Steps Involved in the Plant Breeding Process: 4 Steps

In this article, we will discuss about the four steps involved in the plant breeding process.

The four steps are: (1) Creation of genetic variation by various means (2) Selection (3) Evaluation and Release as a variety and (4) Seed multiplication and distribution among farmers.

1. Domestication:

Domestication is the process of growing plants and keeping animals under human care and management. This is the very first step aimed at increasing food production.

 

 

2. Germplasm Collection:

a. Germplasm is the sum of all the genes present in a crop, and it includes

(i) All the wild species related to the crop species,

(ii) Cultivated improved varieties,

(iii) Improved varieties that are no more cultivated, and

(iv) Old local or ‘desi’ varieties.

b. Collection of germplasm from different sources is an essential first step in any breeding work.

c. Germplasm is usually stored at a low temperature in the form of seeds.

d. Germplasm collection is done from within the country or from Other countries.

3. Plant Introduction:

a. It is the process of introducing plants or germplasms either from a foreign country or introducing plants or germplasm from one region to other regions of the same country.

b. Plant introduction is followed by acclimatisation, i.e., the adaptation of an individual plant or a population of plants, under the changed climate.

c. Introduction of plants from a foreign country is called intercontinental plant introduction.

For example:

(i) Groundnut has been introduced in India from Brazil,

(ii) Rubber has been introduced from South and Central America to India,

(iii) Mexican wheat varieties have been introduced from Mexico to India.

d. Examples of introduced ornamental plants are innumerable, such as Jacaranda, Bougainvillea, Salvia, Cosmos, Dianthus, Antirrhinum etc.

e. Introduction of plants from one state of a country to another state of the same country is called interstate plant introduction. For example, N.P. wheat varieties were introduced from Delhi to different states of India.

f. Purposes of Plant Introduction

(i) For use in agriculture, forestry and industry.

(ii) For genetical improvement of economical crops.

(iii) For studying the origin, distribution, classification and evolution of the plants.

g. Plant Introduction in India:

Following agencies carry out plant introduction in India:

(i) Plant Introduction Division of IARI, New. Delhi,

(ii) Forest Research Institute, Dehradun.

(iii) Botanical Survey of India.

(iv) Some universities, gardens and agricultural departments also play an important role in introducing plants.

Disadvantages of Plant Introduction:

Along with economically important plants, introduction of harmful crop diseases, insect pests and weeds also occurs sometimes.

Diseases Introduced:

(i) Late blight of potato (Phytophthora infestans)

(ii) Fire blight of apple and pear (Erwinia amylovora)

Insect Pests Introduced:

(i) Potato tuber moth

(ii) Woolly aphis

Weeds Introduced:

(i) Argemone mexicana, Lantana.

All introductions are subjected to quarantine, i.e., they are examined for the presence of insects, weeds and disease-causing organisms, and only those introductions that are free from the above are allowed to enter a country.

4. Hybridization:

Hybridization may be defined as “The mating or crossing of two plants or lines of dissimilar genotype.” The chief objective of hybridization is to create genetic variation. When two genotypically different plants are crossed, the genes from both the parents are brought together in F₁ generation.

Segregation and recombination produce many new gene combinations in F₂ and the later generations, i.e., segregating generations. The degree of variation produced in the segregating generations would, therefore, depend on the number of heterozygous genes in the F₁. This will, in turn, depend upon the number of genes for which the two parents differ.

If the two parents are closely related, they are likely to differ for a few genes only. But if they are not related, or are only distantly related, they may differ for several, even a few hundred genes. However, it is most unlikely that the two parents will ever differ for all the genes. Therefore, when it is said that the F₁ is 100 percent heterozygous, it has reference only to those genes for which two parents differ. The aim of hybridization may be transfer of one or few qualitative characters, improvement of one or more quantitative characters, or use of the F₁as a hybrid variety.

Technique of Hybridization:

Before performing hybridisation, a plant breeder should have all the information about the time of flowering, the time when the anther and. stigma are ready for pollination, how long do the pollen grains remain viable, etc.

The actual technique consists of the following steps:

(i) The first step is the selection of parents from the available material possessing desired characters.

(ii) Second step is the selfing of plants to obtain homozygosity in desired characters. This step is not practiced in self pollinated crops because they are already homozygous.

(iii) Third step is emasculation. In this, the anthers are removed before they mature and have shed their pollens. Purpose of emasculation is to prevent self-pollination. It is not practiced in unisexual crops.

(iv) Bagging, tagging and labeling of males as well as females to be used in crosses, is done.

(v) Fifth step is the crossing, in which the pollen from bagged males are dusted on to the bagged female plant.

(vi) Lastly, seeds are collected from the crossed plants after maturity. Seeds are maintained separately and sown in the coming season to raise F₁ generation.

Hybridization in Self-Pollinated Crops:

Several methods, like pedigree method, bulk method and back-cross method are in practice.

Pedigree Method:

In this method, F₁ hybrids possessing desirable characters are selected. The seeds from each plant are collected and grown separately to raise F₂ generation. This process is repeated for a number of generations. Finally, the plants with desired characters are recommended for cultivation.

Bulk Method:

In this method, F₁ hybrids rather than being grown separately are grown in bulk. Seeds, from F₂ plants are also sown together and the process continued for 5-6 generations till homozygosity is obtained.

Back-cross Method:

F₁ hybrids, in this method, are crossed by one of the above mentioned methods. This method is useful in the improvement of both self and cross-pollinated crops.

Hybridization in Cross-Pollinated Crops:

Several methods like single cross, double cross, top cross and synthetic cross.

Single Cross:

It is a cross between two inbreeds. For example, A x B or C x D. The hybrids are distributed directly to fanners for cultivation.

Double Cross:

It is a cross between F₁ hybrids of two different single crosses.

Three-way Cross:

It is a cross between F₁ hybrid of a single cross and a third parent which is used as a male parent. For example,

Top Cross:

It is a cross between an inbred and an open-pollinated variety.

Variety x Inbred

Synthetic Cross:

A number of inbreeds are crossed in order to combine different desirable characters into one variety.

Types of Hybridization:

Two types:

(i) Interspecific and

(ii) Inter-generic.

(i) Interspecific Hybridization:

Here the plants of two different species belonging to the same genus are crossed together. It is also known as intra-generic hybridisation (within the same genus). All the disease, insect, drought and frost resistant varieties in wheat, tomato, sugarcane, etc., have been evolved by this method.

(1) Sugarcane:

Two species of sugarcane are cultivated in India-Saccharum officianarum in Central and South India, while S. barberi is grown in northern India. Both these species are susceptible to red rot, lodging and drought. A wild species of sugarcane, S. spontaneum has genes for resistance to diseases to which cultivated species are susceptible.

Therefore, to introduce the genes for resistance to diseases, both of the cultivated species were crossed with the wild species S. spontaneum individually and thus high-yielding, disease resistant canes have been evolved.

(50% characters of cultivated species, 50% characters of wild species)

It was found that the hybrid shows some bad features of wild species, like no sugar content. So bad characters have to be got rid off. For this, back crossing is carried out. In back crossing, hybrid is crossed with the cultivated species, which has the characters tor sugar content.

So by continued backcrossing, canes with high sugar content have been obtained.

(2) Potato:

Solanum rybinii is a wild diploid species of potato and is resistant to frost and virus infection. Another species S. tuberosum is cultivated and tetraploid species. The characters of wild species can be introduced into the cultivated one by hybridisation. But the two species do not cross with each other as such, because of different ploidy levels. So, S. rybinii is first raised to tetraploid level by auto-polyploidy and then crossed with S. tuberosum.

(ii) Inter-generic Hybridization:

Crosses made between plants belonging to two different genera constitute inter-generic hybridisation.

(1) Cross between Sugarcane and Sorghum:

Sugarcane takes about 9 months to ripe and so no other crop can be grown. Sorghum, on the other hand, is a short duration crop (3 to 4 months). So early maturing sugarcane varieties have been evolved by crossing with sorghum. But since sorghum has less sugar content, the cross results in a hybrid with less sugar content. Therefore, by repeated backcrossing of the hybrid with sugarcane early maturing varieties having normal sugar contents have been evolved.

(2) Cross between Triticum and Secale:

By crossing Triticum (wheat) with Secale, inter-generic hybrid Triticale has been evolved.

5. Heterosis (Hybrid Vigour) and Inbreeding Depression:

In cross-pollinated species and species reproducing asexually which are highly heterozygous, inbreeding leads to severe reduction in fertility and vigour — phenomenon known as inbreeding depression. On the other hand, hybridization between unrelated strains generally results in increased vigour and fertility—a phenomenon called hybrid vigour or heterosis. Most of the improved varieties of the crops are either hybrids or composites, both availing the advantages of heterosis.

Inbreeding Depression:

In cross-pollinated species, inbreeding leads to loss of vigour and fertility; this phenomenon is known as inbreeding depression. There is a general reduction in size of various parts and the yield. In many species, harmful recessive alleles appear in varying frequencies. Many lines may be lost due to severe reduction in vigour and fertility.

The degree of inbreeding depression varies considerably from one species to another. Some species, such as onion, cucurbits etc., show little or no inbreeding depression; in species like maize and bajra there is moderate inbreeding depression, while in some species, such as alfalfa and carrot, the inbreeding depression is very severe. Self-pollinated species, on the other hand, are adapted to inbreeding and do not show any inbreeding depression.

Heterosis (Hybrid Vigour):

Hybrid vigour as the name connotes is increased size, yield, general vegetative luxuriance, resistance to diseases and to insects etc., observed in the F₁ generation of certain crosses as compared to the parents. A.F. Shull (1914) attributes vigour to “the effect of a changed nucleus and relatively unchanged cytoplasm upon each other.”

Hybrid vigour has been exploited in commercial crops such as maize, sorghum, bajra, rice, sugar-beet, tomato, petunia, zinnia, cabbage and cucumber. F₁ hybrids of maize have shown 30 to 50 per cent higher yields than those of the original open-pollinated cultivars from which the inbred lines were derived. Heterosis is lost by inbreeding. Thus to maintain optimum heterosis, seeds must be produced every year by crossing the pure parental lines, which are constantly maintained.

Vegetatively reproducing crop plants are best suited for maintaining hybrid vigour because once a desired hybrid has been produced there are fewer changes of losing it. It is because of this reason that hybrid varieties of mango, apple, guava, rose, dahlia and chrysanthemum are so popular and available everywhere.

6. Synthetic Varieties:

Synthetic varieties are produced by crossing in all possible combinations of a number of lines that combine well with each other and are maintained by open pollination in isolation. The lines that serve as parents of synthetic varieties may be clones, inbreeds, synthetic or other populations.

The steps in production of synthetic verities are:

(i) Evaluation of parental lines using polycross test;

(ii) Production of synthetic varieties by mixing seeds of all parental lines and harvesting open- pollinated seed from the resulting crop, or by making all possible single crosses and mixing their seed and then multiplication of seed thus produced.

Synthetic Varieties Offer Many Advantages Over Hybrid Varieties:

They are practically feasible means of exploiting heterosis in the species where pollination control is difficult; seed production is simpler and cheaper; farmer can save his own seed ; they serve as germplasm reservoir and they may be expected to perform better than hybrids in a variable environment.

7. Mutation Breeding:

Mutations are sudden unpredictable heritable changes without any intermediate stage in characteristics of organism. In molecular terms, mutation is defined as the permanent and relatively rare change in the sequence of nucleotides. Mutations may be chromosomal, cytoplasmic or gene mutation (or point mutation).

Mutation was first discovered by Wright in 1791 in male lamb which had short legs. Later on mutation was discovered and studied in Oenothera by Hugo de Vries in 1900 Morgan in Drosophilla (white-eyed mutant) in 1910, and by several others in various organisms. However, the term “mutation” was coined by de Vries.

Mutations can be induced by some physical and chemical agents, called mutagens. Mutagens greatly enhance the frequency of mutations. Mutagenic action of X-ray was first discovered by Muller in 1927, and that of nitrogen mustards by Averbach and Robson in 1946. Based on their effect on survival, mutations are classified into four groups: lethal, sub-lethal, sub-vital, and vital. Mutation breeding utilizes vital mutations only.

Physical Mutagens:

They include various types of short-wave, electromagnetic radiations (Ultra violet irradiation, X-rays, Cosmic rays) and ionizing radiations (gamma rays obtained from radioactive isotopes ⁶⁰Cobalt and ¹³⁷Caesium. Alpha particles, Beta particles, Fast and Thermal neutrons).

Chemical Mutagens:

They include chemicals like mustard gas, hydrazine, Ethyl Methane sulphonate (EMS), Dimethyl Nitrosoamine (DMN) and maleic hydrazide etc.;

Stage at which mutation occurs:

Mutation can occur at any stage during the life cycle of a living organism.

1. Before the formation of gametes

2. In gametes

3. In zygotes

4. In normal body cell or somatic cell

Frequency of Mutation:

The frequency at which gene mutate is called mutation rate. The mutation rate depends upon the position and nature of the genes. The genes with relatively low mutation rate are known as stable genes and those with high mutation rate as unstable genes. General range of frequency of mutation is 1 in 20,000 to 1 in 2, 00,000.

Rate of mutation is influenced by:

1. Mutator gene (this gene increases the rate of mutation), supressor gene (this gene decreases the rate of mutation).

2. Virus can increase the mutation rate (e.g., Zea mays)

3. Environment (temperature, different radiations and chemicals)

Chromosomal Mutation:

The change in chromosome structure is known as chromosomal mutation. It is also known as chromosomal aberration.

It may be due to the following:

(a) Deletion:

Loss of part of chromosome.

(b) Duplication:

Addition or increase in a part of chromosome

(c) Inversion:

Reversal in order of genes in a part of chromosome.

It is of two types:

(1) Paracentric inversion (inversion segment does not carry centromere)

(2) Pericentric inversion (inversion segment with centromere).

(d) Translocation:

Exchange of genes between non-homologous chromosomes.

Gene Mutation:

The change of nitrogenous base sequence in DNA or gene is known as Gene or Point mutation. In other words, change in the chemical structure of gene at the molecular level is also known as gene mutation. Phenotypic changes which are produced by gene mutation are reversible, whereas due to structural and numerical changes in chromosome are irreversible.

Gene mutations are of two types:

1. Substitute mutation:

The change of base pair or nucleotide pair in a DNA segment or cistron is called substitute mutation.

It is of two types:

(a) Transition:

Exchange of purine base by purine base or pyrimidine by pyrimidine base in a DNA segment or cistron is known as transition.

(b) Trans-version:

The substitution of a purine base by pyrimidine base or vice-versa is known as trans-version.

2. Frame shift mutation:

Insertion or deletion of single nitrogenous base in DNA chain is known as frame shift or gibberish mutation. For example, if the antisense strand of DNA is

TAG AAA GGG GCC AAG AGA

Its DNA transcript will be:

AUG UUU GCC GGG UUG UGG UGU

Translated message will be

Methionine – Phenyl alanine – Proline, – Glycin – Phenyl alanine – Serine.

If a single base ‘G’ is inserted in between G and U of first codon then a new protein will be produced.

AUG GUU UGG GGG GUU GUG GUC.

Methionine – Valine – Serine – Arginine – Valine – Leucine – Leucine.

Achievements and Limitations of Mutation Breeding:

1. A number of crop varieties have been developed through mutation breeding.

2. The first commercial success with induced mutations was reported in 1934 with the release of a new tobacco cultivar ‘Chlorina’ through X-ray irradiation.

The Indian dwarf wheat’s which contain the dwarfing gene was from a Japanese cultivar ‘Norin- 10’, which itself was a mutant.

3. Many varieties of barley contain artificially mutated genes which contribute to reduction in height, increase in yield, insensitivity to day length and resistance to mildew diseases. Sharbati Sonara and Pusa Lerma are two amber grain colour mutants of wheat produced from the red grained Sonara 64 and Lerma Rojo 64A, respectively. A mutant gene that induces semi-dwarfing in rice has been produced by X-ray treatment.

Induced mutations have also become recently important in developing parents useful in hybridization programmes. Forty-five rice cultivars have been developed by the year 1982, either by direct radiation or by crossing with induced mutants.

4. Many crop plants are propagated vegetatively even though they can bear seed. Potato, tapioca and sugarcane are classical examples of such crops. In these, genetic improvement is carried out using sexual reproduction but the maintenance of the improved varieties is by cloning. For examples, potatoes are multiplied by tubers, apples by cuttings, and strawberries by runners.

5. Spontaneous mutations in somatic cells of a vegetatively propagated plant are commonly referred to as SPORTS. Such desirable sports occurring in well-adapted, asexually reproducing plants may result in quick improvements such as the colour sports in many apple varieties and superior shrub types in coffee plants.

6. The characters improved through mutation breeding include flowering time, flower shape, fruit shape, changes in oil content, and protein quality.

7. Some of the important limitations of the use of mutation breeding for crop improvement are:

(i) Most induced mutations are undesirable and have no value to the breeder. Many induced mutations are lethal.

(ii) The mutation rate is extremely low and a very large number of plants must be screened to identify the few individuals that may have desirable mutations. It is equally difficult to grow such useful mutants and include them in breeding programmes.

(iii) The stability of a mutant must be thoroughly tested as some mutants have a tendency to revert.

(iv) Most induced mutations are recessive; these have to be in double dose to be expressed phenotypically.

(v) Unless mutations are induced in gametes—especially in pollen—they will not be easily incorporated into the breeding line.

8. Polyploidy:

1. Any organism in which the number of complete chromosome set is higher than the diploid number is called POLYPLOID and the phenomenon is known as polyploidy.

Characteristics of Polyploids:

Polyploids are characterized by:

1. Leaves large, thick and deep green.

2. Increase in number of floral parts but poor flowering.

3. Formation of large pollen grains.

4. Fruits and seeds much larger.

5. Increase in cell size with more prominent nuclei.

6. Increase osmotic pressure of cell sap.

7. High conc. Of Ca, Mg & K.

Types of Polyploids:

There are two types of polyploids:

(i) Euploids are those forms in which the chromosome number has changed in such away that an organism has an exact multiple of haploid number, such as triploids (3n), tetraploids (4n), pentaploids (5n), hexaploids (6n) and so on.

(ii) Aneuploids or heteroploids are those forms in which the chromosome number has changed in such a way that an organism does not have an exact multiple of the haploid number. For example, 2n-1 (monosomics), 2n-2 (nullisomics), 2n+1 (trisomic), 2n+2 (tetrasomic), and likewise.

2. Euploidy has been used in plant breeding and improvement work.

3. Euploids are of two types: autopolyploids and allopolyploids.

4. In autopolyploids, there is an exact multiplication of one and the same genome (i.e., within a species), as shown below:

5. In allopolyploids, there is a cross between two diploids of different genomes, as in inter­specific hybridization.

6. Segmental allopolyploids are intermediate between auto-and allopolyploids.

7. In these allopolyploids, the different genomes which are present are not quite different from one another.

8. Chromosomes from different genomes do pair to some extent and multivalent are formed.

9. Stebbins called such allopolyploids as segmental allopolyploids.

Induction of Polyploidy:

1. Polyploidy in plants can be induced by colchicine treatment.

2. Colchicine is an alkaloid obtained from the corms of Colchicum autumnale (Liliaceae).

3. Colchicine inhibits the formation of the spindle in the dividing cells and hence chromosomes do not separate at anaphase. Thus, a restitution nucleus (it is a nucleus in which the chromosomes have divided but could not separate into two daughter nuclei) is formed. Effect of colchicine is temporary. As a cell recovers from treatment, a new spindle is formed and the restitution nucleus undergoes normal mitosis as a polyploid cell.

Characters of Autopolyploids:

1. Autoploids are characterized by the presence of same characters as the diploid parent, except that they are the large replica of the diploid parent. They show large flowers and fruits.

2. Cell size, stomata size, nuclei, etc., are also larger.

3. Chloroplasts are more in polyploid cells than in the diploid cells.

4. Cytologically, autoploids are identified by the presence of multivalent.

5. Autotriploids are highly sterile because of the random segregation of three chromosomes of each trivalent produced.

6. Autotetraploids also show sterility to some extent because of irregular segregation of quadrivalents and by lagging of the univalent.

Characters of Allopolyploids:

1. Alloploids show a combination of, the characters of the two parental forms. There is nothing like gigantic effect as seen in autoploids.

2. F₁ hybrid is completely or partially sterile. Chromosome doubling in F₁ hybrid restores fertility.

Role of Autopolyploid in Plant Improvement:

1. Autoploidy has been used to increase seed size in cereals and pulses, root size in root crops, flower size in ornamental plants, seedlessness in fruits and the quantity of active ingredients in medicinal plants.

2. In some fruits, seedless varieties are desired as in grapes, guava and watermelon. Since triploids are sterile, triploidy is used in such cases for developing seedless varieties.

Role of Allopolyploids in Plant Improvement:

1. Allopolyploidy is important in interspecific and inter-generic hybridisation.

2. Allopolyploid cultivated plants include wheat, sugarcane, cotton, tobacco and sesame.

3. Genera raised through inter-generic allopolyploidy include Triticale (Triticum x Secale) and Raphanobrassica (Raphanus x Brassica).

(1) Origin of Cultivated Hexaploid Bread Wheat:

The wild and cultivated wheats form a series of diploid (AA, 2n = 14), tetraploid (AABB, 4n = 28) and hexaploid (AABBDD, 6n – 42) types with basic number = 7. The bread wheat belongs to hexaploid group and has originated as follows:

(2) Origin of Cotton:

American cultivated species of cotton have 2n = 52, and the Asiatic cottons and wild American species of cotton have 2n = 26. It has been shown that American cultivated cottons were allotetraploids and arose as a result of crossing between two forms, followed by chromosome doubling in the hybrid. The long staple Narma cotton (Gossypium hirsutum) is an allopolyploid obtained by crossing Indian species G. herbaceum with the American species G. raimondii.

(3) Origin of Tobacco:

There is evidence that Nicotiana tabacum (2n = 48) is an aliotetraploid between N. sylvestris (2n = 24) and N. tomentosa or some allied species (2n = 24). N. tabacum is known only under cultivation and does not occur as a natural species.

(4) Origin of Triticale:

It is a man-made cereal, an allopolyploid between Triticum (wheat) and Secale (rye). The released varieties of Triticale are hexaploid (2n = 42) and have been synthesized by doubling the chromosome complements of sterile hybrids between T. turgidum (durum wheat, 2n = 28) and S. cereale (rye, 2n = 14). (5) Origin of a new genus allotetraploid:

Raphanobmssica (2n = 36) from diploid parents, viz, Raphanus sativus (2n = 18) and Brassica oleracia (2n = 18).

9. Selection:

Selection is one of the oldest methods for crop improvement. It can be natural or artificial and is possible only if there exist variation in the crop. Natural Selection acts as a sieve in favour of the well adapted strains and varieties. Natural selection is a rule in nature and has resulted in evolution, according to which only the fittest can survive. All local varieties of crops are the result of natural selection.

Many differences between species and sub-species have arisen due to this selection pressure. It is always operating in nature and is one of the natural factors which creates variations in the already existing varieties of crops. Artificial selection involves picking out of the plants having desired combination of characters from a mixed population where the individuals differ in characters.

The various methods of artificial selection are:

a. Mass Selection:

It is practiced in those plants which are cross-pollinated like Zea, Brassica. In this method, plants are selected based on the phenotypic expression from the mixed population of a crop. Then, the seeds of these selected plants are obtained. All the seeds are mixed in a single lot and therefore, the method is known as mass selection. The seeds so obtained are used for raising the next crop. Again from these plants, selection is made as earlier. This process is continued till the plants show uniformity in the desired characters.

b. Pure Line Selection:

It is practiced in self-pollinated crops such as wheat, barley, rice, legumes. Here also the selection is made on the basis of phenotypic expression. But the seeds of one plant are not mixed with the seeds of another. So, it involves testing the progeny of single individual plant separately. Selection is again made from the progenies arising from the seeds of a single individual. This method of selection from a single individual is continued till a true breeding type is obtained.

c. Thus, a breeder by pure line selection renders a particular type, more or less homozygous. Unlike mass selection, here the progeny consists of a uniform population. Pure line lacks variability.

d. Clonal Selection:

This method is practiced in vegetatively propagated crops such as banana, potato, onion, citrus, etc. Clones are plants propagated vegetatively from a single individual. The genotypic constitution of plants propagated in this way is not likely to change. Herein, superior clones are selected on the basis of their phenotypic characters. The selection is always between clones and never within a clone, as all the individuals of a clone have the same genetic constitution.

10. Plant Tissue Culture in Crop Improvement Programme:

Lately, the tissue culture technology has played a very crucial role in crops improvement programme. Essentially the methodology of tissue culture consists of separating cells, tissues or organs of a plant and growing them aseptically in suitable containers on a nutrient medium under controlled conditions of temperature and light. The cultured parts (termed explants) require a source of energy (usually sucrose), salts, providing macro-and microelements, a few vitamins and generally the amino acid, glycine, in the nutrient medium.

The amounts and the nature of salts used vary as there are several formulations developed by different scientists. Hormones and mixtures of substances such as yeast extract, coconut water, bean seed extract are included in the medium by some workers. An excised embryo or a shoot bud may develop into a whole plant. Pollinated ovaries have also been grown to mature fruits. Nevertheless, portions of organs or tissues generally give rise to an unorganized mass of cells called CALLUS.

In the early 1950’s Skoog and Miller showed that shoot or roots can be induced in the callus (organogenesis) by an appropriate balance of amounts of cytokinin and auxin in the medium. We now know that the type of growth response in tissue cultures depends on the source of the explants, composition of the medium and conditions in the culture room.

The following are the benefits of tissue culture in crop improvement:

1. Rapid multiplication of desired plants (Micro propagation)

2. Multiplication of rare plants which reproduce through seeds with great difficulty.

3. To rescue embryos which fail to reach maturity.

4. Multiplication of sterile hybrids.

5. Production of virus-free plants.

6. Protoplast fusion or somatic hybridization.

7. To shortern the period for development of new varieties of plants.

8. To induce weedicides resistance in plants.

9. Induction and selection of mutants.

10. Somoclonal variation and DNA recombinant technology.

There are a few other uses of plant tissue culture such as production of artificial seeds, and germplasm storage and exchange.

11. Genetic Engineering and Biotechnology in Plant Breeding:

The latest interest in crop improvement is not to involve whole genome (as in conventional plant breeding or in protoplast fusion). The objective of genetic engineering or recombinant DNA technology is to introduce one or more genes into an organism that normally does not possess them. This requires isolation of a fragment of DNA corresponding to a desirable character (or function), hooking it to a vector (such as the plasmid in a bacterium, Agrobacterium tumifaciens), and transferring it to a cell.

Genetic transformation is also possible through co-cultivation (incubating recipient protoplast with purified DNA), electroporation (by applying high electric potential for a few micro-seconds to change the porosity of protoplast to take up DNA) and by micro-injection of DNA into the cell by fine needles. Although the above account may sound simplistic and exciting, there are several obstacles in realizing the objectives.

Successful genetic engineering requires identification of the desired genes, their transfer to the cells of a target crop plant, their integration and expression. We know a good deal about genome organisation in a prokaryotic organism such as E. coli However, the genetic material of the eukaryotes is quite complex. Our present knowledge of the location and function of the specific genes in crop plants is so poor that genetic engineering is still very problematic. Each crop plant contains one to ten million genes. Detailed study of genome organisation is needed for major crops and their wild relatives.

Transgenic Breeding:

Individuals which are developed through genetic engineering are called transgenic. A transgenic may be a plant, an animal or a microbe. Foreign genes present in a modified organism is called transgene. Transgenic plants contain transgenes. Using techniques of genetic engineering and biotechnology, useful genes can now be transferred into plants from a wide range of organisms including unrelated plant species, microbes, animals and from DNA synthesized in the laboratory. In the development of transgenic, sexual process is bypassed.

Transgenic plants have been developed in various field crops, such as wheat, barley, oat, maize sugarcane, rapeseed, soybean, peanut, cotton, tobacco, tomato, potato, sunflower etc. BT-cotton, a transgenic, is now successfully grown by farmers in India. BT-cotton is insect resistant and high yielding. Some of the most outstanding limitations of transgenic breeding are that polygenic characters cannot be manipulated, instable performance, low frequency and costly method of crop improvement. In-spite of many limitations and practical difficulties, genetic engineering offers immense possibilities for improving crops that were unthinkable before.

12. Improved Seed:

The primary objective of plant breeding is to develop superior varieties of crops. The benefits from superior varieties can only be realized when they are grown commercially on a large scale. Seeds of improved varieties must be multiplied at a large scale in order to make them available to farmers for large scale cultivation. Here the word “seed” refers to seed or any other propagating material used for raising a crop. For example, grain produced for general consumption is not seed; only grain produced for raising a crop would be known as ‘seed’.

On the other hand potato tubers produced for planting a new crop are known as seed potato. During multiplication of varieties for use as seed, it is essential that genetic purity of the variety must be maintained. If the genetic purity is not maintained, superiority of the variety is likely to be lost. In addition, for best results the farmer should use new pure seed every year in case of self-pollinated crops, and every year (hybrid varieties) or every few years (composite and synthetic varieties) in case of cross-pollinated crops. This would require maintenance of seeds of superior varieties in genetically pure state, which would be multiplied every year to supply new seed to the farmers.

The improved seed has four classes:

(1) Breeder seed,

(2) Foundation seed,

(3) Registered seed, and

(4) Certified seed.

The seed produced by the breeder who developed the variety, or by the institution where the variety was developed is the breeder seed. Foundation seed is the progeny of the breeder seed and is used to produce registered seed or certified seed. Certified seed is grown by various agencies and is certified for use as seed by the State Seed Certification Agency.

The requirements of good seed are:

(1) Genetic purity,

(2) Physical purity,

(3) Good germination,

(4) Freedom from weed seeds,

(5) Freedom from diseases, and

(6) An optimum moisture level.

The minimum standards for certification vary to some extent from one crop to another. To ensure availability of pure seed of different crops to farmers, elaborate seed programmes (production and distribution) exist in most of the countries. Our country also has a well-organized seed production and distribution programme in the form of National Seeds Corporation (NSC), State Seeds Corporation (SSC) and State seed certification Agency (SSCA). These organizations are responsible for seed certification and its distribution.