Hijacking of the nucleolar protein fibrillarin by TGB1 is required for cell‐to‐cell movement of Barley stripe mosaic virus

Barley stripe mosaic virus (BSMV) Triple Gene Block1 (TGB1) is a multifunctional movement protein with RNA‐binding, ATPase and helicase activities which mainly localizes to the plasmodesmata (PD) in infected cells. Here, we show that TGB1 localizes to the nucleus and the nucleolus, as well as the cytoplasm, and that TGB1 nuclear‐cytoplasmic trafficking is required for BSMV cell‐to‐cell movement. Prediction analyses and laser scanning confocal microscopy (LSCM) experiments verified that TGB1 possesses a nucleolar localization signal (NoLS) (amino acids 95–104) and a nuclear localization signal (NLS) (amino acids 227–238). NoLS mutations reduced BSMV cell‐to‐cell movement significantly, whereas NLS mutations almost completely abolished movement. Furthermore, neither the NoLS nor NLS mutant viruses could infect Nicotiana benthamiana systemically, although the NoLS mutant virus was able to establish systemic infections of barley. Protein interaction experiments demonstrated that TGB1 interacts directly with the glycine–arginine‐rich (GAR) domain of the nucleolar protein fibrillarin (Fib2). Moreover, in BSMV‐infected cells, Fib2 accumulation increased by about 60%–70% and co‐localized with TGB1 in the plasmodesmata. In addition, BSMV cell‐to‐cell movement in fib2 knockdown transgenic plants was reduced to less than one‐third of that of non‐transgenic plants. Fib2 also co‐localized with both TGB1 and BSMV RNA, which are the main components of the ribonucleoprotein (RNP) movement complex. Collectively, these results show that TGB1–Fib2 interactions play a direct role in cell‐to‐cell movement, and we propose that Fib2 is hijacked by BSMV TGB1 to form a BSMV RNP which functions in cell‐to‐cell movement.     

Rice stripe virus activates the bZIP17/28 branch of the unfolded protein response signalling pathway to promote viral infection

The unfolded protein response (UPR) plays important roles in plant virus infection. Our previous study has proved that rice stripe virus (RSV) infection elicits host UPR. However, the mechanism on how the UPR is triggered upon RSV infection remains obscure. Here, we show that the bZIP17/28 branch of the UPR signalling pathway is activated upon RSV infection in Nicotiana benthamiana. We found that membrane-associated proteins NSvc2 and NSvc4 encoded by RSV are responsible for the activation of the bZIP17/28 branch. Ectopic expression of NSvc2 or NSvc4 in plant leaves induced the proteolytic processing of NbbZIP17/28 and up-regulated the expression of UPR-related genes. Silencing NbbZIP17/28 significantly inhibited RSV infection. We show that RSV can specifically elicit the UPR through the bZIP17/28 branch, thus promoting virus infection of N. benthamiana plants.     

Laodelphax striatellus Atg8 facilitates Rice stripe virus infection in an autophagy-independent manner

Rice stripe virus(RSV)is the causative agent of rice stripe disease and is completely dependent on insect vectors for its plant-to-plant transmission.Laodelphax striatellus is the major insect vector for RSV.In this study,we explored the interactions be-tween RSV infection and L.striatellus autophagy,a potential intrinsic antiviral mechanism in insects.We found that L.striatellus autophagic activity did not affect RSV infection;however,the autophagy related-8(Atg8)gene significantly enhanced virus infection.Dur-ing RSV initial infection within the L.striatellus midgut,silencing of Atg8 expression significantly decreased the phosphorylation of c-Jun N-terminal kinase(p-JNK);however,when RSV infection is absent,silencing of Atg8 did not alter p-JNK levels.Thesc results indicated that Atg8 might activate the JNK machinery by allowing more virus infection into cells.We further revealed that Atg8-deficiency significantly decreased RSV accumu-lation on the surface of the insect midgut epithelial cells,suggesting a receptor trafficking function of the y-aminobutyric acid receptor-associated protein family.Using the RSV ovary entry as a model,in which vitellogenin receptor(V gR)mediates RSV cell entry,we clarified that Atg8-deficiency decreased the abundance of V gR localizing on the cytomem-brane and disturbed the attachment of RSV in the germarium zones.Collectively,these results revealed an autophagy-independent function of L.striatellus Atg8 that enhances RSV initial infection by increasing virus attachment on the infection sites.     

与蚕豆萎蔫病毒2号胞间运动和长距离转运相关的寄主细胞超微结构研究

电镜超微结构观察和免疫金标记显示:受蚕豆萎蔫病毒2号(Broad bean wilt virus 2,BBWV 2)中国分离物B935侵染的豌豆(Pisum sativum)和蚕豆(Vicia faba)叶细胞中膜结构增生,形成膜结构增生区,病毒以结晶体和管状体形式存在于细胞质中。在病变早期,叶肉细胞的胞间连丝处连接有小管结构,病毒样颗粒呈纵列排在小管中,穿越胞间连丝的小管能被BBWV 2的金标记抗体特异性标记。维管束组织的薄壁细胞、伴胞及转移细胞内存在膜增生区及病毒管状体,在筛管壁附近存在的病毒样颗粒能被BBWV 2金标记抗体特异性标记。实验结果表明BBWV 2胞间运动形式与豇豆花叶病毒(CPMV)相似,以完整粒子通过在胞间连丝处形成的小管结构穿越胞间连丝;细胞质中存在的直径160nm管状体只是一种病毒聚集体,与胞间运动无直接关系;该病毒在筛管中可能也是以完整粒子形式进行长距离转运的。