The application of genetic principles to improve cultivated plants. New varieties of cultivated plants can result only from genetic reorganization that gives rise to improvements over the existing varieties in particular characteristics or in combinations of characteristics. Thus, plant breeding can be regarded as a branch of applied genetics, but it also makes use of the knowledge and techniques of many aspects of plant science, especially physiology and pathology. Related disciplines, like biochemistry and entomology, are also important, and the application of mathematical statistics in the design and analysis of experiments is essential.Genetics
The cornerstone of all plant breeding is selection, or the picking out of plants with the best combinations of agricultural and quality characteristics from populations of plants with a variety of genetic constitutions. Seeds from the selected plants are used to produce the next generation, from which a further cycle of selection may be carried out if there are still differences. Conventional breeding is divided into three categories on the basis of ways in which the species are propagated. First come the species that set seeds by self-pollination; that is, fertilization usually follows the germination of pollen on the stigmas of the same plant on which it was produced. The second category of species sets seeds by cross-pollination; that is, fertilization usually follows the germination of pollen on the stigmas of different plants from those on which it was produced. The third category comprises the species that are asexually propagated; that is, the commercial crop results from planting vegetative parts or by grafting. The procedures used in breeding differ according to the pattern of propagation of the species. Several innovative techniques have been explored to enhance the scope, speed, and efficiency of producing new, superior cultivars. Advances have been made in extending conventional sexual crossing procedures by laboratory culture of plant organs and tissues and by somatic hybridization through protoplast fusion.
The essential attribute of self-pollinating crop species, such as wheat, barley, oats, and many edible legumes, is that, once they are genetically pure, varieties can be maintained without change for many generations. When improvement of an existing variety is desired, it is necessary to produce genetic variation among which selection can be practiced. This is achieved by artificially hybridizing between parental varieties that may contrast with each other in possessing different desirable attributes. This system is known as pedigree breeding, and it is the method most commonly employed, and can be varied in several ways.
Another form of breeding often employed with self-pollinating species involves backcrossing. This is used when an existing variety is broadly satisfactory but lacks one useful and simply inherited trait that is to be found in some other variety. Hybrids are made between the two varieties, and the first hybrid generation is crossed, or backcrossed, with the broadly satisfactory variety which is known as the recurrent parent. Backcrossing has been exceedingly useful in practice and has been extensively employed in adding resistance to diseases, such as rust, smut, or mildew, to established and acceptable varieties of oats, wheat, and barley.Plant pathology
Natural populations of cross-pollinating species are characterized by extreme genetic diversity. No seed parent is true-breeding, first because it was itself derived from a fertilization in which genetically different parents participated, and second because of the genetic diversity of the pollen it will have received. In dealing with cultivated plants with this breeding structure, the essential concern in seed production is to employ systems in which hybrid vigor is exploited, the range of variation in the crop is diminished, and only parents likely to give rise to superior offspring are retained.
Plant breeders have made use either of inbreeding followed by hybridization or of some form of recurrent selection. During inbreeding programs normally cross-pollinated species, such as corn, are compelled to self-pollinate by artificial means. Inbreeding is continued for a number of generations until genetically pure, true-breeding, and uniform inbred lines are produced. During the production of the inbred lines, rigorous selection is practiced for general vigor and yield and disease resistance, as well as for other important characteristics. To estimate the value of inbred lines as the parents of hybrids, it is necessary to make tests of their combining ability. The test that is used depends upon the crop and on the ease with which controlled cross-pollination can be effected.
Breeding procedures designated as recurrent selection are coming into limited use with open-pollinated species. In theory, this method visualizes a controlled approach to homozygosity, with selection and evaluation in each cycle to permit the desired stepwise changes in gene frequency. Experimental evaluation of the procedure indicates that it has real possibilities. Four types of recurrent selection have been suggested: on the basis of phenotype, for general combining ability, for specific combining ability, and reciprocal selection. The methods are similar in the procedures involved, but vary in the type of tester parent chosen, and therefore in the efficiency with which different types of gene action (additive and nonadditive) are measured.
Varieties of asexually propagated crops consist of large assemblages of genetically identical plants, and there are only two ways of introducing new and improved varieties: by sexual reproduction and by the isolation of somatic mutations. (A very few asexually propagated crop species are sexually sterile, like the banana, but the majority have some sexual fertility.) The latter method has often been used successfully with decorative plants, such as chrysanthemum, and new forms of potato have occasionally arisen in this way. When sexual reproduction is used, hybrids are produced on a large scale between existing varieties; the small number that have useful arrays of characters are propagated vegetatively until sufficient numbers can be planted to allow agronomic evaluation.
Cell technologies have been used to extend the range and efficiency of asexual plant propagation. For example, plant cell culture involves the regeneration of entire mature plants from single cells or tissues excised from a source plant and cultured in a nutrient medium. In micropropagation and cloning, tissues are excised from root, stem, petiole, or seedling and induced to regenerate plantlets. All regenerants from tissues of one source plant constitute a clone. Microspore or anther culture is the generation of plants from individual cells with but one set of chromosomes, haploid cells, as occurs in the development of pollen. Microspores are isolated from anthers and cultured on nutrient media, or entire anthers are cultured in this manner. Doubling of chromosomes that may occur spontaneously or can be induced by treatment with colchicine leads to the formation of homozygous dihaploid plants.Plant propagation Pollen
Breeding for new, improved varieties of crop plants is most often based on cross-pollination and hybrid production. Such breeding is limited to compatible plants, and compatibility lessens with increasing distance in the relationship between plants. Breeding would benefit from access to traits inherent in sexually noncompatible plants. Biotechnological techniques such as in vitro fertilization and embryo rescue (the excision and culture of embryos on nutrient media) have been employed to overcome incompatibility barriers, as have somatic hybridization and DNA technologies. Somatic hybridization involves enzymatic removal of walls from cells of leaves and seedlings to furnish individual naked cells, that is, protoplasts, which can then be fused to produce hybrids. Similarity of membrane structure throughout the plant kingdom permits the fusion of distantly related protoplasts. Cell fusion may lead to nuclear fusion, resulting in amphi-diploid somatic hybrid cells. Fusion products of closely related yet sexually incompatible plants have been grown to flowering plants; the most famous example is the potato + tomato hybrid = pomato (Solanum tuberosum + Lycopersicon esculentum). DNA technologies enable the isolation of desirable genes from bacteria, plants, and animals (genes that confer herbicide resistance or tolerance to environmental stress, or encode enzymes and proteins of value to the processing industry) and the insertion of such genes into cells and tissues of target plants by direct or indirect uptake has led to the genetic transformation of plant cells. The regeneration of transformed plant cells and tissues results in new and novel genotypes (transgenic plants). Contrary to hybrids obtained by cross-pollination, such plants are different from their parent by only one or two single, defined traits.Genetic engineering