A putative circular single-stranded DNA (ssDNA) virus was recovered from Hypericum japonicum collected in Vietnam. The viral isolate was tentatively named Hypericum japonicum-associated circular DNA virus (HJasCV). HJasCV shares 58.7-65.4% nucleotide sequence identity with Sclerotinia sclerotiorum hypovirulence-associated DNA virus 1 (SsHADV-1) and SsHADV-1-like viruses. Like this group of viruses, the genome of HJasCV (2 200 nt) has two large ORFs, one in the virion-sense and the other in the complementary-sense DNA. The proteins encoded in the virion-sense and complementary-sense ORFs share 39-46 % and 45-67 % amino acid sequence identity with the putative capsid and replication-associated proteins (Reps), respectively, of SsHADV-1 and SsHADV-1-like viruses. The putative Rep of HJasCV contains all of the motifs related to rolling-circle replication. Its 111-bp intergenic region (IR) contains a hairpin structure with a geminivirus-like nonanucleotide sequence, TAATGTTAT, at the apex of the loop. Phylogenetic analysis revealed that HJasCV forms a monophyletic clade with SsHADV-1 and SsHADV-1-like viruses.

Tomato leaf curl Guangdong virus (ToLCGdV) is a begomovirus associated with a Tomato yellow leaf curl disease (TYLCD) epidemic in Guangdong province, China. Being the least conserved protein among geminivirus proteins, the function of C4 during ToLCGdV infection has not been elucidated. In this study, the infectious clones of ToLCGdV and a ToLCGdV mutant (ToLCGdV mC4 ) with disrupted C4 ORF were constructed. Although ToLCGdV and ToLCGdV mC4 could infect Nicotiana benthamiana and tomato plants, ToLCGdV mC4 elicited much milder symptoms compared with ToLCGdV. To further verify the role of C4 in viral pathogenesis, C4 was expressed in N. benthamiana from Potato virus X (PVX) vector. The results showed that ToLCGdV C4 enhanced the pathogenicity of PVX and induced more severe developmental abnormalities in plants compared with PVX alone or PVX-mC4. In addition, ToLCGdV C4 suppresses systemic gene silencing in the transgenic N. benthamiana line 16c, but not local gene silencing induced by sense GFP in wild-type N. benthamiana plants. Moreover, C4 suppresses transcriptional gene silencing (TGS) by reducing the DNA methylation level of 35S promoter in 16c-TGS N. benthamiana plants. Furthermore, C4 could also interact with the receptor-like kinase (RLK) BARELY ANY MERISTEM 1 (BAM1), suggesting that C4 may suppress gene silencing by interfering with the function of BAM1 in the cell-to-cell spread of RNAi. All these results suggest that C4 is a pathogenic determinant of ToLCGdV, and C4 may suppress post-transcriptional gene silencing (PTGS) by interacting with BAM1.

Ralstonia solanacearum species complex is a devastating phytopathogen with an unusually wide host range, and new host plants are continuously being discovered. In June 2016, a new bacterial wilt on Cucurbita maxima was observed in Guangdong province, China. Initially, in the adult plant stage, several leaves of each plant withered suddenly and drooped; the plant then wilted completely, and the color of their vasculature changed to dark brown, ultimately causing the entire plant to die. Creamy-whitish bacterial masses were observed to ooze from crosscut stems of these diseased plants. To develop control strategies for C. maxima bacterial wilt, the causative pathogenic isolates were identified and characterized. Twenty-four bacterial isolates were obtained from diseased C. maxima plants, and 16S rRNA gene sequencing and pathogenicity analysis results indicated that the pathogen of C. maxima bacterial wilt was Ralstonia solanacearum. The results from DNA-based analysis, host range determination and bacteriological identification confirmed that the 24 isolates belonged to R. solanacearum phylotype I, race 1, and eight of these isolates belonged to biovar 3, while 16 belonged to biovar 4. Based on the results of partial egl gene sequence analysis, the 24 isolates clustered into three egl- sequence type groups, sequevars 17, 45, and 56. Sequevar 56 is a new sequevar which is described for the first time in this paper. An assessment of the resistance of 21 pumpkin cultivars revealed that C. moschata cv. Xiangyu1 is resistant to strain RS378, C. moschata cv. Xiangmi is moderately resistant to strain RS378, and 19 other pumpkin cultivars, including four C. maxima cultivars and 15 C. moschata cultivars, are susceptible to strain RS378. To the best of our knowledge, this is the first report of C. maxima bacterial wilt caused by R. solanacearum race 1 in the world. Our results provide valuable information for the further development of control strategies for C. maxima wilt disease.

RNA silencing is a conserved mechanism in plants that targets viruses. Viral small RNAs (vsiRNAs) can be generated from viral double-stranded RNA replicative intermediates within the infected host, or from host RNA-dependent RNA polymerases activity on viral templates. The abundance and profile of vsiRNAs in viral infections have been reported previously. However, the involvement of vsiRNAs during infection of the Geminiviridae family member cotton leaf curl virus (CLCuD), which causes significant economic losses in cotton growing regions, remains largely uncharacterized. Cotton leaf curl Multan virus (CLCuMuV) associated with a betasatellite called Cotton leaf curl Multan betasatellite (CLCuMuB) is a major constraint to cotton production in South Asia and is now established in Southern China. In this study, we obtained the profiles of vsiRNAs from CLCuMV and CLCuMB in infected upland cotton (Gossypium hirsutum) plants by deep sequencing. Our data showed that vsiRNA that were derived almost equally from sense and antisense CLCuD DNA strands accumulated preferentially as 21- and 22-nucleotide (nt) small RNA population and had a cytosine bias at the 5′-terminus. Polarity distribution revealed that vsiRNAs were almost continuously present along the CLCuD genome and hotspots of sense and antisense strands were mainly distributed in the Rep proteins region of CLCuMuV and in the C1 protein of CLCuMuB. In addition, hundreds of host transcripts targeted by vsiRNAs were predicted, many of which encode transcription factors associated with biotic and abiotic stresses. Quantitative real-time polymerase chain reaction analysis of selected potential vsiRNA targets showed that some targets were significantly down-regulated in CLCuD-infected cotton plants. We also verified the potential function of vsiRNA targets that may be involved in CLCuD infection by virus-induced gene silencing (VIGS) and 5′-rapid amplification of cDNA end (5′-RACE). Here, we provide the first report on vsiRNAs responses to CLCuD infection in cotton.

Wild tomato mosaic virus (WTMV), a potyvirus, has been reported in Laichau, Vietnam, infecting Solanum torvum (wild tomato) in 2008 (3), and Kanchanaburi, Thailand, infecting Capsicum spp. in 2013 (KF250353). In mid-May 2013, Nicotiana tabacum showing yellowing, mosaic, and/or ringspot symptoms were found in natural tobacco fields of Nanxiong, Guangdong Province, China. Total RNA was extracted from symptomatic leaves and reverse transcribed with M4T (5′-GTTTTCCCAGTCACGAC (T)15-3′) as the 3′ anchoring primer (1). The cDNA was used as template in a PCR assay using primers M4: 5′-GTTTTCCCAGTCACGAC-3′ and Sprimer: 5′-GGXAAYAAYAGYGGXCAZCC-3′, which amplifies a region comprising part of the NIb protein gene, the entire coat protein (CP) gene and the 3′ nontranslated region (UTR) of a potyvirus (1). A ~1,700-bp product was amplified from the cDNA derived from three of the five diseased plants. The product (KF639967) showed 87% and 84% nucleotide sequence identities with those of WTMV isolates KAN and Laichau, respectively. The CP deduced from the sequence of the product shared 87% and 86% nucleotide and 94% and 93% amino acid sequence identities with those of WTMV isolates KAN and Laichau, respectively. The 3′-UTR of the putative virus shared 93% and 92% nucleotide sequence identities to those of WTMV isolates KAN and Laichau, respectively. Thus, according to the molecular criteria for potyvirus species demarcation (2), the virus we identified should be an isolate of WTMV (isolate GD1). One of the diseased samples was homogenized in 0.1 mol/liter phosphate buffer (pH 7.0) and used to inoculate the potyvirus to healthy, two to four leaf-stage Capsicum annuum L., N. tabacum, and N. benthamiana. The inoculated, as well as mock-treated plants, which were inoculated only with phosphate buffer, were grown in soil under 12 h day/12 h night at 25°C. All inoculated N. tabacum and N. benthamiana plants developed yellowing and mosaic symptoms by 14 days post inoculation (dpi). For N. benthamiana, the symptom became very severe by 21 dpi and some diseased plants died prematurely. About 10% of inoculated C. annuum L. developed very mild veinal chlorosis 18 dpi. Cloning and sequencing experiments showed that all the symptomatic plants tested were WTMV positive, but Cucumber mosaic virus, Tobacco mosaic virus, and Tobacco etch virus negative. To our knowledge, this is the first report of WTMV in China. Also, it is the first report that WTMV infects Nicotiana spp. Although further experiments are needed to definitively attribute the disease observed in the field to WTMV, our results indicate that WTMV, which forms a monophyletic clade with a number of other potyviruses infecting Solanaceae species in phylogenetic analysis, is widely distributed, or is spreading in Southeast Asia. It may pose a threat to Solanaceae species cultivation in this region. References: (1) Chen et al. Arch. Virol. 146:757, 2001. (2) Adams et al. Arch. Virol. 150:459, 2005. (3) Ha et al. Arch. Virol. 153:25, 2008.