Cloning vectors for higher plants

The most important single tool for the genetic engineering of plants is the Ti plasmid. This is found naturally in the soil bacterium Agrobacterium tumefaciens, which infects plants at wound sites, and leads to a condition called crown gall disease. The important feature of this plasmid is that part of it, called the T-DNA, can integrate into the host plant's chromosomes, and be expressed along with host genes (Figure 12.13). Geneticists were quick to spot the potential of the Ti plasmid, replacing tumour-forming genes with foreign genes, and having them expressed in plant tissues. The recombinant A. tumefaciens is used to infect protoplasts, which can be regenerated into a whole plant, every cell of which will contain the integrated foreign gene. The Ti plasmid system has been used in the successful transfer of genes for insect- and herbicide-resistance into economically significant crop plants.

Plant viruses have very limited usefulness as vectors. Only the caulimoviruses and the geminiviruses have DNA rather than RNA as their genomic material, and a variety of problems, including instability of inserts and narrow host range, have been encountered with the use of these.

Cytokinin

Cytokinin

Figure 12.13 Agrobacterium tumefaciens. (a) The Ti plasmid. The T-DNA contains genes for tumour production and the synthesis of opines, unusual amino acid derivatives that serve as nutrients for A. tumefaciens. (b) Crown gall formation by A. tumefaciens. The T-DNA integrates into the DNA of the host, where its genes are expressed. This ability has been exploited in order to transfer foreign genes into plant cells. From Reece, RJ: Analysis of Genes and Genomes, John Wiley & Sons, 2003. Reproduced by permission of the publishers

Figure 12.13 Agrobacterium tumefaciens. (a) The Ti plasmid. The T-DNA contains genes for tumour production and the synthesis of opines, unusual amino acid derivatives that serve as nutrients for A. tumefaciens. (b) Crown gall formation by A. tumefaciens. The T-DNA integrates into the DNA of the host, where its genes are expressed. This ability has been exploited in order to transfer foreign genes into plant cells. From Reece, RJ: Analysis of Genes and Genomes, John Wiley & Sons, 2003. Reproduced by permission of the publishers

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