Detection and epidemic dynamic of ToCV and CCYV with Bemisia tabaci and weed in Hainan of China.

BACKGROUND: In recent years, two of the crinivirus, Tomato chlorosis virus (ToCV) and Cucurbit chlorotic yellows virus (CCYV) have gained increasing attention due to their rapid spread and devastating impacts on vegetable production worldwide. Both of these viruses are transmitted by the sweet potato whitefly, Bemisia tabaci (Gennadius), in a semi-persistent manner. Up to now, there is still lack of report in Hainan, the south of China. METHODS: We used observational and experimental methods to explore the prevalence and incidence dynamic of CCYV and ToCV transmitted by whiteflies in Hainan of China. RESULTS: In 2016, the chlorosis symptom was observed in the tomato and cucumber plants with a large number of B. tabaci on the infected leaves in Hainan, China, with the incidence rate of 69.8% and 62.6% on tomato and cucumber, respectively. Based on molecular identification, Q biotype was determined with a viruliferous rate of 65.0% and 55.0% on the tomato and cucumber plants, respectively. The weed, Alternanthera philoxeroides near the tomato and cucumber was co-infected by the two viruses. Furthermore, incidence dynamic of ToCV and CCYV showed a close relationship with the weed, Alternanthera philoxeroides, which is widely distributed in Hainan. CONCLUSION: Our results firstly reveal that the weed, A. philoxeroides is infected by both ToCV and CCYV. Besides, whiteflies showed a high viruliferous rate of ToCV and CCYV. Hainan is an extremely important vegetable production and seed breeding center in China. If the whitefly can carry these two viruses concurrently, co-infection in their mutual host plants can lead to devastating losses in the near future.

Double Subgenomic Promoter Control for a Target Gene Superexpression by a Plant Viral Vector.

Several new deconstructed vectors based on a potexvirus genome sequence for efficient expression of heterologous proteins in plants were designed. The first obtained vector (AltMV-single), based on the Alternanthera mosaic virus (AltMV) strain MU genome, bears a typical architecture for deconstructed plant viral vectors, i.e. a triple gene block was deleted from the viral genome and the model gene of interest was placed under control of the first viral subgenomic promoter. To enhance the efficiency of expression, maintained by the AltMV-single, another vector (AltMV-double) was designed. In AltMV-double, the gene of interest was controlled by two viral subgenomic promoters located sequentially without a gap upstream of the target gene. It was found that AltMV-double provided a significantly higher level of accumulation of the target protein in plants than AltMV-single. Moreover, our data clearly show the requirement of the presence and functioning of both the subgenomic promoters for demonstrated high level of target protein expression by AltMV-double. Taken together, our results describe an additional possible way to enhance the efficiency of transient protein expression maintained in plants by a plant viral vector.

Effect of host species on the distribution of mutational fitness effects for an RNA virus.

Knowledge about the distribution of mutational fitness effects (DMFE) is essential for many evolutionary models. In recent years, the properties of the DMFE have been carefully described for some microorganisms. In most cases, however, this information has been obtained only for a single environment, and very few studies have explored the effect that environmental variation may have on the DMFE. Environmental effects are particularly relevant for the evolution of multi-host parasites and thus for the emergence of new pathogens. Here we characterize the DMFE for a collection of twenty single-nucleotide substitution mutants of Tobacco etch potyvirus (TEV) across a set of eight host environments. Five of these host species were naturally infected by TEV, all belonging to family Solanaceae, whereas the other three were partially susceptible hosts belonging to three other plant families. First, we found a significant virus genotype-by-host species interaction, which was sustained by differences in genetic variance for fitness and the pleiotropic effect of mutations among hosts. Second, we found that the DMFEs were markedly different between Solanaceae and non-Solanaceae hosts. Exposure of TEV genotypes to non-Solanaceae hosts led to a large reduction of mean viral fitness, while the variance remained constant and skewness increased towards the right tail. Within the Solanaceae hosts, the distribution contained an excess of deleterious mutations, whereas for the non-Solanaceae the fraction of beneficial mutations was significantly larger. All together, this result suggests that TEV may easily broaden its host range and improve fitness in new hosts, and that knowledge about the DMFE in the natural host does not allow for making predictions about its properties in an alternative host.

Colour break in reverse bicolour daffodils is associated with the presence of Narcissus mosaic virus.

BACKGROUND: Daffodils (Narcissus pseudonarcissus) are one of the world's most popular ornamentals. They also provide a scientific model for studying the carotenoid pigments responsible for their yellow and orange flower colours. In reverse bicolour daffodils, the yellow flower trumpet fades to white with age. The flowers of this type of daffodil are particularly prone to colour break whereby, upon opening, the yellow colour of the perianth is observed to be 'broken' into patches of white. This colour break symptom is characteristic of potyviral infections in other ornamentals such as tulips whose colour break is due to alterations in the presence of anthocyanins. However, reverse bicolour flowers displaying colour break show no other virus-like symptoms such as leaf mottling or plant stunting, leading some to argue that the carotenoid-based colour breaking in reverse bicolour flowers may not be caused by virus infection. RESULTS: Although potyviruses have been reported to cause colour break in other flower species, enzyme-linked-immunoassays with an antibody specific to the potyviral family showed that potyviruses were not responsible for the occurrence of colour break in reverse bicolour daffodils. Colour break in this type of daffodil was clearly associated with the presence of large quantities of rod-shaped viral particles of lengths 502-580 nm in tepals. Sap from flowers displaying colour break caused red necrotic lesions on Gomphrena globosa, suggesting the presence of potexvirus. Red necrotic lesions were not observed in this indicator plant when sap from reverse bicolour flowers not showing colour break was used. The reverse transcriptase polymerase reactions using degenerate primers to carla-, potex- and poty-viruses linked viral RNA with colour break and sequencing of the amplified products indicated that the potexvirus Narcissisus mosaic virus was the predominant virus associated with the occurrence of the colour break. CONCLUSIONS: High viral counts were associated with the reverse bicolour daffodil flowers that were displaying colour break but otherwise showed no other symptoms of infection. Narcissus mosaic virus was the virus that was clearly linked to the carotenoid-based colour break.

Diverse and recombinant DNA betasatellites are associated with a begomovirus disease complex of Digera arvensis, a weed host.

Weeds are considered as a source of new viruses and reservoirs of economically important viruses but are often neglected during diversity studies. Here, we report the complete nucleotide sequences and phylogenetic analyses of the components of a begomovirus disease complex associated with yellow vein disease of Digera arvensis, a common weed. The begomovirus associated with the disease showed 98% nucleotide sequence identity with Cotton leaf curl Rajasthan virus. Two species of betasatellite were identified. The first betasatellite species was an isolate of Ageratum yellow leaf curl betasatellite. The second was a recombinant consisting for the most part of sequence derived from a Tobacco leaf curl betasatellite but with the satellite conserved region (SCR) and some sequence between the SCR and adenine-rich (A-rich) region derived from a Cotton leaf curl Multan betasatellite. The alphasatellite isolated from this weed was near identical to an isolate recently characterized from potato. The presence of multiple and recombinant betasatellites in D. arvensis indicates that weeds can be important sources of multiple begomovirus components that affect crop plants. Furthermore, the presence of a recombinant betasatellite suggested that weeds are likely vessels for recombination and evolution of components of begomovirus complexes.

Beet soil-borne mosaic virus RNA-3 is replicated and encapsidated in the presence of BNYVV RNA-1 and -2 and allows long distance movement in Beta macrocarpa.

Beet soil-borne mosaic virus (BSBMV) and Beet necrotic yellow vein virus (BNYVV) belong to the Benyvirus genus. BSBMV has been reported only in the United States, while BNYVV has a worldwide distribution. Both viruses are vectored by Polymyxa betae and possess similar host ranges, particle number and morphology. BNYVV and BSBMV are not serologically related but they have similar genomic organizations. Field isolates usually consist of four RNA species but some BNYVV isolates contain a fifth RNA. RNAs 1 and 2 are essential for infection and replication while RNAs 3 and 4 play important roles in plant and vector interactions, respectively. Nucleotide and amino acid analyses revealed that BSBMV and BNYVV are sufficiently different to be classified as two species. Complementary base changes found within the BSBMV RNA-3 5' UTR made it resemble to BNYVV 5' RNA-3 structure whereas the 3' UTRs of both species were more conserved. cDNA clones were obtained, and allowed complete copies of BSBMV RNA-3 to be trans-replicated, trans-encapsidated by the BNYVV viral machinery. Long-distance movement was observed indicating that BSBMV RNA-3 could substitute BNYVV RNA-3 for systemic spread, even though the p29 encoded by BSBMV RNA-3 is much closer to the RNA-5-encoded p26 than to BNYVV RNA-3-encoded p25. Competition occurred when BSBMV RNA-3-derived replicons were used together with BNYVV-derived RNA-3 but not when the RNA-5-derived component was used. Exploitation of the similarities and divergences between BSBMV and BNYVV should lead to a better understanding of molecular interactions between Benyviruses and their hosts.

Natural infection of Alternanthera tenella (Amaranthaceae) by a new potyvirus.

A virus was isolated from joyweed (Alternanthera tenella Colla-Amaranthaceae), a common weed in tropical and sub-tropical regions. Examination by electron microscopy showed long flexuous particles with an average length of 756 nm in crude sap. Serological results showed positive reaction with antisera to PVY-O. A fragment of 1772 nucleotides was sequenced. The CP sequence shares 76% of identity with the CP of Potato virus Y strain NTN. These results confirm that the virus is a new potyvirus infecting A. tenella, and the name Alternanthera mild mosaic virus (AltMMV) is proposed.

Identification and full sequence of an isolate of Alternanthera mosaic potexvirus infecting Phlox stolonifera.

A potexvirus was isolated from creeping phlox (Phlox stolonifera) plants from a commercial nursery in Pennsylvania. The virus was serologically related to clover yellow mosaic virus, plantain virus X, potato virus X, and potato aucuba mosaic virus, and was most closely related to papaya mosaic virus (PapMV). The sequence of a PCR fragment obtained with potexvirus group-specific primers was distinct from that of PapMV; the coat protein (CP) gene and 3' untranslated region (UTR) were closely related to Alternanthera mosaic virus (AltMV), previously reported only from Australia. The host range was similar to that of the Australian isolate (AltMV-Au), and the phlox isolate reacted strongly with antiserum to AltMV-Au. The full sequence of the phlox isolate was more closely related to PapMV throughout the genome than to any potexvirus other than AltMV-Au, for which only the CP and 3'UTR sequences are available. The phlox isolate was therefore named AltMV-PA (for Pennsylvania), and the full 6607 nt sequence is presented(1). Additional AltMV isolates from creeping phlox (AltMV-BR and AltMV-SP) and trailing portulaca (Portulaca grandiflora; AltMV-Po) were also isolated, suggesting that AltMV may be widespread, and may have been mis-diagnosed in the past as PapMV. AltMV has the potential to spread to other ornamental crops.

The p92 polymerase coding region contains an internal RNA element required at an early step in Tombusvirus genome replication.

The replication of positive-strand RNA viral genomes involves various cis-acting RNA sequences. Generally, regulatory RNA sequences are present at or near genomic termini; however, internal replication elements (IREs) also exist. Here we report the structural and functional characterization of an IRE present in the readthrough portion of the p92 polymerase gene of Tomato bushy stunt virus. Analysis of this element in the context of a noncoding defective interfering RNA revealed a functional core structure composed of two noncontiguous segments of sequence that interact with each other to form an extended helical conformation. IRE activity required maintenance of several base-paired sections as well as two distinct structural features: (i) a short, highly conserved segment that can potentially form two different and mutually exclusive structures and (ii) an internal loop that contains a critical CC mismatch. The IRE was also shown to play an essential role within the context of the viral genome. In vivo analysis with novel RNA-based temperature-sensitive genomic mutants and translationally active subgenomic viral replicons revealed the following about the IRE: (i) it is active in the positive strand, (ii) it is dispensable late in the viral RNA replication process, and (iii) it is functionally inhibited by active translation over its sequence. Together, these results suggest that IRE activity is required in the cytosol at an early step in the viral replication process, such as template recruitment and/or replicase complex assembly.