Plant Breeding: Steps and Methods of Plant Breeding for Disease Resistance!

Plant Breeding: Steps and Methods of Plant Breeding for Disease Resistance!

Green Revolution:

Traditional farming can only yield limited food for humans and animals. Better management can increase yield but only to a limited extent.

But plant breeding as a technology increased yield to a very large extent. In India, “Green Revolution” was responsible for our country to not only meet our requirements in food production but also helped us to export it.

Monkambu Sambasivan Swaminathan (M.S. Swaminathan) initi­ated collaboration with Dr. Borlaug which reached the highest point into the “Green Revo­lution” through introduction of Mexican varieties of wheat in India. Green Revolution de­pended mainly on plant breeding techniques for high yielding and disease resistant varieties in wheat, rice, maize, etc.

1. What is Plant Breeding?

Plant breeding is the genetic improvement of the crop in order to create desired plant types that are better suited for cultivation, give better yields and are disease resistant. Conventional plant breeding is in practice from 9,000-11,000 years ago. Most of our major food crops are derived from the domesticated varieties.

But now due to advancements in genetics, molecular biology and tissue culture, plant breeding is being carried out by using molecular genetics tools. Classical plant breeding includes hybridization (crossing) of pure lines, artificial selection to produce plants with desirable characters of higher yield, nutrition and resistance to diseases.

When the breeders wish to incorporate desired characters (traits) into the crop plants, they should increase yield and improve the quality. Increased tolerance to salinity, extreme temperatures, drought, resistance to viruses, fungi, bacteria and increased tolerance to insect pests should also be the desired traits in these crop plants.

Various Steps Required For Developing New Varieties:

The various steps required for developing new varieties are as follows:

(i) Collection of Germplasm:

Germplasm is the sum total of all the alleles of the genes present in a crop and its related species. The germplasm of any crop species consists of the following types of materials:

(i) cultivated improved varieties,

(ii) improved varieties that are no more in cultivation,

(iii) old local or ‘desi’ varieties, (iv) pure lines produced by plant breeders, and (v) wild species related to the crop species.

The entire collection (of plants/seeds) having all the diverse alleles for all genes in a given crop is called germplasm collection. A good germplasm collection is essential for a success­ful breeding programme.

(ii) Evaluation and Selection of Parents:

The germplasm is evaluated to identify plants with desirable combination of characters. Selection of parents is picking up seeds of only those plants for multiplication which have the desired traits. For example, grain length in rice is variable— longer grains, intermediate grains and shorter grains. If we select the seeds of the longest grains and sow them to grow the next generation, the selected population of rice plants will have on average, longer grains than the original population.

(iii) Cross-Hybridization among Selected Parents:

Hybridisation is the most com­mon method of creating genetic variation. Hybridisation is crossing of two or more types of plants for bringing their traits together in the progeny. It brings about useful genetic/ heritable variations of two or more lines together. Line is a group of individuals related to descent and have similar genotype. The individuals or lines used in hybridisation are called parents. Hybridisation takes a lot of time.

As stated earlier a wheat variety HUW 468 took 12 years to develop. Hybridisation may involve a single cross (two plants) or multiple cross (more than two plants). Wheat variety C-306 was developed through multiple cross between C-591 (Reagent 1974 x Ch₂-3) and hybrid of P-19 x C-281. Hybridisation may further be:

(i) intravarietal,

(ii) intervarietal (= intraspecific) or

(iii) interspecific and

(iv) intergeneric. Intervarietal hybridisation is the process of crossing individuals of different lines or varieties of the same species to produce hybrid, e.g., different varieties of wheat are mated. Interspecific hybridisation is the process of crossing individuals of two different species to produce a hybrid. Examples of interspecific hybridisation are the development of rice variety ADT-37 from a cross between Oryza japonica and O. indices and all the sugarcane varieties being cultivated today. In intergeneric hybridisation, the cross is between two different genera.

The procedure of hybridisation involves the following steps.

(a) Selection of Parents with Desired Characters:

All the desirable traits which are required in the new crop variety are first selected.

(b) Selfing:

The selected plants as parents are allowed to undergo self breeding to bring about homozygosis of the desired traits.

(c) Emasculation:

The removal of anthers (male parts) from a bisexual flower, before the anthers mature is called emasculation. This prevents self-pollination in these flowers.

(d) Bagging:

The emasculated flowers are immediately covered by paper, plastic or polythene bags. The process is called bagging. It prevents unwanted pollen to come in contact with emasculated flowers. This prevents contamination from foreign pollen grains.

(e) Tagging:

The emasculated and bagged flowers must be tagged by writing every step with date and time. The bagging and pollination is incomplete without tagging.

(f) Artificial Pollination (= Crossing):

Pollen grains are collected from the covered flowers of the ‘male’ parents in clean sterile paper/polythene bags or test tubes. The collected pollen grains can be stored for later use. When the stigma of the emasculated flower of ‘female’ parent matures, the covering bag is removed for a short while. The stigma is dusted with pollen grains by means of a clean brush. Controlled pollination by bringing selected pollen grains in contact with a stigma through human efforts is called artificial pollination.

After pollination, the emasculated flower is covered again till the stigma remains receptive. Bags are discarded when fruits begin to develop. The seeds produced by these flowers of the female parent are the hybrid or F_t Seeds. These seeds are stored for testing. These seeds are sown in the next season. There will be segregation, independent assortment and recombination in the F₂ and later generations are obtained from these F, seeds.

(iv) Selection and Testing of Superior Recombinants:

This step comprises selecting, among the progeny of the hybrids, those plants that have the desired character combination. The selection process yields plants that are superior to both of the parents. These plants are self-pollinated for several generations till they come to a state of uniformity (homozygosity) so that the characters will not separate in the progeny. Selection is of two types— self pollinated and cross pollinated.

(a) Selection in Self-pollinated Crops:

The degree of cross pollination is less than 5%. There is repeated self pollination of selected plants till superior homozygous genotypes are obtained. The best one is used as new variety. The self-pollinated progeny of homozy­gous plant constitutes a pure line. All the plants in pure line have identical genotype. The wheat variety HUW 468 is a good example of pure line. Variation appearing later in such a pure line is due to environment.

(b) Selection in Cross-pollinated Crops:

The cross-pollinated crops are heterozy­gous for most of their genes and their population contains plants of several different genotypes. Some of these genotypes are superior but many are inferior. Superior genotype plants are selected and are allowed to crossbreed (these plants are not allowed to self breed) so that heterozygosity is also maintained. Selection can be continued in a few successive genera­tions of cross-pollinated crops.

(v) Testing, Release and Commercialisation of New Cultivars:

The newly selected lines are evaluated for their yield and other agronomic traits of quality, disease resistance, etc. This evaluation is done by growing these in the research field and recording their perfor­mance under ideal fertiliser (application), irrigation, etc. After the evaluation in the research fields, the testing of the materials is done in the farmer’s fields, for at least three growing seasons at different locations in the country, representing all the agro climatic zones. The material is evaluated in comparison to the best available crop cultivar. Thus the seeds of new variety are multiplied and made available to the farmers.

Examples of some improved varieties:

(1) Wheat— Kalyan Sona, Sonalika.

(2) Rice— Jaya and Ratna

(3) Sugarcane— Saccharum barberi, Sachharum officinarum

(4) Rapeseed mustard Brassica— Pusa swarnim

High Yielding Varieties (HYVs):

India is an agricultural country. Agriculture contributes about 33 per cent of India’s GDP and gives employment to about 62 per cent of the population. After India’s independence, one of the main challenges faced by the country was enough food production for the increasing population. The development of several high yielding varieties of wheat and rice in 1960 increased yields per unit area. This phase is often called the Green Revolution. Some high yielding varieties (HYVs) of Indian hybrid crops are given in the figure 9.15.

(i) Wheat:

In 1960 to 2000 wheat production increased from 11 million tonnes to 75 million tonnes while rice production increased from 35 million tonnes to 89.5 million tonnes. It was due to the development of semi-dwarf varieties of wheat and rice. Nobel Prize winner Norman E. Borlaug of International Centre for Wheat and Maize Improvement in MEXICO developed semi-dwarf wheat.

In 1963, many lines like Sonalika and Kalyan Sona were selected from these that were high yielding and disease resistant. They were introduced all over the wheat growing areas of India. Some more improved varieties of wheat are (i) Lerma Roja 64-A, (ii) Sonora 64-Early, (iii) Safed Lerma, (iv) Chhoti Lerma, (v) Sharbati Sonora.

(ii) Rice:

Semi-dwarf rice varieties were developed from IR-8 at International Rice Research Institute (IRRI), Philippines and Taichung Native-1 from Taiwan. The developed varieties were introduced in 1966. Later on better yielding semi dwarf varieties Jaya and Ratna were developed in India. As stated earlier M.S. Swaminathan contributed much for Green Revolution in India.

(iii) Sugarcane:

Saccharum barberi was originally grown in North India, but had poor sugar content and yield. However, Saccharum officinarum had higher sugar content and thicker stems but did not grow well in North India. These two species were crossed to have sugar cane varieties combining the desirable qualities of high sugar, high yield, thick stems and ability to grow in the sugarcane belt of North India.

(iv) Millets:

Plants producing a large crop of small seeds are called millets. Hybrid bajara, jowar and maize have been developed in India. From hybrid varieties, the development of several high yielding varieties resistant to water stress has been possible.

2. Plant Breeding for Disease Resistance:

Fungal, bacterial, viral and nematode pathogens attack the cultivated crops. Crop losses can be upto 20-30 per cent. In such situation if the crops are made disease resistant, food production is increased and use of fungicides and bactericides would also be reduced. Before breeding, it is important to know the causative organism and the mode of transmission. Some fungal diseases are rusts, e.g., brown rust of wheat, red rot of sugarcane and late blight of potato; by bacteria— black rot of crucifers and some viral diseases are tobacco mosaic, turnip mosaic, etc.

Disease is an abnormal unhealthy condition produced in an individual due to defective nutrition, defective heredity, unfavourable environment or infec­tion. Disease causing organism is called pathogen. The individual in which a disease is caused by a patho­gen is called host. The development of disease in a plant depends on three factors: (i) host genotype, (ii) patho­gen genotype and (iii) the environ­ment as shown in the figure 9.16.

Some host genotypes possess the ability to prevent a pathogen strain from producing disease. Such host lines are called resistant, and this ability is called resistance or disease resistance. The term strain has a similar meaning for the pathogen as line has for the host.

Those lines of a host that are not resistant to the pathogen are called susceptible. A successful breeding for disease resistance depends mainly on the following two factors: (i) a good source of resistance, and (ii) a dependable disease test. In disease test, all the plants are grown under conditions in which a susceptible plant is expected to develop disease. Therefore, disease resistant crop plants should be produced to avoid infec­tion.

Methods of Breeding for Disease Resistance:

Breeding is carried out either by conventional breeding techniques described earlier or by mutation breeding. The conventional method of breeding for disease resistance is hybrid­ization and selection. The various sequential steps are: screening germplasm for resistance sources, hybridization of selected parents, selection and evaluation of hybrids and testing and release of new varieties. Some of the released crop varieties bred by hybridization and selection for disease resistance to fungal, bacterial and viral diseases are given below:

Some released crop varieties bred by hybridization and selection, for disease resistance to fungi, bacteria and viral diseases.

Conventional breeding is often constrained by the availability of limited number of disease resistance genes that are present and identified in various crop varieties. Inducing mutations in plants sometimes leads to desirable genes being identified. Plants having these desirable characters can either be multiplied directly or can be used in breeding. Other breeding methods that are used are mutation, selection among somaclonal variants and genetic engi­neering.

Polyploidy in Crop Improvement (Polyploidy Breeding):

An organism which has more than two sets of chromosomes or genomes per cell is called polyploidy and this condition is known as polyploidy. Most important crops having polyploidy condition are wheat, bananas, cotton, potatoes, sugarcane and tobacco. Polyp­loidy occurs in nature due to the failure of chromosomes to separate at the time of anaphase either due to non-disjunction or due to non-formation of spindle. It can be artificially induced by application of colchicine.

Depending upon the number of genomes present in a polyploid, it is known as triploid (3n), tetraploid (4n), pentaploid (5n), hexaploid (6n), etc. Polyploids with odd number of genomes (i.e., triploids, pentaploids) are sexually sterile because the odd chromosomes do not form synapsis. They are, therefore, propagated vegetatively, e.g., Ba­nana, Pineapple. Polyploids also do not cross-breed freely with diploids.

Polyploidy is of two types— autopolyploidy and allopolyploidy.

(i) Auto polyploidy:

It is a type of polyploidy in which there is a numerical increase of the same genome, e.g., autotriploid (AAA), autotetraploid (AAAA). Some of the crop and garden plants are autopolyploids, e.g., Maize, Rice, Gram. Autopolyploidy induces gigas effect.

(ii) Allopolyploidy:

It has developed through hybridisation between two species followed by doubling of chromosomes (e.g., AABB). Allotetraploid is the common type. Allopolyploids function as new species, e.g., Wheat, American Cotton, Nicotiana tabacum. Two recently produced allopolyploids are Raphanobrassica and Triticale. Thus Triticale is a hybrid of wheat (Triticum turgidum) and rye (Secale cereale). Among artificially produced allopolyploidy, Triticale is the first man made crop derived by crossing wheat and rye.

Autoallopolyploidy is a type of allopolyploidy in which one genome is in more than diploid state. Commonly autoallopolyploids are hexaploids (AAAABB), e.g., Helianthus tuberosus.

Mutation Breeding:

Mutation is a sudden and heritable change in a character of an organism. Mutation can be due to a change in any one of the following: (a) base sequence of the concerned gene, (b) chromosome structure and chromosome number.

Spontaneous Mutations:

Mutations occurring naturally are called spontaneous muta­tions. They are both germinal and somatic. Useful somatic mutations can be incorporated in crop improvement only in vegetatively propogated plants, e.g., seedless grape, naval orange, Bhaskara banana. Vegetative propagation is also useful in maintaining germinal varia­tion got through sexual reproduction, e.g., apple, mango, potato, sugarcane. Thus sponta­neous mutations are the source of all the genetic variations occurring in all living things today.

Mutagens and Induced Mutations:

Rate of spontaneous mutations is very low. There­fore, rate of mutation is increased by means of certain agents called mutagens. Mutagens are of two types (a) chemical and (b) physical mutagens. Chemical mutagens are some chemicals such as ethylmethane sulphonate (EMS) and sodium azide, that induce mutations.

Physical mutagens are different kinds of radiation like X-rays, gamma-rays, ultraviolet rays, etc., that cause mutations. These mutagens induce changes in DNA and chromosomes, which produce mutations. Mutations produced in response to mutagens are known as induced mutations. They were first produced by Muller (1927) with the help of X-rays on Drosophila and by Stadler in maize. Use of induced mutations in plant breeding to develop improved varieties is called mutation breeding.

In India, over 200 varieties have been developed through mutation breeding.

Selection amongst Somaclonal Variation:

Genetic variation present among plant cells during tissue culture is called somaclonal variation. The term somaclonal variation is also used for the genetic variation present in plants regenerated from a single culture. This variation has been used to develop several useful varieties.

Some of the somaclonal variations are stable and useful, e.g., resistance to diseases and pests, stress tolerance, male sterility, early maturation, better yield, better quality, etc. Thus somaclonal variations have produced wheat tolerant to rust and high temperature, Rice to leaf ripper and Tungro virus, Potato to Phytophthora infestans (late blight of Potato), etc. Other useful variations include high protein content of Potato, short duration Sugarcane and increase shelf life of Tomato.

Genetic Engineering (Recombinant DNA Technology):

This is a process in which the alteration of the genetic makeup of cells is done by deliberate and artificial means. This process involves transfer or replacement of genes to create recombinant DNA.

This is done by cutting DNA molecules at specific sites to get fragments containing desirable and useful genes from one type of cell. Thereafter, these genes can be inserted into a suitable carrier or vector. Now, these recombinant DNA can be put into completely differ­ent cell of a bacterium or plant or animal cell. By this method, they acquire useful characters, such as disease resistance or to make useful enzymes, hormones, vaccines, etc.

This process involves manipulation or engineering of the DNA (genes), therefore, the term ‘genetic engineering’ has been used. The recombinant DNA molecules can be cloned and amplified to an unlimited extent.

3. Plant Breeding for Developing Resistance to Insect Pests:

Insects and pest infestation are two major causes for large destruction of crop plant and crop. Insect resistance in host crop plants is due to morphological, biochemical or physi­ological characters. Hairy leaves of many plants are associated with resistance to insect pests.

For example, resistance to jassids in cotton and cereal leaf beetles in wheat. Solid stems in wheat lead to non-preference by the stem saw fly and smooth leaved and nectar-less cotton varieties does not attract bollworms. Low nitrogen, sugar and high aspartic acid in maize develops resistance to maize stem borers.

Breeding methods for insect pests resistance include the same steps as for any other agronomic character like yield or quality as described above. Sources of resistance genes may be cultivated varieties, germplasm collections of the crop or wild relatives of the crop.

4. Plant Breeding for Improved Food Quality:

It is estimated that more than 840 million people in the world do not have adequate food to meet their daily requirements. Three billion people suffer from protein, vitamins and micronutrient deficiencies or ‘hidden hunger’ because these people cannot afford to buy adequate vegetables, fruits, legumes, fish and meat. Their food does not contain essential micronutrients specially iron, iodine, zinc and vitamin A.

This increases the risk for disease, reduces mental abilities and life span. Breeding of crops with higher levels of vitamins and minerals or higher protein and healthier fats is called biofortification. This is the most practical aspect to improve the health of the people.

Plant breeding is undertaken for improved nutritional quality of the plants. Following are the objectives of improving:

(1) Protein content and quality

(2) Oil content and quality

(3) Vitamin content and

(4) Micronutrient and mineral content.

Maize hybrids that had twice the amount of the amino acids lysine and tryptophan, compared to existing maize hybrids were developed in 2000. Wheat variety with high protein content Atlas 66 has been used as a donor for improving cultivated wheat. It was possible to develop an iron rich variety containing more than five times as much iron as in usually consumed varieties.

There are eight essential amino acids. When these amino acids are present in the protein of our diet in sufficient amount, they constitute protein quality. Proteins of cereals and millets are deficient in two amino acids, i.e., lysine and tryptophan. Whereas pulses are deficient in methionine and cysteine both are sulphur containing amino acids.

Indian Agricultural Research Institute (IARI), New Delhi, has also developed many vegetable crops that are rich in minerals and vitamins. For example, vitamin A enriched carrots, pumpkin, spinach, vitamin С enriched bitter gourd, Bathua, tomato, mustard, cal­cium and iron enriched spinach and bathua; and protein enriched beans (broad lablab, French and garden peas).

Single Cell Protein (SCP):

As we know demand of food is increasing due to increase in human and animal popu­lation, the shift from grain to meat diets does not solve the problem as it takes 3-10 kg of grain to produce 1 kg of meat by animal farming. More than 25 per cent of human population is suffering from hunger and malnutrition. One of the alternate sources of proteins for animal and human nutrition is single cell protein (SCP).

Microorganisms are used for the preparation of fermented foods (e.g., cheese, butter, idlis, etc.). Some microorganisms (e.g., blue green algae- Spirulina and mushrooms- fungi) are being used as human food. Now efforts are being made to produce microbial biomass using low cost substrates. Microbes like Spirulina can be grown on waste water from potato processing plants (containing starch), straw, molases, animal manure and even sewage, to produce food rich in proteins, minerals, fat, carbohydrates and vitamins. This biomass is used as food by humans.

The cells from microorganisms such as bacteria, yeasts, filamentous algae, treated in various ways and used as food, are called single cell protein (SCP). The term SCP does not indicate its actual meaning because the biomass is not only obtained from unicellular microorganisms but also from multicellular microorganisms.

Thus SCP is produced using bacteria, algae, fungi (yeasts, etc). The substrates used for SCP production range from C0₂ (used by algae) through industry effluents like whey (water of curd), etc. to low-cost organic materials like saw dust and paddy straw. Commercial production of SCP is mostly based on yeasts and some other fungi, e.g., Fusarium graminearum. In most cases, SCP has to be processed to remove the excess of nucleic acids. SCP is rich in high quality protein and is poor in fats. Both high quality of protein and low quantity of fats constitute good human food.

It has been estimated that a 250 kg cow produces 200 g of protein per day. In the same period 250 g of a microorganism like Methylophilus methylotrophus because of its high content of biomass production and growth, can produce about 25 tonnes of protein.

Some Common Microbes as SCP producers:

(i) Cyanobacteria – Spirulina

(ii) Bacteria – Methylophilus methylotrophus

(iii) Yeasts – Candida utilis

(iv) Filamentous fungi – Fusarium gramiearum

Advantages of SCP:

(i) It is rich in high quality protein and poor in fat content,

(ii) It reduces the pressure on agricultural production systems for the supply of the required proteins,

(iii) SCP production is based on industrial effluents so it helps to minimise environ­mental pollution,

(iv) SCP can be produced in laboratories throughout the year.

Role of Plant Breeding:

Plant breeding has played an important role in enhancing food production:

(i) Triticale is a man-made alloploid developed from Triticum turgidum and Secale cereale.

(ii) Lysine-rich maize varieties like Shakti, Rattan and Protina have been developed.

(iii) Through mutation breeding, more than 200 varieties of crops have been developed.

(iv) Disease resistance in plants has been introduced through breeding.

(v) All the sugarcane varieties that are cultivated today are interspecific hybrids.

(vi) Plant breeding has also given us improved varieties of crops like Sonora-64 of wheat and Taichung Native -1 of rice.