In plants, RNA silencing (RNA interference) is an efficient antiviral system, and therefore successful virus infection requires suppression of silencing. Although many viral silencing suppressors have been identified, the molecular basis of silencing suppression is poorly understood. It is proposed that various suppressors inhibit RNA silencing by targeting different steps. However, as double-stranded RNAs (dsRNAs) play key roles in silencing, it was speculated that dsRNA binding might be a general silencing suppression strategy. Indeed, it was shown that the related aureusvirus P14 and tombusvirus P19 suppressors are dsRNA-binding proteins. Interestingly, P14 is a size-independent dsRNA-binding protein, while P19 binds only 21-nucleotide ds-sRNAs (small dsRNAs having 2-nucleotide 3 overhangs), the specificity determinant of the silencing system. Much evidence supports the idea that P19 inhibits silencing by sequestering silencing-generated viral ds-sRNAs. In this study we wanted to test the hypothesis that dsRNA binding is a general silencing suppression strategy. Here we show that many plant viral silencing suppressors bind dsRNAs. Beet yellows virus Peanut P21, clump virus P15, Barley stripe mosaic virus ␥B, and Tobacco etch virus HC-Pro, like P19, bind ds-sRNAs size-selectively, while Turnip crinkle virus CP is a size-independent dsRNA-binding protein, which binds long dsRNAs as well as ds-sRNAs. We propose that size-selective ds-sRNA-binding suppressors inhibit silencing by sequestering viral ds-sRNAs, whereas size-independent dsRNA-binding suppressors inactivate silencing by sequestering long dsRNA precursors of viral sRNAs and/or by binding ds-sRNAs. The findings that many unrelated silencing suppressors bind dsRNA suggest that dsRNA binding is a general silencing suppression strategy which has evolved independently many times.RNA silencing (termed RNA interference [RNAi] in animals) is an RNA-based eukaryotic gene regulatory system that plays essential roles in many biological processes. RNA silencing is induced by accumulation of double-stranded RNAs (dsRNAs). dsRNAs are first processed by an RNase III-like nuclease called DICER (in plants it is termed DICER-LIKE [DCL]) into (21-to 25-nucleotide [nt]) small dsRNAs (ds-sRNAs) having 2-nt 3Ј overhangs, and then these sRNAs incorporate into different silencing effector complexes. In the active effector complexes sRNAs are present as single-stranded molecules, which guide these complexes to the complementary nucleic acids for suppression (2, 3, 22, 61).In plants, different dsRNA precursors are processed by distinct DCLs into functionally different short (21-to 22-nt) and long (23-to 25-nt) sRNAs (24, 25, 67). Short sRNAs guide a multicomponent nuclease (RNA-induced silencing complex [RISC]) to homologous mRNAs for suppression. RISC cleaves targeted mRNA in the case of (near) perfect base pairing between mRNA and guide RNA. When the guide RNA is only partially complementary to the mRNA, RISC mediates translational repression. Short sRNAs could also provide ...

Nonsense-mediated mRNA decay (NMD) is a eukaryotic quality control mechanism that identifies and eliminates aberrant mRNAs containing a premature termination codon (PTC). Although, key trans-acting NMD factors, UPF1, UPF2 and UPF3 are conserved in yeast and mammals, the cis-acting NMD elements are different. In yeast, short specific sequences or long 3′-untranslated regions (3′-UTRs) render an mRNA subject to NMD, while in mammals' 3′-UTR located introns trigger NMD. Plants also possess an NMD system, although little is known about how it functions. We have elaborated an agroinfiltration-based transient NMD assay system and defined the cis-acting elements that mediate plant NMD. We show that unusually long 3′-UTRs or the presence of introns in the 3′-UTR can subject mRNAs to NMD. These data suggest that both long 3′-UTR-based and intron-based PTC definition operated in the common ancestors of extant eukaryotes (stem eukaryotes) and support the theory that intron-based NMD facilitated the spreading of introns in stem eukaryotes. We have also identified plant UPF1 and showed that tethering of UPF1 to either the 5′- or 3′-UTR of an mRNA results in reduced transcript accumulation. Thus, plant UPF1 might bind to mRNA in a late, irreversible phase of NMD.

Nonsense-mediated mRNA decay (NMD) is a quality control system that degrades mRNAs containing premature termination codons. Although NMD is well characterized in yeast and mammals, plant NMD is poorly understood. We have undertaken the functional dissection of NMD pathways in plants. Using an approach that allows rapid identification of plant NMD trans factors, we demonstrated that two plant NMD pathways coexist, one eliminates mRNAs with long 3 0 UTRs, whereas a distinct pathway degrades mRNAs harbouring 3 0 UTR-located introns. We showed that UPF1, UPF2 and SMG-7 are involved in both plant NMD pathways, whereas Mago and Y14 are required only for intron-based NMD. The molecular mechanism of long 3 0 UTR-based plant NMD resembled yeast NMD, whereas the intron-based NMD was similar to mammalian NMD, suggesting that both pathways are evolutionarily conserved. Interestingly, the SMG-7 NMD component is targeted by NMD, suggesting that plant NMD is autoregulated. We propose that a complex, autoregulated NMD mechanism operated in stem eukaryotes, and that despite aspect of the mechanism being simplified in different lineages, feedback regulation was retained in all kingdoms.

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RNA silencing is a conserved eukaryotic gene regulatory system in which sequence specificity is determined by small RNAs. Plant RNA silencing also acts as an antiviral mechanism; therefore, viral infection requires expression of a silencing suppressor. The mechanism and the evolution of silencing suppression are still poorly understood. Tombusvirus open reading frame (ORF) 5-encoded P19 is a size-selective double-stranded RNA (dsRNA) binding protein that suppresses silencing by sequestering double-stranded small interfering RNAs (siRNAs), the specificity determinant of the antiviral silencing system. To better understand the evolution of silencing suppression, we characterized the suppressor of the type member of Aureusviruses, the closest relatives of the genus Tombusvirus. We show that the Pothos latent virus (PoLV) ORF 5-encoded P14 is an efficient suppressor of both virus-and transgene-induced silencing. Findings that in vitro P14 binds dsRNAs and double-stranded siRNAs without obvious size selection suggest that P14, unlike P19, can suppress silencing by sequestering both long dsRNA and double-stranded siRNA components of the silencing machinery. Indeed, P14 prevents the accumulation of hairpin transcript-derived siRNAs, indicating that P14 inhibits inverted repeat-induced silencing by binding the long dsRNA precursors of siRNAs. However, viral siRNAs accumulate to high levels in PoLV-infected plants; therefore, P14 might inhibit virus-induced silencing by sequestering double-stranded siRNAs. Finally, sequence analyses suggest that P14 and P19 suppressors diverged from an ancient dsRNA binding suppressor that evolved as a nested protein within the common ancestor of aureusvirus-tombusvirus movement proteins.RNA silencing (also termed posttranscriptional gene silencing in plants and RNA interference in animals) is a conserved eukaryotic gene inactivation system that plays regulatory roles in many biological processes including development, maintenance of genome stability, and antiviral responses (2,6,12,25,54). RNA silencing is induced by accumulation of doublestranded RNAs (dsRNAs). dsRNAs are first processed by an RNase III-like nuclease called DICER (in plants termed DICER-LIKE, or DCL) into short (21 to 25 nucleotide [nt]) RNAs, and then these short RNAs incorporate and guide different silencing effector complexes to homologous nucleic acids for suppression (2,6,12,16,25,54). In plants, RNA silencing acts at both single-cell (cell-autonomous silencing) and at whole-plant (systemic silencing) levels. Cell-autonomous silencing inactivates genes in the cells in which dsRNAs accumulated. Moreover, cell-autonomous silencing generates mobile silencing signals that confer suppression of homologous mRNAs in neighboring cells (short distance) and in distant tissues (long-distance systemic silencing) (29,31,32,56).DICERs can process dsRNAs into two functionally different small RNAs, micro-RNAs (miRNAs) and small interfering RNAs (siRNAs). miRNAs are involved in the control of many endogenous protein-encoding mRNAs...