Making transgenic plants

Transgenesis means putting a foreign gene (a transgene) into an organism's genome, so that it is expressed and passed on. The results are transgenic organisms, or GMOs. The first were Boyer and Cohen's bacterium (1973), Jaenisch and Mintz's mice (1974), and the Flavr Savr tomato (1994).

First a vector is built, carrying the gene with a promoter and a terminator. Then the gene is moved into plant cells, indirectly or directly.

Indirect (vector-mediated) transfer, using Agrobacterium tumefaciens, the 'natural genetic engineer'. In nature, this soil bacterium infects plants and moves a piece of DNA, the T-DNA of its Ti plasmid, into the plant cells. The T-DNA carries genes for the plant hormones auxin and cytokinin, which make the cells grow into a tumour, the crown gall. Scientists replace these tumour genes with the gene of interest, plus an antibiotic-resistance marker. This disarmed plasmid no longer causes tumours, but still delivers the gene.

After infection, the plant cells are grown on a regeneration medium containing two things: a bacteriostatic agent, which stops Agrobacterium growing, and the marker antibiotic, which kills any plant cells that did not take up the gene. The surviving cells grow shoots, then roots in two to three weeks, and the young plantlets are moved to soil. Plant viruses, such as the caulimoviruses, can also be used as vectors, but less often.

Direct (vector-less) transfer:

• Particle bombardment (biolistics): a gene gun fires tiny gold or tungsten particles coated with DNA into plant cells. It works for both monocots and dicots, and is especially useful for cereals, where Agrobacterium works poorly. It can even put genes into chloroplasts, giving high levels of expression.
• Protoplasts: plant cells whose walls have been removed can take up DNA directly, helped by chemicals such as PEG and dextran sulphate, or by electroporation. They then grow new walls and are regenerated into whole plants.