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| Plant Domestication and Breeding |
Plant domestication and breeding are the processes by which wild plants are intentionally raised to meet human food, fiber, shelter, medicinal, or aesthetic needs.
No one knows exactly when the first crop was cultivated, but most authorities believe that it occurred at some time between eight and ten thousand years ago. For centuries prior to that time, humans had known that some wild plants and plant parts (such as fruits, leaves, and roots) were edible. These plants appeared periodically (usually annually) and randomly throughout a given region.
Eventually humans discovered that these wild plants grew from seeds and that the seeds from certain wild plants could be collected, planted, and later gathered for food. This most likely occurred at about the same time in both the Sumerian region between the Tigris and Euphrates Rivers and in Mexico and Central America.
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While the earliest attempts at domesticating plants were primarily to supplement the food supply provided by hunting and gathering, people soon improved their ability to domesticate and breed plants to the point that they could depend on an annual supply of food. This food supply allowed the development of permanent settlements.
Early Crop Domestication
By six thousand years ago, agriculture was firmly established in Asia, India, Mesopotamia, Egypt, Mexico, Central America, and South America. Before recorded history, these areas had domesticated some of the world’s most important food (corn, rice, and wheat) and fiber (cotton, flax, and hemp) crops.
The place of origin of wheat is unknown, but many authorities believe that it may have grown wild in the Tigris and Euphrates Valleys and spread from there to the rest of the Old World. Wheat was grown by Stone Age Europeans and was reportedly produced in China as far back as 2700 b.c.e. Wheat is now the major staple for about 35 percent of the people of the world.
The earliest traces of the human utilization of corn date back to about 5200 b.c.e. It was probably first cultivated in the high plateau region of central or southern Mexico and represented the basic food plant of all pre-Columbian advanced cultures and civilizations, including the Inca of South America and the Maya of Central America.
Botanists believe that rice originated in Southeast Asia. Rice was being cultivated in India as early as 3000 b.c.e. and spread from there throughout Asia and Malaysia.
Today rice is one of the world’s most important cereal grains and is the principal food crop of almost half of the world’s people. Hemp, most likely the first plant cultivated for its fiber, was cultivated for the purpose of making cloth in China as early as the twenty-eighth century b.c.e.
It was used as the cordage or rope on almost all ancient sailing vessels. Linen, made from flax, is one of the oldest fabrics. Traces of flax plants have been identified in archaeological sites dating back to the Stone Age, and flax was cultivated in Mesopotamia and Egypt five thousand years ago.
Cotton has been known and highly valued by people throughout the world for more than three thousand years. From India, where a vigorous cotton industry began as early as 1500 b.c.e., the cultivation of cotton spread to Egypt and then to Spain and Italy.
In the West Indies and South America, a different species of cotton was grown long before the Europeans arrived. Other important plants that have been under domestic cultivation since antiquity include dates, figs, olives, onions, grapes, bananas, lemons, cucumbers, lentils, garlic, lettuce, mint, radishes, and various melons.
Modern Plant Breeding
Genetic variability is prevalent in plants and other organisms that reproduce sexually and thereby produce spontaneous mutants. Throughout most of history, plant domestication and breeding were primarily based on the propagation of mutants.
When a grower observed a plant with a potentially desirable mutation (such as a change that produced bigger fruit, brighter flowers, or increased insect resistance), the grower would collect seeds or take cuttings and produce additional plants with the desirable characteristic.
Advances in the understanding of genetics in the early part of the twentieth century made it possible to breed some of the desirable characteristics resulting from mutation into plants that previously had lacked the characteristic.
The obvious advantages of producing plants with improved characteristics such as higher yield made plant breeding very desirable. As human populations continued to grow, there was a need to select and produce higher-yielding crops.
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The development and widespread use of new high-yield varieties of crop plants in the 1960’s is often referred to as the Green Revolution. Basic information supplied by biological scientists allowed plant breeders to fuse a variety of characteristics from different plants to produce new, higher-yielding varieties of numerous crops, particularly seed grains.
When a plant characteristic is identified as desirable, it is studied both morphologically and biochemically to determine the mechanism of inheritance. If it is determined that the mechanism is transferable, attempts are made to incorporate the trait into the target plant.
If the plants are closely related, traditional breeding techniques are used to crossbreed the plant with the desirable trait with the plant that lacks the characteristic. Although this process is often tedious, it is based on a fairly simple concept.
Basically, pollen from one of the plant types is used to fertilize the other plant type. This process often requires specialized handling techniques to ensure that only the pollen from the plant with the desired characteristic is allowed to fertilize the eggs of the recipient plant.
Sometimes this process involves the use of bags or other materials to isolate the recipient flowers, which are then pollinated by hand. Another technique involves the introduction of a gene for male sterility into the recipient plant.
In these cases, only pollen from another plant can be used to fertilize the egg. Once plantswith the desirable characteristics are developed, the lines are often inbred to maintain large numbers of progeny with the desired traits.
In many cases, inbred lines will lose vigor after several generations. When this occurs, two inbred lines may be crossed to produce hybrids. A majority of the hybrid offspring will still contain the desired characteristics but will be more vigorous.
Recombinant Technology
Until recently, the use of traditional breeding techniques between two very closely related species was the only means of transferring heritable characteristics from one to the other. The advent of recombinant technologies in the manipulation of deoxyribonucleic acid (DNA), however, made it possible to transfer genetic characteristics from any plant (or from any organism) to any other.
The simplest method for accomplishing this transfer involves the use of a vector, usually a piece of circular DNA called a plasmid. The plasmid is removed from a microorganism such a bacterium and cut open by an enzyme called a restriction endonuclease, or restriction enzyme.
A section of DNA from the plant donor cell that contains the gene for an identified desirable trait is cut from the donor cell DNA by the same restriction endonuclease. The section of plant donor cell DNA with the gene for the characteristic of interest is then combined with the open plasmid DNA, and the plasmid closes with the new gene as part of its structure.
The recombinant plasmid (DNA from two sources) is placed back into the bacterium, where it will replicate and code for protein just as it did in the donor cell. The bacteriumis then used as a vector to transfer the gene to another plant, where it will also be transcribed and translated.
