Principles of Plant Genetics and Breeding. George Acquaah

there is a general perception that such creations derive from crossing different plants. This introductory chapter is devoted to presenting a brief overview of plant breeding, including its benefits to society and some historical perspectives. After completing this chapter, the student should have a general understanding of:

      1 The need and importance of plant breeding to society.

      2 The goals of plant breeding.

      3 The art and science of plant breeding.

      4 Trends in plant breeding as an industry.

      5 Selected milestones and accomplishments of plant breeders.

      6 The future of plant breeding in society.

1.1 What is plant breeding?

Plant breeding is a deliberate effort by humans to nudge nature, with respect to the heredity of plants, to an advantage. The changes made in plants are permanent and heritable. The professionals who conduct this task are called plant breeders. This effort at adjusting the status quo is instigated by a desire of humans to improve certain aspects of plants to perform new roles or enhance existing ones. Consequently, the term “plant breeding” is often used synonymously with “plant improvement” in modern society. It needs to be emphasized that the goals of plant breeding are focused and purposeful. Even though the phrase “to breed plants” often connotes the involvement of the sexual process in effecting a desired change, modern plant breeding also includes the manipulation of asexually reproducing plants (plants that do not reproduce through the sexual process). Breeding is hence about manipulating plant attributes, structure, and composition to make them more useful to humans. It should be mentioned at the onset that it is not every plant character or trait that is readily amenable to manipulation by breeders. However, as technology advances, plant breeders are increasingly able to accomplish astonishing plant manipulations – needless to say not without controversy, as is the case involving the development and application of biotechnology to plant genetic manipulation. One of the most controversial of these modern technologies is transgenesis, the technology by which gene transfer is made across natural biological barriers.

      Plant breeders specialize in breeding different groups of plants. Some focus on field crops (e.g. soybean, cotton), horticultural food crops (e.g. vegetables), ornamentals (e.g. roses, pine trees), fruit trees (e.g. citrus, apple), forage crops (e.g. alfalfa, grasses), or turf species (e.g. Bluegrass, fescue). More importantly, breeders tend to specialize in or focus on specific species in these groups (e.g. corn breeder, potato breeder). This way, they develop the expertise that enables them to be most effective in improving the species of their choice. The principles and concepts discussed in this book are generally applicable to breeding all plant species.

1.2 The goals of plant breeding

      The plant breeder uses various technologies and methodologies to achieve targeted and directional changes in the nature of plants (nudge nature to the advantage of humans). As science and technology advance, new tools are developed while old ones are refined for use by breeders. Before initiating a breeding project, clear breeding objectives are defined based on factors such as producer needs, consumer preferences and needs, and environmental impact. Breeders aim to make the crop producer's job easier and more effective in various ways. They may modify plant structure so it would resist lodging and thereby facilitate mechanical harvesting. They may develop plants that resist pests so the farmer does not have to apply pesticides, or apply smaller amounts of these chemicals. Not applying pesticides in crop production means less environmental pollution from agricultural sources. Breeders may also develop high yielding varieties (or cultivars) so the farmer can produce more for the market to meet consumer demands while improving his or her income. The term cultivar is reserved for variants deliberately created by plant breeders and will be introduced more formally later in the book. It will be the term of choice in this book.

      When breeders think of consumers, they may, for example, develop foods with higher nutritional value and that are more flavorful. Higher nutritional value means reduced illnesses in society (e.g. nutritionally related ones such as blindness, rickettsia) caused by the consumption of nutrient‐deficient foods, as occurs in many developing regions where staple foods (e.g. rice, cassava) often lack certain essential amino acids or nutrients. Plant breeders may also target traits of industrial value. For example, fiber characteristics (e.g. strength) of fiber crops such as cotton can be improved, while oil crops can be improved to yield high amounts of specific fatty acids (e.g. high oleic content of sunflower seed). Latest advances in technology, specifically genetic engineering technologies, are being applied to enable plants to be used as bioreactors to produce certain pharmaceuticals (called biopharming or simply pharming).

      The technological capabilities and needs of societies of old restricted plant breeders then to achieving modest objectives (e.g. product appeal, adaptation to production environment). It should be pointed out that these “older” breeding objectives are still important today. However, with the availability of sophisticated tools, plant breeders are now able to accomplish these genetic alterations in novel ways that are sometimes the only option, or are more precise and more effective. Furthermore, as previously indicated, they are able to undertake more dramatic alterations that were impossible to attain in the past (e.g. transferring a desirable gene from a bacterium to a plant!). Some of the reasons why plant breeding is important to society are summarized next.

1.3 The concept of genetic manipulation of plant attributes

      The work of Gregor Mendel and further advances in science that followed his discoveries established that plant characteristics are controlled by hereditary factors or genes that consist of DNA (deoxyribose nucleic acid, the hereditary material). These genes are expressed in an environment to produce a trait. It follows then that in order to change a trait or its expression, one may change the nature or its genotype, and/or modify the nurture (environment in which it is expressed). Changing the environment essentially entails modifying the growing or production conditions. This may be achieved through an agronomic approach, for example the application of production inputs (e.g. fertilizers, irrigation). Whereas this approach is effective in enhancing certain traits, the fact remains that once these supplemental environmental factors are removed, the expression of the plant trait reverts to status quo. On the other hand, plant breeders seek to modify plants with respect to the expression of certain selected attributes by modifying the genotype (in a desired way by targeting specific genes). Such an approach produces an alteration that is permanent (i.e. transferable from one generation to the next).

1.4 Why breed plants?

The reasons for manipulating plant attributes or performance change according to the needs of society. Plants provide food, feed, fiber, pharmaceuticals, and shelter for humans. Furthermore, plants are used for esthetic and other functional purposes in the landscape and indoors.

      1.4.1 Addressing world food and feed quality needs

Food is the most basic of human needs. Plants are the primary producers in the ecosystem (a community of living organisms including all the nonliving factors in the environment). Without them, life on earth for higher organisms would be impossible. Most of the crops that feed the world are cereals (Table 1.1). Plant breeding is needed to enhance the value of food crops, by improving their yield and the nutritional quality of their products, for healthy living of humans. Certain plant foods are deficient in certain essential nutrients to the extent that where these foods constitute the bulk of a staple diet, diseases associated with nutritional deficiency are often common. Cereals tend to be low in lysine and threonine, while legumes tend to be low in cysteine and methionine (both sulfur‐containing amino acids). Breeding is needed to augment the nutritional quality of food crops. Rice, a major world food, lacks pro‐vitamin A (the precursor of vitamin