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Introduction
Over the last three decades, advances in plant biotechnology have had a significant impact on plant breeding and the plant industry. Notably over the past five years, numerous crop varieties have been marketed that used some form of plant biotechnology in their development. According to the International Seed Trade Federation estimates, the area cultivated with transgenic crops jumped from 2.8 million in 1996 to 39.9 million ha in 1999.
Plant biotechnology comprises several different technologies that are currently used in plant breeding and production. These are explained in the following sections:
In vitro Technology
In vitro technology (tissue culture) is the rapid and large-scale propagation of genetically identical plant material in glass or plastic jars that takes place in specialized laboratories. The objective is to accelerate multiplication as well as obtaining disease-free starting material. It has developed to such an extent that the tissue culture is now considered relatively 'low-tech' and routine. More advanced tissue culture methods such as anther or embryo culture, somatic hybridization, single cell regeneration and the use of somaclonal variation, expand the genetic variability useful for breeding and help to generate and select superior material.
Marker Technology
Marker technology has been successfully exploited currently in plant breeding strategies. A 'marker' is a protein or more often a short sequence of DNA, the presence of which is linked to a desired trait such as resistance to a particular pest. By using such markers as a flag for regions of genetic material (DNA), complex or difficult-to-assay traits can more easily be evaluated and selected in a process called marker-assisted breeding. This speeds up and improves the accuracy of selection. A considerable part of current plant biotechnology research is aimed at identifying suitable markers for 'traits of interest' in breeding material. There are many different kinds of DNA markers such as RAPD, RFLP and AFLP. The ideal marker is the Single Nucleotide Polymorphism (SNP), a single difference in the basic building blocks of DNA. Several novel technologies have been and are being developed that target SNPs in crops. Marker technology is also used in other fields of plant science such as in establishing genetic relationships in ecosystems.
Gene Technology
Most of the challenges, opportunities and discussions about plant biotechnology are focused on gene technology. Genetic engineering (GE) allows the transfer of a piece of functional DNA (gene) into a host organism. There are several ways to introduce such novel genetic information in a plant. The most common method is to use the bacterium (Agrobacterium tumefaciens) that transfers DNA to host plant cells to modify them for its own needs. Another method is particle bombardment, a transformation method based on physical principle where DNA fragments are bound to the surface of minute metal particles and shot into plant cells using specially developed devices. There are several other approaches for gene delivery such as chemical methods (CaCl2 or polyethyleneglycol) and electroporation.
The key feature of GE is that the origin of the gene-of-interest is not important. Classical plant breeding is limited to the genes that occur in related, crossable plant populations. Such relatives may not carry genes for desired traits. Using GE, the donor organism may be a plant, but can also be a bacterium or even a fish: the concept of '(wild) relative' is of little relevance for plant biotechnology. When the donor is a wild, crossable plant relative, GE can make extensive backcrossing unnecessary and speed up breeding. Therefore, GE enlarges the gene pool available for crop improvement and breeding dramatically.
Aims of Gene Technology in Plants
Nowadays, GE is important in many areas of academic research to elucidate the basic biology of plants. In its applications, GE basically aims at the same improvements of crops as conventional breeding has done and still does: improved crop varieties that appeal to farmers, industries, retailers and consumers. The aim and outcome of GE is still not so very different from hybridization, but GE widens the scope of plant breeding and (potentially) accelerates the delivery of results. GE has resulted in the introduction of genetically modified (GM) crops in farmers' fields and on the consumer markets. Notably in the USA, GM crops are planted on a large scale and the adoption rate has been remarkable. GE has seen a development from input traits to output traits and there is a trend to go from simple single genes to complex multiple gene traits.
Input traits
These affect the agronomic performance of a crop such as yield and costs of production, but they do not necessarily change the final product, hence farmer's advantage. Input traits which may be assisted by GE, comprise:
improved weed control: herbicide tolerance
biotic stress: resistance against pests such as fungi, bacteria, viruses, insects, nematodes
abiotic stress: tolerance against drought, salinity, mineral deficiency, heat, cold, freezing, environmental pollution
increased efficiency of production male sterility, seed quality (dormancy, vigor)
reduced environmental load in crop production: less spraying, less energy use, less waste, less losses
intrinsic yield: more efficiency of plant growth in terms of mineral uses, nitrogen fixation, photosynthesis, increased seed set
Important input traits that are currently used in GM crops are herbicide tolerance and pest resistances (virus, insect, nematode). Many improvements can be expected in the understanding and use of resistance genes in enhancing the resistance of crops notably broad spectrum resistance. Future GE of input traits may allow us to change plants in such a way that low or suboptimal environments can be used for productive agriculture.
Output traits
In contrast, these traits improve the quality or economic value of the final product, rather than the agronomic performance of the crop, hence to the consumer's advantage. Output traits to be affected by GE include:
nutritional value: vitamin content, health promoting substances, nutraceuticals
product quality: longer shelf life, reduced allergenicity, reduced toxicity, increased consumer appeal
specialty chemicals: specific oil, starch or fiber components for existing or innovative industrial applications, biodegradable plastics, cosmetics
pharmaceuticals: edible vaccines, blood proteins, expensive medicines
The goals of GE for output traits go far beyond the aims of classical breeding. GM may turn plants into chemical factories for added-value compounds: the plant as a plant. By modifying metabolic pathways, common crops could be redesigned into units that produce virtually anything, from commodity chemicals to pharmaceuticals and cosmetics. Basically it is only human capacity that is limiting the range of possibilities.
Current Status of GM Crops
The GM crops now marketed generally contain a single added gene that confers a single desired trait, such as resistance to specific pests or tolerance to a specific type of herbicide.
For some time breeders argued that most important traits such as yield or quality were essentially quantitative traits determined by complex polygenic networks that would never be amenable to GE. Results from quantitative trait mapping, however, suggest that also in complex quantitative traits, relatively few genes exert the largest part of the effect. If so, GE may also be of use in improving quantitative traits.
These more advanced applications will require the stacking of several genes. The golden rice is currently the best example of multiple GE. Rice with two genes from daffodil, and a gene from a bacterium is able to accumulate b-carotene, which the human body can convert to vitamin A. The expectation is that numerous multiple genes will be stacked in varieties of crops. Future applications will also give more attention to the precise control of introduced genes. Now often so-called constitutive expression is used, but it may be advantageous to limit the expression of a given gene to specific cells or specific phases in a plant's life.
The application of GE to plant breeding is straightforward and ambitious: for any trait desired, somewhere there will be an organism that is able to do the job. The challenge is to find that organism, identify the genes involved, and transfer those genes into a suitable crop. The identification and evaluation of properties in organisms for potential application in crops, is currently a major activity in plant GE. Botanical gardens, national parks or national plant communities may be as useful as the diversity in local farming communities. Any organism in any ecosystem may contain genes that may have value in crop improvement.
In addition, GM can create novel diversity in the laboratory that has no equivalent in nature. New genes that encode improved proteins may emerge out of random recombination of existing genes. The created gene can result in a protein that has an activity far exceeding the specifications of the parental proteins. This laboratory-based accelerated evolution can be considered as the molecular equivalent of heterosis in plant breeding.
Continuing developments in large-scale and high-throughput (and high-cost) techniques, collectively known as 'genomics', will further speed up the identification and use of novel genes. Rapidly growing databases of genes, expression patterns, proteins and metabolites will speed up breeding and further expand the possibilities of GE and of plant biotechnology.
Pandora's Box or the Philosopher's Stone?
Despite the many possibilities for improvement and innovation, the GE is looked upon with considerable distrust, concern and even fear. Notably in Western Europe, GM crops are having a difficult time and Europe is trying to set the stage for the rest of the world.
The technical concerns over GE focus on safety: for example are GM crops safe to grow and consume? What will be the impacts on ecology both on farm and for biodiversity at large, and how about the toxicological characteristics of food and feed? How to determine safety and impact? Such issues can only be assessed on a case-by-case basis. Unfortunately, in all discussions, the issue of case-by-case is often overlooked. For the GM crop products now on the market, the conclusion that they are as safe as their non-GM counterparts seems well substantiated.
There is also considerable debate on the socio-economic impact of GM crops. A key issue introduced to plant breeding is 'ownership' of GE materials. The current organization of plant biotechnology in a few multinational corporations that set goals and aim to define markets may result in the situation that too few control too much. This is a legitimate concern. It should be pointed out however, that the current load of regulation, comes with a considerable cost and delay that may only be affordable by very large companies. The (over?) regulation of GM crops may therefore favors this concentration of technology that is considered by many to be undesirable.
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