Andrzej Czaplicki, Jaroslava Ovesna, Gert E. de Vries
The improvement of plants for food production and the use of conservation methods have been in practice as long as humans settled down to rely on agriculture for sustenance. The techniques and successes of traditional plant breeding have, in the past, evolved gradually. With the advent of genetics, the advancements have accelerated and allowed us in the 1960s to greatly increase the productivity of our land. This enabled many countries, for the first time, to provide adequate food supply to their growing population. Essential knowledge and understanding of cell function and heredity combined with new possibilities to modify and transfer DNA between organisms is only a few decades old. These advancements have resulted in the development of efficient vaccines and pharmaceuticals, new food technologies and many other products improving the overall standard of life. This is also true of agriculture where genetic engineering of crops can complement traditional plant breeding to suit the needs of today's society. Most of these advances can be grouped under the term "biotechnology", which aims to use organisms, cells or part of cells in technical/industrial processes.
When humans realised that they could grow their own crops and breed animals with improved characteristics, they effectively started to use biotechnology. The first biotechnological processes that were recorded in a written form concern food production around 5000 BC. Our more recent ancestors were producing bread, wine and beer and used fermentation, all natural microbiological processes, in part to prevent foodstuffs from spoiling, in part for health and pleasure. Many of our current food products are still using the same methods of fermentation that were discovered thousands of years ago. Plant species have been improved by selection and crossing to obtain more yielding varieties with better nutritional traits.
The roots of modern biotechnology can be dated back to the work of Louis Pasteur, Robert Koch and Gregor Mendel, approximately 100 years ago. Pasteur and Koch founded the basis for the current science of microbiology. When Pasteur discovered the efficacy of weakened rabies virus to prevent the incidence of the same disease, the technique of preventing infection diseases by means of vaccination was established - having been pioneered by Edward Jenner in the case of smallpox. Mendel described the laws of inheritance, the transfer of parental properties (genes) to offspring. It was Karl Ereky in 1919 who used the term biotechnology for the first time in order to describe the interaction of biology and human technology. Thus, one definition of biotechnology is that it is a method through which life forms (organisms) can be manipulated to provide desirable products.
A common misconception today is that biotechnology refers only to genetic modification (also referred to as genetic engineering, genetic manipulation, gene splicing, recombinant DNA or gene technology, etc.). Genetic modification, however, is only one of many techniques used to derive products from microorganisms, plants or possibly animals that may be used in the biotechnological industry. A list of areas covered by the term biotechnology would more properly also include plant tissue culture, mammalian cell culture, enzyme systems, plant breeding, immunology, fermentation and others. In this chapter the term "modern biotechnology" will be used when speaking about techniques using genetic modification.
Genetic modification can be defined as the technique that involves the isolation of genetic material, splicing, altering, recombining and transferring it from one organism to another. These techniques can be performed at various levels: from whole genome manipulations through chromosome manipulations to precise modification of single genes. Genetic modification has come to include the manipulation and alteration of the genetic material of an organism in such a way as to allow it to produce proteins with properties different from those normally produced, or to produce entirely foreign proteins altogether.
It is now a routine practice to combine, exchange or mix genetic material. If thought useful, DNA can be isolated from one organism, combined with DNA from another (so called recombinant DNA) and placed into cells of a third organism. As a result, it is for example possible to have bacterial cells produce human proteins, such as human insulin. Therapeutic compounds (e.g. interferon, inter-leukin) that were unavailable previously can now be synthesised in a variety of systems, ranging from micro-organisms to plant or animal cells. Biotechnology can be also used to produce artificial transplant materials such as blood-vessel or skin tissue. Basic knowledge, as well as novel pharmaceuticals from biological origin, contribute significantly in treatments for cardiovascular diseases, cancer, stroke, etc. Biotechnology is also applied in other areas such as the environment and food. Micro-organisms have been used to efficiently extract minerals from ores and to clean up pollutants. Genetically improved microorganisms are being used in the food industry, and give us yoghurts, beer or cheese with improved texture, taste or quality. It is clear, that biotechnology contributes to many fields of man's activities. In agriculture, it has brought us a range of methodologies for high quality plant breeding that result in both fast results and superior varieties.
Plant Breeding and Plant Biotechnology
Almost without exception, crop plants in use today have been adapted to the needs of mankind. The development of new plant varieties through selective breeding has been improving agriculture and food production for thousands of years. The genetic make-up of crop plants has been changed by mankind's selection of naturally occurring variants. What has come to be called biotechnology and the genetic enhancement of agricultural products may be one of the oldest human activities. Traditional plant breeding involves crossing of different plants with useful characteristics, and this has been very useful in improving crop plants. Since the entire gene pool of each of the two partners is mixed and useful trait(s) must be selected upon successive rounds of crossing and selection, plant breeding is a time-consuming process. Improving crops through traditional methods is also subject to restrictions imposed by sexual compatibility, which limits the diversity of useful genetic material.
Undoubtedly however, classical breeding has dramatically increased the productivity of the plants we grow for food, fibre and other purposes. The so-called Green Revolution during the years 1950-1965 involved the simultaneous effects of breeding efforts and altered agricultural practices. New crop varieties and the availability of pesticides and fertilisers greatly increased the needed crop yields in developing countries. Mexico, for instance, was importing half of its wheat consumption in 1944. In 1956 the country was self-supporting, and by 1964 Mexico was exporting 0.5 million tons of wheat. Apart from crop yield, significant progress was made in improving the quality of plant products such as proteins, carbohydrates, fibre or oil content.
Technological developments, which allowed the manipulation of plant cells, were a major requirement before plant biotechnology really could take off. Although it had been shown around 1900 that it would be possible to regenerate an entire plant from just a small part - omnipotency - it would take many years before this finding could be put to practice. Efficient in vitro cloning and propagation techniques, sterile ways to reproduce identical, healthy plants of valuable genotypes in tubes, were only recently established for most plant species. The technology can now be used to maintain disease-free plants prior to large-scale production. Ornamental plants, high quality banana seedlings or potato micro-tubers are produced using these techniques.
Although still a relatively young scientific discipline, genetic modification is being applied in a broad variety of ways, mainly in biomedical research, but also in agriculture and food production. With the help of genetic modification, it is now possible to transfer a gene from a bacterium to a plant so that the transgenic plant produces the corresponding bacterial protein in its cells. Thus genetic modification enables the targeted transfer of individual traits that are encoded in genes to a given plant, allowing the transfer of only one or a few desirable genes to develop crops with specific traits. Genetic modification is partly seen as complementing and extending traditional plant breeding. Others perceive it as a completely new way of generating plant varieties.
Plant Biotechnological Techniques
While the DNA molecule (deoxyribonucleic acid) from a micro-organism, a plant, an animal or a human is chemically identical, the sequence of its building blocks differs significantly. Genes are units of sequences,which bear resemblance in different organisms, and their cohesion and collaboration is the basis of life. A gene is a blueprint for the synthesis of a specific protein. The genetic code in the blueprint is universal, therefore any gene can be made understood in each organism. This is the basis for genetic transformation,whereby genes can be taken from any source (plant, animal or microbe) and inserted into another organism where it can be expressed as a normal part of the genome. Plants that were made resistant to herbicides or produce advanced pharmaceuticals serve as an example.
Advances in genetic modification are based on a long history of scientific enquiry. One of the key events was the discovery of the fundamental principles of heredity by Gregor Mendel at the end of the nineteenth century. He established the basic laws of genetics that led to our understanding of the inheritance of traits and the role of genes in transferring these traits to offspring of plants and animals. With the understanding of the laws of heredity, plant breeding protocols were devised whereby selection was accompanied by deliberate crossing. Subsequent investigations advanced our understanding of the location, composition and function of genes.
Mendel described the laws of heredity following his pioneering work with pea plants. While his rules for the segregation of traits are generally valid, exceptions can always be observed in the form of enhanced linkage of genes. Later, such groups of genes were found to be located on the same chromosomes and as a consequence have a greater chance to be inherited together. These linkages provided breeders with the possibility of detecting the presence of one gene by the detection of a neighbouring gene, a genetic marker. Easily detectable markers, such as flower colour or seed shape, have facilitated the tracking of other, more complicated, traits such as stress resistance or crop yield in breeding programmes. Four types of genetic markers can been used, which can be divided according to the method of analysis that is based on morphology, cytology, protein chemistry or the detection of DNA sequences. Such genetic markers therefore function as indicators for the possible presence of linked, and desirable,stretches of chromosomal DNA or individual genes.
A morphological marker is a trait that can be seen directly in plants growing in the field, examples are flower colour, aberrant stem length, leaf shape etc. The choice for such a marker has two serious drawbacks: the number of useful morphological traits is limited, and the phenotype itself is often undesirable. Cyto-logical markers rely on regions of the chromosome, which can be preferentially stained to reveal the presence or absence of certain sequences in the genome. Unfortunately, cytological techniques are time-consuming and too inefficient to be used routinely in a plant-breeding program. The use of protein markers relies on the ability to detect specific proteins in extracts from plant tissues. This method is relatively inexpensive and efficient but the number of known and useful markers is very limited.
DNA markers offer the greatest potential. In 1980, it was suggested that such markers could be identified by fragmenting the DNA with restriction endo-
nuclease enzymes (enzymes that can cut DNA at specific locations) and pinpointing individual traits to specific fragments. Restriction fragment length polymorphism (RFLP) became an important technique in the analysis of genomes from microbes to humans and has been used extensively to develop genetic maps. The practice has now been revised by the use of a new technique, the polymerase chain reaction (PCR). This method is based on the ability to detect DNA segments through amplification by a process of DNA synthesis. Various genetic marker systems for crop improvement have now been developed using PCR-based strategies. The technique is almost ideal because genetic markers can be developed for most regions of the genome and they are highly polymorphic (variable), which means that even individual gene variants can be tracked. Since the technology involved is simple, the use of molecular genetic markers have become routine in most breeding stations and an established tool in plant breeding. The molecular techniques, although they are used in the process of genetic modification as well, are themselves no subject of public opposition since they are only a tool to accelerate plant breeding and selection.
Manipulation of Plant Genomes
Plant genes are organised on chromosomes, which are normally located in the nucleus of plant cells as sets of pairs to form the plant genome. The paired chromosomes originate from the parent plant(s), one from each parental diploid set. The number of chromosomes in a plant cell varies from species to species, and the whole genome may have undergone multiplication, leading to polyploidy. Polyploidy plays a major role in evolution and has also been shown to improve certain characteristics of cultured plants. Geneticists have therefore increased the ploidy level of certain crops to achieve greater vegetative production, larger flowers and improved seed. More than one third of all cultivated plants in use are now polyploid.
Some plant species are natural hybrids between two separate species and thus carry two genomes, each of which originates from a distinct diploid species. Plant breeders have used such anomalous genetic processes and, in their search for the combinations of best qualities, produced new plant genome combinations through interspecific crosses. In the 20th century, scientists succeeded in the production of a wide range of hybrids by the use of tissue culture techniques. Using these techniques it also became possible to generate large numbers of genetically identical plants (clones) in a period of only a few weeks or months. Somatic hybridisation is a more advanced method that is used in the laboratory to produce interspecific hybrids between plants that normally would not cross or produce viable offspring. The technique relies on the fusion of plant cells lacking cell walls, called protoplasts, followed by growth in tissue culture and plant formation. Somatic hybridisation offers the possibility to introduce genetic material from non-compatible wild species into cultivated species.
Evidence has been found that some species evolved through interspecific crosses and hybridisation between species that possess dissimilar genomes.Viable (sometimes barely) offspring may be produced with more or fewer and rearranged chromosomes than the present in the parental species. Aberrant plant genomes may also arise when complete chromosome(s) of one species are replaced with equivalent chromosome(s) of another variety or related species. The instability of the resulting plant genomes increases with degree of evolutionary distance of the parent plants.
The deleterious effect of the loss or the gain of a full chromosome carrying unwanted as well as necessary genes, can be overcome by natural rearrangements, breakage or deletion of segments in such a chromosome. Chromosome breaks may occur naturally at meiosis, but in the hands of a researcher they can also be induced by certain chemical and irradiation treatments. Broken chromosomes have the ability to fuse with each other to produce translocated, or rear-ranged chromosomes, which, with luck, may provide the plant with improved characteristics. Although advanced techniques are now available to introduce whole chromosomes or chromosomal segments in plant cells, the limitation of this strategy (as is also true in conventional plant breeding) is the laborious procedure to select and further develop desired genotypes from the progeny.
How Can Genes be Altered?
So far we have discussed the evolution of crop improvement from domestication to deliberate plant breeding and how overall, uncontrolled, genetic changes can be achieved. Genetic improvements have long been based on simple selection in the field. Relatively recent efforts combine planned crosses between defined parent plants, combined with a targeted selection for defined improvements. These processes rely on natural or induced recombinations of genetic material to reshuffle genes. A major limitation in this approach is that the desired trait or phenotype must be present in the parent plants that can be crossed. What if a desired variation does not exists among the available plant varieties?
In 1901 Hugo de Vries published his mutation theory, arguing that progressive mutation could bring about change within a species. While many mutants had been produced in a number of crop plant species, most had changes for the worse. Moreover it was found difficult to select those mutant plants which had acquired improved characteristics. Several thousands groups of plants had to be screened to find a rare desirable type and this was considered too random a process for the time. Today there are new methods of inducing mutations and the improvements in selection and detection (such as the use of genetic markers) has given new impulse to mutation-based breeding strategies. The power of the selection of mutations for breeding purposes can be illustrated by an example of maize lines that were mutated to gain resistance to a herbicide compound. By itself this an unremarkable event since such lines had been produced before in other crops. But a remarkable finding was that herbicide resistance was due to a single amino acid replacement in the enzyme targeted by the herbicide. This example demonstrates that unexpected properties may arise from seemingly trivial changes in the genetic makeup of an organism that cannot be predicted. It is clear that, while a proper method for inducing mutations is important, an essential step is the design of a clever system to select the desired plant lines.
Genetic transformation is a technique whereby genes can be taken from a selected organism (microbe, plant or animal) to be introduced into another organism where they can be expressed as a normal part of the genome. The first step in this process is to splice a selected gene segment from the DNA sequences of a donor, using restriction enzymes. This DNA fragment is then linked to other segments of DNA that contain marker and selection genes. Several methods are used to actually introduce the prepared donor DNA into cells of the target organism. A no-nonsense method just shoots gold particles, coated with the target DNA, at random into a mass of cells. In a only few cells will what is aimed for happen: the donor DNA is taken up and incorporated into DNA material of the target organism. Thanks to the presence of selection genes,successful transformed cells will grow and stay alive on a growth medium that prevents the development of untransformed cells. Subsequent analyses will reveal whether the selected gene functions correctly in the target organism.
The story of genetic modification in plants started 1980, when it was demonstrated that a soil bacterium, Agrobacterium, caused tumours in plants after transferring a small but distinct DNA fragment to a plant cell, where it would be incorporated in the nucleus and change the physiology of the local tissue. In 1983 the system was put to use and GM Agrobacteria were used to transfer an antibiotic resistance gene into a tobacco plant. Today advanced methods for the genetic transformation in a wide range of plants have been developed that are based on this natural phenomenon.
It should be noted that the term "transformation" is a bit of a misnomer. Most transformation work in crop plants involves the transfer of single genes. The resultant plants are not radically changed, new species are not produced but rather the crop plant is modified by an acquired new trait.
Plant biotechnology can complement traditional plant breeding to suit the needs of today's society. However, it has not often been recognised that plant bio technology consists of many techniques and that only some require transfer of genes between unrelated organisms. Since the initial successes of genetically modifying plants, a wide range of applications have been investigated to solve existing problems in agriculture such as problems with insects, plant pathogens and weeds as well as abiotic stresses. Edible vaccines for infectious diseases like cholera, hepatitis B, and diarrhoea, when produced by GM plants and administered as a food component, may in certain instances help to efficiently fight diseases in the Third World. GM plants may have a role in cleaning polluted soil (phytoremediation) and may also be suitable as fuels in power plants (see Sects. 5.1 to 5.3).
In spite of the promising contribution of genetic modification of plants, many points of criticism have been raised by non-governmental organisations, activist groups, politicians and individuals. Opponents argue that genetic modification is not required to feed the world or to solve problems of agriculture production. Problems in the world are primarily caused by the economic balance of power, war, drought, mismanagement or poverty and not by lack of resources. Perceived environmental or health risks when using GM organisms have also motivated European citizens to protest against the use of GM plants and biotechnology.
Agrifood Awareness Australia, How plant breeding works. http://www.afaa.com.au/pdf/ 4-Plant_breeding.pdf
Australian Office of the Gene Technology Regulator, World agricultural biotechnology on GMOs: What Is biotechnology? What is gene technology? http://www.ogtr.gov.au/pdf/public/ factwhatis.pdf
Barth R et al, Genetic engineering and organic farming. German Federal Environmental Agency.
http://www.oeko.de/bereiche/gentech/documents/gruene_gentech_en.pdf Conko G, The benefits of biotech. http://www.cato.org/pubs/regulation/regv26n1/v26n1-4.pdf European biotechnology: development and outlook. http://biozine.kribb.re.kr/special/1-3-03.html
European Plant Biotechnology Network. http://www.epbn.org/
FAO Comm Genetic Resources Food Agric, The status of the draft code of conduct on biotechnology. http://www//ext-ftp.fao.org/ag/cgrfa/cgrfa9/r9w18e.pdf Information on biopharmaceutical production. http://www.ul.ie/~biopharm/ James C (2002) Global status of commercialised transgenic crops: 2002. ISAAA Briefs 27
http://www.botanischergarten.ch/UNIDO/ISAAA_Briefs_No._27.pdf Levy A et al, The dynamic plant genome. http://www/avraham_levy.pdf Rodemeyer M, Future uses of agricultural biotechnology. Pew Initiative. http://pewagbiotech.
org/research/harvest/harvest.pdf USDA, US-information on GMOs in agriculture. http://www.usda.gov/usda.htm
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