Identification of two novel poleroviruses and the occurrence of Tobacco bushy top disease causal agents in natural plants.

Tobacco bushy top disease (TBTD) is a devastating tobacco disease in the southwestern region of China. TBTD in the Yunnan Province is often caused by co-infections of several plant viruses: tobacco bushy top virus (TBTV), tobacco vein distorting virus (TVDV), tobacco bushy top virus satellite RNA (TBTVsatRNA) and tobacco vein distorting virus-associated RNA (TVDVaRNA). Through this study, two new poleroviruses were identified in two TBTD symptomatic tobacco plants and these two novel viruses are tentatively named as tobacco polerovirus 1 (TPV1) and tobacco polerovirus 2 (TPV2), respectively. Analyses of 244 tobacco samples collected from tobacco fields in the Yunnan Province through RT-PCR showed that a total of 80 samples were infected with TPV1 and/or TPV2, and the infection rates of TPV1 and TPV2 were 8.61% and 29.51%, respectively. Thirty-three TPV1 and/or TPV2-infected tobacco samples were selected for further test for TBTV, TVDV, TBTVsatRNA and TVDVaRNA infections. The results showed that many TPV1 and/or TPV2-infected plants were also infected with two or more other assayed viruses. In this study, we also surveyed TBTV, TVDV, TBTVsatRNA and TVDVaRNA infections in a total of 1713 leaf samples collected from field plants belonging to 29 plant species in 13 plant families and from 11 provinces/autonomous regions in China. TVDV had the highest infection rates of 37.5%, while TVDVaRNA, TBTV and TBTVsatRNA were found to be at 23.0%, 12.4% and 8.1%, respectively. In addition, TVDV, TBTV, TBTVsatRNA and TVDVaRNA were firstly detected of co-infection on 10 plants such as broad bean, pea, oilseed rape, pumpkin, tomato, crofton weed etc., and 1 to 4 of the TBTD causal agents were present in the samples collected from Guizhou, Hainan, Henan, Liaoning, Inner mongolia and Tibet autonomous regions. The results indicated that TBTD causal agents are expanding its host range and posing a risk to other crop in the field.

[Progress of studies on Dicer structure and function].

Dicer is responsible for producing small interfering RNA and microRNA as an essential component of RNA interference. The character of these small RNAs is determined by the structure of Dicer protein, which contains RNA HELICs, PAZ, RNaseand dsRNA binding domain. Different organisms have different numbers of Dicers with distinct but redundant functions. Evolutionally, the amount of Dicer in organism decreases, but its functions become multiple and extremely important. Studies on structure and function of Dicer will improve our understanding of the mechanism, as well as the evolutional rule and character, of RNAi and relative pathways. Here, we review the structural and functional character of Dicer.

[Current situation and prospect of studies on genus Umbravirus].

Umbraviruses are a group of imperfectly characterized plant viruses, which are distinguished from most other viruses by their genomes lack of a gene for coat protein (CP) , and as a result umbraviruses do not form conventional virus particles. Umbraviruses are mechanically transmissible, and can be aphid transmitted in the persistent manner by an unrelated assistor virus, which is always a member of the family Luteoviridae . In nature, each umbravirus depends for survival on one particular luteovirus. The genus Umbravirus comprises seven distinct virus specieses and three tentative members. Only Tobacco bushy top virus (TBTV) has been reported in China as an umbravirus. Tobacco bushy top disease, caused by TBTV and its helper, Tobacco vein distorting virus(TVDV), which resulted in severe tobacco losses in western of Yunan. Umbraviruses had a restricted host range in nature, and their infectivity and longevity in vitro are not so stable. Plants infected umbraviruses contain abundant double-stranded RNA (dsRNA), and some umbraviruses possess one or more additional dsRNA species associated with the presence of a satellite RNA. The genomes of the umbraviruses consist of one linear segment of positive sense single-stranded RNA(ssRNA), and the nucleotide sequences possess ORFs for four potential non-structural protein products. The umbravirus-encoded ORF3 proteins play essential roles in stabilization of viral RNA and mediation of its long-distance movement. The current research progresses have been reviewed detailly, and the future research tendency and research fields about umbraviruses and umbravirus-caused diseases are put forward.

[Relationship between genetic lineage and pathotype of Pyricularia grisea in Yunnan Plateau].

Rice blast caused by Pyricularia grisea is the most destructive disease in Yunnan Plateau, China. In order to elucidate the relationship between genetic lineage and pathotype of P. grisea of Yunnan Plateau as well as the variability of the fungus at DNA level,the repetitive element-based PCR (rep-PCR) of Pot2, an element found in approximately 100 copies in the fungus genome,was exploited. Two hundred and thirty-six isolates of P. grisea collected from 15 main rice-growing counties of Yunnan Plateau were fingerprinted by using rep-PCR. A linkage graph of the rep-PCR fingerprints from 134 representative isolates was generated using an unweighted pair-grouped average program (UPGMA) of the STATISTICAL 5.0 software. The isolates were classified into 8 genetic lineages (G1 approximately G8) at the level of 1.75 genetic linkage distance, of which the G1, G2 and G4 were the dominant lineages. The isolates in a certain area generally belonged to one correspondent genetic lineage and the isolates from the same plot and host rice variety mostly shared one linkage group though different genetic lineages within one lesion. Furthermore, 29 isolates representing the eight genetic lineages were inoculated on 33 rice cultivars of Yunnan at the stage of 3 approximately 4 leaves in greenhouse. The isolates were divided into 6 pathotype groups (P1 approximately P6) according to its compatibility, which demonstrated that some isolates of one genetic lineage sharing two or three pathotype groups, alternatively, one or four pathotype groups. The isolates from each genetic lineage, however, may share one pathotype group such as P2. The preliminary results implicated that the relationship between genetic lineages and pathotype groups of P. grisea in Yunnan Plateau was complicated rather than simple. On the other hand,2 rice cultivars including HeXi 16 and JingGuo 92 were resistant to the 29 isolates but YunJing 20 and HeXi 30 both susceptible to all of them, which was helpful for deploying the blast-resistant genes in rice production of Yunnan Plateau. Therefore, the rice blast-resistance spectrum of the tentative new rice cultivars should be evaluated before its release considering the blast-resistant rice breeding and the practice of rice production in Yunnan Plateau.