Plant viruses are complexes of protein and nucleic acid (DNA or RNA) that rely mostly on host cells for their propagation. Most economically important crops are infected with viruses, causing serious diseases that are responsible for significant decreases in both the yield and quality of harvests worldwide. Unlike other plant pathogens, viruses cannot be controlled effectively by the application of chemicals, and current control measures rely mostly on preventative measures and insect vector control. New breeding technologies, most notably CRISPR/Cas technology, are shown to be a powerful alternative to engineer resistance against plant viruses.
The DNA nuclease Cas9 from Streptococcus pyogenes (SpCas9) was the first Cas effector to be adapted as a genome engineering tool. In the natural system, an RNA complex (comprised of a crRNA and tracrRNA) directs a cluster of Cas9 nuclease proteins to cleave invading double-stranded DNA (dsDNA), which essentially results in target interference. For most applications in genome editing, the crRNA and tracrRNA are fused into a single guide-RNA (sgRNA) with a specific 20 nucleotide spacer sequence complementary to a DNA target. In essence, genome editing relies on this RNA-guided Cas nuclease to create a double-stranded break at the target site, and the error-prone endogenous DNA repair mechanisms of the host cell, to introduce a single or multiple insertion/deletion (indel) mutation in the targeted DNA. Such indels can disrupt the reading frame of a gene and thus lead to non-functional proteins.
Since the discovery of the SpCas9 system, related systems in many different bacterial and archaeal species have been discovered. Using primarily the functionality of the effector molecules, these systems have been grouped into different classes, types and subtypes. For the sake of simplicity, the systems can be separated into Cas used for DNA editing (e.g., Cas9 and Cas12) and Cas for RNA editing (e.g., RCas9, FnCas9, and Cas13).
CRISPR/Cas can be used for the introduction of virus resistance following one of two broad strategies: firstly, by targeting the DNA or RNA virus genomes directly; or secondly by targeting plant genes, called susceptibility (S) factors. S factors are host proteins that viruses use to replicate and complete their lifecycle in the plant. By modifying these S genes with genome editing, we can limit their availability and therefore mitigate the pathogenicity of viruses in plants.
While a number of successes for the use of CRISPR/Cas to combat plant viruses have been reported, several aspects need to be improved; one of which is the delivery of CRISPR/Cas components into cells. Ironically, the use of viruses for this purpose is an exciting possibility which have so far been applied in a few cases. A major advantage of this approach is that editing can be affected in a GMO-free way since foreign or additional DNA is not inserted.
We believe that CRISPR/Cas technology will soon play a major role in generating disease resistant food crops. In doing so, a significant contribution would be made in securing the sustainable food supplies urgently needed, particularly in the light of world population expansion projected for the middle of this century.
Gaëlle Robertson, Johan Burger and Manuela Campa published this article in Molecular Plant Pathology:
TITLE IMAGE: Plant viruses infect most economically important crops and are responsible for signficant worldwide production losses. The lower image demonstrates CRISPR/Cas strategies which could be used to target plant viruses. All images used with permission of the author.