vsiRNA18 derived from tobacco curly shoot virus can regulate virus infection in Nicotiana benthamiana

Virus-derived small interfering RNAs (vsiRNAs) play important roles in regulating host endogenous gene expression to promote virus infection and induce RNA silencing to suppress virus infection. However, to date, how vsiRNAs affect geminivirus infection in host plants has been less studied. In this study, we found that tobacco curly shoot virus (TbCSV)-derived vsiRNA18 (TvsiRNA18) can regulate TbCSV infection in Nicotiana benthamiana plants. The virus-mediated small RNA expression system and stable transformation technique were used to clarify the molecular role of TvsiRNA18 in TbCSV infection. The results indicate that TvsiRNA18 can aggravate disease symptoms in these plants and enhance viral DNA accumulation. ATP-dependent RNA helicase (ATP-dRH) was proven to be a target of TvsiRNA18, and down-regulation of ATP-dRH in plants was shown to induce virus-like leaf curling symptoms and increase TbCSV infection. These results suggest that TvsiRNA18 is an important regulator of TbCSV infection by suppressing ATP-dRH expression. This is the first report to demonstrate that TbCSV-derived vsiRNA can target host endogenous genes to affect symptom development, which helps to reveal the molecular mechanism of symptom occurrence after the virus infects the host.

Temporal Dynamic of the Ratio between Monopartite Begomoviruses and Their Associated Betasatellites in Plants, and Its Modulation by the Viral Gene βC1

The begomovirus-betasatellite complex constantly threatens crops in Asia. However, the quantitative relationship between begomoviruses and betasatellites remains largely unknown. The quantities of tobacco curly shoot virus (TbCSV) and its betasatellite (TbCSB) and their ratio varied significantly in initial infection, and thereafter, the ratio tended to become constant. The TbCSB/TbCSV ratio in agrobacteria inoculum significantly affected that in plants in the initial infection but not thereafter. Null-mutation of βC1 that encodes a multifunctional protein important for pathogenesis in TbCSB significantly reduced the TbCSB/TbCSV ratio in plants. Viral inoculum plants with higher TbCSB/TbCSV ratios promoted whitefly transmission of the virus. The expression of AV1 encoded by TbCSV, βC1 encoded by TbCSB and the βC1/AV1 ratio varied significantly in the initial infection and thereafter the ratio tended to become constant. Additionally, the temporal dynamics of the ratio between another begomovirus and its betasatellite was similar to that of TbCSV and was positively regulated by βC1. These results indicate that the ratio between monopartite begomoviruses and betasatellites tend to become constant as infection progresses, and is modulated by βC1, but a higher betasatellite/begomovirus ratio in virally inoculated plants promotes virus transmission by whiteflies. Our findings provide novel insights into the association between begomoviruses and betasatellites.

Characterization of Pathogenicity-Associated V2 Protein of Tobacco Curly Shoot Virus

V2 proteins encoded by some whitefly-transmitted geminiviruses were reported to be functionally important proteins. However, the functions of the V2 protein of tobacco curly shoot virus (TbCSV), a monopartite begomovirus that causes leaf curl disease on tomato and tobacco in China, remains to be characterized. In our report, an Agrobacterium infiltration-mediated transient expression assay indicated that TbCSV V2 can suppress local and systemic RNA silencing and the deletion analyses demonstrated that the amino acid region 1-92 of V2, including the five predicted α-helices, are required for local RNA silencing suppression. Site-directed substitutions showed that the conserved basic and ring-structured amino acids in TbCSV V2 are critical for its suppressor activity. Potato virus X-mediated heteroexpression of TbCSV V2 in Nicotiana benthamiana induced hypersensitive response-like (HR-like) cell death and systemic necrosis in a manner independent of V2's suppressor activity. Furthermore, TbCSV infectious clone mutant with untranslated V2 protein (TbCSV∆V2) could not induce visual symptoms, and coinfection with betasatellite (TbCSB) could obviously elevate the viral accumulation and symptom development. Interestingly, symptom recovery occurred at 15 days postinoculation (dpi) and onward in TbCSV∆V2/TbCSB-inoculated plants. The presented work contributes to understanding the RNA silencing suppression activity of TbCSV V2 and extends our knowledge of the multifunctional role of begomovirus-encoded V2 proteins during viral infections.

Suppression of nbe-miR1919c-5p Expression in Nicotiana benthamiana Enhances Tobacco Curly Shoot Virus and Its Betasatellite Co-Infection

MicroRNAs (miRNAs) are non-coding but functional RNA molecules of 21-25 nucleotides in length. MiRNAs play significant regulatory roles in diverse plant biological processes. In order to decipher the relationship between nbe-miR1919c-5p and the accumulations of tobacco curly shoot virus (TbCSV) and its betasatellite (TbCSB) DNAs, as well as viral symptom development, we investigated the function of nbe-miR1919c-5p during TbCSV and TbCSB co-infection in plants using a PVX-and a TRV-based short tandem target mimic (STTM) technology. Suppression of nbe-miR1919c-5p expression using these two technologies enhanced TbCSV and TbCSB co-infection-induced leaf curling symptoms in Nicotiana benthamiana plants. Furthermore, suppression of nbe-miR1919c-5p expression enhanced TbCSV and TbCSB DNA accumulations in the infected plants. Our results can advance our knowledge on the nbe-miR1919c-5p function during TbCSV and TbCSB co-infection.

Iterons Homologous to Helper Geminiviruses Are Essential for Efficient Replication of Betasatellites

Betasatellites associated with geminiviruses can be replicated promiscuously by distinct geminiviruses but exhibit a preference for cognate helper viruses. However, the cis elements responsible for betasatellite origin recognition have not been characterized. In this study, we identified an iteron-like repeated sequence motif, 5'-GAGGACC-3', in a tobacco curly shoot betasatellite (TbCSB) associated with tobacco curly shoot virus (TbCSV). Competitive DNA binding assays revealed that two core repeats (5'-GGACC-3') are required for specific binding to TbCSV Rep; TbCSB iteron mutants accumulated to greatly reduced levels and lost the cognate helper-mediated replication preference. Interestingly, TbCSV also contains identical repeated sequences that are essential for specific Rep binding and in vivo replication. In order to gain insight into the mechanism by which TbCSB has acquired the cognate iterons, we performed a SELEX (systematic evolution of ligands by exponential enrichment) assay to identify the high-affinity Rep binding ligands from a large pool of randomized sequences. Analysis of SELEX winners showed that all of the sequences contained at least one core iteron-like motif, suggesting that TbCSB has evolved to contain cognate iterons for high-affinity Rep binding. Further analyses of various betasatellite sequences revealed a region upstream of the satellite conserved region replete with iterative sequence motifs, including species-specific repeats and a general repeat (5'-GGTAAAT-3'). Remarkably, the species-specific repeats in many betasatellites are homologous to those in their respective cognate helper begomoviruses, whereas the general repeat is widespread in most of the betasatellite molecules analyzed. These data, taken together, suggest that many betasatellites have evolved to acquire homologous iteron-like sequences for efficient replication mediated by cognate helper viruses.IMPORTANCE The geminivirus-encoded replication initiator protein (Rep) binds to repeated sequence elements (also known as iterons) in the origin of replication that serve as essential cis elements for specific viral replication. Betasatellites associated with begomoviruses can be replicated by cognate or noncognate helper viruses, but the cis elements responsible for betasatellite origin recognition have not been characterized. Using a betasatellite (TbCSB) associated with tobacco curly shoot virus (TbCSV) as a model, we identify two tandem repeats (iterons) in the Rep-binding motif (RBM) that are required for specific Rep binding and efficient replication, and we show that identical iteron sequences present in TbCSV are also necessary for Rep binding and the replication of helper viruses. Extensive analysis of begomovirus/betasatellite sequences shows that many betasatellites contain iteron-like elements homologous to those of their respective cognate helper begomoviruses. Our data suggest that many betasatellites have evolved to acquire homologous iteron-like sequences for efficient replication mediated by cognate helper viruses.