Researchers at the University of California, Riverside, with colleagues at the University of Florida and at UC Davis, have uncovered how viruses circumvent the immune response of plants.

The findings were published in the Nov. 2 issue of the Proceedings of the National Academy of Sciences in a paper titled “Three Distinct Suppressors of RNA Silencing Encoded By a 20-kb Viral RNA Genome.” UC Riverside Associate Professor of Plant Pathology Shou-Wei Ding, at the Center for Plant Cell Biology, and UCR colleagues Rui Lu, Wan-Xiang Li and Michael Shintaku, co-authored the paper with Bryce W. Falk at UC Davis and William O. Dawson at the University of Florida Citrus Research and Education Center.

RNA silencing is a recently discovered defense mechanism against virus infection in plants and invertebrates. For successful infection to occur, viruses must be able to suppress the RNA silencing’s antiviral response.
“Our results demonstrate that citrus tristeza virus (CTV) produces three proteins that are suppressors of RNA silencing and each inhibits RNA silencing in a distinct manner,” said Ding.

CTV is one of the most important virus pathogens affecting citrus worldwide, causing significant economic losses not only from disease, but also from the need to remove CTV-infected trees. Since viral suppressors are also known to interfere with plant development, further analyses of the CTV suppressors will explain why CTV is capable of such destructive effects. One approach for the control of CTV in a number of labs is to genetically engineer virus-resistant citrus trees.

“Our findings will help improving the efficacy of this approach, e.g., by directly targeting the CTV suppressor genes,” Ding said.

“Our work indicates for the first time that viruses may have to produce more than one suppressor of RNA silencing to overcome the antiviral immunity,” he added. “Secondly, one of the CTV suppressors identified is mechanistically novel as it inhibits spread of RNA silencing without interfering with intracellular RNA silencing.”

As a result, that type of suppressor cannot be identified by the methods in wide use today by labs around the world.

The California Citrus Research Board, the UC-Biostar program and the U.S. Department of Agriculture funded the research.

The University of California, Riverside is a major research institution and a national center for the humanities. Key areas of research include nanotechnology, genomics, environmental studies, digital arts and sustainable growth and development. With a current undergraduate and graduate enrollment of nearly 17,000, the campus is projected to grow to 21,000 students by 2010. Located in the heart of inland Southern California, the nearly 1,200-acre, park-like campus is at the center of the region's economic development.

ABSTRACT of the article

Viral infection in both plant and invertebrate hosts requires a virus-encoded function to block the RNA silencing antiviral defense. Here, we report the identification and characterization of three distinct suppressors of RNA silencing encoded by the 20-kb plus-strand RNA genome of citrus tristeza virus (CTV). When introduced by genetic crosses into plants carrying a silencing transgene, both p20 and p23, but not coat protein (CP), restored expression of the transgene. Although none of the CTV proteins prevented DNA methylation of the transgene, export of the silencing signal (capable of mediating intercellular silencing spread) was detected only from the F₁ plants expressing p23 and not from the CP- or p20-expressing F₁ plants, demonstrating suppression of intercellular silencing by CP and p20 but not by p23. Thus, intracellular and intercellular silencing are each targeted by a CTV protein, whereas the third, p20, inhibits silencing at both levels. Notably, CP suppresses intercellular silencing without interfering with intracellular silencing. The novel property of CP suggests a mechanism distinct to p20 and all of the other viral suppressors known to interfere with intercellular silencing and that this class of viral suppressors may not be consistently identified by Agrobacterium coinfiltration because it also induces RNA silencing against the infiltrated suppressor transgene. Our analyses reveal a sophisticated viral counter-defense strategy that targets the silencing antiviral pathway at multiple steps and may be essential for protecting CTV with such a large RNA genome from antiviral silencing in the perennial tree host.

PDF version of the article: http://www.pnas.org/cgi/reprint/101/44/15742.pdf