Plant biotechnology : The promise and the objections
Plant biotechnology can be defined as the use of tissue culture and genetic engineering techniques to produce genetically modified plants that exhibit new or improved desirable characteristics. The desirable characteristics include, among others, better yields, better quality, and greater resistance to adverse factors, including diseases, pests, and environmental conditions such as freezes, drought, and salinity.
Plant biotechnology also makes possible the production in plants of useful proteins coded by microbial, animal, or human genes. Plant biotechnology has shown that all of these goals are attainable, at least in the kinds of plants on which they have been attempted. The number of crop, ornamental, and forest plants that have been modified genetically and released by university and industry scientists around the world is in the thousands and continues to grow.
There are numerous cases in which plant biotechnology is used successfully to produce crop plants that avoid or resist certain plant pathogens. Some plants have been rendered resistant to specific pathogens by genetically engineering (transforming) them with isolated specific genes that provide resistance against these pathogens. Transformed plants become resistant by coding for enzymes that mobilize other enzymes that carry out numerous defensive functions, such as breaking down the structural compounds of the pathogen. Several of the enzymes produce compounds in the plant that are toxic to or otherwise inhibit the growth and spread of the pathogen both through the plant and to other plants.
Other plants have been transformed with animal (mouse) genes that code for antibodies (plantibodies) against a coat protein of the pathogen. Genetic engineering has been particularly effective in producing plants resistant to viruses by incorporating viral genes in the crop plants that code for virus coat protein, for altered movement protein, or by incorporating in the plant noncoding segments of virus nucleic acid or even segments of the nonsense strand of the virus nucleic acid. Many of these crop plants have been tested for resistance in the field with excellent results.
Practical examples of successful genetic engineering of disease-resistant plants include melon, squash, tomato, tobacco, and papaya crops that are protected from a variety of viral diseases. The success of genetically engineered papaya for resistance to papaya ring spot virus has saved the papaya as a crop in Hawaii and in the Far East. Numerous other cases are still under development. For example engineering tobacco with a chimeric transgene containing sequences from two different viruses (turnip mosaic and tomato spotted wilt) resulted in new plants resistant to both viruses.
Similarly, engineering tomato plants with a truncated version of the gene coding for the DNA replicase of one of the very destructive gemini viruses resulted in plants resistant not only to the virus from which the transgene was obtained, but also to three other viruses. In other work, potato plants engineered with a chimeric gene encoding two insect proteins exhibiting antimicrobial activities showed significant resistance to the late blight oomycete and their tubers were protected in storage from infection by the soft rot-causing bacteria. In other work, raspberry plants engineered with the gene coding for the common plant polygalacturonase-inhibiting protein (PGIP) became resistant to the gray mold fungus Botrytis cinerea, although the transgene in raspberry, but not in other plants, is expressed only in immature green fruit.
In addition to helping us engineer plants resistant to disease, molecular biology and biotechnology have made possible the development and use of nontoxic chemical substances that, when applied to plants externally, stimulate the plants and elicit the activation of their natural defense mechanisms, i.e., activation of the localized defense mechanism (hypersensitive response) and systemic-aquired resistance (SAR). Two such chemical substances that have been proven effective and are used commercially are Actigard, where one application increases the plants’ resistance against some bacterial and some fungal diseases for several weeks, and Messenger, derived from the fire blight bacterium gene coding for the protein harpin, which elicits a hypersensitive response and SAR in plants. Messenger, which also promotes plant growth, is effective against a variety of diseases of several crops, including strawberry, tomato, and cotton.
In transforming plants for disease resistance or for any other characteristic, it is necessary to modify their nucleic acid by adding genetic material from another plant or, rarely, from an animal or a pathogen. In most cases, these nucleic acids are or become active, producing in the plant compounds that may be toxic to pathogens or pests and, possibly, to humans. In addition, some of this nucleic acid may find its way, through cross-pollination or through transfer by microorganisms, into weeds or other wild plants, making these plants also resistant to the pathogen or pest. Several kinds of plants have been engineered to produce toxins against certain insects; to produce vaccines against certain human pathogens; to produce animal or human growth hormones; or to produce pharmaceutical compounds that can be used to treat diseases of humans and animals.
The fear by some people that some or all of these products will get into the human diet or in the animal food chain and cause allergies and other adverse health effects has resulted in significant unfavorable publicity for such products and for biotechnology. That type of publicity has, in turn, led many large buyers to refuse to buy and use products produced by genetically modified organisms (GMO). Following the adverse publicity, several governments, especially in Europe, passed laws and raised barriers to the importation of products derived from genetically modified organisms.
In addition to the argument against introducing into crops, through genetic engineering, new proteins that may cause allergic reactions in some people there have also been arguments against biotechnology because it takes possession of patents and monopolizes genetic material that was previously available and free to everybody.
It replaces the numerous sustainable local varieties with a few genetically engineered ones, the seed of which the farmers must buy from large companies every year; it threatens the development of pests and pathogens that can resist or overcome the transformed resistant crops.
It threatens to lead to the use of larger amounts of herbicides with crops like those made herbicide resistant while the weeds are still susceptible; it threatens unknown numbers of non target organisms that may be affected adversely by the protein; it threatens to upset the plant balance and through it the entire biotic balance of the environment by having such new genes transferred naturally to non target plants and their proteins harmless or not consumed by microorganisms, animals and humans unaccustomed to such proteins; it threatens the occurrence of accidents in which crops transformed for the production of pharmaceuticals, vaccines, and so on become mixed with edible crops.