The glycoprotein gene of Chrysanthemum stem necrosis virus and Zucchini lethal chlorosis virus and molecular relationship with other tospoviruses

The complete genome of the tospovirus Zucchini lethal chlorosis virus

BACKGROUND

Zucchini lethal chlorosis virus (ZLCV) causes significant losses in the production of cucurbits in Brazil. This virus belongs to the genus Tospovirus (family Bunyaviridae) and seems to be exclusively transmitted by Frankliniella zucchini (Thysanoptera). Tospoviruses have a tripartite and single-stranded RNA genome classified as S (Small), M (Medium) and L (Large) RNAS. Although ZLCV was identified as a member of the genus Tospovirus in 1999, its complete genome had not been sequenced until now.

FINDINGS

We sequenced the full-length genome of two ZLCV isolates named ZLCV-SP and ZLCV-DF. The phylogenetic analysis showed that ZLCV-SP and ZLCV-DF clustered with the previously reported isolate ZLCV-BR09. Their proteins were closely related, except the non-structural protein (NSm), which was highly divergent (approximately 90 % identity). All viral proteins clustered similarly in our phylogenetic analysis, excluding that these ZLCV isolates have originated from reassortment events of different tospovirus species.

CONCLUSION

Here we report for the first time the complete genome of two ZLCV isolates that were found in the field infecting zucchini and cucumber.

Genetic and serological characterization of chrysanthemum stem necrosis virus, a member of the genus Tospovirus

Chrysanthemum stem necrosis virus (CSNV) is a member of a tentative tospovirus species. In this study, the complete genomic sequence of the Japanese CSNV isolate TcCh07A was determined. The L RNA is 8960 nt long and encodes the 331.0-kDa RNA-dependent RNA polymerase. The M RNA is 4828 nt long and encodes the 34.1-kDa movement protein (NSm) and the 127.7-kDa glycoprotein precursor (Gn/Gc). The S RNA is 2949 nt long and encodes the 52.4-kDa silencing suppressor protein (NSs) and the 29.3-kDa nucleocapsid (N) protein. The N protein of CSNV-TcCh07A was purified from virus-infected plant tissues and used for production of a rabbit polyclonal antiserum (RAs) and a monoclonal antibody (MAb). Results of serological tests by indirect ELISA and western blotting using the prepared RAs and MAb and a previously produced RAs against the N protein of tomato spotted wilt virus (TSWV) indicated that CSNV-TcCh07A, TSWV, tomato chlorotic spot virus, groundnut ringspot virus, alstroemeria necrotic streak virus and impatiens necrotic spot virus are serologically related.

The complete nucleotide sequence of chrysanthemum stem necrosis virus

The complete genome sequence of chrysanthemum stem necrosis virus (CSNV) was determined using Roche 454 next-generation sequencing. CSNV is a tentative member of the genus Tospovirus within the family Bunyaviridae, whose members are arthropod-borne. This is the first report of the entire RNA genome sequence of a CSNV isolate. The large RNA of CSNV is 8955 nucleotides (nt) in size and contains a single open reading frame of 8625 nt in the antisense arrangement, coding for the putative RNA-dependent RNA polymerase (L protein) of 2874 aa with a predicted Mr of 331 kDa. Two untranslated regions of 397 and 33 nt are present at the 5' and 3' termini, respectively. The medium (M) and small (S) RNAs are 4830 and 2947 nt in size, respectively, and show 99 % identity to the corresponding genomic segments of previously partially characterized CSNV genomes. Protein sequences for the precursor of the Gn/Gc proteins, N and NSs, are identical in length in all of the analysed CSNV isolates.

A New Tospovirus sp. in Cucurbit Crops in Mexico

During the 2007 growing season, melon (Cucumis melo) samples from the state of Guerrero in Mexico showing mosaic and other virus-like symptoms were collected for analysis. Electron microscopic examination of negatively stained leaf-dip extracts revealed the presence of abundant virus-like particles with features characteristic of the family Bunyaviridae. No other viral particles were observed in these preparations. However, enzyme-linked immunosorbent assays (ELISAs) specific for the most common Tospovirus spp. gave negative results. Antibodies raised against purified nucleocapsids reacted specifically with the infected leaf extracts in Western blots and double-antibody sandwich ELISA. The viral RNA was used as a template for a cDNA library, and nucleotide sequence analysis identified cloned cDNAs representing sequences corresponding to the three Tospovirus genome segments. Sequence comparisons showed that the new virus had the highest similarity to Chrysanthemum stem necrosis virus (CSNV). Phylogenetic analysis of two genome regions confirmed that this virus, provisionally named Melon severe mosaic virus (MeSMV), is a previously undescribed Tospovirus sp. belonging to the "new world" clade of Tospovirus spp. An initial survey of various cucurbit crops in various states of Mexico confirmed the widespread occurrence of this virus.

Biological and molecular characterization of tospoviruses in Thailand

Twenty-eight isolates of tospoviruses associated with tomato, pepper, cucurbits, peanut, and Physalis plants collected from fields in different regions of Thailand were characterized. On the basis of N gene and protein sequence relationships, three tospoviruses were identified, namely Watermelon silver mottle virus (WSMoV), Capsicum chlorosis virus (CaCV), and Melon yellow spot virus (MYSV). CLUSTAL analysis of selected N protein sequences showed different isolates of CaCV in three distinct clades. Based on necrosis symptoms on tomato and their 93% identity to CaCV isolates in the other two clades, CaCV-TD8, CaCV-AIT and CaCV-KS16-Thailand tomato tospovirus were designated as CaCV-tomato necrosis strain. A phylogenetic tree based on the 413-amino-acid Gc fragment of the CaCV-Pkk isolate supported the existence of three distinct CaCV clades. Vigna unguiculata produced concentric rings useful for discriminating the Thai CaCV peanut isolates from tomato or pepper isolates. By using reverse transcription polymerase chain reaction with species-specific primers, the three tospoviruses could be detected in mixed infections in watermelon and Physalis, as well as in the bodies of thrips vectors, Thrips palmi and Scirtothrips dorsalis, collected from fields.