Plant Breeding for Disease Resistance | Essay | Plants | Biology

Here is an essay on ‘Plant Breeding for Disease Resistance’ for class 9, 10, 11 and 12. 

The main hindrance to higher yield is the susceptibility of plants towards disease caused by fungi, bacteria and viruses and to insects and pests. There are various physical, chemical and biological methods for disease control but the cheapest and the most convenient method is to produce resistant varieties.

It has been studied that resistance to diseases, insects and pests are genetically controlled characters. So, it is possible to transfer these characteristics to susceptible and desirable variety through crossing. Resistant varieties not only enhance yield but also stabilize it.

Crop loss may be up to 30% due to attack of pathogens. Moreover, due to development of disease resistant varieties use of fungicides and bactericides is reduced considerably. This happens due to changed genetic set up of disease resistant plants.

Plant viruses cause serious losses to yield and quality in most crops. Substantial crops losses occur due to attacks by insect pests and microbial pathogens. Genetic transformation can boost plant breeding efforts for developing disease resistant varieties.

Some of the disease resistant varieties developed are given in Table 9.14.

Methods of Plant Breeding for Disease Resistance:

Breeding is done either by conventional methods or by mutation breeding. A conventional method of breeding for disease resistance usually follows the same steps as that of normal hybridization programme.

Sequential steps will be:

(i) Screening of germplasm for resistance sources.

(ii) Hybridization followed by selection and evaluation of hybrids.

(iii) Testing and release of newly formed disease resistant hybrid variety.

However above methods is confined to availability of disease resistant genes in various crops and other related plants.

Other methods employed to get variety of genes including disease resistant genes are:

1. Mutation Breeding.

2. Genetic Engineering.

Many wild varieties of present day crops have been identified exhibiting some resistant characteristics. However their yield in very low. Such resistant genes are introduced into high yielding varieties by hybridization e.g., resistance to yellow mosaic virus in bhindi (Abelmoschus esculentus) was transferred from its wild relative, A. manihot. As a result yellow mosaic virus resistant variety of bhindi was developed named as Parbhani Kranti.

(1) Mutation Breeding:

Darwin’s theory (1859) was based on hypothesis that natural selection acted upon the hereditary variability present in a species to preserve only those types which were better adapted to environment. In support of his theory, Darwin gathered numerous examples called “sports” which suddenly appeared e.g., tailless cats, short-legged varieties of dogs and cats etc. However, Darwin himself was of the view that evolution proceeded in a more gradual way than in large sudden steps.

The idea of mutation first originated from the observations of a dutch botanist Hugo de Vries (1880) on variation in plants of Oenothera lamarckiana (evening primrose) de Vries collected seeds from Oenothera plants, raised plants, from them, and analyses the progeny for transmission of traits showing variations.

He found 834 mutated plants in a population of 54343 plants. This view of evolutionary changes was supported by Baeston (1894) and Krorjinsky (1899). Hugo de Vries (1901, 1903) introduced the term mutation for large discrete changes in genotype in his book Mutation theory.

The mutation can be defined as sudden, stable discontinuous and inheritable variations which appear in organism due to permanent change in their genotype.

The product of mutation is called mutant and could be genotype cell, a polypeptide chain or an individual. Mutations are responsible for the origin of new traits in the race and therefore the mutations are the source of all variations.

Mutation can occur in various kinds of cells, but most of them probably have little effect on individual and are ordinarily remain undetected. The mutations may occur in somatic cells as well as in germinal cells. The somatic mutations produce mutated buds and germinal mutation give rise to mutated seeds in plants. Bud sports in fruit trees are somatic mutations and are of practical importance, since they can be propagated by grafting.

Germ mutations are important from the stand point of heredity as they are passed on to the next generation. Mutation rates are very low for individual genes, but there are so many genes, that the likelihood of a mutation in one or another of them is very high.

Mutations produce different types of effects e.g., deleterious, lethal, advantageous and neutral, visible or invisible. Most of the mutations occur in recessive genes and are not expressed, while some take place in dominant genes and appear in mutants.

Mutations have been occurring in nature also and are called spontaneous mutations which numerous physical and chemical agents are used to increase the frequency of mutations, they are called induced mutations.

Artificial Induction of Mutations (Induced Mutations):

The spontaneous mutation rate is very low at all the loci in all the organisms. Muller’s experiments decided that the mutation rate is much higher in the progency of Drosophila when treated with X-rays. Any physical or chemical agent which is used in artificial induction of mutations is called mutagen.

(i) Chemical Mutagens:

Chemical mutagens are used in the following ways:

(a) Incorporation of Base Analogues:

The two commonly used base analogues are 5-Bromouracil (5BU) and 5 Fluorouracil (5FU). Both are analogues bases to thymine of DNA, but 5BU or 5FU pairs with guanine instead of thymine’s natural pair adenine, thus producing 5BU-G to 5FU-G pairing rather than T-A pairing. It disturbs replication, transcription and translation mechanisms of DNA molecule.

(b) Methylating Agent:

Some of chemicals like RN (CH₂Cl)₂ or nitrogen mustards cause addition of methyl group to the nitrogen bases of DNA. For example, cytosine on methylation forms 5-methyl cytosine which prevents the separation of DNA strands of replication and transcription.

(c) Acridine Dyes:

Certain organic dyes such as acridine orange and proflavine causes insertion or deletion of nitrogen bases in a gene. Acridines may get inserted in between nitrogen bases of DNA, as a result, frameshift of the genetic code takes place and thus changing the whole lot of genetic information (codon). These are also known as Gibberish or frameshift mutations because such mutation forms non-sense polypeptide chain or codon.

(d) Deamination of Bases:

Some chemicals like nitrous oxide, deaminate (removal of NH^ or amino group) the nitrogen basis and therefore, change the codon of DNA. Nitrous oxide deaminates adenine to form hypoxaminate which has guanine like properties. As a result of which T-A pairing in DNA molecule will be replaced by G-C pairing thus causing gene mutation. Nitrous oxide can also change cytosine to uracil and guanine to xanthine. As a result of deamination the replication, transcription and translation are disturbed.

(ii) Physical Mutagens:

Physical mutagens are used in the form of high energy radiation and high or low temperature treatment.

High Energy Radiations:

All types of energy that can change the chemical structure of genes or chromosomes include mutations. For example. X-rays, alpha rays, gamma rays, beta rays, cosmic rays, known as inonizing effect on DNA molecules. They distort or break DNA duplex and disturb the replication. Ultraviolet rays are non-ionizing radiations and produce thymine dimers.

Other Aspects of Mutation Breeding:

Conventional plant breeding in time-consuming and labour intensive. It is also limited by natural pollination barriers between species that mean desirable traits cannot be easily introduced from one species to another. By mutation new genetic variations are created through changes in the base sequence with in genes. The mutation helped in changing the genotype and phenotype of plants at any time according to need. The creation of mutation at will and their utilization for the production of new crop varieties is known as mutation breeding.

Several mutant varieties have been developed in pulses, millets, vegetables, fruit plants and cereals. Main mutation breeding work has been carried upon wheat, rice and barley varieties.

In mung bean, resistances to yellow mosaic virus and powdry mildew were induced by mutations. First mutant variety of white mustard (Brassica hirta) was released in 1950 followed by Ragina II, Summer Rape (B. campestris) in 1953.

Mutations occur most frequently in cross fertilized plants than in self fertilized plants. Mutational variations are not always useful, rather many are recessive or lethal, but in few cases they are of great advantage. Many of best varieties have resulted from the selection of sports from field populations.

Mutations are artificially induced by mutagenic agents called mutagens. Some of the mutagens are short wave electro-magnetic radiations (UV radiation. X-rays, cosmic rays), ionising radiations especially gamma rays (obtained-from ₆₀cobalt and ₁₃₇caesium) and chemicals like mustard gas, nitrous acid, phenol, EMS (ethyl methyl sulphonate) etc.

Practical applications of induced mutations (mutation breeding):

1. Many varieties of barley contain artificially mutated genes due to which increase in yield, insensitivity to day length, resistance to mildew and reduction in height has resulted.

2. Two amber grain coloured mutants i.e., sharbati sonora and pusa lerma were produced from the red grained sonora 64 and lerma rajo 64 A.

3. With the X-ray treatment semi-dwarf variety in rice has been produced. More than forty-five cultivars in rice were developed by the year 1982. Gamma rays treatment in Indonesia of rice line pelita-1 produced a high yielding cultivar called atomita-2. This release is resistant to brown plant hopper and tolerates saline conditions in soil.

4. In Carolina, groundnuts with thick shells, less liable to cracks in transit, have been produced as a result of mutation breeding.

5. Somatic mutations are of much importance in vegetatively propagated plants like potato, sugarcane, mango, etc.

Limitations of Mutation Breeding:

1. Most of the mutations are lethal and may lead to death of the organism.

2. Large number of plants is to be employed to get a desirable amount.

3. Many mutations get reverted and are not stable.

4. Usually mutations are recessive and are thus expressed only in homozygous recessive condition.

5. Mutations are inherited only if they are induced in gametes particularly in pollen.

(2) Genetic Engineering:

The methods of artificial synthesis of new genes and their subsequent transplantation in the genome of an organism or a method of correcting the defective genes is called genetic engineering. With the genetic techniques available a modest beginning has been made. Terms such as gene surgery, gene therapy, gene manipulation, gene transplantation, gene intervention and algeny have been synonymously used for genetic engineering.

Achievements and prospects of genetic engineering:

(i) It may become possible to produce plants and animals with specific characters at will with the help of genetic engineering.

(ii) Genetic engineering has introduced a new form of cure system called gene therapy which may be used for treating crippling hereditary diseases like haemophilia, phenylketonuria, etc.

(iii) Gene coding for vitamins, hormones, antibiotics in bacteria etc., is also possible, which will help to produce chemicals which are impossible to get or synthesise.

1. High yielding variety of rice was produced in India by introduction dwarfing genes called De-goo-woo- gen from Taiwan.

2. High yielding variety of wheat was produced from dwarfing genes of Norin-10 from Japan in India.

3. The seeds of dwarf Mexican wheats Sonora-634 and Lerma Rajo-64 were introduced in India in 1963.

4. Germplasm is the genetic material mainly within reproductive cells.

5. When fused protoplasms of two cells develop its own wall, they are called somatic hybrid cells.

Plant Breeding For Developing Resistance to Insect Pests:

Plant breeding has been found to be successful to develop insect resistant plants. Insect attack and pest infestation cause large scale damage to crop plants.

Insect and pest resistance in some plants may be due to morphological; biochemical and physiological characters.

For example:

(i) Plants are insect pests resistant due to presence of hairy leaves as cotton is resistant to jassids and wheat is resistant to cereal leaf beetle.

(ii) Smooth leaved and vector less cotton variety is not attacked by hollworms.

(iii) Stem sawfly does not cause damage to wheat with solid stems.

(iv) Maize stem borers do not cause damage to maize plants with high aspartic acid, low nitrogen and sugar contents.

Sources of resistant genes are cultivated varieties and germplasm collection of their wild relatives. Same breeding techniques are followed as in the case for development of disease resistant varieties.

Hybrids obtained by crossing Gossypium hirsutum (cotton), cultivated varieties SRT 1, Khandwa 2, DNJ 286 and B 10007 and wild cotton varieties like G. tomentosum, G. anomalum and G. arboreum have been found to be jassids resistant.

Few other examples of insect pest resistant crops bred by hybridization are:

(i) Pusa Gaurav variety of Brassica (rapeseed mustard) is resistant to aphids.

(ii) Pusa Sem 2 and Pusa sem 3 varieties of flat bean are resistant to aphids jarrids, aphids and fruit barer.

(iii) Pusa Sawni and Pusa A 4 varieties of Okra (Bhindi) are resistant to shoot and fruit borer.

Plant breeding for Improving the Food Quality:

Increase in quantity is of less importance if it is not accompanied by better quality. Quality is an important aspect for plant breeders because it determines the suitability of plant produce for various uses. Qualitative characters differ from crop to crop and also vary with the use of plant produce.

For example, shape, size, colour, milling, baking, malting and cooking in foodgrains; size, colour, taste, flavour and nutrition in fruits; keeping quality in fruits and vegetables; protein content in cereals and pulses; strong, long and fine fibres in cotton; shape, size and colour patterns in leaves and flowers of ornamental plants are some of the characters desired by the plant breeder for improvement.

Breeding for anti-nutritional quality is done with objectives of improving:

(i) Protein content and quality.

(ii) Oil content and quality.

(iii) Vitamin content.

(iv) Micronutrient and mineral content.

Antinutritional factors are those compounds which badly affect the human/animal growth and development.

Thus a better quality crop should not contain antinutritional factors.

Some examples of antinutritional factors are:

(i) Glucosinolates in oils and cakes of rapeseed and mustard.

(ii) Neurotoxin found in seeds of Lathyrus sativus (Khesari).

It is noteworthy that humans cannot synthesize eight amino acids. The quantity of essential amino acids in proteins relative to contents required in human diet represents the protein quality. Pulses lack methionine and cysteine (sulphur containing amino acids).

Cereals and millets do not contain amino acids, lysine and tryptophan. A combination of pulses and cereals, are able to provide all essential amino acids. ‘Shakti’, ‘Rattan’ and ‘Protina’ are the newly developed varieties of maize which are rich in amino acid lysine.

The long chain saturated fatty acids and polyunsaturated fatty acid contents of various oils constitute oil quality.

Physical nature of fat is determined by the chain length of fatty acids and whether they are saturated or unsaturated. Fatty acids are called saturated if they do not bear double bonds between the carbon atoms of the molecular chain e.g. palmitic acid (16C) and stearic acid (18C). On the other hand, unsaturated fatty acids have one or more double bonds as oleic acid (18C), linoleic acid and linolenic acid with 1, 2 and 3 double bonds respectively.

The terminal carboxyl group is polar and is water soluble while the hydrocarbon group is non-polar and insoluble. Fatty acids become less and less water soluble as the length of their hydrocarbon increases. Unsaturated fatty acids have lower melting points than saturated fatty acids.

We usually find advertisements in newspapers about vegetable oils –“rich in polyimsaturates.” Indirectly, it tells us that fatty acids in the said vegetable oil contain more than one double bond. Persons having high blood cholesterol and other cardiovascular diseases are advised to go for such type of oils instead of hydrogenated vanaspati ghee and margarine.

Because vanaspati ghee becomes solid fat on hydrogenation of unsaturated fatty acids. These unsaturated fatty acids turn into saturated fatty acids on hydrogenation. By using vegetable oil, polyunsaturated fatty acids diet reduces the high blood cholesterol level without raising the fat. Long chain fatty acids like euricic acid is not good for human health. Serious efforts are being made to increase the number and quantity of polyunsaturated fatty acids in oil seeds.

Dr. M.S. SWAMINATHAN:

Dr. Monkombu Sambasivan Swaminathan was born in August 1925 in Kombakonam in Tamil Nadu. He got his master degree in Botany from Madras University. Swaminathan et. al. developed early maturing high yielding varieties of rice including scented basmati.

Dr. M.S. Swaminathan (Fig. 9.16), a leading agricultural scientist and pioneer radiation breeder under whose guidance, significant mutation breeding work has been done at IARI, New Delhi. Padam Vibhushan Dr. Swaminathan has played the major role for bringing green revolution in India. With his collaboration with Dr. Borlang, he brought green revolution in India. He has been recognised as initiator of Lab to land, food security and many other environmental improvement programmes.

He has been awarded with several prizes and distinctions:

(i) He has been described as ‘Father of Economic Ecology’ by UNEP (United Nations Environmental Programme).

(ii) Winner of World Food Prize for his pioneer work for fighting against hunger.

(iii) Prof. Swaminathan has been given the honour of three most influential Indians and twenty Asians of 20th century (A survey by TIME magazine).

(iv) Selected for chair in UNESCO, Ecotechnology.

(v) He is the founder of M.S. Swaminathan Research Foundation, Chennai.

(vi) He is known for the development of concept of crop cafeteria, crop scheduling and genetically improving the yield and quality.

He has been holding several key posts like:

(i) Director, IARI.

(ii) Director General, ICAR, New Delhi.

(iii) D.G. International Rice Research Institute, Marula. In India, the science of Plant Breeding was started in the beginning of present century about 85 years ago. Real impetus and encouragement came from the work of Prof. M.S. Swaminathan.

Single Cell Protein:

Single cell proteins (SCP) refers to any microbial biomass produced by uni-and multicellular micro­organisms and can be used as food or feed additives.

SCP contains about 45 to 55% protein, although those of certain bacteria the protein content is as high as 80%.

Advantages of SCP:

(a) SCP is rich in proteins but is rather poor in fats.

(b) These can be produced throughout the year.

(b) Large quantities of SCP from very small land area due to rapid growth of microbes.

(c) Substrates used may be cheaper or even wastes.

(d) Substrates used may be cheaper or even wastes.

(e) Some SCPs are good sources of B-complex vitamins.

So SCP is expected to solve the problem of protein deficiency in the children of developing countries. But SCP may contain toxic compounds produced by some microbes and may lead to indigestin and allergenic reactions.

Process of production of SCP involves following steps:

(a) Preparation of suitable medium with suitable carbon source (e.g., methanol for Methylophilus bacterium; sulphite liquor for Paecilomyces; CO for Chlorella and Spirulina; etc.).

(b) Addition of certain salts and gaseous ammonia to the carbon source to promote the growth of micro­organisms.

(c) Inoculation of pure strain of selected micro­organism.

(d) Proper aeration and cooling of the medium.

(e) The SCP is recovered from the medium by a variety of methods like filtration or centrifugation.