Engineering Virus-Resistant Plants

Based on the reported results, “I believe that the CRISPR-Cas9 system can be extensively applied as a new weapon at a molecular level to protect plants from DNA viruses,” Jianwei Zhang of the University of Arizona and the Arizona Genomics Institute, who was not involved in the work, wrote in an email to The Scientist.

Magdy Mahfouz, an assistant professor at the Center for Desert Agriculture at King Abdullah University, and his colleagues used CRISPR to target the tomato yellow leaf curl virus (TYLCV), a single-stranded DNA virus that infects tomato plants. Thus far, there are no adequate ways to control TYLCV, the most destructive tomato pathogen, according to Zhang. The present study, while not the first to use the CRISPR genome-editing tool to target and degrade plant viral DNA, is the first to demonstrate the ability to curtail TYLCV infection.

“We [show] the portability of the CRISPR/Cas9 system to confer molecular immunity in plants against invading viruses,” Mahfouz wrote in an email. “Two [other] reports just came out with similar conclusions, which is quite reassuring to the community.”

“That these three studies targeting CRISPR/Cas9 against three different DNA viruses all come to the same conclusion suggests that this is a broadly applicable system,” said Daniel Voytas, director of the Center for Genome Engineering at the University of Minnesota, a coauthor on one of the other studies.

Mahfouz’s team delivered single guide RNA (sgRNA) targeting TYLCV sequences or a control sequence using a tobacco rattle virus infection into N. benthamiana plants that overexpress the Cas9 endonuclease. After challenging these plants with TYLCV using two different methods, plants expressing the virus-targeted sgRNA had few accumulated TYLCVs compared to those plants expressing a sgRNA targeting a control sequence. Rather than just interfering with viral DNA replication, the researchers confirmed that the CRISPR/Cas9 editing resulted in cleavage of the viral genome, through sgRNA-directed endonuclease activity. Transfection of an RNA virus expressing two TYLCV-targeting sgRNAs resulted in further reduction in TYLCV and plant disease symptoms after inoculation with TYLCV. “This system allows targeting of multiple viruses simultaneously,” Mahfouz wrote.

According to study coauthor Zahir Ali, a postdoc in the Mahfouz laboratory, the team is now working to create plant lines that can pass on the CRISPR-mediated viral resistance to the next generation. The researchers are also testing the expression of the CRISPR/Cas9 machinery in tomato plants. “We are now applying this technology in key crop species, including tomato, for virus resistance,” Mahfouz noted.

The current process introduces foreign DNA into the plant genome, which may classify such plants as genetically modified organisms (GMOs). “We have thought about ways to do this in a non-GMO way but, for now, these are not obvious because you need all of the elements in the plant, poised and ready to take on an infecting virus,” said Voytas.

Both Yang and Voytas see clear benefit in improving food sources using CRISPR. “This is a strategy that could increase productivity, save billions in crop losses, and provide sustainable food for those around the world,” said Voytas.

Z. Ali et al., “CRISPR/Cas9-mediated viral interference in plants,” Genome Biology, doi:10.1186/s13059-015-0799-6, 2015. 

N.J. Baltes et al., “Conferring resistance to geminiviruses with the CRISPR–Cas prokaryotic immune system,” Nature Plants, doi:10.1038/nplants.2015.145, 2015.

X. Ji et al., “Establishing a CRISPR–Cas-like immune system conferring DNA virus resistance in plants,” Nature Plants, doi:10.1038/nplants.2015.144, 2015.