The greening of virology: Plant Virology Satellite Symposium, American Society for Virology, UC-Davis, July 12, 2003.

The American Society for Virology (ASV) annual meeting serves as a yearly gathering point for U.S. and international scientists conducting molecular studies on plant viruses. This meeting has recently been enhanced by the inclusion of a satellite symposium highlighting advances in plant virology that precedes the formal meeting. This year’s day-long symposium, held on July 12, 2003 and attended by some 200 virologists, covered host defense mechanisms, novel insights into viral symptom induction, virus replication and movement, and the use of plants to produce edible vaccines. The symposium began with a lecture by Dr. George Bruening (UC-Davis), who reviewed his laboratory’s studies of the 359-nt satellite (sat) RNA of Tobacco ringspot virus. Infected tissue accumulates monomeric and oligomeric linear and circular forms of both plus and minus polarities, with linear molecules being encapsidated. The circular forms suggested rolling circle transcription for both polarities of the RNA, with cleavage of oligomeric transcripts to the unit length and ligation to form circles as other steps in the replication scheme. Ligation of plus polarity RNA was not observed in vitro and was studied in vivo by expression from a geminivirus (single-stranded DNA virus) vector. On the other hand, ligation of minus RNA and cleavage of both polarities of oligomeric RNA were observed in vitro. Furthermore, these steps were shown to be RNA-mediated, with no protein requirement. Mutational analyses revealed that a few dozen nucleotides are sufficient for each reaction. These sequences are sources of the hammerhead and hairpin ribozymes now being applied in gene regulation and antiviral strategies in animal and plant systems. This is an excellent example of an advance that originated in plant virology but is now widely applied in many areas of research and biotechnology. RNA interference (RNAi) and its relationship to plant virus infections has over the last few years been one of the most rapidly developing fields in plant virology and was a focus of this year’s symposium. Dr. Jozsef Burgyan (Agricultural Biotechnology Center, Godollo, Hungary) presented the results of a survey of cloned 21-nt-long interfering RNAs (siRNAs) present in plants infected with Cymbidium ringspot tombusvirus. Most of the siRNAs (80%) were derived from the positive-strand viral RNA, with a distribution that did not randomly represent the entirety of the viral RNA. This led to the provocative suggestion that siRNAs in this system predominantly originate by direct Dicer (the ribonuclease responsible for the generation of siRNAs) cleavage of imperfect duplexes in the most folded regions of the positive-sense genomic RNA. This is analogous to the production of miRNAs and contrasts with other recent interpretations in which siRNAs arise from double-stranded RNA that is either produced by viral transcription events or by the action of host RNA-dependent RNA polymerase. Dr. Burgyan also described studies on the properties of p19, the viral protein that suppresses the silencing response of the host. Biochemical and X-ray crystallographic experiments indicate that p19 binds perfect and near-perfect 21-nt siRNA duplexes. This sequesters the RNAs that are central to the silencing response, as indicated by the observation that almost all siRNAs present in infected plants were found in p19–siRNA complexes. Dr. Hong-Wei Li (laboratory of Dr. Show-Wei Ding, UC-Riverside) described the application of experience gained with plant viruses to uncover virus-induced gene silencing and the existence of a viral silencing suppressor in * Corresponding author. Department of Microbiology, 220 Nash Hall, Oregon State University, Corvallis, OR 97331-3804, USA. Fax: 1-541737-0496. E-mail address: theo.dreher@orst.edu (T.W. Dreher) . R Available online at www.sciencedirect.com