Development of Model Systems for Plant Rhabdovirus Research

Plant virology: an RNA treasure trove

Key principles pertaining to RNA biology not infrequently have their origins in plant virology. Examples have arisen from studies on viral RNA-intrinsic properties and the infection process from gene expression, replication, movement, and defense evasion to biotechnological applications. Since RNA is at the core of the central dogma in molecular biology, how plant virology assisted in the reinforcement or adaptations of this concept, while at other instances shook up elements of the doctrine, is discussed. Moreover, despite the negative effects of viral diseases in agriculture worldwide, plant viruses can be considered a scientific treasure trove. Today they remain tools of discovery for biotechnology, studying evolution, cell biology, and host-microbe interactions.

Interspecies/Intergroup Complementation of Orthotospovirus Replication and Movement through Reverse Genetics Systems

Orthotospoviruses, the plant-infecting bunyaviruses, cause serious diseases in agronomic crops and pose major threats to global food security. The family of Tospoviridae contains more than 30 members that are classified into two geographic groups, American-type and Euro/Asian-type orthotospovirus. However, the genetic interaction between different species and the possibility, during mixed infections, for transcomplementation of gene functions by orthotospoviruses from different geographic groups remains underexplored. In this study, minireplicon-based reverse genetics (RG) systems have been established for Impatiens necrotic spot virus (INSV) (an American-type orthotospovirus) and for Calla lily chlorotic spot virus and Tomato zonate spot virus (CCSV and TZSV) (two representative Euro/Asian orthotospoviruses). Together with the earlier established RG system for Tomato spotted wilt virus (TSWV), a type species of the Orthotospovirus American-clade, viral replicase/movement proteins were exchanged and analyzed on interspecies transcomplementation. Whereas the homologous RNA-dependent RNA polymerase (RdRp) and nucleocapsid (N) protein supported the replication of orthotospoviruses from both geographic groups, heterologous combinations of RdRp from one group and N from the other group were unable to support the replication of viruses from both groups. Furthermore, the NSm movement protein (MP), from both geographic groups of orthotospoviruses, was able to transcomplement heterologous orthotospoviruses or a positive-strand Cucumber mosaic virus (CMV) in their movement, albeit with varying efficiency. MP from Rice stripe tenuivirus (RSV), a plant-infecting bunyavirus that is distinct from orthotospoviruses, or MP from CMV also moves orthotospoviruses. Our findings gain insights into the genetic interaction/reassortant potentials for the segmented plant orthotospoviruses. IMPORTANCE Orthotospoviruses are agriculturally important negative-strand RNA viruses and cause severe yield-losses on many crops worldwide. Whereas the emergence of new animal-infecting bunyaviruses is frequently associated with genetic reassortants, this issue remains underexposed with the plant-infecting orthotospovirus. With the development of reverse genetics systems for orthotospoviruses from different geographic regions, the interspecies/intergroup replication/movement complementation between American- and Euro/Asian-type orthotospoviruses were investigated. Genomic RNAs from American orthotospoviruses can be replicated by the RdRp and N from those of Euro/Asia-group orthotospoviruses, and vice versa. However, their genomic RNAs cannot be replicated by a heterologous combination of RdRp from one geographic group and N from another geographic group. Cell-to-cell movement of viral entity is supported by NSm from both geographic groups, with highest efficiency by NSm from viruses belonging to the same group. Our findings provide important insights into the genetic interaction and exchange ability of viral gene functions between different species of orthotospovirus.

Reflections on a Career in Plant Virology: A Chip Floating on a Stream

At the time I entered college and for a few years afterward, I had very few concrete goals. Hence, my progress was more a matter of luck than planning and was somewhat analogous to a small wood chip floating down a slow stream, bumping into various objects tossed and turned hither and thither, all the while being surrounded by larger and more appealing chips. I have been extremely lucky to have been associated with numerous helpful and knowledgeable mentors, colleagues, postdocs, students, and coworkers whose advice had major impacts on my life. Therefore, throughout this article, I have attempted to acknowledge central individuals who contributed to my progress in academia and to highlight the positive bumps to my chip on the steam that affected the directions of my career.

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.

Rice yellow stunt virus activates polyamine biosynthesis to promote viral propagation in insect vectors by disrupting ornithine decarboxylase antienzyme function

Intracellular polyamines (putrescine, spermidine, and spermine) have emerged as important molecules for viral infection; however, how viruses activate polyamines biosynthesis to promote viral infection remains unclear. Ornithine decarboxylase 1 (ODC1) and its antienzyme 1 (OAZ1) are major regulators of polyamine biosynthesis in animal cells. Here, we report that rice yellow stunt virus (RYSV), a plant rhabdovirus, could activate putrescine biosynthesis in leafhoppers to promote viral propagation by inhibiting OAZ1 expression. We observed that the reduction of putrescine biosynthesis by treatment with difluormethylornithine (DFMO), a specific nontoxic inhibitor of ODC1, or with in vitro synthesized dsRNAs targeting ODC1 mRNA could inhibit viral infection. In contrast, the supplement of putrescine or the increase of putrescine biosynthesis by treatment with dsRNAs targeting OAZ1 mRNA could facilitate viral infection. We further determined that both RYSV matrix protein M and ODC1 directly bind to the ODC-binding domain at the C-terminus of OAZ1. Thus, viral propagation in leafhoppers would decrease the ability of OAZ1 to target and mediate the degradation of ODC1, which finally activates putrescine production to benefit viral propagation. This work reveals that polyamine-metabolizing enzymes are directly exploited by a vector-borne virus to increase polyamine production, thereby facilitating viral infection in insect vectors.

Development of a Reverse Genetic System for Studying Rose Rosette Virus in Whole Plants

Rose rosette virus (RRV) is a negative-sense RNA virus with a seven-segmented genome that is enclosed by a double membrane. We constructed an unconventional minireplicon system encoding the antigenomic (ag)RNA1 (encoding the viral RNA-dependent RNA polymerase [RdRp]), agRNA3 (encoding the nucleocapsid protein [N]), and a modified agRNA5 containing the coding sequence for the iLOV protein in place of the P5 open reading frame (R5-iLOV). iLOV expression from the R5-iLOV template was amplified by activities of the RdRp and N proteins in Nicotiana benthamiana leaves. A mutation was introduced into the RdRp catalytic domain and iLOV expression was eliminated, indicating RNA1-encoded polymerase activity drives iLOV expression from the R5-iLOV template. Fluorescence from the replicon was highest at 3 days postinoculation (dpi) and declined at 7 and 13 dpi. Addition of the tomato bushy stunt virus (TBSV) P19 silencing-suppressor protein prolonged expression until 7 dpi. A full-length infectious clone system was constructed of seven binary plasmids encoding each of the seven genome segments. Agro-delivery of constructs encoding RRV RNAs 1 through 4 or RNAs 1 through 7 to N. benthamiana plants produced systemic infection. Finally, agro-delivery of the full-length RRV infectious clone including all segments produced systemic infection within 60 dpi. This advance opens new opportunities for studying RRV infection biology.

Transmission of the Bean-Associated Cytorhabdovirus by the Whitefly Bemisia tabaci MEAM1

The knowledge of genomic data of new plant viruses is increasing exponentially; however, some aspects of their biology, such as vectors and host range, remain mostly unknown. This information is crucial for the understanding of virus–plant interactions, control strategies, and mechanisms to prevent outbreaks. Typically, rhabdoviruses infect monocot and dicot plants and are vectored in nature by hemipteran sap-sucking insects, including aphids, leafhoppers, and planthoppers. However, several strains of a potentially whitefly-transmitted virus, papaya cytorhabdovirus, were recently described: (i) bean-associated cytorhabdovirus (BaCV) in Brazil, (ii) papaya virus E (PpVE) in Ecuador, and (iii) citrus-associated rhabdovirus (CiaRV) in China. Here, we examine the potential of the Bemisia tabaci Middle East-Asia Minor 1 (MEAM1) to transmit BaCV, its morphological and cytopathological characteristics, and assess the incidence of BaCV across bean producing areas in Brazil. Our results show that BaCV is efficiently transmitted, in experimental conditions, by B. tabaci MEAM1 to bean cultivars, and with lower efficiency to cowpea and soybean. Moreover, we detected BaCV RNA in viruliferous whiteflies but we were unable to visualize viral particles or viroplasm in the whitefly tissues. BaCV could not be singly isolated for pathogenicity tests, identification of the induced symptoms, and the transmission assay. BaCV was detected in five out of the seven states in Brazil included in our study, suggesting that it is widely distributed throughout bean producing areas in the country. This is the first report of a whitefly-transmitted rhabdovirus.

List of non-EU viruses and viroids infecting potato (Solanum tuberosum) and other tuber-forming Solanum species

The European Commission requested a pest categorisation of the non-EU viruses and viroids of potato (hereafter referred to as viruses). As a first step, a systematic literature and database search was carried out to identify the viruses reported to naturally infect Solanum tuberosum and other tuber-forming Solanum spp (hereafter referred to as potato). Based on the global distribution and on the prevalence inside the European Union (EU), the Panel identified 40 non-EU viruses known to occur only outside the EU or with only a limited presence in the EU (reported in only one or few Member States (MSs) and/or with restricted distribution, outbreaks). Twenty-seven viruses were identified as having a significant presence in the EU (known to occur in several MSs, frequently reported in the EU, widespread in several MSs) or reported only from the EU so far, and will be excluded from further categorisation in the frame of the present mandate. Five viruses remained with an undetermined standing because the available information did not allow their allocation to one of the above groups. The viruses considered non-EU and those with undetermined standing will be further categorised if not addressed by EFSA in previous scientific opinions. Seven viruses for which non-European isolates are specifically regulated in Annex I of directive 2000/29/EC will be categorised separately. The main knowledge gaps and uncertainties of this grouping concern the natural host status of potato, the taxonomy, and/or information on the geographical distribution and prevalence of some of the analysed viruses.

Diversity and epidemiology of plant rhabdoviruses

Plant rhabdoviruses are recognized by their large bacilliform particles and for being able to replicate in both their plant hosts and arthropod vectors. This review highlights selected, better studied examples of plant rhabdoviruses, their genetic diversity, epidemiology and interactions with plant hosts and arthropod vectors: Alfalfa dwarf virus is classified as a cytorhabdovirus, but its multifunctional phosphoprotein is localized to the plant cell nucleus. Lettuce necrotic yellows virus subtypes may differentially interact with their aphid vectors leading to changes in virus population diversity. Interactions of rhabdoviruses that infect rice, maize and other grains are tightly associated with their specific leafhopper and planthopper vectors. Future outbreaks of vector-borne nucleorhabdoviruses may be predicted based on a world distribution map of the insect vectors. The epidemiology of coffee ringspot virus and its Brevipalpus mite vector is illustrated highlighting the symptomatology and biology of a dichorhavirus and potential impacts of climate change on its epidemiology.

Pest categorisation of non-EU viruses and viroids of potato

Following a request from the EU Commission, the Panel on Plant Health has addressed the pest categorisation of those viruses and viroids (hereafter referred to as viruses) of Solanum tuberosum and other tuber-forming Solanum spp. (hereafter referred to as potato) which are considered to be either non-EU or of undetermined standing based on a previous EFSA opinion. These viruses belong to different families and genera and either have an established identity or produce consistent symptoms. Plants for planting is the main pathway for entry for all categorised viruses as they can all be transmitted by vegetative propagation. Several categorised viruses have a relatively wide host range and/or are vector-transmitted, increasing the potential for entry. The information currently available on geographical distribution, biology, epidemiology, impact and potential entry pathways has been evaluated with regard to the criteria to qualify as potential Union quarantine pest or as Union regulated non-quarantine pest (RNQP). Since this opinion addresses specifically the non-EU potato viruses, in general these viruses do not meet the criteria assessed by EFSA to qualify as potential Union regulated non-quarantine pests. The following viruses meet the criteria to qualify as potential Union quarantine pest: APLV, APMMV, APMoV, ChiLCV, CYSDV, PAMV, PBRSV, PVH, PVP, PVT, PYDV, PYMV, PYV, PYVV, RCVMV, SALCV, SB26/29, ToCV, ToLCNDV, ToMHaV, ToMoTV, ToSRV and ToYVSV. With the exception of the criterion regarding the potential for consequences in the EU territory, for which the Panel is unable to conclude because of lack of information, AVB, CPSbV, PaLCrV, PapMV, PVB, PVU, SB41 and TVBMV meet all the other criteria to qualify as potential Union quarantine pest. PotLV and WPMV do not qualify as potential Union quarantine pest, since they are not reported to have any impact. For most of the categorised viruses, the conclusions of the Panel have inherent uncertainties, due to the lack of quantitative data on their impact and/or absence or limited availability of information on the biology, epidemiology and geographical distribution.

Cell Biology During Infection of Plant Viruses in Insect Vectors and Plant Hosts

Plant viruses typically cause severe pathogenicity in plants, even resulting in the death of plants. Many pathogenic plant viruses are transmitted in a persistent manner via insect vectors. Interestingly, unlike in the plant hosts, persistent viruses are either nonpathogenic or show limited pathogenicity in their insect vectors, while taking advantage of the cellular machinery of insect vectors for completing their life cycles. This review discusses why persistent plant viruses are nonpathogenic or have limited pathogenicity to their insect vectors while being pathogenic to plants hosts. Current advances in cell biology of virus-insect vector interactions are summarized, including virus-induced inclusion bodies, changes of insect cellular ultrastructure, and immune response of insects to the viruses, especially autophagy and apoptosis. The corresponding findings of virus-plant interactions are compared. An integrated view of the balance strategy achieved by the interaction between viral attack and the immune response of insect is presented. Finally, we outline progress gaps between virus-insect and virus-plant interactions, thus highlighting the contributions of cultured cells to the cell biology of virus-insect interactions. Furthermore, future prospects of studying the cell biology of virus-vector interactions are presented.

Plant Viruses in Plant Molecular Pharming: Toward the Use of Enveloped Viruses

Plant molecular pharming has emerged as a reliable platform for recombinant protein expression providing a safe and low-cost alternative to bacterial and mammalian cells-based systems. Simultaneously, plant viruses have evolved from pathogens to molecular tools for recombinant protein expression, chimaeric viral vaccine production, and lately, as nanoagents for drug delivery. This review summarizes the genesis of viral vectors and agroinfection, the development of non-enveloped viruses for various biotechnological applications, and the on-going research on enveloped plant viruses.