Plant breeding is an applied science, in which knowledge of genetics, pathology, plant physiology, statistics, and molecular biology are used to modify plant species for human needs or preferences. Although plant breeding dates back to the domestication of plants by early humans, the underlying principles were not well understood until the beginning of the 20th century. Discovery of the laws of genetics and subsequent developments in that science has led to plant breeding being seen as the process of changing the frequencies of genes that influence important plant characteristics. New techniques in genetic engineering and biotechnology provide new and exciting tools for plant breeding.
Most plant breeding programs start by creating new genotypes (genetic combinations) through hybridization or mutagenesis, and conclude by evaluating and selecting some superior combinations. Although the basic steps are the same, details of the programs vary considerably with the longevity of the species, their primary mode of reproduction and the genetic structure of the cultivars that are released for commercial production. For most economically important crops grown in Canada, release of a new cultivar can take 10-15 years.
With the advent of genetic engineering, the ability to transfer genes from unrelated species is now possible and can be used to incorporate desirable traits not currently found in crop cultivars. For example, "Golden Rice" was genetically engineered to biosynthesize the precursors of beta-carotene (pro-vitamin A) by inserting genes from daffodil and Erwinia uredovora, a naturally occurring soil bacterium. Some corn, cotton and potato cultivars have been engineered to express a gene isolated from Bacillus thuringiensis and produce a protein that kills invading insect larvae. Producers grow Bt-cultivars as an alternative to spraying insecticides (see pesticides) for control of insect pests. The majority of canola cultivars grown in Western Canada have been genetically engineered to resist some herbicides. Using these specific herbicides has reduced the amount of chemical needed for weed control in canola fields.
Self-fertilized species include wheat, oats, barley, flax, tobacco, tomatoes, peas, peanuts and rice. These species usually have "perfect" flowers, which means that the stigma is pollinated with pollen produced within the same flower. Self-fertilization leads to high levels of inbreeding. Commercial or cultivated cultivars are normally highly inbred, and therefore produce progeny with essentially the same genotype.
In self-fertilizing species, plant breeders must force hybridization between plants from different cultivars. Hybridization (crossing) is usually accomplished by manually removing the anthers from the flowers of the designated female parent and subsequently transferring mature pollen from the designated male parent. The resulting hybrid seed carries genetic information for characteristics of both parents.
Hybridization is usually followed by 6-8 generations of self-fertilization, which produces a population of many genetically distinct plants, each capable of producing genetically uniform progeny. Selection for desirable traits occurs throughout the inbreeding generations, with a shift from highly heritable traits in early generations, to those traits whose expression is strongly influenced by the environment in later generations. New genotypes must be tested in various locations for several years and evaluated for characteristics relating to productivity, processing quality, storability and marketability, as well as for environmental and health-related characteristics. Of the many new genotypes evaluated in each population, none or only a few may possess the characteristics required of a new cultivar.
Cross-fertilized species include maize (Indian corn), rye, bromegrass, timothy, alfalfa and clover. Commercial cultivars usually are mixtures of genotypes. New genotypes may occur naturally as the result of cross-fertilization among different genotypes within a commercial cultivar or among genotypes from different cultivars grown in mixtures. In perennial species, individual plants can be evaluated on the basis of their own performance, the average performance of clones produced from them, or the average performance of progeny produced by cross-fertilization or forced self-fertilization. Selected plants may be combined to form a new cultivar, either by mixing their seeds and allowing the new cultivar to maintain itself by cross-fertilization, or by mixing seeds from the parents each time a new planting is required. The latter method can be used only if some form of vegetative propagation is available to maintain the parents as a seed source.
Asexually Reproducing Species
Some annual species and many perennials can be reproduced asexually from vegetative tissue such as stems, modified stems (rhizomes, tubers, corms and bulbs), leaves or roots, and, in some species, by apomictic seed production (ie, independent of fertilization).
In those species that also reproduce sexually, genetic variation can be generated through hybridization. In species in which seed production is difficult, new genotypes may occur naturally by spontaneous mutations or may be created by using mutagenic agents (eg, radiation or chemical mutagens). Selection of new genotypes is similar to that in other species, except that any superior genotype can be maintained by cloning.
Commercial cultivars of maize and many garden vegetables consist of highly uniform plants produced by crossing two or more inbred lines. This practice is economically feasible only when the cost of seed is small relative to the value of the crop, or where the cost of producing hybrid seed can be minimized by using a system that combines genetic male sterility with fertility restoration. Hybrid seed usually result in more vigorous plants than those produced by inbred lines, a genetic phenomenon called "hybrid vigour."
Biotechnology has recently allowed the development of systems that produce hybrid seeds on female plants that carry a gene for male sterility coupled with resistance to a specific herbicide. Application of herbicide kills all male plants ensuring that only seeds produced by the female plants are harvested. Such techniques have already been applied to canola and will increase the number of species for which it is economically feasible to produce hybrid seeds.
Backcrossing is used to transfer one or a few desirable genes from a donor parent to an otherwise acceptable recipient parent. Backcrossing requires repeated crossing of new hybrids to the desirable "recurrent" parent and selection of the desired trait from the donor parent. In cereal grains (see cereal crops), resistance to rust and other obligate parasites has been achieved and maintained primarily through the use of backcrossing.
Application of Genomics to Plant Breeding
Genomics is the study of the DNA sequences, including structural genes, regulatory sequences, and noncoding DNA segments, in the chromosomes of an organism. Understanding the genes that influence desirable traits has the potential to increase the efficiency of selection. Recent advances in molecular biology have made it possible to acquire extensive knowledge of the genome of plants. The complete sequencing of the rice and Arabidopsis thaliana (mouseear cress) genomes has allowed the identification of plant genes involved in the expression of some economically important traits. For crops lacking genome sequence information, genetic mapping experiments are now common, and are used to identify chromosome regions that contain genes that maximize the expression of a desirable trait.
Molecular marker assisted selection (MMAS) is a complementary technology that is often used in conjunction with conventional methods of genetic selection. This technology relies on the visualization of small DNA fragments that are either genetically linked to the expression of a trait, or are the genes themselves. Once a fingerprint or gene has been identified, a plant breeder can use molecular techniques to select only those plants that possess the desirable allele of that gene. Although single gene controlled traits have received most attention, progress has also been made using MMAS for selection of multiple gene traits.
Plant Breeding Applications
Plant breeding has been used to improve productivity, quality and disease resistance of most agricultural crops. Severe epidemics of rust and other fungal plant diseases have become infrequent because breeders and pathologists have succeeded in incorporating stable genetic resistance into new cultivars. For instance, the pasta quality of durum wheat has been modified to meet changing requirements of export markets and the baking quality of wheat has been altered to make it better suited to continuous-flow baking methods. Similarly, enzyme concentrations in barley have been increased to meet the needs of newer brewing techniques.
Plant breeding has played a major role in altering crop species (eg, maize, sunflowers, soybean) so they can be grown over a wider area in Canada. The ability of winter wheat to survive the harsh winters of the prairies has been improved, allowing production of this crop with minimal damage from winter kill. The development of rapeseed as a significant export commodity stands as a major accomplishment of Canadian plant breeders working with chemists, pathologists and agronomists. The fatty acid composition of flax seed has been altered to produce healthier oils for human consumption. Hardiness and persistence of many perennial forages have been improved through breeding, creeping-rooted alfalfa being one important example. In addition, efforts to reduce the alkaloid content of reed canary grass, the coumarin content of white and yellow sweet clover, and the levels of bloat-causing agents in alfalfa have all been successful. Breeding crops specifically for the production of environmentally friendly biofuels is a reality.
In horticulture, plant breeding has been used effectively to improve productivity and quality of fruits (eg, strawberries, apples) and vegetables. Dwarf cherry trees were developed by plant breeding. The desirable fruit characteristics of European raspberry and grape cultivars have been combined with the hardiness of native Canadian species. Winter hardiness and adaptation to a short growing season are important characteristics of ornamentals as well as fruits. Significant improvements in these characteristics have been achieved in flowering crabapples, poplars, lilacs, roses, junipers and willows.
Plant breeding techniques have been used to improve the cooking and chipping quality of potatoes and to develop cultivars that are better adapted to short growing seasons. Improvements in the quality and maturity of sweet corn have made this species a part of home gardens throughout the country. Hybrids have been developed in crops such as tomatoes and cucumbers to provide more vigorous plants for home and commercial growers.
Early plant breeding was carried out by individuals as a hobby associated with farming or horticulture. Now most plant breeding is carried out by private or government institutions. Agriculture and Agri-Food Canada, which has research stations in all provinces, has active plant breeding programs in cereal crops, oilseeds, forages, tobacco, vegetables, flowers, fruits and ornamental shrubs. The federal government also has breeding programs for various shelterbelt and commercial tree species. Plant breeding is carried out in the agricultural departments or colleges of several Canadian universities funded in part by provincial departments of agriculture, by several farmer co-operatives and by seed companies.
A network of committees has traditionally played coordinating and regulatory roles in the development and licensing of new cultivars. Committees whose members share expertise on grain breeding, grain diseases and grain quality evaluate and make recommendations on applications for licensing candidate cereal, oilseed, and pulse cultivars. A similar system of evaluation and approval exists for forage crops and certain horticultural crops.