Scientists at the John Innes Centre (JIC), Norwich have today explained the control of an important biological process (so-called RNA-interference) that contributes to plants’ ability to regulate genes and to defend themselves against virus attack.

“RNA-interference (RNAi)[1] is a relatively new discovery in biology that we know is very important in controlling plant and animal development and in plant resistance to virus diseases” says Dr Robert Sablowski (leader of the research project at the JIC). “Our discovery shows us an unexpected way in which this important process can be switched on”.

Active genes produce short-lived multiple copies of themselves (rather like photocopying a recipe) These copies are molecules called messenger-RNA (mRNA). The information in the mRNA is read by the cell to make a product and that product will then have an effect on the biology of the cell. If the mRNA copy carries a mistake or is damaged, it could make a defective product. To prevent this from happening cells have quality control systems that check mRNA and destroy defective copies.

Cells may also need to get rid of mRNAs made by viruses, to stop them reproducing themselves. This defence mechanism, based on RNAi, is triggered by an unusual type of RNA (double stranded RNA) which is made by viruses when they reproduce themselves.

RNAi is sometimes triggered when genes are introduced into plants by genetic modification. When this happens, the mRNA made by the gene is destroyed, thus the product of the gene cannot be made and so the gene can have no effect in the cell – although the gene is present it is silenced. It is not clear why the plant recognises some genes as ‘foreign’ and a target for RNAi.

The JIC team have found that gene silencing can be triggered by mRNAs that are targeted for destruction by the natural mRNA quality control mechanism[2]. If too many defective copies of a particular mRNA accumulate, then the cell turns them into double stranded RNA and this triggers RNAi. Thus plants can silence ‘foreign’ genes if their mRNA does not function smoothly along with the rest of the plant's mRNAs. 

Although this work was done in plants, it has implications beyond plant biology. RNAi has become an important tool because it allows scientists to switch off genes at will and so see what their role is. Although RNAi was originally discovered in plants, it also functions in animals, including humans, and has potential therapeutic applications. 

The discovery is reported in this weeks edition of the international science journal Science[3].

Notes

[1] RNA-interference describes the phenomenon where double-stranded RNAs cause the targeted breakdown of homologous mRNA. This is now known to occur in a wide range of organisms, both animals and plants.

[2] The JIC scientists’ findings demonstrate that de-capped mRNA is the likely template used by the cell to make the double-stranded RNA that triggers RNAi. The work originated from finding mutant plants that silenced a foreign gene at high frequency. The mutated gene was XRN4, which encodes an RNAse implicated in the turnover of de-capped mRNAs. Thus, when plants cannot efficiently destroy de-capped mRNAs, they are diverted into the RNAi process.

[3] A Link Between mRNA Turnover and RNA Interference in Arabidopsis
S. Gazzani, T. Lawrenson and R. Sablowski. Published in Science on Friday, 05 November, 2004.

The John Innes Centre (JIC), Norwich, UK is an independent, world-leading research centre in plant and microbial sciences.  The JIC has over 850 staff and students.  JIC carries out high quality fundamental, strategic and applied research to understand how plants and microbes work at the molecular, cellular and genetic levels.  The JIC also trains scientists and students, collaborates with many other research laboratories and communicates its science to end-users and the general public.  The JIC is grant-aided by the Biotechnology and Biological Sciences Research Council.