Introduction
Plant breeding has been practiced for thousands of years, perhaps even dating back to the domestication of cereal grains by early humans. This essay provides only the most rudimentary description of plant breeding procedures. For the purposes of this article, biotechnology and plant breeding are considered to be distinct techniques. Specifically, our definition of plant breeding excludes genetic modifications achieved through the use of radiation, chemicals, viruses, etc., although these methods are briefly mentioned.
Irrespective of the specific method utilized, all breeding efforts face the challenge of distinguishing between environmental versus genetic factors—plants that demonstrate greater vigor may simply be located in a more favorable site, which can cause better performance even in the absence of superior genes. In order to detect and isolate plants that are likely to carry a superior hereditary endowment, experimental methods are useful for controlling for non-genetic factors, including environmental, that could influence outcomes.
Plant Breeding Techniques
Mass Selection
Mass selection is perhaps the most practical plant breeding method for devotee farm communities—it is also the oldest technique, going as far back as the dawn of agriculture, as recorded by material historians. Within a given field or cropped area, when a plant was observed to be superior, the seeds of that plant were saved and sown. The expectation is that the following generations (progeny) would possess the same or similar degrees of excellence as the parent (progenitor) plant. If a cropped area is eventually populated with only the superior plants, then it is highly likely that pollination would occur only among these improved varieties.
As the seeds of the strongest plants are repeatedly selected for planting across generations, the result would be cultivated forms possessing hereditary makeups that differ sharply from the original progenitors that existed in the wild. It is important to note that the cultivated forms, however desirable they may be in other respects, may not be able to survive in the wild, to the extent that they require human care and a controlled growing environment. Since the wild progenitors are in fact capable of surviving in more natural and more hostile environments, it is important to avoid discarding their seeds altogether, as they contain valuable genetic material that could still be needed.
Backcrossing or Introgression Breeding
One form of crop breeding utilizes a process known as backcrossing. A plant that has a valuable trait, e.g. resistance to a certain pest, is crossed / mated with a plant that does not have that strength, but is desirable in all other respects. Crop breeders ensure that the only change to the original variety is the desired trait. For example, a high-yielding bean can be crossed with a pest-resistant bean. When this crossed variety matures and produces seeds, these seeds are then planted to produce the next generation plant, referred to as the progeny. All progeny that are still pest resistant are then crossed to their high-yielding parent. This process is repeated for a number of generations, always crossing back to the high-yielding parent, and selecting the pest-resistant progeny. This process ensures the subsequent generations are in most ways similar to the high-yielding parent while adding the pest-resistant trait from the other parent.
Inbreeding and Hybrid Breeding
A number of species are self-pollinated, i.e. on a given plant, the pollen is transferred to it from a flower of the same plant. Wheat, barley, rice, peas, beans, lettuce, and tomatoes are largely “selfers”, i.e., self-pollinating. However, it is estimated that about half of the more important cultivated plants are naturally cross-pollinated (outbreeders), wherein the flowers of a given plant are pollinated from flowers of a different plant.
By controlling the pollination process to ensure that a plant only fertilizes itself, a breeder can produce an inbred variety, which preserves its original traits from one generation to the next. A researcher may develop a variety of inbred lines—in this case, although differences would exist across various inbred lines, within each line there would exist a high degree of uniformity due to their pure-breeding nature.
Given a number of different inbred lines, a breeder may select 2 different inbred varieties to function as parents of hybrids. Thus, the hybrid would be developed by crossing 2 different inbred lines (parents), resulting in offspring that demonstrates superior qualitative and quantitative characteristics than either inbred parent.
Mutation Breeding
In the absence of human intervention, naturally occurring genetic mutations occur throughout the global agricultural landscape. These “random” mutations broaden the range of genetic diversity, thereby augmenting the existing stock of genetic resources that can be used to breed new varieties.
Induced Mutations
Prolonged use of traditional breeding methods tends to lead to an increasingly narrow genetic base of cultivars. Rather than allowing naturally occuring changes in a structure of a gene (combined with cross pollination) to effectively re-introduce biodiversity, laboratory-based mutations can be artificially induced by exposing plants to chemicals or through gamma irradiation.
Molecular Marker-Assisted Selection
Breeding can be a slow process even for annual plants but particularly for trees. Moreover, it is not always clear that observed differences in performance are a result of genetic rather than environmental factors. Scientists have attempted to link individual genes to adaptive traits. With this approach, the plant selection process would be guided by an understanding of the specific genes that are present in the plant, before utilizing the classical, backcrossing, or inbreeding and hybridization methods. Gene maps can reduce the uncertainty that is inherent in efforts to develop new varieties and possibly reduce long breeding cycles.
Genetic Engineering
Genetics techniques can also be utilized to directly alter or insert genetic material, resulting in transgenic plants that are referred to as genetically modified organisms (GMOs). Relatively recent gene editing techniques makes use of proteins that can cut DNA at specific locations, allowing scientists to cut out or insert genes that are linked to very specific plant characteristics with precision.
Unfortunately, genetic alterations that are induced outside the boundaries of natural ecosystems — e.g., through irradiation or gene insertion carried out using viruses or gene editing — may lead to potentially catastrophic unintended consequences. Supplementary information on biotechnology is available in “Hazards of Biotechnology”. This is a vast and growing field, as biotechnological innovations continue to advance rapidly, without an adequate understanding of the health and environmental impacts of these innovations.