Cytoplasmic and nuclear Sw-5b NLR act both independently and synergistically to confer full host defense against tospovirus infection.

Plant intracellular nucleotide-binding leucine-rich repeat (NLR) receptors play critical roles in mediating host immunity to pathogen attack. We use tomato Sw-5b::tospovirus as a model system to study the specific role of the compartmentalized plant NLR in dictating host defenses against the virus at different infection steps. We demonstrated here that tomato NLR Sw-5b distributes to the cytoplasm and nucleus, respectively, to play different roles in inducing host resistances against tomato spotted wilt orthotospovirus (TSWV) infection. The cytoplasmic-enriched Sw-5b induces a strong cell death response to inhibit TSWV replication. This host response is, however, insufficient to block viral intercellular and long-distance movement. The nuclear-enriched Sw-5b triggers a host defense that weakly inhibits viral replication but strongly impedes virus intercellular and systemic movement. Furthermore, the cytoplasmic and nuclear Sw-5b act synergistically to dictate a full host defense of TSWV infection. We further demonstrated that the extended N-terminal Solanaceae domain (SD) of Sw-5b plays critical roles in cytoplasm/nucleus partitioning. Sw-5b NLR controls its cytoplasm localization. Strikingly, the SD but not coil-coil domain is crucial for Sw-5b receptor to import into the nucleus to trigger the immunity. The SD was found to interact with importins. Silencing both importin α and ß expression disrupted Sw-5b nucleus import and host immunity against TSWV systemic infection. Collectively, our findings suggest that Sw-5b bifurcates disease resistances by cytoplasm/nucleus partitioning to block different infection steps of TSWV. The findings also identified a new regulatory role of extra domain of a plant NLR in mediating host innate immunity.

The Sw-5b NLR nucleotide-binding domain plays a role in oligomerization, and its self-association is important for activation of cell death signaling.

Plant and animal intracellular nucleotide-binding and leucine-rich repeat (NLR) receptors play important roles in sensing pathogens and activating defense signaling. However, the molecular mechanisms underlying the activation of host defense signaling by NLR proteins remain largely unknown. Many studies have determined that the coil-coil (CC) or Toll and interleukin-1 receptor/resistance protein (TIR) domain of NLR proteins and their dimerization/oligomerization are critical for activating downstream defense signaling. In this study, we demonstrated that, in tomato, the nucleotide-binding (NB) domain Sw-5b NLR alone can activate downstream defense signaling, leading to elicitor-independent cell death. Sw-5b NB domains can self-associate, and this self-association is crucial for activating cell death signaling. The self-association was strongly compromised after the introduction of a K568R mutation into the P-loop of the NB domain. Consequently, the NBK568R mutant induced cell death very weakly. The NBCΔ20 mutant lacking the C-terminal 20 amino acids can self-associate but cannot activate cell death signaling. The NBCΔ20 mutant also interfered with wild-type NB domain self-association, leading to compromised cell death induction. By contrast, the NBK568R mutant did not interfere with wild-type NB domain self-association and its ability to induce cell death. Structural modeling of Sw-5b suggests that NB domains associate with one another and likely participate in oligomerization. As Sw-5b-triggered cell death is dependent on helper NLR proteins, we propose that the Sw-5b NB domain acts as a nucleation point for the assembly of an oligomeric resistosome, probably by recruiting downstream helper partners, to trigger defense signaling.

Development of a Mini-Replicon-Based Reverse-Genetics System for Rice Stripe Tenuivirus.

Negative-stranded RNA (NSR) viruses include both animal- and plant-infecting viruses that often cause serious diseases in humans and livestock and in agronomic crops. Rice stripe tenuivirus (RSV), a plant NSR virus with four negative-stranded/ambisense RNA segments, is one of the most destructive rice pathogens in many Asian countries. Due to the lack of a reliable reverse-genetics technology, molecular studies of RSV gene functions and its interaction with host plants are severely hampered. To overcome this obstacle, we developed a mini-replicon-based reverse-genetics system for RSV gene functional analysis in Nicotiana benthamiana. We first developed a mini-replicon system expressing an RSV genomic RNA3 enhanced green fluorescent protein (eGFP) reporter [MR3(-)eGFP], a nucleocapsid (NP), and a codon usage-optimized RNA-dependent RNA polymerase (RdRpopt). Using this mini-replicon system, we determined that RSV NP and RdRpopt are indispensable for the eGFP expression from MR3(-)eGFP. The expression of eGFP from MR3(-)eGFP can be significantly enhanced in the presence of four viral suppressors of RNA silencing (VSRs), NSs, and P19-HcPro-γb. In addition, NSvc4, the movement protein of RSV, facilitated eGFP trafficking between cells. We also developed an antigenomic RNA3-based replicon in N. benthamiana. However, we found that the RSV NS3 coding sequence acts as a cis element to regulate viral RNA expression. Finally, we made mini-replicons representing all four RSV genomic RNAs. This is the first mini-replicon-based reverse-genetics system for monocot-infecting tenuivirus. We believe that the mini-replicon system described here will allow studies of the RSV replication, transcription, cell-to-cell movement, and host machinery underpinning RSV infection in plants. IMPORTANCE Plant-infecting segmented negative-stranded RNA (NSR) viruses are grouped into three genera: Orthotospovirus, Tenuivirus, and Emaravirus. Reverse-genetics systems have been established for members of the genera Orthotospovirus and Emaravirus. However, there is still no reverse-genetics system available for Tenuivirus. Rice stripe virus (RSV) is a monocot-infecting tenuivirus with four negative-stranded/ambisense RNA segments. It is one of the most destructive rice pathogens and causes significant damage to the rice industry in Asian countries. Due to the lack of a reliable reverse-genetics system, molecular characterizations of RSV gene functions and the host machinery underpinning RSV infection in plants are extremely difficult. To overcome this obstacle, we developed a mini-replicon-based reverse-genetics system for RSV in Nicotiana benthamiana. This is the first mini-replicon-based reverse-genetics system for tenuivirus. We consider that this system will provide researchers a new working platform to elucidate the molecular mechanisms dictating segmented tenuivirus infections in plants.

Molecular Co-Chaperone SGT1 Is Critical for Cell-to-Cell Movement and Systemic Infection of Tomato Spotted Wild Virus in Nicotiana benthamiana.

Tospovirus is a tripartite negative stranded RNA virus and is considered as one of the most devastating plant viruses. Successful virus infection in plant requires many host factors. To date, very few host factors have been identified as important in Tospovirus infection in plants. We reported earlier that NSm protein encoded by Tomato spotted wilt virus (TSWV), a type species of the genus Orthotospovirus, plays critical roles in viral cell-to-cell and long-distance movement. In this study, we determined that molecular co-chaperone NbSGT1 interacted with TSWV NSm in Nicotianabenthamiana. TSWV infection significantly upregulated the expression of NbSGT1 gene and transient overexpression of NbSGT1 in N.benthamiana leaves accelerated TSWV infection. In contrast, silencing the NbSGT1 gene expression using a virus-induced gene silencing (VIGS) approach strongly inhibited TSWV NSm cell-to-cell movement, as well as TSWV local and systemic infection in N.benthamiana plants. Furthermore, NbSGT1 was found to regulate the infection of both American and Euro/Asia type tospoviruses in N.benthamiana plant. Collectively, our findings presented in this paper and the results published previously indicated that molecular co-chaperone NbSGT1 plays important roles in modulating both positive stranded and tripartite negative stranded RNA virus infection in plants.

An Improved Brome mosaic virus Silencing Vector: Greater Insert Stability and More Extensive VIGS.

Virus-induced gene silencing (VIGS) is used extensively for gene function studies in plants. VIGS is inexpensive and rapid compared with silencing conducted through stable transformation, but many virus-silencing vectors, especially in grasses, induce only transient silencing phenotypes. A major reason for transient phenotypes is the instability of the foreign gene fragment (insert) in the vector during VIGS. Here, we report the development of a Brome mosaic virus (BMV)-based vector that better maintains inserts through modification of the original BMV vector RNA sequence. Modification of the BMV RNA3 sequence yielded a vector, BMVCP5, that better maintained phytoene desaturase and heat shock protein70-1 (HSP70-1) inserts in Nicotiana benthamiana and maize (Zea mays). Longer maintenance of inserts was correlated with greater target gene silencing and more extensive visible silencing phenotypes displaying greater tissue penetration and involving more leaves. The modified vector accumulated similarly to the original vector in N. benthamiana after agroinfiltration, thus maintaining a high titer of virus in this intermediate host used to produce virus inoculum for grass hosts. For HSP70, silencing one family member led to a large increase in the expression of another family member, an increase likely related to the target gene knockdown and not a general effect of virus infection. The cause of the increased insert stability in the modified vector is discussed in relationship to its recombination and accumulation potential. The modified vector will improve functional genomic studies in grasses, and the conceptual methods used to improve the vector may be applied to other VIGS vectors.

Maize Elongin C interacts with the viral genome-linked protein, VPg, of Sugarcane mosaic virus and facilitates virus infection.

The viral genome-linked protein, VPg, of potyviruses is involved in viral genome replication and translation. To determine host proteins that interact with Sugarcane mosaic virus (SCMV) VPg, a yeast two-hybrid screen was used and a maize (Zea mays) Elongin C (ZmElc) protein was identified. ZmELC transcript was observed in all maize organs, but most highly in leaves and pistil extracts, and ZmElc was present in the cytoplasm and nucleus of maize cells in the presence or absence of SCMV. ZmELC expression was increased in maize tissue at 4 and 6 d post SCMV inoculation. When ZmELC was transiently overexpressed in maize protoplasts the accumulation of SCMV RNA was approximately doubled compared with the amount of virus in control protoplasts. Silencing ZmELC expression using a Brome mosaic virus-based gene silencing vector (virus-induced gene silencing) did not influence maize plant growth and development, but did decrease RNA accumulation of two isolates of SCMV and host transcript encoding ZmeIF4E during SCMV infection. Interestingly, Maize chlorotic mottle virus, from outside the Potyviridae, was increased in accumulation after silencing ZmELC expression. Our results describe both the location of ZmElc expression in maize and a new activity associated with an Elc: support of potyvirus accumulation.

Differential requirements for Tombusvirus coat protein and P19 in plants following leaf versus root inoculation.

Traditional virus inoculation of plants involves mechanical rubbing of leaves, whereas in nature viruses like Tomato bushy stunt virus (TBSV) are often infected via the roots. A method was adapted to compare leaf versus root inoculation of Nicotiana benthamiana and tomato with transcripts of wild-type TBSV (wtTBSV), a capsid (Tcp) replacement construct expressing GFP (T-GFP), or mutants not expressing the silencing suppressor P19 (TBSVΔp19). In leaves, T-GFP remained restricted to the cells immediately adjacent to the site of inoculation, unless Tcp was expressed in trans from a Potato virus X vector; while T-GFP inoculation of roots gave green fluorescence in upper tissues in the absence of Tcp. Conversely, leaf inoculation with wtTBSV or TBSVΔp19 transcripts initiated systemic infections, while upon root inoculation this only occurred with wtTBSV, not with TBSVΔp19. Evidently the contribution of Tcp or P19 in establishing systemic infections depends on the point-of-entry of TBSV in the plants.

Rationale for developing new virus vectors to analyze gene function in grasses through virus-induced gene silencing.

The exploding availability of genome and EST-based sequences from grasses requires a technology that allows rapid functional analysis of the multitude of genes that these resources provide. There are several techniques available to determine a gene's function. For gene knockdown studies, silencing through RNAi is a powerful tool. Gene silencing can be accomplished through stable transformation or transient expression of a fragment of a target gene sequence. Stable transformation in rice, maize, and a few other species, although routine, remains a relatively low-throughput process. Transformation in other grass species is difficult and labor-intensive. Therefore, transient gene silencing methods including Agrobacterium-mediated and virus-induced gene silencing (VIGS) have great potential for researchers studying gene function in grasses. VIGS in grasses already has been used to determine the function of genes during pathogen challenge and plant development. It also can be used in moderate-throughput reverse genetics screens to determine gene function. However, the number of viruses modified to serve as silencing vectors in grasses is limited, and the silencing phenotype induced by these vectors is not optimal: the phenotype being transient and with moderate penetration throughout the tissue. Here, we review the most recent information available for VIGS in grasses and summarize the strengths and weaknesses in current virus-grass host systems. We describe ways to improve current virus vectors and the potential of other grass-infecting viruses for VIGS studies. This work is necessary because VIGS for the foreseeable future remains a higher throughput and more rapid system to evaluate gene function than stable transformation.

Functions of rice NAC transcriptional factors, ONAC122 and ONAC131, in defense responses against Magnaporthe grisea.

NAC (NAM/ATAF/CUC) transcription factors have important functions in regulating plant growth, development, and abiotic and biotic stress responses. Here, we characterized two rice pathogen-responsive NAC transcription factors, ONAC122 and ONAC131. We determined that these proteins localized to the nucleus when expressed ectopically and had transcriptional activation activities. ONAC122 and ONAC131 expression was induced after infection by Magnaporthe grisea, the causal agent of rice blast disease, and the M. grisea-induced expression of both genes was faster and higher in the incompatible interaction compared with the compatible interaction during early stages of infection. ONAC122 and ONAC131 were also induced by treatment with salicylic acid, methyl jasmonate or 1-aminocyclopropane-1-carboxylic acid (a precursor of ethylene). Silencing ONAC122 or ONAC131 expression using a newly modified Brome mosaic virus (BMV)-based silencing vector resulted in an enhanced susceptibility to M. grisea. Furthermore, expression levels of several other defense- and signaling-related genes (i.e. OsLOX, OsPR1a, OsWRKY45 and OsNH1) were down-regulated in plants silenced for ONAC122 or ONAC131 expression via the BMV-based silencing system. Our results suggest that both ONAC122 and ONAC131 have important roles in rice disease resistance responses through the regulated expression of other defense- and signaling-related genes.

Analysis of gene function in rice through virus-induced gene silencing.

Virus-induced gene silencing (VIGS) is a powerful RNA-silencing based technology adapted for the study of host-gene function. VIGS functions through the expression of a host gene from a virus vector. Both the virus-encoded host sequence and the homologous host target messenger RNA are destroyed or made inactive through a host surveillance system. Here, we describe procedures for the use of a new virus vector for VIGS in monocotyledonous hosts and, in particular, in rice (Oryza sativa), a species for which no VIGS vector was previously available.

Characterization of a Brome mosaic virus strain and its use as a vector for gene silencing in monocotyledonous hosts.

Virus-induced gene silencing (VIGS) is used to analyze gene function in dicotyledonous plants but less so in monocotyledonous plants (particularly rice and corn), partially due to the limited number of virus expression vectors available. Here, we report the cloning and modification for VIGS of a virus from Festuca arundinacea Schreb. (tall fescue) that caused systemic mosaic symptoms on barley, rice, and a specific cultivar of maize (Va35) under greenhouse conditions. Through sequencing, the virus was determined to be a strain of Brome mosaic virus (BMV). The virus was named F-BMV (F for Festuca), and genetic determinants that controlled the systemic infection of rice were mapped to RNAs 1 and 2 of the tripartite genome. cDNA from RNA 3 of the Russian strain of BMV (R-BMV) was modified to accept inserts from foreign genes. Coinoculation of RNAs 1 and 2 from F-BMV and RNA 3 from R-BMV expressing a portion of a plant gene to leaves of barley, rice, and maize plants resulted in visual silencing-like phenotypes. The visual phenotypes were correlated with decreased target host transcript levels in the corresponding leaves. The VIGS visual phenotype varied from maintained during silencing of actin 1 transcript expression to transient with incomplete penetration through affected tissue during silencing of phytoene desaturase expression. F-BMV RNA 3 was modified to allow greater accumulation of virus while minimizing virus pathogenicity. The modified vector C-BMV(A/G) (C for chimeric) was shown to be useful for VIGS. These BMV vectors will be useful for analysis of gene function in rice and maize for which no VIGS system is reported.

The Tobacco mosaic virus 126-kDa protein associated with virus replication and movement suppresses RNA silencing.

Systemic symptoms induced on Nicotiana tabacum cv. Xanthi by Tobacco mosaic virus (TMV) are modulated by one or both amino-coterminal viral 126- and 183-kDa proteins: proteins involved in virus replication and cell-to-cell movement. Here we compare the systemic accumulation and gene silencing characteristics of TMV strains and mutants that express altered 126- and 183-kDa proteins and induce varying intensities of systemic symptoms on N. tabacum. Through grafting experiments, it was determined that M(IC)1,3, a mutant of the masked strain of TMV that accumulated locally and induced no systemic symptoms, moved through vascular tissue but failed to accumulate to high levels in systemic leaves. The lack of M(IC)1,3 accumulation in systemic leaves was correlated with RNA silencing activity in this tissue through the appearance of virus-specific, approximately 25-nucleotide RNAs and the loss of fluorescence from leaves of transgenic plants expressing the 126-kDa protein fused with green fluorescent protein (GFP). The ability of TMV strains and mutants altered in the 126-kDa protein open reading frame to cause systemic symptoms was positively correlated with their ability to transiently extend expression of the 126-kDa protein:GFP fusion and transiently suppress the silencing of free GFP in transgenic N. tabacum and transgenic N. benthamiana, respectively. Suppression of GFP silencing in N. benthamiana occurred only where virus accumulated to high levels. Using agroinfiltration assays, it was determined that the 126-kDa protein alone could delay GFP silencing. Based on these results and the known synergies between TMV and other viruses, the mechanism of suppression by the 126-kDa protein is compared with those utilized by other originally characterized suppressors of RNA silencing.