Bioengineering Increases Yield and Research Opportunities

Someday soon, much of the produce available in grocery stores will be the result of genetic engineering-a synthesis of molecular biology and plant genetics to produce crops that yield greater harvests and are better able to resist insects, diseases, and environmental conditions such as drought and frost. Research efforts in engineering genes in major crop plants are not restricted to one segment of the scientific community. These investigations are being actively pursued by governments worldwide, major corporations, and private institutions. And as research activity in this field blossoms, so will opportunities for molecular biologists and plant geneticists. What makes these future advances noteworthy is that they will be accomplished not through the time-honored technique of genetic cross-breeding, but by methods that introduce new genetic material directly into a plant's DNA. "The tools to transfer genes between plants are being developed now, and they are very powerful," says Randall Niedz of the Agricultural Research Service (ARS) Horticultural Research Laboratory in Orlando, Fla. "People just need to realize that these tools are useless without the basic understanding of the biochemistry." The genetic code in plants is similar to that in animals, but some species of plants also contain a characteristic known as totipotency-the potential to regenerate a complete plant from a single cell. This quality is very important to genetics engineers because it simplifies the process required to introduce new DNA into plants. Using cell cultures, researchers can isolate and manipulate a gene or several genes that control a specific characteristic. These genes can then be introduced into a new cell that lacks the characteristic, and, ideally, the altered cell will grow into a whole plant that will carry the trait to future generations. To date, the most successful genetic manipulation of crop plants has involved those categorized as dicots (such as tobacco, tomatoes, and potatoes). Dicot cells can be genetically altered using Agrobacterium tumefaciens-a plant pathogen that causes crown gall disease in plants-to transfer new DNA into the cells' genetic makeup. Agrobacterium contains a large tumorinducing (Ti) plasmid with a segment of DNA known as T-DNA. (transferred DNA). When this pathogen infects a plant cell, the T-DNA is transferred from the Ti plasmid into the cell. Through bacterial genetics, researchers deactivate the disease causing portion of Ti and introduce a new gene that expresses a particular desirable trait. Scientists have already used Agrobacterium to successfully transform tomato and tobacco plants. Tobacco plants were the first species to be genetically altered through this method. The resulting prototype plants proved resistant to caterpillars and are currently being field tested. More recently, transgenic species of tomatoes and potatoes have been engineered for resistance to viral infections. While plants have always been crossbred within species to develop resistance t disease, genetic engineering introduces the possibility that resistance factors can be transferred between species that do not usually interbreed. Agrobacterium makes up a 1989 research front identified by the Philadelphia-based Institute for Scientific Information (ISI) from its Science Citation Index database. This specialty area is composed of 158 papers published in 1989 and the seven papers that each consistently listed as references. All seven of these articles were published in the 1980s: the earliest in 1981, the most recent three in 1986. The seven core papers appeared in a variety of journals, such as Annual Review of Plant Physiology and Plant Molecular Biology, and Nitrogen Fixation, Nature, Science, and Microbiological Reviews. The interdisciplinary nature of this research-in particular, the molecular biology/plant biology connection-is even more pronounced in the 158 papers from 1989. They were published in journals as diverse as Annual Review of Genetics, Nucleic Acids Research, Journal of Chromatography, Journal of Bacteriology, Journal of Biological Chemistry, and Applied and Environmental Microbiology. Plant engineers at Monsanto Co., based in St. Louis, have also used Agrobacterium to develop tomatoes that resist larvae of insects in the Lepidoptera order-moths and butterflies. This latter transformation involves a specific gene from the Bacillus thuringiensis bacteria-a naturally occurring insect pathogenetic soil microbe-to produce transgenic tomato plants that poison the larvae that feed on them. These plants are concurrently undergoing field testing. The use of Bacillus thuringiensis is highlighted by two 1989 specialty areas in ISI's research front database. One front indicates the development an duse of various strains of the bacteria. The second front concentrates on the metabolic effects of bacteria on host organisms. The fronts combined contain 97 articles published in 1989 (79 in one front, 18 in the other) that cited eight and two core papers, respectively. Bacillus thurigiensis was first successfully introduced to tobacco plants in 1987 by Plant Genetic Systems, a Belgian biotechnology company. The company developed plants that killed feeding Manduca sexta larvae. Monocotic plants, which include the major cereal grains-barley, corn, rice, rye, and wheat-are also the focus of intense genetic scrutiny. Most monocots, however, are immune to the Agrobacterium pathogen. As a result, researchers have had to implement alternative methods to transfer material to these crops. One promising technique uses high velocity microprojectile technology-or particles guns-to shoot small metal particles coated with DNA through a plant cell's wall and into its cytoplasm at speeds of one to several hundred meters per second (The Scientist, Sept. 18, 1989, page 25). So far, scientists have successfully used this technology to introduce new genetic material into corn cells. In these experiments, a team led by Monsanto's Michael Fromm, formerly of the ARS- University of California Plant Gene Expression Center in Albany, Calif., coated tungsten particles with various marker genes and inserted them into a cluster of corn cells produced by Charles Armstrong at monsanto. These marker genes did not express traits useful to corn, but, rather, produced characteristics that the researchers could easily trace; for example, a gene taken from a firefly and inserted in the corn cells caused them to glow faintly. Researchers at the Boyce Thompson Institute for Plant Research in Ithaca, N.Y., have conducted similar experiments using firefly genes and plants. Fromm believes that this research may provide scientists with the information needed to insert genes into cereals such as wheat. Finding viable methods for genetically engineering cereal grains like wheat is of major importance to researchers-not only to increase crop yield by improving resistance to undesirable environmental factors, but also to increase nutritional value. Recent interest has focused on oats, which contain betaglucans, the soluble fiber believed to reduce cholesterol in humans. David Peterson of the United States Department of Agriculture-ARS Central Crops Research, University of Wisconsin, Madison, and Darrell Wesenberg of USDA-ARS Small Grains and Potato Germplasm Research in Aberdeen, Idaho, are working on methods to screen the 21,000 oat genotypes currently available at the National Small Grains Collection in Aberdeen for varieties high in betaglucans. They also investigating the possibility of a link between the genes that control synthesis of betaglucans and the growth cycle of the oat plant. Eventually, they hope to identify the genes involved in betaglucans synthesis. Another way to genetically engineer plants involves the transformation of plants protoplasts by the introduction of DNA, which can be achieved through the use of calcium phosphate precipitation, polyethylene glycol, electroporation, or a combination of these techniques. This method appears promising-already scientists have successfully regenerated whole plants from altered rice protoplasts. Still other techniques include the transfer of genes into pollen and the injection of DNA directly into the reproductive organs of the plant to be altered. In the long run, however, the method used to transfer DNA depends on the biology (biochemistry, for example) of the crop as well as the trait to be introduced. No one method of introducing new genes in plants can be used to express all characteristics of a species. Genetic characteristics in plants are generally classified as either qualitive or quantative. Quantative characteristics, such as those that promote viral and insect resistance, are relatively easy to isolate because they are controlled by one or a few genes, and are seldom affected by environmental conditions. These multigenic characteristics, which are much more difficult for molecular biologists to isolate, tend to control some of the more critical characteristics that breeders hope to change. One such quantative characteristic is resistance to cold. To produce food crops, such as citrus,that are sensitive to cold, scientists are combining traditional and new methodologies. In 1963, plant geneticists C. Jack Hearn at the ARS Horticultural Research Laboratory in Orlando began breeding a cross of sweet orange, mandarin, and grapefruit in an attempt to create a tree that was more resistant to the could than existing citrus trees, but still bore edible fruit. It took Hearn 16 years to develop the cold-hardy and sweet-tasting Ambersweet orange hybrid that was finally released in 1989. But says Hearn's colleague Niedz, " `cold -hardy' is a relative term. Ambersweet is hardier than most citrus that grow in central Florida, but what breeders really need is a citrus that can be grown in northern Florida or even Georgia." Breeders will probably turn to another citrus called Poncirus trifoliata for the cold-hardiness trait. "It's the only citrus that will grow all the way up in Pennsylvania. The fruit is inedible, but once we understand why the tree is cold-hardy, we can just splice those genes [cold-hardy] out and leave the poor fruit quality behind." Although the mechanism that makes some trees resistant to cold has not yet been identified, classical breeders are using Poncirus trifoliata as root-stock for grafting. Molecular biologists and plant geneticists are also trying other approaches to solve the secret of cold-resistance. For example, researchers may study cold-hardy deciduous plants to try to identity what genes make these plants drop their leaves. This action may be linked to the plant's resistance to cold temperatures. Alternatively, they first may work out the mechanism for cold-hardiness in tobacco or tomato plants, which are better understood and easier to work with, and then extend these findings and adapt the methods to citrus. In spite of the complex nature of work involved, Niedz claims he claims he has "no doubt that traits such as cold-hardiness will someday be cleared up." Crops such as cotton, corn, rice, soybeans, sugar beets, tomatoes, and alfalfa, engineered for qualitative or single gene traits, should be commercially available by the end of the 20th century. In particular , scientists hope to produce soybeans, cotton, corn, and sugar beets that are resistant to the herbicides used to control weeds. Genetic engineering can not only increase crop yields, but also help decrease crop production costs. Researchers may be able to improve a particular crop's yield by increasing its resistance to various environmental pests and conditions. This engineering, in turn, could reduce a farmer's dependence on chemicals, thus lowering the cost of bringing the crop to market. Whatever method is used to improve plant characteristics, however, the ultimate goal of genetic engineering in crops today is the increased production of better quality food to meet the needs of the growing world population. Abigail Grissom is a freelance writer based in Drexel Hill, Pa.

Bioengineering Increases Yield and Research Opportunities

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