George Acquaah - Principles of Plant Genetics and Breeding

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 biopharmingor 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.

The work of Gregor Mendel and further advances in science that followed his discoveries established that plant characteristics are controlled by hereditary factors or genesthat 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).

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.

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 A). The Golden Rice project currently underway at the International Rice Research Institute (IRRI) in Philippines and other parts of the world, is geared toward developing, for the first time ever, a rice cultivar with the capacity to produce pro‐vitamin A (Golden rice 2, with a 20‐fold increase in pro‐vitamin A, has been developed by Syngenta's Jealott's Hill International Research Centre in Berkshire, UK). An estimated 800 million people in the world, including 200 million children, suffer chronic undernutrition, with its attendant health issues. Malnutrition is especially prevalent in developing countries.

Table 1.1Twenty‐five major food crops of the world.

Source: Extracted from Harlan (1976). The ranking is according to total tonnage produced annually.

Breeding is also needed to make some plant products more digestible and safer to eat by reducing their toxic components and improving their texture and other qualities. High lignin content of plant material reduces its value for animal feed. Toxic substances occur in major food crops, such as alkaloids in yam, cyanogenic glucosides in cassava, trypsin inhibitors in pulses, and steroidal alkaloids in potatoes. Forage breeders are interested, among other things, in improving feed quality (high digestibility, high nutritional profile) for livestock.

In spite of a doubling of the world population in the last three decades, agricultural production rose at an adequate rate to meet world food needs. However, an additional 3 billion people will be added to the world population in the next three decades, requiring an expansion in world food supplies to meet the projected needs. The world population is estimated to reach 9 billion by 2050. Even though crop acreage in the US, for example, increased from 160 million acres (in 1926) to 225 acres (in 2015), arable land globally, including in the US, is on a steady decline due to a variety of factors, including the adverse impacts of climate change, surrendering of land to urban development, and salinization. Consequently, as the world population increases, there is a need for an agricultural production system that is apace with population growth but will be conducted on less land. Generally, it is estimated that about 40–60% of agricultural productivity is attributed to plant breeding efforts, through development of improved varieties. This calls for improved and high‐yielding cultivars to be developed by plant breeders. With the aid of plant breeding, the yields of major crops have dramatically changed over the years. Another major concern is the fact that most of the population growth will occur in developing countries where food needs are currently most serious, and where resources for feeding the people are already most severely strained, because of natural or human‐made disasters, or ineffective political systems.