Higher plants employ a homology-dependent RNA-degradation system known as posttranscriptional gene silencing (PTGS) as a defense against virus infection. Several plant viruses are known to encode proteins that can suppress PTGS. Here we show that P0 of beet western yellows virus (BWYV) displays strong silencing suppressor activity in a transient expression assay based upon its ability to inhibit PTGS of green fluorescent protein (GFP) when expressed in agro-infiltrated leaves of Nicotiana benthamiana containing a GFP transgene. PTGS suppressor activity was also observed for the P0s of two other poleroviruses, cucurbit aphid-borne yellows virus and potato leafroll virus. P0 is encoded by the 5-proximal gene in BWYV RNA but does not accumulate to detectable levels when expressed from the genome-length RNA during infection. The low accumulation of P0 and the resulting low PTGS suppressor activity are in part a consequence of the suboptimal translation initiation context of the P0 start codon in viral RNA, although other factors, probably related to the viral replication process, also play a role. A mutation to optimize the P0 translation initiation efficiency in BWYV RNA was not stable during virus multiplication in planta. Instead, the P0 initiation codon in the progeny was frequently replaced by a less efficient initiation codon such as ACG, GTG, or ATA, indicating that there is selection against overexpression of P0 from the viral genome.Most if not all plants possess a defense mechanism against virus infection known as posttranscriptional gene silencing (PTGS; see reference 44 for a recent review). PTGS is a homology-dependent RNA degradation system to recognize and degrade viral RNA (and any homologous transgene mRNA) in the cytoplasm. Mechanistically similar phenomena exist in animals and fungi (reviewed in references 4, 13, and 35). The mechanism by which a virus infection triggers PTGS in plants is not fully understood, but double-stranded RNA is a strong inducer of PTGS (43) and such a form is produced during replication of an RNA virus. Production of small RNAs of ϳ21 to 23 nucleotides, called small interfering RNAs (siRNAs), corresponding to both the plus and minus strands of the target RNA is associated with PTGS (12,13,45). In plants, gene silencing at one site can trigger PTGS in distant tissues and across a graft union (29, 39a). The mobile signal, which must consist at least in part of homologous RNA, is thought to move from cell to cell through plasmodesmata and systemically via the vasculature. Some plant viruses have been shown to encode proteins which can counteract PTGS (8,22,39,40,42). These silencing suppressor proteins may act at different steps in the PTGS pathway. Thus, (i) the potyvirus helper component-proteinase (HCPro) interferes with initiation and maintenance of silencing at a step coincident with or upstream of siRNA production (1,7,16,23,24), (ii) the 2b protein of cucumber mosaic virus (CMV) prevents initiation of PTGS in new growth by inhibiting the long-range PTGS signaling acti...

The full-length nucleotide sequence of the genomic RNA of a new cytorhabdovirus infecting lettuce was determined. Six open reading frames were found in the antigenomic sequence of the 12,926-nt negative-sense viral RNA genome. The genomic organisation was similar to that of lettuce necrotic yellows virus (LNYV), the type member of the genus Cytorhabdovirus: 3'-N-P-3-M-G-L-5', where N is the capsid protein gene, P the putative phosphoprotein gene, 3 a gene coding for a putative protein of unknown function, M the putative matrix protein gene, G the glycoprotein gene, and L the putative polymerase gene. Amino acid sequence comparison with the corresponding sequences of other rhabdoviruses revealed the closest relationship to LNYV, with identities ranging from 41% for the matrix proteins and 65% for the L polymerase proteins. These results indicate that this virus may be a member of a new cytorhabdovirus species, for which the name Lettuce yellow mottle virus (LYMoV) is proposed.

We recently noticed that some figures contained Northern and Western blot images comprising spliced lanes and nonuniform contrast adjustments that were not indicated as such. Upon closer inspection of the published figures and the original material that was still available, we also realized that some lanes of one Western blot and one Northern blot were reused in assembling different figures. In addition, some figures presented signs of inappropriate manipulation. The details of these issues are as follows. Page 6818: In Fig. 3B, lower panel, the 5th lane is a duplication and inversion of the 2nd lane. Pages 6819 and 6821: The first three lanes of Fig. 4B are reused in Fig. 6B. Pages 6820 and 6821: Some lanes of Fig. 6D are reused in Fig. 5C. In Fig. 5C, 6C, and 6D, some lanes with background are duplicated. Page 6821: In Fig. 6C, the contrast is not evenly adjusted. Figures 6C and 6D present spliced lanes that are not indicated as such. While some of these issues were genuine mounting mistakes and could have been corrected by making new figures with the original blots, we could not retrieve all of the original data. Since the publication of the article, we have repeated the experiments and confirmed that its conclusion, P0 protein being a suppressor of RNA silencing, remains valid. Moreover, the data have also been confirmed independently by several other laboratories. Nonetheless, given the nature and extent of some of the manipulations, we decided that the best course of action is to retract this publication. S. Pfeffer was responsible for the mounting of the problematic figures and takes full responsibility for the errors that were made. We apologize for and deeply regret the errors in this publication.