Intercellular movement of plant RNA viruses: Targeting replication complexes to the plasmodesma for both accuracy and efficiency

Zn2+-dependent association of cysteine-rich protein with virion orchestrates morphogenesis of rod-shaped viruses

The majority of rod-shaped and some filamentous plant viruses encode a cysteine-rich protein (CRP) that functions in viral virulence; however, the roles of these CRPs in viral infection remain largely unknown. Here, we used barley stripe mosaic virus (BSMV) as a model to investigate the essential role of its CRP in virus morphogenesis. The CRP protein γb directly interacts with BSMV coat protein (CP), the mutations either on the His-85 site in γb predicted to generate a potential CCCH motif or on the His-13 site in CP exposed to the surface of the virions abolish the zinc-binding activity and their interaction. Immunogold-labeling assays show that γb binds to the surface of rod-shaped BSMV virions in a Zn2+-dependent manner, which enhances the RNA binding activity of CP and facilitates virion assembly and stability, suggesting that the Zn2+-dependent physical association of γb with the virion is crucial for BSMV morphogenesis. Intriguingly, the tightly binding of diverse CRPs to their rod-shaped virions is a general feature employed by the members in the families Virgaviridae (excluding the genus Tobamovirus) and Benyviridae. Together, these results reveal a hitherto unknown role of CRPs in the assembly and stability of virus particles, and expand our understanding of the molecular mechanism underlying virus morphogenesis.

Emergence of two distinct spatial folds in a pair of plant virus proteins encoded by nested genes

Virus genomes may encode overlapping or nested open reading frames that increase their coding capacity. It is not known whether the constraints on spatial structures of the two encoded proteins limit the evolvability of nested genes. We examine the evolution of a pair of proteins, p22 and p19, encoded by nested genes in plant viruses from the genus Tombusvirus. The known structure of p19, a suppressor of RNA silencing, belongs to the RAGNYA fold from the alpha+beta class. The structure of p22, the cell-to-cell movement protein from the 30K family widespread in plant viruses, is predicted with the AlphaFold approach, suggesting a single jelly-roll fold core from the all-beta class, structurally similar to capsid proteins from plant and animal viruses. The nucleotide and codon preferences impose modest constraints on the types of secondary structures encoded in the alternative reading frames, nonetheless allowing for compact, well-ordered folds from different structural classes in two similarly-sized nested proteins. Tombusvirus p22 emerged through radiation of the widespread 30K family, which evolved by duplication of a virus capsid protein early in the evolution of plant viruses, whereas lineage-specific p19 may have emerged by a stepwise increase in the length of the overprinted gene and incremental acquisition of functionally active secondary structure elements by the protein product. This evolution of p19 toward the RAGNYA fold represents one of the first documented examples of protein structure convergence in naturally occurring proteins.

Rubisco small subunit (RbCS) is co-opted by potyvirids as the scaffold protein in assembling a complex for viral intercellular movement

Plant viruses must move through plasmodesmata (PD) to complete their life cycles. For viruses in the Potyviridae family (potyvirids), three viral factors (P3N-PIPO, CI, and CP) and few host proteins are known to participate in this event. Nevertheless, not all the proteins engaging in the cell-to-cell movement of potyvirids have been discovered. Here, we found that HCPro2 encoded by areca palm necrotic ring spot virus (ANRSV) assists viral intercellular movement, which could be functionally complemented by its counterpart HCPro from a potyvirus. Affinity purification and mass spectrometry identified several viral factors (including CI and CP) and host proteins that are physically associated with HCPro2. We demonstrated that HCPro2 interacts with both CI and CP in planta in forming PD-localized complexes during viral infection. Further, we screened HCPro2-associating host proteins, and identified a common host protein in Nicotiana benthamiana-Rubisco small subunit (NbRbCS) that mediates the interactions of HCPro2 with CI or CP, and CI with CP. Knockdown of NbRbCS impairs these interactions, and significantly attenuates the intercellular and systemic movement of ANRSV and three other potyvirids (turnip mosaic virus, pepper veinal mottle virus, and telosma mosaic virus). This study indicates that a nucleus-encoded chloroplast-targeted protein is hijacked by potyvirids as the scaffold protein to assemble a complex to facilitate viral movement across cells.

The phosphorylation of the movement protein TGBp1 regulates the accumulation of the Bamboo mosaic virus

Phosphorylation and dephosphorylation of viral movement proteins plays a crucial role in regulating virus movement. Our study focused on investigating the movement protein TGBp1 of Bamboo mosaic virus (BaMV), which is a single-stranded positive-sense RNA virus. Specifically, we examined four potential phosphorylation sites (S15, S18, T58, and S247) within the TGBp1 protein. To study the impact of phosphorylation, we introduced amino acid substitutions at the selected sites. Alanine substitutions were used to prevent phosphorylation, while aspartate substitutions were employed to mimic phosphorylation. Our findings suggest that mimicking phosphorylation at S15, S18 and T58 of TGBp1 might be linked to silencing suppressor activities. The phosphorylated form at these sites exhibits a loss of silencing suppressor activity, leading to reduced viral accumulation in the inoculated leaves. Furthermore, mimicking phosphorylation at residues S15 and S18 could diminish viral accumulation at the single-cell level, while doing so at residue T58 could influence virus movement. However, mimicking phosphorylation at residue S247 does not appear to be relevant to both functions of TGBp1. Overall, our study provides insights into the functional significance of specific phosphorylation sites in BaMV TGBp1, illuminating the regulatory mechanisms involved in virus movement and silencing suppression.

A sword or a buffet: plant endomembrane system in viral infections

The plant endomembrane system is an elaborate collection of membrane-bound compartments that perform distinct tasks in plant growth and development, and in responses to abiotic and biotic stresses. Most plant viruses are positive-strand RNA viruses that remodel the host endomembrane system to establish intricate replication compartments. Their fundamental role is to create optimal conditions for viral replication, and to protect replication complexes and the cell-to-cell movement machinery from host defenses. In addition to the intracellular antiviral defense, represented mainly by RNA interference and effector-triggered immunity, recent findings indicate that plant antiviral immunity also includes membrane-localized receptor-like kinases that detect viral molecular patterns and trigger immune responses, which are similar to those observed for bacterial and fungal pathogens. Another recently identified part of plant antiviral defenses is executed by selective autophagy that mediates a specific degradation of viral proteins, resulting in an infection arrest. In a perpetual tug-of-war, certain host autophagy components may be exploited by viral proteins to support or protect an effective viral replication. In this review, we present recent advances in the understanding of the molecular interplay between viral components and plant endomembrane-associated pathways.

Plant virus movement proteins originated from jelly-roll capsid proteins

Numerous, diverse plant viruses encode movement proteins (MPs) that aid the virus movement through plasmodesmata, the plant intercellular channels. MPs are essential for virus spread and propagation in distal tissues, and several unrelated MPs have been identified. The 30K superfamily of MPs (named after the molecular mass of tobacco mosaic virus (TMV) MP, the classical model of plant virology) is the largest and most diverse MP variety, represented in 16 virus families, but its evolutionary origin remained obscure. Here, we show that the core structural domain of the 30K MPs is homologous to the jelly-roll domain of the capsid proteins (CPs) of small RNA and DNA viruses, in particular, those infecting plants. The closest similarity was observed between the 30K MPs and the CPs of the viruses in the families Bromoviridae and Geminiviridae. We hypothesize that the MPs evolved via duplication or horizontal acquisition of the CP gene in a virus that infected an ancestor of vascular plants, followed by neofunctionalization of one of the paralogous CPs, potentially through the acquisition of unique N- and C-terminal regions. During the subsequent coevolution of viruses with diversifying vascular plants, the 30K MP genes underwent explosive horizontal spread among emergent RNA and DNA viruses, likely permitting viruses of insects and fungi that coinfected plants to expand their host ranges, molding the contemporary plant virome.

Interaction between Movement Proteins of Hibiscus green spot virus

Movement proteins (MPs) of plant viruses enable the translocation of viral genomes from infected to healthy cells through plasmodesmata (PD). The MPs functions involve the increase of the PD permeability and routing of viral genome both to the PD entrance and through the modified PD. Hibiscus green spot virus encodes two MPs, termed BMB1 and BMB2, which act in concert to accomplish virus cell-to-cell transport. BMB1, representing an NTPase/helicase domain-containing RNA-binding protein, localizes to the cytoplasm and the nucleoplasm. BMB2 is a small hydrophobic protein that interacts with the endoplasmic reticulum (ER) membranes and induces local constrictions of the ER tubules. In plant cells, BMB2 localizes to PD-associated membrane bodies (PAMBs) consisting of modified ER tubules and directs BMB1 to PAMBs. Here, we demonstrate that BMB1 and BMB2 interact in vitro and in vivo, and that their specific interaction is essential for BMB2-directed targeting of BMB1 to PAMBs. Using mutagenesis, we show that the interaction involves the C-terminal BMB1 region and the N-terminal region of BMB2.

Distinct Mechanisms of Endomembrane Reorganization Determine Dissimilar Transport Pathways in Plant RNA Viruses

Plant viruses exploit the endomembrane system of infected cells for their replication and cell-to-cell transport. The replication of viral RNA genomes occurs in the cytoplasm in association with reorganized endomembrane compartments induced by virus-encoded proteins and is coupled with the virus intercellular transport via plasmodesmata that connect neighboring cells in plant tissues. The transport of virus genomes to and through plasmodesmata requires virus-encoded movement proteins (MPs). Distantly related plant viruses encode different MP sets, or virus transport systems, which vary in the number of MPs and their properties, suggesting their functional differences. Here, we discuss two distinct virus transport pathways based on either the modification of the endoplasmic reticulum tubules or the formation of motile vesicles detached from the endoplasmic reticulum and targeted to endosomes. The viruses with the movement proteins encoded by the triple gene block exemplify the first, and the potyviral system is the example of the second type. These transport systems use unrelated mechanisms of endomembrane reorganization. We emphasize that the mode of virus interaction with cell endomembranes determines the mechanism of plant virus cell-to-cell transport.

The N-Terminal α-Helix of Potato Virus X-Encoded RNA-Dependent RNA Polymerase Is Required for Membrane Association and Multimerization

Positive-sense single-stranded RNA viruses replicate in virus-induced membranous organelles for maximum efficiency and immune escaping. The replication of potato virus X (PVX) takes place on the endoplasmic reticulum (ER); however, how PVX-encoded RNA-dependent RNA polymerase (RdRp) is associated with the ER is still unknown. A proline-kinked amphipathic α-helix was recently found in the MET domain of RdRp. In this study, we further illustrate that the first α-helix of the MET domain is also required for ER association. Moreover, we found that the MET domain forms multimers on ER and the first α-helix is essential for multimerization. These results suggest that the RdRp of PVX adopts more than one hydrophobic motif for membrane association and for multimerization.

Palmitoylation of γb protein directs a dynamic switch between Barley stripe mosaic virus replication and movement

Viral replication and movement are intimately linked; however, the molecular mechanisms regulating the transition between replication and subsequent movement remain largely unknown. We previously demonstrated that the Barley stripe mosaic virus (BSMV) γb protein promotes viral replication and movement by interacting with the αa replicase and TGB1 movement proteins. Here, we found that γb is palmitoylated at Cys-10, Cys-19, and Cys-60 in Nicotiana benthamiana, which supports BSMV infection. Intriguingly, non-palmitoylated γb is anchored to chloroplast replication sites and enhances BSMV replication, whereas palmitoylated γb protein recruits TGB1 to the chloroplasts and forms viral replication-movement intermediate complexes. At the late stages of replication, γb interacts with NbPAT15 and NbPAT21 and is palmitoylated at the chloroplast periphery, thereby shifting viral replication to intracellular and intercellular movement. We also show that palmitoylated γb promotes virus cell-to-cell movement by interacting with NbREM1 to inhibit callose deposition at the plasmodesmata. Altogether, our experiments reveal a model whereby palmitoylation of γb directs a dynamic switch between BSMV replication and movement events during infection.

Bioinformatic Analysis Predicts a Novel Genetic Module Related to Triple Gene and Binary Movement Blocks of Plant Viruses: Tetra-Cistron Movement Block

Previous studies have shown that the RNA genomes of some plant viruses encode two related genetic modules required for virus movement over the host body, containing two or three genes and named the binary movement block (BMB) and triple gene block (TGB), respectively. In this paper, we predict a novel putative-related movement gene module, called the tetra-cistron movement block (TCMB), in the virus-like transcriptome assemblies of the moss Dicranum scoparium and the Antarctic flowering plant Colobanthus quitensis. These TCMBs are encoded by smaller RNA components of putative two-component viruses related to plant benyviruses. Similar to the RNA2 of benyviruses, TCMB-containing RNAs have the 5′-terminal coat protein gene and include the RNA helicase gene which is followed by two small overlapping cistrons encoding hydrophobic proteins with a distant sequence similarity to the TGB2 and TGB3 proteins. Unlike TGB, TCMB also includes a fourth 5′-terminal gene preceding the helicase gene and coding for a protein showing a similarity to the double-stranded RNA-binding proteins of the DSRM AtDRB-like superfamily. Additionally, based on phylogenetic analysis, we suggest the involvement of replicative beny-like helicases in the evolution of the BMB and TCMB movement genetic modules.

Strawberry Vein Banding Virus Movement Protein P1 Interacts With Light-Harvesting Complex II Type 1 Like of Fragaria vesca to Promote Viral Infection

Chlorophyll a/b-binding protein of light-harvesting complex II type 1 like (LHC II-1L) is an essential component of photosynthesis, which mainly maintains the stability of the electron transport chain. However, how the LHC II-1L protein of Fragaria vesca (FvLHC II-1L) affects viral infection remains unclear. In this study, we demonstrated that the movement protein P1 of strawberry vein banding virus (SVBV P1) interacted with FvLHC II-1L in vivo and in vitro by bimolecular fluorescence complementation and pull-down assays. SVBV P1 was co-localized with FvLHC II-1L at the edge of epidermal cells of Nicotiana benthamiana leaves, and FvLHC II-1L protein expression was upregulated in SVBV-infected F. vesca. We also found that FvLHC II-1L effectively promoted SVBV P1 to compensate for the intercellular movement of movement-deficient potato virus X (PVX^ΔP25) and the systemic movement of movement-deficient cucumber mosaic virus (CMV^ΔMP). Transient overexpression of FvLHC II-1L and inoculation of an infectious clone of SVBV showed that the course of SVBV infection in F. vesca was accelerated. Collectively, the results showed that SVBV P1 protein can interact with FvLHC II-1L protein, which in turn promotes F. vesca infection by SVBV.

Optimizing RNAi-Target by Nicotiana benthamiana-Soybean Mosaic Virus System Drives Broad Resistance to Soybean Mosaic Virus in Soybean

Soybean mosaic virus (SMV) is a prevalent pathogen of soybean (Glycine max). Pyramiding multiple SMV-resistance genes into one individual is tedious and difficult, and even if successful, the obtained multiple resistance might be broken by pathogen mutation, while targeting viral genome via host-induced gene silencing (HIGS) has potential to explore broad-spectrum resistance (BSR) to SMV. We identified five conserved target fragments (CTFs) from S1 to S5 using multiple sequence alignment of 30 SMV genome sequences and assembled the corresponding target-inverted-repeat constructs (TIRs) from S1-TIR to S5-TIR. Since the inefficiency of soybean genetic transformation hinders the function verification of batch TIRs in SMV-resistance, the Nicotiana benthamiana-chimeric-SMV and N. benthamiana-pSMV-GUS pathosystems combined with Agrobacterium-mediated transient expression assays were invented and used to test the efficacy of these TIRs. From that, S1-TIR assembled from 462 bp CTF-S1 with 92% conservation rate performed its best on inhibiting SMV multiplication. Accordingly, S1-TIR was transformed into SMV-susceptible soybean NN1138-2, the resistant-healthy transgenic T₁-plants were then picked out via detached-leaf inoculation assay with the stock-plants continued for progeny reproduction (T₁ dual-utilization). All the four T₃ transgenic progenies showed immunity to all the inoculated 11 SMV strains under individual or mixed inoculation, achieving a strong BSR. Thus, optimizing target for HIGS via transient N. benthamiana-chimeric-SMV and N. benthamiana-pSMV-GUS assays is crucial to drive robust resistance to SMV in soybean and the transgenic S1-TIR-lines will be a potential breeding source for SMV control in field.

Variability, Functions and Interactions of Plant Virus Movement Proteins: What Do We Know So Far?

Of the various proteins encoded by plant viruses, one of the most interesting is the movement protein (MP). MPs are unique to plant viruses and show surprising structural and functional variability while maintaining their core function, which is to facilitate the intercellular transport of viruses or viral nucleoprotein complexes. MPs interact with components of the intercellular channels, the plasmodesmata (PD), modifying their size exclusion limits and thus allowing larger particles, including virions, to pass through. The interaction of MPs with the components of PD, the formation of transport complexes and the recruitment of host cellular components have all revealed different facets of their functions. Multitasking is an inherent property of most viral proteins, and MPs are no exception. Some MPs carry out multitasking, which includes gene silencing suppression, viral replication and modulation of host protein turnover machinery. This review brings together the current knowledge on MPs, focusing on their structural variability, various functions and interactions with host proteins.

Diversity of Plant Virus Movement Proteins: What Do They Have in Common?

The modern view of the mechanism of intercellular movement of viruses is based largely on data from the study of the tobacco mosaic virus (TMV) 30-kDa movement protein (MP). The discovered properties and abilities of TMV MP, namely, (a) in vitro binding of single-stranded RNA in a non-sequence-specific manner, (b) participation in the intracellular trafficking of genomic RNA to the plasmodesmata (Pd), and (c) localization in Pd and enhancement of Pd permeability, have been used as a reference in the search and analysis of candidate proteins from other plant viruses. Nevertheless, although almost four decades have passed since the introduction of the term “movement protein” into scientific circulation, the mechanism underlying its function remains unclear. It is unclear why, despite the absence of homology, different MPs are able to functionally replace each other in trans-complementation tests. Here, we consider the complexity and contradictions of the approaches for assessment of the ability of plant viral proteins to perform their movement function. We discuss different aspects of the participation of MP and MP/vRNA complexes in intra- and intercellular transport. In addition, we summarize the essential MP properties for their functioning as “conditioners”, creating a favorable environment for viral reproduction.