PLANT BREEDING

Plant Breeders Training Course

Lesson Structure

There are 7 lessons in this course:

1. The Scope and Nature of the Plant Breeding Industry
   • What is Plant Breeding
   • Scope of the Modern Industry
   • Sources of Genetic Material
   • Germplasm Preservation
   • Botanic Gardens, Plant Breeding Organisations, Research Bodies
2. Introduction to Genetics
   • Review of Plant Genetics Linkage and Crossing Over
   • DNA
   • Homologous Chromosomes
   • Cell Biology -cell components, cell wall, nucleus
   • Protein Synthesis
   • Plant Anatomy
   • Plant Genetics, Mendel's Principles and Experiment
   • Genetic Terminology
   • Gene Linkages
3. Gamete Production, Pollination and Fertilisation in Plants
   • Phases of Plant Reproduction
   • Gamete Production
   • Gene Mutation
   • Sources of Genetic Variation: Polyploidy, Bud Sports and Chimeras
   • Male Sterility
   • Effect of Environment
   • Terminology
   • Use of Pollination Biology in Plant Breeding: Pollination Process, Pollination Requirements, Cross pollination, Fertilisation, Male/Female Recognition, Overcoming incompatibility, Post Fertilisation, Pollen Selection, Floral Introduction etc.
   • Mitosis and Meiosis
   • Genes
   • Sexual Structures in Plants: Flowers, Fruit, Seed
4. Mono Hybrid and Dihybrid Inheritance in Plants
   • Mono hybrid Crosses
   • Dihybrid Crosses
   • Gene Linkages
   • Crossing Over
   • Recombination
   • Quantitative Traits
   • Terminology
5. Systematic Botany and Floral Structures
   • Systematic Botany
   • Plant Morphology
   • Type Specimens
   • Floral Diagrams
   • International Botanical Code
   • Binomial System; Genus and species
   • Hybrids, Varieties, Cultivars
   • Name Changes
   • Nomenclature of hybrids
   • Using Botanical Keys
6. Practical Plant Breeding Techniques
   • Plant Breeding Programs
   • Breeding Self Pollinated Crops
   • Pure Line Breeding
   • Mass Selection
   • Pedigree Breeding
   • Bulk Population Breeding
   • Breeding Cross Pollinated Crops
   • Single Plant Selection
   • Mass Selection
   • Progeny Selection
   • Line Breeding
   • Recurrent Selection
   • Backcross Breeding
   • Induced Polyploidy
   • Hybrid Seed Production
   • Dormancy Factors Affecting Germination (eg. hard seeds, impermeability to water, Chemical inhibitors, Undeveloped embryos, etc)
7. Current Developments in Plant Genetics
   • Plant Biotechnology
   • Genetic Engineering
   • DNA Markers
   • Somatic Hybridisation
   • Micro Propagation
   • Plant Breeders Rights
   • Trade Marks, Patents

Aims

• Explain the results of mono hybrid and dihybrid inheritance in plants.
• Describe gamete production in plants.
• Investigate the role of systematic botany in horticulture.
• Explain a variety of different plant breeding techniques.
• Describe the commercial and scientific nature of the modern plant breeding industry, on a global basis
• Describe the structure and function of genetic material
• Review current developments in plant breeding.

Working as a Plant Breeder

A plant breeder produces new cultivars of plants that have a valuable commercial potential, and they licence production nurserymen to grow and sell those plants. Schemes (e.g. Plant Variety Rights) operate in some countries, which allow plant breeders to register ownership of a plant they bred, whilst other countries simply do not operate such schemes.

Plant breeders create new types of plants by:

Where Do They Work?
Plant breeders may work in specialised laboratories operated by large commercial companies, research institutions, or universities. They may also work for nurseries or botanic gardens, or specialised government departments.

Opportunities
In some developed countries, plant breeders can earn as much or more profit on plant sales than profits earned by production nurserymen or retailers. Plant breeding can be a risky business, though, if you do not have an adequate level of plant knowledge - as well as horticultural skills. It can also involve an element of luck.  You need to produce a cultivar that is unlike anything else available (or differs significantly), and has the potential to sell at a high price and in large numbers. Then it needs to be promoted well and have an organised and effective distribution system - so that it is available to all who want it.

Some types of plants can be bred, grown on, selected, and propagated in very large numbers and then launched to the public all within less than two or three years. However, for other types of plants - breeding, developing and launching a new cultivar can take five, ten, or even more years.

What Is Needed?
A plant breeder needs an intimate understanding of plant botany, genetics, plant propagation skills and above all, great plant knowledge. Plant breeders are plant specialists who can readily see the subtle differences between different genera, species and varieties of plants. They need to have a heightened awareness of their industry, the plants that are already available, the characteristics which are most favoured in plants and the things that the industry desires and cannot readily find in existing cultivars. They need to set trends and recognise changing trends set by others.

Some plant breeders are self-taught enthusiasts who have worked for so long with a genus they are passionate about that they know more about it than anyone else i.e. other horticulturists, scientists or nurserymen. Others have studied science in depth, they have learnt about plant breeding, genetics and propagation as a foundation, and then studied particular plant types that they are keen to breed.

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How to Breed Plants? 

There are a number of different methods of selection which the breeder can use, and the one which is chosen will depend upon:

1. Objectives of the breeder program.
2. The inheritance patterns of traits to be improved.
3. The parental population and their mode of reproduction.

The conventional method of plant breeding involves hybridising plants by transferring pollen from one plant to another, usually of the same species. The resulting progeny are grown under test conditions to identify their characteristics. In most cases, many crosses and backcrosses must be made before desirable varieties can be selected, tested and released for commercial use.
 
New technologies including micropropagation and genetic engineering have facilitated and revolutionised the techniques of plant breeding. While conventional plant breeding is still the main technique used to create new plant varieties, plant biotechnology is increasingly playing an important role in breeding programs.

Genetic Improvements

The basis of modern plant breeding is genetics - the inheritance of characteristics from one generation to the next. Anyone wishing to breed plants must have an understanding of the fundamental principles of plant reproduction and genetic inheritance.

The genetic improvement of plants has been practised since the beginning of agriculture. Early farmers and gardeners selected plants with desirable characteristics such as faster growth, larger fruit, increased yields and resistance to diseases and pests. Unknowingly, those early farmers were creating strains of genetically improved plants, but without any knowledge of genetics or the mechanics of plant breeding it was impossible for them to transfer desirable characteristics from one line to another.

It was only after Gregor Mendel's work with breeding peas in the 1860s that scientists began to understand the role of heredity in plant improvement. Mendel's experiments provided the basis for modern breeding programs, enabling scientists to artificially hybridise plants with the aim of improving specific traits.

The greatest impact of plant breeding has been on agronomic crops, such as wheat, corn, barley and rice. Since the second half of the twentieth century, crop yields have increased by up to 300%; mostly as a result of selective breeding programs (but also due to chemical fertilisers and other modern farming practices).

In recent years, plant breeders have also targeted ornamental plants, selecting and breeding for improved flowers, pest and disease resistance, improved environmental tolerances, and a range of desirable growth forms. Today the introduction and marketing of new, different ornamental cultivars is a very important aspect of the nursery industry.
 
The conventional method of plant breeding involves hybridising plants by transferring pollen from one plant to another, usually of the same species. The resulting progeny are grown under test conditions to identify their characteristics. In most cases, many crosses and backcrosses must be made before desirable varieties can be selected, tested and released for commercial use.

New technologies including micropropagation and genetic engineering have facilitated and revolutionised the techniques of plant breeding. While conventional plant breeding is still the main technique used to create new plant varieties, plant biotechnology is increasingly playing an important role in breeding programs.
 
PLANT BREEDING PROGRAMS

Plant breeding has been defined as 'the art and science of improving the genetic pattern of plants in relation to their economic use'. It basically involves:
 
a) creating variability, by breeding new plants from two different parents, or by causing mutations to occur;
 
b) selecting what you want from the variation that occurs in the new generation of plants.
 
In this way, plant breeders are able to create plants with desirable characteristics, ranging from improved flower colour, shape and size for the ornamental plant industry, to crops with superior yields and improved environmental tolerances.
 
There are a number of choices which a plant breeder needs to make such as:

• choice of parental plants;
• choice of breeding methods;
• choice of selection criteria;
• testing procedures;
• final choice of cultivars for commercial use.  

There are a number of different methods of selection which the breeder can use, and the one which is chosen will depend upon:

Before starting a breeding program, it is essential to know the plant's pollination requirements - whether it is self or cross pollinated - and how it behaves when it is inbred or crossbred.
 
 
Breeding Self-Pollinated Crops
 
The genetic effect of continued self fertilisation in self-pollinated plants is to reveal the dominant and recessive genes. As Mendel's experiments show, heterozygosity is reduced by one half in each generation, so that after six or seven generations of selfing, a population will consist almost entirely of equal numbers of homozygotes. In this way, selection of characters by continued selfing results in pure lines - these plants are said to be 'pure breeding' or breeding 'true to type'.
  
The following methods are used to breed self-pollinated crops.
 
Pure-line Breeding
 
In pure-line breeding (also known as 'single plant selection') the new variety is made of the progeny of a single pure line. It involves three steps:
 
1. Selecting a large number of superior individuals from a genetically variable population.
 
2. Raising the self progeny of each of these over several years, preferably in different environments. Unsuitable lines are eliminated in each generation. When the breeder can no longer select superior lines by observation only, the third step is commenced.
 
3. Replicating the trials to compare the remaining selections. This is done over several seasons (at least three years) to compare them with each other and with existing commercial varieties.
 
 
Mass Selection
 
In mass selection the progeny of many pure lines are used to form the new variety. Unlike pure-line selection where the derived type consists of a single pure line, in mass selection the majority of selected lines are likely to be retained.
 
It is not as rigorous as pure-line breeding - obviously inferior plants are destroyed before flowering but overall many lines are kept and contribute to the genetic base. This gives the advantage of retaining the best features of an original variety and avoids the extensive testing required in step 3 of pure-line breeding.
 
 
Pedigree Breeding
 
This is the most widely used method of breeding in self-pollinated plants. Superior types are selected in successive segregating generations (as in pure-line breeding) and a record is kept of all parent-progeny relationships. It starts with the crossing of two varieties which complement each other with respect to one or more desirable characters. In the F2 generation a single plant selection is made of the individuals the breeder thinks will produce the best progeny. In the F3 and F4 generations, many loci become homozygous and family characteristics begin to appear. By the F5 and F6 generations, most families are homozygous at most loci; hence selection with families is no longer very effective, only between them.
 
Its main advantage is that the plant breeder is able to exercise his/her skill in selecting plants to a greater degree than other self-pollinating breeding methods. A disadvantage is the limitation it has on the amount of material one breeder can handle.
 
 
Bulk Population Breeding
 
In this method the F2 generation is planted out in large numbers (hundreds to thousands of plants), harvested in bulk and the seeds sown in similar numbers the following year. This process is repeated as many years as desired by the breeder. Natural selection reduces or eliminates those that have poor survival value, while artificial selection is practised to rogue out obviously inferior types.
 
It is only suitable for the commercial breeding of small grains and bean crops. It has the advantage of avoiding the labour required in pure line and pedigree breeding.
 
 
Backcross Breeding
 
The purpose of backcross breeding is to improve a variety by transferring a desirable characteristic from another less desirable variety. It involves making a series of backcrosses of the inferior (donor) variety to the superior one (recurrent parent), selecting for the desired characteristic at each generation.
At the end of backcrossing the gene or genes being transferred are heterozygous, but the other genes are homozygous. Selfing after the last backcross results in homozygosity for the gene pair, producing a plant that is identical to its recurrent parent, except that it also has the characteristic of the donor variety.
A successful back cross program depends on the following:

1. A satisfactory recurrent (superior) parent must exist.
    
2. The desired trait must be able to maintain its intensity through several back crosses.
    
3. Sufficient back crosses must be carried out to ensure the genotype of the recurrent parent is recovered - a minimum of six back crosses is used. The method is popular because it gives the breeder a precise way of improving varieties that already excel in a number of characteristics. 
   

Breeding Cross-Pollinated Crops
 
Each cross-pollinated plant is heterozygous for many genes and continued inbreeding often results in loss of vigour and fertility, known as 'inbreeding depression'. It is essential therefore for a breeding program to maintain heterozygosity.
 
Single Plant Selection
 
Single plant selection (see 'Pure Line Breeding' above) can only be practised in a modified form to avoid inbreeding depression. It may be possible to inbreed for a while, selecting the best phenotypes, but then these must be intercrossed to re-establish a degree of heterozygosity.