Plant breeding

MAIN STEPS IN PLANT BREEDING

The main steps in breeding a new genetic variety of a crop are the following:

Collection of Variability :

Genetic variability is the root of any breeding programme.

In many crops preexisting genetic variability is available from wild relatives of the crop.

Germplasm is collected from within country (IC – Indigenous collection) and also from other countries (EC – Exotic collection).

The germplasm collections are usually maintained at low temperature in form of propagules.

The stored propagules are periodically grown in the field to obtain fresh propagules.

In fruit trees, the germplasm is maintained as trees grown in the field.

Collection and preservation of all the different wild varieties, species and relatives of the cultivated species is a pre-requisite for effective exploitation of natural genes available in the population.

Germplasm is the sum total of all the alleles for all the genes present in a crop and its related 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.

Therefore, germplasm collection is the most practical and effective answer to problem like loss of germplasm due to expansion of agriculture, industries and other human activities.

Evaluation and Selection of Parents :

The germplasm is evaluated to identify plants with desirable combination of characters.

Selection of parents is done by picking up seeds of only those plants for multiplication which have the desired traits.

Selected plants are multiplied and used in the process of hybridisation. Pure lines are created wherever desirable and possible.

Common Methods of Crop Improvement

(a) Selection :

Selection is the oldest method of crop improvement.

Almost all our present day crops are the result of selections carried out by prehistoric human beings.

During selection, the individual plants or groups of plants having desired characters are picked up from population, eliminating the undesirable ones.

The selected parents are allowed to reproduce for setting their seeds. The seeds are collected and again a new crop is developed. Selection is again done in this new crop.

It is of three types:

(i) Mass selection :

Simplest and oldest method mainly for cross pollinated crops and is based on phenotypic characters.

It is most common method of crop improvement.

In this, large number of plants of a species with similar phenotypic characters are selected and their seeds are mixed together at time of harvesting.

These seeds are sown in field for progeny testing. Progeny selection is done by eliminating the undesirable one and saving the best.

(ii) Pure line / Inbreed selection :

Involves isolation of desirable homozygous individuals and is done for self pollinating crops.

A pure line is the progeny of a single homozygous self pollinated plant. In this method, a plant with desired characters is selected from a genetically mixed population.

The single selected plant is self pollinated during subsequent generations to pick out a true breeding species with desired characters.

Once homozygosity is confirmed, the individual is multiplied on a large scale and recommended to farmers. e.g., Wheat varieties like : PV - 18; HUW - 468, Kalyan Sona

(iii) Clonal selection :

It is used for vegetatively reproducing crops. Progeny of a single vegetatively propagated plant is called clone. Here selection is made between clones and not within clone e.g., Kufri safed potato.

(b) Hybridisation :

Hybridisation is the process of making a cross between two genetically diverse parents to obtain a progeny with desired superior traits.

The desired characters very often combines from different parents. For example high protein quality of one parent may need to be combined with disease resistance from another parent.

When a hybridisation is performed between two plants it is called single cross and when it is performed between more than two plants, it is called multiple cross e.g., development of C-306 wheat variety and IR-36 rice variety.

It is not necessary that the hybrids do combine the desirable characters, only one in few hundred to a thousand crosses shows the desirable combination. The primary aim of hybridization is to produce variations.

First natural hybridisation was reported in corn (Maize). First artificial hybrid was obtained by crossing sweet william and carnation by Thomas Fair child and is known as fairchild mule.

Hybridisation was first practically utilised in crop improvement by Kolreuter.

(c) Mutation Breeding :

Mutation is sudden, stable and inheritable change which alter the gene expression of organism. This results in a new character not found among parents.

The mutations, which are artificially induced by treatment with certain physical or chemical agent are called induced mutations.

The application of induced mutations for crop improvement is called mutation breeding.

The agents which are used to induce mutations are called mutagens.

Various chemicals such as HNO₂ base analogues, alkylating agents, acridine dyes, and physical agents such as X-rays, UV-rays and gamma rays are used to induce mutations which produce desirable qualities.

It has been used commonly in self pollinating crops.

A number of new varieties have been developed e.g.,

(i) Sharbati Sonora and Pusa Lerma varieties of wheat (formed basis of green revolution in India).

(ii) Reimei, Atomita-2 and Jagannath varieties of rice.

(iii) Erectiferum and Erectoids varieties of barley.

(iv) Aruna variety of castor.

(v) Thick shell in ground nut (TGI)

(vi) Wheat - NP 836

(vii) Cotton - lndore-2

(viii) In mung bean, resistance to yellow mosaic virus and powdery mildew.

(ix) Peppermint (Mentha piperita) - Todd's Mitcham variety, (high oil content and disease resistant).

(d) Polyploid Breeding :

Organisms having more than two sets of chromosomes are called polyploid.

Induced polyploidy is used by plant breeders for improving yield of forage and other crops.

Many of modern day crops such as wheat, rice, sugarcane, potato and cotton are natural polyploids.

The allopolyploids have been used for obtaining fertile hybrids between different species and genera.

Allopolyploidy immediately forms new species.

Odd-numbered polyploids (3x, 5x etc.) are usually sterile, while even-numbered polyploids are fertile.

Allopolyploids are produced in two steps, as follows - First of all, two different species are hybridised to produce F₁. The F₁ is usually sterile.

Therefore, in the second step, the chromosome number of the F₁ is doubled.

The resulting allopolyploid is usually, at least, partially fertile and forms a new species.

Many alloployploids have been produced in nature, some of which have succeeded as crops e.g., wheat.

Humans have produced a new alloployploid crop called Triticale in the following manner.

Allotetraploid wheat (Triticum turgidum) was hybridised with rye (Secale cereale; a diploid grass).

The chromosome number of the resulting F₁ was doubled to produce Triticale. Triticale is cultivated in some areas of Punjab and in the hilly regions of the country.

Selection and Testing of Superior Recombinations :

This step comprises of selecting, among the progeny of the hybrids, i.e. those plants that have the desired character combination.

This is the crucial step for the success of breeding experiment, so require careful scientific evaluation of progeny.

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 (hornozygosity) so that the characters will not separate in the progeny.

Testing, Release and Commercialization 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 fertilizer, irrigation etc.

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 agroclimatic zones.

The material is evaluated in comparison to the best available local crop cultivar - a check or reference cultivar.

Finally the seeds of desirable plants are certified by National Seed Corporation (NSC) for marketing.

GREEN REVOLUTION

India is an agricultural country. Agriculture contributes about 33% of India's GDP and gives employment of about 62% of the population.

The development of several high yielding varieties of wheat and rice in mid 1960's increased the yield per unit area. This phase is often referred to as the Green Revolution.

From 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.

This was due to the development of semi-dwarf varieties of wheat and rice.

Dwarf Wheat

A dwarfing gene Norin-10 was reported in Japan.

American plant breeders produced single dwarf wheat. N. Borlaug developed triple dwarf wheats in Mexico, popularly known as Mexican Wheats.

These had high yield, resistance for lodging, common pathogens and pests, photoinsensitive, fertilizer responsive and have smaller growth period.

Sonora-64 and Lerma Rojo-64 were brought to India and modified through gamma mutations so that these can become part of Indian Agriculture.

In 1963, many varieties like Sonalika and Kalyan sona were introduced in all wheat growing belts of India.

Dwarf Rice.

A dwarfing gene dee-geo-woo-gen, was reported in Taiwan.

It was introduced in rice varieties by IRRI, Philippines in varieties IR-8, IR-24.

Taichung Native-1 was developed in Taiwan. Later better yielding semi dwarf varieties Jaya and Ratna were developed in India.

Sugarcane :

Saccharum barberi was originally grown in North India, but had poor sugar content and yield.

Tropical canes grown in South India i.e. Saccharum officinarum had higher sugar content and thicker stems but did not grow well in North India.

These two species were crossed to get sugarcane varieties combining the desirable qualities of high sugar, high yield, thick stem and ability to grow in the sugarcane belt of North India.

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 were taken over.

PLANT BREEDING FOR DISEASE RESISTANCE

Fungal, bacterial, viral and nematode pathogens attack the cultivated crops. So crop loss can be upto 20-30 percent, sometimes even total.

In such situation if the crops are made disease resistant, food production is increased and use of fungicides and bacteriocides would be reduced.

(1) The development of diseases in a plant depends on the interactions among following factors :

(a) Host genotype

(b) Pathogen genotype

(c) The environment

(2) 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.

(3) 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 :

(a) A good source of resistance

(b) A dependable disease test. In disease test, all the plants are grown under conditions in which a susceptible plant is expected to develop disease.

Some Important Diseases of Economically Important Plants (A Brief Information)

(a) Late Blight of Potato :

The disease is famous as it caused Ireland famine of 1845.

The disease occurs in all potato growing areas of the world. It is most destructive under cool and moist conditions.

The pathogen is Phytophthora infestans. The disease kills the foliage of crop because of which the yield is reduced.

The disease not only infects potato tubers in the field, but also continues to advance inside the tubers under storage conditions.

Symptoms : They appear first as water soaked or hydrotic areas along the margin and tips of lower leaflets. The spots enlarge rapidly, become necrotic, turn brown and then blackish or blighted. The infected leaves become limp, appear blighted and rot away, producing a characteristic odour.

Control Measures.

(i) Seed tubers should be free from infection.

(ii) Disease Resistant varieties.

(iii) Fungicides. In susceptible environment and disease prone areas, the crop should be sprayed or dusted with fungicides.

(b) Loose Smut of Wheat

Causal organism : Ustilago nuda tritici.

Symptoms : It is recognized as soon as the effected inflorescence emerges from leaf sheaths. In the smutted deformed inflorescence, spikelets are completely filled with black, dry, powdery mass of chlamydospores (brand spores), the skin of fruit wall bursts, soon exposing the spores

Control measures : Disease resistant varieties and fungicides. In susceptible environment and disease prone areas, the crop should be sprayed or dusted with fungicides.

(c) Black Rust of Wheat

Causal organism : Puccinia graminis tritici.

Symptoms : Black rust or stem rust of wheat is seen on the stem and leaf sheaths. Both uredosori and teleutosori are seen. Uredospores are brownish, spherical or oblong and teleutospores are black and elongated. As this rust is heteroecious, uredosori and teleutosori are found on Wheat, whereas some other stages are found on other host, Barberry.

Control measures : Disease resistant varieties, fungicides. In susceptible environment and disease prone areas, the crop should be sprayed or dusted with fungicides.

(d) Bacterial Blight of Rice :

It is caused by bacterium Xanthomonas oryzae.

The infected tissues collapses and are digested by bacteria. Lesions enlarge and become necrotic.

The stems and the leaves of infected plant give blighted or burnt up appearance.

Control measures include rogueing, 3-year crop rotation, spray of agrimycin plus copper oxy-chloride and antibiotics besides sowing disease resistant varieties.

(e) Cucumber Mosaic Disease / Beans Mosaic Disease

It is world-wide disease caused by an isometric single stranded RNA virus called Cucumovirus having fragmented genome.

The virus seems to have the widest range of hosts, attacking different types of plants like cucumbers, melons, squash, gladioli, crucifers, banana, celery, spinach, pepper, beans, chickpea (gram) etc.

It attacks plants which are 5-6 weeks old. Infected plants develop bunched or bushy appearance.

There is mottling, distortion, wrinkling, curling and dwarfing of leaves. Later on leaves fall down. Flowers and fruits also develop distortions.

It is transmitted by a variety of methods including aphids, agricultural implements, farm workers, air, water and other mechanical means.

Control measure : Disease resistant varieties.

(f) Root Knot of Tomato and Brinjal

The disease is quite common in all vegetable growing areas especially where the climate is warm with short or mild winters.

It devitalizes root tips, develop swelling over roots and reduces flow of water and minerals to the aerial parts.

As a result, yield and quality of fruits are affected. The disease is caused by root knot nematode Meloidegyne incognita.

Symptoms : The main as well as lateral roots develop a number of spherical to elliptical swellings or galls. The diameter is 2 to several times the diameter of the root. They include reduced growth, smaller, fewer yellow leaves which tend to wilt. Flowering is reduced. Fruits are fewer and of poor quality.

Control.

(i) Biological control. Nematode infested soil is inoculated with spores of bacterium Pesteuria penetrans, fungus Dactylella oviparasitica and VAM fungus.

(ii) Resistant varieties. It is always preferred to grow varieties resistant to root knot nematodes.

(iii) Flooding of the fields eliminates the root knot nematodes.

(iv) Steam sterilization and fumigation with nematicides free the soil of nematodes.

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 hybridisation and selection.

Conventional breeding is often constrained by availability of limited number of resistance genes identified in crop.

The various steps are:

(1) Screening germplasm for resistance sources

Wild plants can be a best source because they survive without getting protection from humans in any environment. Their characters are useful for plants breeders; for example potato has got

(i) Resistance to potato virus X and potato leaf roll virus from Solanum acaule.

(ii) Resistance to five races of cyst nematodes and to fungus Fusarium coeruleum from Solanum spegazzini.

(iii) Resistance to potato virus from Solanum stoloniferum.

(iv) Resistance to Phytophthora infestans from S. demissum.

(v) Resistance to red rot of sugarcane and adverse environment in noble sugarcane (Saccharum officinarum) was conferred from the wild Saccharum spontaneum.

(2) Hybridization of selected parents

(3) Selection and evaluation of hybrids

(4) Testing and release of new varieties.

Some released crop varieties bred by hybridisation and selectionfor disease resistance to fungi, bacteria and viral diseases

New varieties having these desirable characters can either be multiplied directly or can be used in breeding.

Constraints of conventional breeding like the limited availability of limited number of disease resistance genes can be overcome by this method.

Other useful breeding methods like mutation, selection among somaclonal variants and genetic engineering.

In mung bean resistance to YMV and powdery mildew were induced by mutations.

Resistance to YMV in bhindi (Abelmoschus esculentus) was transferred from a wild species and resulted in a new variety of A. esculentus called Prabhani Kranti.

Plant Breeding for Developing Resistance to Insects / Pests

Insects and pest infection is another major cause for large destruction of crop plant and crop produce. Insect resistance in host crop plants is due to morphological, biochemical or physiological characters.

Solid stem in wheat lead to non-preference by the stem saw fly.

Smooth leaved and nectar-less cotton varieties do not attract bollworms. Low nitrogen, sugar and high aspartic acid in maize develops resistance to maize stem borers.

Some released crop varieties bred by hybridisation and selection for insect pest resistance

Sources of resistance genes may be cultivated varieties, germplasm collections of the crop or wild relatives of the crop.

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 are suffering from 'hidden hunger' as their diet shows deficiencies in proteins, vitamins and micronutrient because these people cannot afford to buy adequate vegetables, fruits, legumes, fish and meat.

Breeding of crops with higher levels of vitamins and minerals or higher protein and healthier fats is called biofortification.

Plant breeding is undertaken for improved nutritional quality of the plants, with the objectives of improving:

(i) Protein content and quality

(ii) Oil content and quality

(iii) Vitamin content

(iv) Micronutrient and mineral content

Cereals are commonly deficient in lysine and tryptophan, while legumes are commonly deficient in sulphur containing amino acids like methionine and cysteine.

In 2000, maize hybrids that had twice the amount of lysine and tryptophan were developed as compared to existing varieties.

Three lysine rich maize varieties have been developed in India are Protina, Shakti and Rattan.

Wheat variety Atlas 66 with high protein content has been used as a donor for improving cultivated wheat.

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 C enriched bitter gourd, bathua, tomato, mustard, calcium and iron enriched spinach, bathua and protein enriched beans (broad lablab, french and garden peas).

Breeding for Anti-nutritional Factors

Nutritional quality of a crop generally means improvement in its produce which can be suitable for human (animal nutrition).

Anti-nutritional factors are compounds present in foods and have adverse effect on animal and human growth.