Cloning of plants

Categories: Biotechnology; economic botany and plant uses; genetics

The term “clone” is derived from the Greek word klōn, meaning a slip or twig. Hence, it is an appropriate choice. Plants have been “cloned” from stem cuttings or whole-plant divisions for many centuries, perhaps dating back as far as the beginnings of agriculture.

Historical Background

In 1838 German scientists Matthias Schleiden and Theodor Schwann presented their cell theory, which states, in part, that all life is composed of cells and that all cells arise from preexisting cells. This theory formed the basis for the concept of totipotency, which states that since cells must contain all of the genetic information necessary to create an entire, multicellular organism, all of the cells of a multicellular organism retain the potential to re-create, or regenerate, the entire organism. Thus was the basis for plant cell culture research.

The first attempt at culturing isolated plant tissues was by Austrian botanist Gottlieb Haberlandt at the beginning of the twentieth century, but it was unsuccessful. In 1939 Roger Jean Gautheret and his colleagues demonstrated the first successful culture of isolated plant tissues as a continuously dividing callus tissue. The term callus is defined as an unorganized mass of dividing cells, such as in a wound response. It was not until 1954, however, that the first whole plant was regenerated, or cloned, from a single adult plant cell by W. H. Muir, Albert Hildebrandt, and Albert Riker. Thereafter, an increased understanding of plant physiology, especially the role of plant hormones in plant growth and development, contributed to rapid advances in plant cell and tissue culture technologies in the 1970s and 1980s. Many plant species have been successfully cloned from single cells, thus demonstrating and affirming the concept of totipotency.

Horticulture

By far, the greatest impact of cloning plants in vitro (Latin for “in glass,” meaning in the laboratory or outside the plant) has been on the horticultural industry. In the 1980s plant tissue culture technologies propagated and produced many millions of plants. Since then, many economically important plants have been commonly propagated via tissue culture techniques, including vegetable crops (such as the potato), fruit crops (strawberries and dates), floriculture species (orchids, lilies, roses, Boston ferns), and even woody species (pines and grapes).

The advantages of plant cell, tissue, and organ culture technologies include a more rapid production of plants, taking weeks instead of months or years. Much less space is required (square feet instead of field plots). Plants can be produced year-round, and economic, political, and environmental considerations that hamper the propagation of regional or endangered plant species can be reduced. The disadvantages include the high start-up costs for facilities, the skilled labor required, and the need to maintain sterile conditions.

Two other significant considerations must be considered as a result of plant propagation technologies. As illustrated by the Irish Potato Famine of the 1840s, the cultivation of whole fields of genetically identical plants (monoculture) leaves the entire crop vulnerable to pest and disease infestations. The second important consideration when generating entire populations of clones, especially using tissue culture technologies, is the potential for introducing genetic abnormalities, which then are present in the entire population of plants produced, a process termed somaclonal variation.

Biotechnology

An absolute requirement for the genetic engineering of plants is the ability to regenerate an entire plant from a single, genetically transformed cell, thus emphasizing the second major impact of plant cell culture technologies. In 1994 the US Food and Drug Administration (FDA) approved the first genetically modified whole food crop, Calgene’s Flavr Savr tomato. This plant was produced using what is termed anti-sense technology. One of the tomato’s genes involved in fruit ripening was reversed, thus inactivating it and allowing tomatoes produced from it to have significantly delayed ripening. Although no longer commercially marketed, the Flavr Savr demonstrated the impact of genetic engineering in moving modern agriculture from the Green Revolution into what has been termed the Gene Revolution.

Other examples of agricultural engineering exist, such as Roundup Ready soybeans, engineered to resist the herbicide used on weeds where soybeans are grown, and Bt corn, which contains a bacterial gene conveying increased pest resistance. Since 1987, the US Department of Agriculture (USDA) has required field testing of genetically modified crops to demonstrate that their use will not be disruptive to the natural ecosystem. Thousands of field trials have been completed or are in progress for genetically modified versions of various crop species, including potatoes, cotton, alfalfa, canola, and cucumbers.

These studies have not done enough to alleviate consumer concerns, however. Genetically modified foods (produced from genetically modified organisms, or GMOs) have caused major controversy not only over how they might affect nature—in terms of reduced biodiversity, new pests, and other environmental factors—but over how they could affect human health over the long term as well. There has been concern that GMOs could introduce a new allergen, for example. There has also been also concern that a gene transfer of harmful, antibiotic-resistant bacteria could occur, when animals consume GM plants and then humans consume the animal or the plant itself. Consumers have also shown concern over a lack of labeling; a person may unwittingly eat a GMO product when trying to avoid it.

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