May 1, 2000 / Nature Biotechnology
A research team has adapted a technique that employs small
hairpin-shaped molecules made up of RNA and DNA to introduce single base
changes into DNA; the team has successfully generated herbicide-resistant
plants with just a single change in the genetic code.
Pinpoint crop engineering
The problem with traditional genetic engineering techniques in plants is
that they are rather messy. Typically, plant transformation methods
introduce many copies of foreign genes at random positions on the chromosome
leading to variation in the level of foreign gene expression and,
occasionally, triggering suppression (silencing) of similar genes that
naturally occur in the plant. Now, Chris Baszczynski and his colleagues have
adapted a technique, originally developed for gene repair in mammalian
cells, that employs small hairpin-shaped molecules made up of RNA and DNA
(so called chimeric oligonucleotides) to introduce single base changes into
DNA. Using this approach, they have successfully generated
herbicide-resistant plants with just a single change in the genetic code.
Chimeric oligonucleotides consist of a short self-complementary stretch of
double stranded DNA flanked by a longer stretch of RNA to protect it from
degradation in vivo. The DNA sequence corresponds to the region of the gene
to be modified, but with a single nucleotide change. When introduced into
cells, the chimeric oligonucleotide homes in on the gene of interest with
the matching sequence and then triggers the plant's own DNA repair machinery
to substitute the oligonucleotide-encoded sequence with the single base
alteration for the original plant sequence.
In previous work, Basczynski's team used chimeric oligonucleotides to alter
the gene encoding the plant enzyme acetohydroxyacid synthase (AHAS) in
masses of plant cells called calli, which as a result acquired resistance to
the broad-spectrum herbicide imidazolinone. However, it was not clear
whether the results would be reproducible or long lasting in whole plants.
In the new study, they regenerate whole transgenic plants from the calli and
show that the modified AHAS gene is maintained in subsequent generations.
While the results are promising, researchers still have to overcome the low
efficiency of gene conversion (currently only 1 in 10,000 cells are
modified) or improve the screening procedures to identify which cells have
undergone gene conversion. Nevertheless, if the technique can be made more
efficient, it could avoid many of the problems associated with traditional
plant transformation used for GM crops.
(posted without permission)
