George Acquaah - Principles of Plant Genetics and Breeding

The phenomenon of global climatic change that has been occurring over the years is partly responsible for modifying the crop production environment (e.g. some regions of the world are getting drier and others saltier). This means that new cultivars of crops need to be bred for new production environments. Whereas developed economies may be able to counter the effects of unseasonable weather by supplementing the production environment (e.g. by irrigating crops), poor countries are easily devastated by even brief episodes of adverse weather conditions. For example, development and use of drought‐resistant cultivars is beneficial to crop production in areas of marginal or erratic rainfall regimes. Breeders also need to develop new plant types that can resist various biotic (diseases and insect pests) and other abiotic (e.g. salt, drought, heat, cold) stresses in the production environment. Crop distribution can be expanded by adapting crops to new production environments (e.g. adapting tropical plants to temperate regions). Development of photoperiod insensitive crop cultivars would allow the expansion in production of previously photoperiod sensitive species.

Breeders need to produce plant cultivars for different production systems to facilitate crop production and optimize crop productivity. For example, crop cultivars must be developed for rain‐fed or irrigated production, and for mechanized or non‐mechanized production. In the case of rice, separate sets of cultivars are needed for upland production and for paddy production. In organic production systems where pesticide use is highly restricted, producers need insect‐ and disease‐resistant cultivars in crop production.

The ornamental horticultural production industry thrives on the development of new varieties through plant breeding. Esthetics is of major importance to horticulture. Periodically, ornamental plant breeders release new varieties that exhibit new colors and other morphological features (e.g. height, size, shape). Also, breeders develop new varieties of vegetables and fruits with superior yield, nutritional qualities, adaptation, and general appeal.

Processed foods are a major item in the world food supply system. Quality requirements for fresh produce meant for the table are different from those used in the food processing industry. For example, there are table grapes and grapes bred for wine production. One of the reasons why the first genetically modified(GM) crop (produced by using genetic engineering tools to incorporate foreign DNA) approved for food, the “FlavrSavr™” tomato, did not succeed was because the product was marketed as a table or fresh tomato, when in fact the gene of interest was placed in a genetic background for developing a processing tomato variety. Other factors contributed to the demise of this historic product. Different markets have different needs that plant breeders can address in their undertakings. For example, potato is a versatile crop used for food and industrial products. Different varieties are bred for baking, cooking, fries (frozen), chipping, and starch. These cultivars differ in size, specific gravity, and sugar content, among other properties. High sugar content is undesirable for frying or chipping because the sugar caramelizes under high heat to produce undesirable browning of fries and chips.

Plant breeding has come a long way from the cynical view of “crossing the best with best and hoping for the best” to carefully planned and thought‐out strategies to develop high performance cultivars. Plant breeding methods and tools keep changing as technology advances. Consequently, plant breeding approaches may be categorized into two general types: conventionaland unconventional. This categorization is only for convenience.

Conventional approachConventional breeding is also referred to as traditional or classical breeding. This approach entails the use of tried, proven, and older tools. Crossing two plants (hybridization) is the primary technique for creating variability in flowering species. Various breeding methods are then used to discriminate among the variability (selection) to identify the most desirable recombinant. The selected genotype is increased and evaluated for performance before release to producers. Plant traits controlled by many genes (quantitative traits) are more difficult to breed. Age notwithstanding, the conventional approach remains the workhorse of the plant breeding industry. It is readily accessible to the average breeder and is relatively easy to conduct, compared to the unconventional approach.

Unconventional approachThe unconventional approach to breeding entails the use of cutting‐edge technologies for creating new variability that is sometimes impossible to achieve with conventional methods. However, this approach is more involved, requiring special technical skills and knowledge. It is also expensive to conduct. The advent of recombinant DNA (rDNA) technology gave breeders a new set of powerful tools for genetic analysis and manipulation. Gene transfer can now be made across natural biological barriers, circumventing the sexual process (e.g. the Bt products that consist of bacterial genes transferred into crops to confer resistance to the European corn borer). Molecular markers are available for aiding the selection process to make the process more efficient and effective.

Even though two basic breeding approaches have been described, it should be pointed out that they are best considered as complementary rather than independent approaches. Usually, the molecular tools are used to generate variability for selection, or to facilitate the selection process. After genetically modifying plants using molecular tools, it may be used as a parent in subsequent crosses to transfer the desirable genes into adapted and commercially desirable genetic backgrounds, using conventional tools. Whether developed by conventional or molecular approaches, the genotypes are evaluated in the field by conventional methods, and then advanced through the standard seed certification process before the farmer can have access to it for planting a crop. The unconventional approach to breeding tends to receive more attention from funding agencies than the conventional approach, partly because of its novelty and advertised potential, as well as the glamour of the technologies involved.

Regardless of the approach, a breeder follows certain general steps in conducting a breeding project. A breeder should have a comprehensive plan for a breeding project that addresses the following:

ObjectivesThe breeder should first define a clear objective (or set of objectives) for initiating the breeding program. In selecting breeding objectives, breeders need to consider:The producer (grower) from the point of view of growing the cultivar profitably (e.g. need for high yield, disease resistance, early maturity, lodging resistance).The processor (industrial user) as it relates to efficiently and economically using the cultivar as raw materials for producing new product (e.g. canning qualities, fiber strength).The consumer (household user) preference (e.g. taste, high nutritional quality, shelf life).The tomato will be used to show how different breeding objectives can be formulated for a single crop. Tomato is a very popular fruit with a wide array of uses, each calling for certain qualities. For salads, tomato is used whole, and hence the small size is preferred; for hamburgers, tomato is sliced, round large fruits being preferred. Tomato for canning (e.g. puree) requires certain pulp qualities. Being a popular garden species, gardeners prefer a tomato cultivar that ripens over time so harvesting can be spaced. However, for industrial use as in the case of canning, the fruits on the commercial cultivar must ripen together, so the field can be mechanically harvested. Further, whereas appearance of the fruit is not top priority for a processor who will be making tomato juice, the appearance of fruits is critical in marketing the fruit for table use.