Genetically Modified crops
Applications of plant transformation
2.20 GM plants are used or will be used in a number of different ways. Research applications are increasingly important. For example, GM plants are developed to try to identify gene function by simply seeing whether the introduced gene has any observable effects. Transformation is now so routine in some model species that it can be used as a tool to identify which fragment of a plant's DNA contains a gene of interest. When other experiments have narrowed the possibilities down to a stretch of DNA which contains, say, 100 genes, the critical gene may be identified by breaking the fragment into smaller pieces and firing these into the plant to see which have the desired effect. These sorts of applications are not considered here in detail.
2.21 The most important use of GM plants is to accelerate plant breeding: here, native genes and promoters, previously isolated from the target species, are placed directly into an otherwise ideal varietal background. Although this can also be achieved by repeated backcrossing (12), it might take ten generations to meet the purity standards required by the Plant Breeders Rights legislation in the UK. The products of the transgenic strategy will be virtually indistinguishable from those of conventional breeding. However, the stringency of the DUS (13) regulatory procedure means that varieties that have been developed elsewhere may take several years before they can be licensed in the UK, and this applies particularly to the new varieties of oilseed rape that are currently being developed. Quite apart from the issues that are specific to GM plants, these crops will not be available for commercial planting in the UK for several years because of the need, for example, to show that their yield is higher than that of current cultivars (see Box 3.1). One example of using genetic modification to accelerate plant breeding is the manipulation of storage-protein genes in wheat to improve bread-making quality. A further example, soon to be in agricultural use, is the transfer of a bacterial blight resistance gene, Xa21, from a wild relative to cultivated rice where it was found to confer resistance against most, if not all, races of the pathogen.
2.22 Antisense transformation: this technique eliminates the effects of unwanted genes. If a gene is inserted into a plant in reverse (antisense) sequence, the transcribed antisense RNA (ribonucleic acid) product will often interfere with the function of similar native genes. This property can be exploited to remove or suppress the effects of any gene or group of similar genes. In some situations a similar result can be achieved by mutating the target gene and rendering it functionless, but this conventional technique is much slower and requires considerable resources for the necessary screening. An example of the use of antisense transformation is the development of a transgenic tomato with delayed ripening and longer shelf life. In this case, the gene controlling production of an enzyme which promotes cell wall breakdown after ripening was knocked out by use of the tomato gene in an antisense sequence. As a result the tomatoes stay firmer for a longer period.
2.23 Transformation with beneficial genes isolated from other plants: this procedure, also called inter-specific transfer, provides a means of circumventing natural breeding barriers. This application is not in wide usage, simply because the identification and supply of useful genes from other plants is limited. However, the complete DNA sequences of the model plants Arabidopsis and rice will soon be available, so increasing the availability of a large number of plant genes (paragraph 3.41). One of the eventual goals of the plant breeder is transfer of the genes conferring apomixis (the ability to produce seed without going through normal sexual reproduction) to crop plants. Other examples include the use of plant genes to modify starches and oils.
2.24 Transformation using genes isolated from bacteria or viruses: at present this is a widely used approach because many genes have been identified from these sources. Examples include the insect-resistance genes and herbicide-tolerance genes, currently used in the US in the production of corn, cotton, soya and potato varieties. Although these genes are commonly spoken of as bacterial or viral in origin, the genes that are eventually used to transform crop plants are considerably modified. DNA is broadly similar in plants, bacteria and animals. However, even between narrow-leaved and broad-leaved plants, there are some differences between the preferred sequence of the DNA components. To accommodate this variation, transgenes are usually reconfigured, or 'optimised' and resynthesised. As a result, transgenes may bear as little as 60% identity to the original gene, although the differences will not often alter the amino acid sequence of the protein that is produced.
Footnotes12 Backcrossing is the process by which an FI hybrid, made by crossing two parent plants, is crossed back to one of the parents.
13 DUS are the criteria needed for a new inbred variety to be approved for Plant Varieties Rights regulations in the UK. These are: distinctness – is it different from anything already available on the market? uniformity – are all the seeds exactly the same? and stability – is the variety stable over several generations?
14 Fujimoto H, Itoh K, Jamamoto M, Kyozuka J and Shimamoto K (1993) Insect-resistant rice generated by introduction of a modified delta-endotoxin gene of Bacillus thuringiensis (Bt), BioTechnology 11:1151-1155. DNA is made up of base-pairs. Groups of three base-pairs code for individual amino acids. The amino acids are then linked together to form proteins.