Boosting Plant Immunity with CRISPR/Cas 9

Following on from last week’s repost of a piece written by Meristem Journeys on the use of CRISPR technology to assist plants without implanting additional DNA, a Research Highlight appeared in the Genome Biology journal looking at the benefits of and potential limits to this technology, particularly its ability to protect crops against virus-caused damage.

Review of recent study results

The article looks at three recent studies into providing a novel defence against Geminiviridae viruses, a DNA virus with the ability to ruin crops. Current methods of avoiding damage caused by these viruses is restricted to the use of chemical controls against vector insects. However, the three studies reviewed looked at the positive outcomes of using CRISPR genome editing to repress the ability of the virus to replicate in the crops, similar to how the Cas 9 nuclease is naturally used by certain bacteria to defend against disease.

The researchers inserted a piece of DNA into the genome of plants that, when expressed, created the Cas 9 nuclease and a piece of single guide RNA that matches one or more pieces of DNA in the virus genome.

The result is a plant with its own ability to detect the insertion of the virus into its cells by matching the single guide RNA it has created to the matching DNA section in the virus genome. The single guide RNA will pair with virus DNA and attract the Cas 9 nuclease that will create a double stranded DNA break in the virus genome. The chances of the virus recovering and being able to go about using the plant cells to complete its replication and transmission to other parts of the plant have been dealt a heavy blow.

Boosting Plant Immunity Figure 1

Figure 1 from article. The resistant plant creates its own Cas 9 nuclease and single guide RNA to restrict replication of the virus genome within the plant’s infected cells.

The three research papers reviewed used these methods, targeting different parts of the virus genome, with good effect. One research paper targeted a piece of DNA that is conserved across three different viruses and was able to lower the titre of the virus in the plant, an exciting development.

Discussion of the benefits and pitfalls for the techniques studied

But this wasn’t just a review touting the benefits – it also looked at the possible negative consequences. These include the possibility of the mutagenic properties of the Cas 9 nuclease causing breaks at non-target parts of the plant’s own genetic material given enough time. There is the also the possibility of mutations in the code for the single guide RNA which would reduce the effectiveness against the virus but also open the possibility of off-target breaks.

Finally, just as thermodynamics always trumps perpetual motion machines, evolution trumps the long term stability of our treatment of too many diseases. With continual attack on a particular segment of the target virus by the Cas 9 nuclease, the selective pressure on the virus will see any alteration which helps the virus evade the attack proliferate among the virus species.

The conclusion is upbeat, and so it should be. Adapting vulnerable plants to use the CRISPR/Cas 9 system as an immune response has significant potential to improve crop production rates. But reducing the selective pressure on virus evolution and monitoring any defect in the systemic expression of the nuclease are areas that require larger studies to guide our use of this technology.

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