San Francisco, California
March 19, 2008
Public Library of Science
Imagine you are a rice breeder and
one day within a large field you discover a plant that has just
the characteristics you have been looking for. You happily take
your special plant to the laboratory where you find out that the
spontaneous, beneficial event was due to inactivation of a
single gene. This is a great observation; however, there are
many different strains grown in different parts of the world,
well adapted to the particular region they grow in. How can you
now transfer the inactivated gene to other strains of rice?
Conventionally, you would have to go through years and years of
breeding, until you have successfully transferred that single
gene, without affecting all the other genes that are responsible
for the target strains being so well adapted to their local
environment. Would it not be great, if one could do this faster?
Using inactivated genes for rice breeding might sound
far-fetched, but is not unusual. For example, the main change
enabling the green revolution in rice resulted from loss of a
gene that normally makes rice grow tall (and hence prone to
toppling over if a plant makes many heavy rice grains). Thus,
transferring inactivated genes is something rice breeders are
indeed very much interested in.
Researchers at the Max
Planck Institute (MPI) for Developmental Biology in
Tübingen, Germany in collaboration with the International Rice
Research Institute in the Philippines, have now generated a tool
that should greatly speed up this particular aspect of rice
breeding: According to a study published in PLoS ONE this week,
a team led by Norman Warthmann (MPI) successfully demonstrated
highly specific gene silencing using so-called artificial miRNAs
in rice (Oryza sativa).
MicroRNAs are 20-22 bp long RNA molecules. In animals as well as
in plants they have important functions in regulating gene
activity. In plants, they cause highly specific degradation of
sequence-matched messenger RNAs, which encode enzymes,
regulatory factors or other proteins. The end effect is that the
corresponding gene is silenced. With artificial miRNAs
(amiRNAs), this natural silencing pathway can be harnessed to
inactivate genes of interest to the breeder, with unprecedented
Detlef Weigel’s research group at the Max Planck Institute in
Tübingen had initially pioneered this technique in the model
plant Arabidopsis thaliana. The plethora of potential
applications in agriculture now motivated them to try the method
in rice. One of the rice genes they targeted is called Eui1.
When Eui1 is inactive, flowers tend to be fertilized by pollen
from other plants, rather than being self-fertilized. While this
trait would be harmful to a wild rice plant, breeders use this
genetic trick for hybrid seed production. Originally identified
as a spontaneous mutant in a japonica rice variety, the eui1
mutation was introduced into indica varieties by several years
of breeding. With an artificial miRNA targeting the Eui1
messenger RNA, the researchers at the International Rice
Research Institute obtained within weeks plants with the desired
property in two different rice varieties, including the
agronomically important indica variety IR64, the most commonly
grown strain in South-East Asia. Similarly, the researchers also
report successful silencing of two other genes, Pds and SPl11.
Besides allowing the quick transfer of reduced gene function
between different varieties, artificial miRNAs also accelerate
the initial identification of important genes and the discovery
of functions of genes that have not been studied before.
Potential applications in rice breeding are manifold and they
don’t stop at rice genes. By targeting pathogen-derived genes,
for example, it should be possible to enhance virus and insect
resistance. In addition, because they act dominantly, they are
also perfectly suited for hybrid breeding.
MiRNAs have been found in all plant species examined so far. It
should hence be possible to adapt the technique of gene
silencing by artificial miRNAs to other crops and it may provide
an important new avenue to enhance agronomic performance and
nutritional value. Computer software to design the required
oligonucleotide sequences and detailed protocols to produce
amiRNAs are provided free of charge on the authors’ web site, at
http://wmd2.weigelworld.org. Similarly, the artificial miRNA
vector is provided free of charge to colleagues.
Highly Specific Gene Silencing by Artificial miRNAs in Rice
Warthmann N, Chen H, Ossowski S, Weigel D, Hervé P (2008)
Co-authors on the study include: Norman Warthmann, Stephan
Ossowski, Detlef Weigel (Max Planck Institute for Developmental
Biology, Tübingen, Germany) and Hao Chen, Philippe Hervé
(International Rice Research Institute, Los Baños, Philippines).
PLoS ONE 3(3): e1829. doi:10.1371/journal.pone.0001829
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