characters to be combined in crosses can be easily seen and recognized (e.g., awned or awnless characters). This method is quickest and a new variety can be selected and well-tested with their genetic information. On the other hand, this method is very expensive and disadvantageous due to the consumption of more attention and labour for keeping the clear pedigree record of each selected plant separately.

(ii) Bulk Method:
This system of breeding differs from the pedigree method. In that case, the Fg plants are not maintained separately but are bulked together to form a single F3 population. In F3, again the suitable plants are selected, collected, and bulked together. This bulking is done for six generations. In F6, the desired individuals are selected and harvested separately. The products of each plant is kept separate and carried out under comparative trials. The best performers are released as new varieties. The bulk method is a simple, convenient, and inexpensive method. It takes more time but minimizes labour as the plants are not paid any individual attention.

Differences between Pedigree Method and Bulk Method:

Pedigree Method	Bulk Method
1. Individual plants are selected and bulked at the end.	1. The selected plants are bulked and individuals are tested separately at the end.
2. Less time is required in producing a variety.	2. More time is required in producing a variety.
3. It is tedious and complicated.	3. It is simple and convenient.
4. More expensive.	4. Inexpensive.

Some hybrid plants obtained by the pedigree method:

• Paddy: Jaya, Padma (hybrid between Taichung native 1 × 141).
• Wheat (K65): C591 × NP773.
• Tomato: Pusa variety (Maruti x red cloud) variety.

Breeder’s Kit:
During plant breeding, the instruments used are known as the breeder’s kit. These are hand lenses, forceps, scissors, needles, scalpels, brushes, tags, spirit, polybag or muslin cloth, thread, etc.

4. Polyploid Breeding

Polyploid cells and organisms are those containing more than two paired (homologous) sets of chromosomes. Most eukaryotic species are diploid, meaning they have two sets of chromosomes one set inherited from each parent. However, polyploidy is found in some organisms and is especially common in plants. Polyploidy can be induced in plants and cell cultures by some chemicals the best known is colchicine, which can result in chromosome doubling, though its use may have other less obvious consequences as well. Oryzalin also will double the existing chromosome content.

Polyploidy in Plants:
Most of the plants are diploids (2n). i.e., they possess two sets of chromosomes (genome). The plant which contains more than two complete sets of chromosomes, is called polyploid. In nature, polyploids may evolve from diploid plants through chromosome doubling in somatic cells during mitosis that carries irregularities giving rise to meristematic cells that perpetuate the irregularities in the next generation of plants. They may also occur in reproductive cells with an irregular reductional or equational division in which chromosome sets fail to separate themselves completely during anaphase movement. Depending upon the number of chromosome sets, the individuals are given different names monoploids, diploids, triploids, tetraploids, pentaploids, and hexaploids (e.g., wheat). Polyploids are characterized by gigantism or an increase in cell size and hence organ size and thus overall size. These polyploids are used in crop improvement, e.g., triploids are present naturally in different crop plants e.g., an autotetraploid variety of tea in India. Another example of triploids includes seedless bananas, watermelon, apple, pear, etc.

Polyploidy is pervasive in plants and some estimates, suggest that 30-80% of living plant species are polyploid and many lineages show evidence of ancient polyploidy (paleopolyploidy) in their genomes. Huge explosions in angiosperm species diversity appear to have coincided with the timing of ancient genome duplications shared by many species. It has been established that 15% of angiosperm and 31% of fern speciation events are accompanied by a ploidy increase. Polyploid plants can arise spontaneously in nature by several mechanisms, including meiotic or mitotic failures and fusion of unreduced (2n) gamete. Both autopolyploids (e.g., potato) and allopolyploids (e.g., canola, wheat, cotton) can be found among both wild and domesticated plant species. Most polyploids display heterosis relative to their parental species and may display novel variations or morphologies that may contribute to the processes of speciation and eco-niche exploitation.

The mechanisms leading to novel variation in newly formed allopolyploids may include gene dosage effects (resulting from more numerous copies of genome content), the reunion of divergent gene regulatory hierarchies, chromosomal rearrangement, and epigenetic remodeling all of which affect gene content or expression levels. Many of these rapid changes may contribute to reproductive isolation and speciation. There are few naturally occurring polyploid conifers. One example is the giant tree Sequoia sempervirens or Coast Redwood which is a hexaploid (6x) with 66 chromosomes (2n = 6x = 66), although the origin is still unclear. Aquatic plants, especially those in Monocotyledons, include a large number of polyploids.

Polyploid Crops:
The induction of polyploidy is a common technique to overcome the sterility of a hybrid species during plant breeding. For example, Triticale is the hybrid of wheat (Triticum turgidum) and rye (Secale cereale). It combines the sought-after characteristics of the parents, but the initial hybrids are sterile. After polyploidization, the hybrid becomes fertile and can thus be further propagated to become triticale. In some situations, polyploid crops are preferred because they are sterile.

For example, many seedless fruit varieties are seedless as a result of polyploidy. Such crops are propagated using asexual techniques such as grafting. Any agent that interferes with spindle formation during mitosis might result in chromosomal doubling. Induced polyploidy was first demonstrated by subjecting growing plants to a temperature that is higher than the usual optimal temperature. In certain cells, an increase in chromosome number was noticed. Such cells when propagated through germinal tissues, whole polyploid plants were produced, e.g., Maize. Similarly, scar tissues from tomatoes enabled shoots with cells in which chromosome number was found to increase. New polyploid plants could be produced from such cells.

The method of inducing polyploidy in plants that have become popular was developed by A. F. Blakeslee, A. G. Avery, and B. R. Nebel was found that colchicine, an alkaloid extracted from Autumn crocus, Colchicum autumnale, could disturb spindle formation during cell division.

Examples of Polyploid Crops:

• Triploid crops: Apple, banana, citrus, ginger, watermelon.
• Tetraploid crops: Apple, durum or macaroni wheat, cotton, potato, cabbage, leek, tobacco, peanut, kinnow, pelargonium.
• Hexaploid crops: Chrysanthemum, bread wheat, triticale, oat, kiwifruit.
• Octaploid crops: Strawberry, dahlia, pansies, sugarcane, oca (Oxalis tuberosa).

Some crops are found in a variety of ploidies: tulips and lilies are commonly found as both diploid and triploid; daylilies (Hemerocallis cultivars) are available as either diploid or tetraploid; apples and minnows can be diploid, triploid or tetraploid.

Mechanisms of Polyploidy Formation:
There are several spontaneous cytological mechanisms discussed below.

• The non-reduction of gametes during meiosis is called meiotic nuclear restitution (MNR). Here the formed gametes (2n) contain the somatic nuclear condition of the cells. The meiotic aberrations related to spindle formation, spindle function, and cytokinesis have been implicated in this process.
• By another route, during mitosis, there is a formation of polyploids through the somatic doubling of chromosomes.

In nature, the formation of polyploids as a result of mitotic aberrations has been reported in the meristematic tissue of several plant species including tomato, and in non-meristematic tissues of plants such as bean. In addition, an uncommon mechanism of polyploid formation involves polyspermy where one egg is fertilized by several male nuclei as commonly observed in orchids (Ramsey and Schemske, 1998). It is also observed that by the process of diploidization, the polyploid plants revert back to the diploid stage. The major pathways involved in polyploidy formation are represented.

Polyploid Production

Polyploid organisms have three or more complete sets of chromosomes. As might be expected, they are produced by processes that duplicate chromosome sets. They arise from preexisting organ¬isms via the addition of one or more entire extra sets of chromosomes. Diploid organisms have two sets of chromosomes. Haploid organisms have only one. Common bread wheat (Triticum aestivum) is an example of a common amphiploid. This hexaploid came into being about 7,000 years ago from hybridization between rivet wheat (T. turgidum), a tetraploid, and Tausch’s goatgrass (Aegilops tauschii), a diploid.

In fact, Talbert et. al., (1998) say it arose repeatedly as the result of multiple hybridization events. Rivet wheat itself is a much older, natural amphiploid derived from hybridization between two diploid grasses. One was the goatgrass Aegilops speltoides, and the other was either wild einkorn (Triticum bootcut) or another wild wheat, T. urartu. Another natural amphiploid is Plagiomnium medium (medium plagiomnium moss), shown by Wyatt et al. (1988) to be derived from hybridization between P. ellipticum (elliptic plagiomnium moss) and P. insigne (Plagiomnium moss).

Allopolyploids develop due to hybridization between two distinct species followed by the doubling of chromosomes by colchicine. Allotetraploid is the common type. More than half of the cultivated plants are allopolyploids such as wheat, cotton, oats, sugarcane, tobacco, and plums. Common bread wheat (Triticum aestivum) is one of the best examples of allohexaploids. Artificially developed allopolyploid is Triticale which is the first man-made crop derived by crossing wheat (Triticum) and rye (Secale). This new cereal gives better bread and is well adopted in sandy soil where normally wheat cannot be grown.

Applications of Polyploid Breeding

(i) Production of Apomictic Crops:
Apomixis provides another avenue for the use of polyploids in breeding. Apomixis provides an avenue for the production of seeds asexually through parthenogenesis. Most apomictic plants are polyploid but most polyploid plants are not apomictic. In plants capable of both sexual and asexual reproduction, polyploidy promotes the latter. Obligate apomicts are the most desired of hybrids but little gain has been realised towards their development. However, it has been suggested that obligate apomicts may be induced through the development of very high ploidy plants. An example of an obligate apomict achieved at a high ploidy level is the octoploid of the grass, Themeda triandra.

(ii) Disease Resistance Through Aneuploidy:
Aneuploidy has been applied in breeding to develop disease-resistant plants through the addition of an extra chromosome into the progeny genome. An example is the transfer of leaf rust resistance to Triticum aestivum from Aegilops umbellate through backcrossing. In addition, other breeding strategies utilizing aneuploidy have been explored including chromosome deletion, chromosome substitution, and supernumerary chromosomes.

(iii) Seedless Fruits:
The seedless trait of triploids has been desirable, especially in fruits. Commercial use of triploid fruits can be found in crops such as watermelons and are produced artificially by first developing tetraploids which are then crossed with diploid watermelon. In order to set fruit, the triploid watermelon is crossed with a desirable diploid pollen donor.

(iv) Bridge Crossing:
Another breeding strategy that uti¬lizes the reproductive superiority of polyploids is bridge crossing. When sexual incompatibilities between two species are due to ploidy levels transitional crosses can be carried out followed by chromosome doubling to produce fertile bridge hybrids. This method has been used to breed far superior tall fescue grass (E arundinacea) from Italian ryegrass (2n = 2x = 14) and tall fescue (2n = 6x = 42) by using meadow grass (Fescue pratensis) as a bridge species (Acquaah, 2007). The same principle has been applied in fixing heterozygosity in hybrids by doubling the chromosomes in the superior progeny (Comai, 2005).

(v) Industrial Applications of Polyploidy:
Chromosome doubling is reported to have an apparent effect on many physiological properties of a plant. The most discernable of these has been the increase in secondary as well as primary metabolism. The resulting increase in secondary metabolites, in some cases by 100%, after chromosome doubling has been widely exploited in the breeding of narcotic plants such as Cannabis, Datura, and Atropa. In vitro, secondary metabolite production systems that exploit polyploids have also been developed. The production of the antimalarial sesquiterpene artemisinin has been enhanced sixfold by inducing tetraploids of the wild diploid Artemisia annua L. (clone YUT 16). In addition, commercial synthesis of sex hormones and corticosteroids has been improved significantly by artificial induction of tetraploids from diploid Dioscorea zingiberensis native to China. Attempts have been made to improve the production of pyrethrin, a botanical insecticide, by chromosome doubling of Chrysanthemum cinerariifolium.

5. Induced Mutation

A sudden heritable change in the genetic element of the living organism is called a mutation. Mutation can occur due to changes in the Base sequence of the concerned gene, Chromosome structure, and Chromosome number.

In genetics, a mutation is a change in the nucleotide sequence of the genome of an organism, virus or extrachromosomal genetic element. Mutations result from unrepaired damage to DNA or to RNA genomes (typically caused by radiation or chemical mutagens), from errors in the process of replication or from the insertion or deletion of segments of DNA by mobile genetic elements.

The mutation may occur in nature at a very low rate and is called Spontaneous mutation or may be induced artificially and is called induced mutation. The agent causing mutation is called mutagen. Mutagen may be of two types namely Physical mutagen and Chemical mutagen. Mustard gas, nitrous acid, ethyl methane sulphonate (EMS), acridine orange, etc. are examples of chemical mutagens, and X-rays, UV-rays, and gamma-rays are several physical mutagens.

Some examples of crop improvement by mutation breeding are

• The giant variety at gram (Cicer gigass) and of ground nut (Arachis hypogea).
• The Sharbati Sonora, Sonara-64, Pusa Lerma, and Lerma Rojo 64-A are dwarf wheat varieties.
• Semi-dwarf variety of rice is resistant to lodging.
• High-yielding varieties of barley, early maturing varieties of castor plant.
• Seedless fruits like grapes, bananas, oranges, watermelon, etc.
• A new sugarcane variety matures in 10 months instead of 18 months taken by old varieties.

The use of mutation in breeding has given us over 200 crop varieties in our country. Mutation breeding is the process of exposing seeds to chemicals or radiation in order to generate mutants with desirable traits to be bred with other cultivars. Plants created using mutagenesis are sometimes called mutagenic plants or mutagenic seeds. From 1930-2007 more than 2540 mutagenic plant varieties have been released that have been derived either as direct mutants (70%) or from their progeny (30%). Crop plants account for 75% of released mutagenic species with the remaining 25% ornamentals or decorative plants. However, it is unclear how many of these varieties are currently used in agricultural production around the world, as these seeds are not always identified or labelled as being mutagenic or having a mutagenic provenance.

Mutagenic Varieties:

• Japan: Osa Gold Pear.
• United States: Rio Star Grapefruit; Todd’s Mitcham Peppermint; (Verticillium wilt tolerance); Murray Mitcham Peppermint (Verticillium wilt tolerance); Calrose 76 Rice (short height rice induced with gamma rays).
• People’s Republic of China: Purple Orchard 3 sweet potato; Zhefu 802 (rice mutant).
• India: PNR-381 Rice; Sharbati Sonora wheat; ‘MUM 2’, ‘BM 4’, ‘LGG 407’, ‘LGG 450’, ‘Co4’, ‘Dhauli’ (TT9E), ‘Pant moong-1’ black gram (YMC, [Yellow mosaic virus] resistance).
• Italy: Creso Wheat.
• Pakistan: Basmati 370 (short height rice mutant); NLAB-7 (high yielding, heat tolerant, early maturing cotton mutant); CM-72 (high yielding, blight resistant, desi type chickpea mutant created with 150 Gy of gamma rays); NM-28 (short height, uniform and early maturing, high seed yield mungbean mutant); NIAB Masoor 2006 (early maturing, high yield, resistant to disease lentil mutant created with 200 Gy of radiation.
• Thailand: RD 16 and RD 6 (aromatic indica rice mutant created with gamma rays).
• Czech Republic: Diamant barley (high yield, short height mutant created with X-Rays).
• United Kingdom: Golden Promise barley (semi-dwarf, salt-tolerant mutant created with gamma rays) is used to make beer and whiskey.

6. Heterosis

Heterosis refers to the phenomenon that progeny of diverse varieties of a species or crosses between species exhibit greater biomass, speed of development, and fertility than both parents. The superiority of the hybrids over the parents is known as heterosis or hybrid vigour. Hybrid vigour is used as a synonym for heterosis. It is generally agreed upon, that hybrid vigour describes only the superiority of hybrid over parents while heterosis describes other conditions like weak hybrids as well. Whaley (1944) opined that it would be more appropriate to term the developed superiority of the hybrids as hybrid vigour and to refer to the mechanism by which superiority is developed as heterosis.

It is manifested in various forms such as vigorous growth, higher yield, greater resistance to disease and pests, early flowering, and greater length of life. Plant and animal breeders mate two different purebred lines in order to exploit heterosis and inculcate desirable traits in the hybrid progenies. Several varieties of plants exhibit hybrid vigour. Hybrid corn is an excellent illustration,’ of hybrid vigour. Its yield is two or three times that of the pure varieties. Corn heterosis was demonstrated by George H. Shull and Edward M. East after hybrid corn was invented by Dr. William James Beal of Michigan State University based on work begun in 1879. Hybrid vigour has also been exploited in the improvement of sorghum, sugar beet, grasses, tomatoes, onion, squash, cucumber, tobacco, and several other ornamental and cereal plants.

The majority of our fruit trees, vegetables, and garden plants are the result of so many cross-pollinations that they are heterozygous for most of their desirable genes and possess hybrid vigour to a marked degree. They seldom come true by seed because of segregation and recombination and resulting homozygosity for many genes which cause loss of hybrid vigour. For this reason, many of them are propagated vegetatively. Thus apples and pears are multiplied by grafting, peaches and plumes by bud¬ding, grapes by cutting, etc. Ornamental plants like dahl¬ias, Chrysanthemum, ropes, and the farm plants like white potatoes, sweet potatoes, and sugarcane rarely come true by seed because they are hybrid. Hybridization produces valuable plants and vegetative propagation preserves their characters and vigour.

Effects of Heterosis on Plant Breeding

• Increase Yield: Heterosis is generally expressed as an increase in the yield of hybrid and may be measured in terms of grain, fruit, seed, leaf, tubers, etc.
• Increased Reproductive Ability: Hybrids exhibiting heterosis show an increase in fertility or reproductive ability.
• Increase in Size and General Vigour: The hybrids are generally with more vigour, healthier, and faster growing.
• Better Quality: In many cases, hybrid show improved quality, e.g., in Onion keeping quality.
• Earlier Flowering and Maturity: Hybrids have earlier flowering and maturity than the parents, e.g., Tomato.
• Greater Resistance to Disease and Pests: Hybrid exhibits greater resistance to insects and diseases than parents.
• Greater Adaptability: Hybrids are more adapted to environmental changes than inbreds.
• Faster Growth Rate: Hybrids show faster growth rates than their parents but the total size may be comparable to that of the parent.
• Increase in Number of Plant Parts: In some cases, there is an increase in the number of nodes, leaves, and other plant parts, but the total plant size may not be larger.

Use of Heterosis in Plant Breeding:
Heterosis is exploited through the development of a hybrid. It is commercially exploited in seed production of cross-pollinated crops like jowar, maize, bajra, onion, and cucurbits. It has been also used in some self-pollinated species such as rice, wheat, tomato, and brinjal, etc.

7. Gene Technology

The improvement of crops with the use of genetics has been occurring for years. Traditionally, crop improvement was accomplished by selecting the best-looking plants/seeds and saving them to plant for the next year’s crop. Once the science of genetics became better understood, plant breeders used what they knew about the genes of a plant to select specific desirable traits. This type of genetic modification, called traditional plant breeding, modifies the genetic composition of plants by making crosses and selecting new superior genotype combinations. Traditional plant breeding has been going on for hundreds of years and is still commonly used today.

Plant breeding is an important tool, but has limitations. First, breeding can only be done between two plants that can sexually mate with each other. This limits the new traits that can be added to those that already exist in that species. Second, when plants are mated (crossed), many traits are transferred along with the trait of interest including traits with undesirable effects on yield potential. Genetic engineering is a new type of genetic modification. It is the purposeful addition of a foreign gene or genes to the genome of an organism.

A gene holds information that will give the organism a trait. Genetic engineering is not bound by the limitations of traditional plant breeding. Genetic engineering physically removes the DNA from one organism and transfers the gene(s) for one or a few traits into another. Since crossing is not necessary, the ‘sexual’ barrier between species is overcome. Therefore, traits from any living organism can be transferred into a plant. This method is also more specific in the fact that a single trait can be added to a plant.

Step 1. DNA Extraction:
The process of genetic engineering requires the successful completion of a series of five steps. DNA extraction is the first step in the genetic engineering process. In order to work with DNA, scientists must extract it from the desired organism. A sample of an organism containing the gene of interest is taken through a series of steps to remove the DNA.

Step 2. Gene Cloning:
The second step of the genetic engineering process is gene cloning. During DNA extraction, all of the DNA from the organism is extracted at once. Scientists use gene cloning to separate the single gene of interest from the rest of the genes extracted and make thousands of copies of it.

Step 3. Gene Design:
Once a gene has been cloned, genetic engineers begin the third step, designing the gene that will be able to work once inside a different organism. This is done in a test tube by cutting the gene apart with enzymes and replacing it with gene regions that have been separated.

Step 4. Transformation:
The modified gene is now ready for the fourth step in the process, transformation or gene insertion. Since plants have millions of cells, it would be impossible to insert a copy of the transgene into every cell. Therefore, tissue culture is used to propagate masses of undifferentiated plant cells called callus. These are the cells to which the new transgene will be added.

The new gene is inserted into some of the cells using various techniques. Some of the more common methods include the gene gun, Agrobacterium, microfibers, and electroporation. The main goal of each of these methods is to transport the new gene(s) and deliver them into the nucleus of a cell without killing it. Transformed plant cells are then regenerated into transgenic plants. The transgenic plants are grown to maturity in greenhouses and the seed they produce, which has inherited the transgene, is collected. The genetic engineer’s job is now complete. He/she will hand over the transgenic seeds to a plant breeder who is responsible for the final step.

Step 5. Backcross Breeding:
The fifth and final part of producing a genetically engineered crop is backcross breeding. Transgenic plants are crossed with elite breeding lines using traditional plant breeding methods to combine the desired traits of elite parents and the transgene into a single line. The offspring are repeatedly crossed back to the elite line to obtain a high-yielding transgenic line. The result will be a plant with a yield potential close to current hybrids that expresses the trait encoded by the new transgene.

The Process of Plant Genetic Engineering:
The entire genetic engineering process is basically the same for any plant. The length of time required to complete all five steps from start to finish varies depending on the gene, crop species, available resources, and regulatory approval. It can take anywhere from 6-15+ years before a new transgenic hybrid is ready for release and to be grown in production fields.