Invasion of minor veins of tobacco leaves inoculated with tobacco mosaic virus mutants defective in phloem-dependent movement

cmv1-Mediated Resistance to CMV in Melon Can Be Overcome by Mixed Infections with Potyviruses

Resistance to cucumber mosaic virus (CMV) strain LS in melon is controlled by the gene cmv1, which restricts phloem entry. In nature, CMV is commonly found in mixed infections, particularly with potyviruses, where a synergistic effect is frequently produced. We have explored the possibility that this synergism could help CMV-LS to overcome cmv1-mediated resistance. We demonstrate that during mixed infection with a potyvirus, CMV-LS is able to overcome cmv1-controlled resistance and develop a systemic infection and that this ability does not depend on an increased accumulation of CMV-LS in mechanically inoculated cotyledons. Likewise, during a mixed infection initiated by aphids, the natural vector of both cucumoviruses and potyviruses that can very efficiently inoculate plants with a low number of virions, CMV-LS also overcomes cmv1-controlled resistance. This indicates that in the presence of a potyvirus, even a very low amount of inoculum, can be sufficient to surpass the resistance and initiate the infection. These results indicate that there is an important risk for this resistance to be broken in nature as a consequence of mixed infections, and therefore, its deployment in elite cultivars would not be enough to ensure a long-lasting resistance.

Keep in contact: multiple roles of endoplasmic reticulum-membrane contact sites and the organelle interaction network in plants

Functional regulation and structural maintenance of the different organelles in plants contribute directly to plant development, reproduction and stress responses. To ensure these activities take place effectively, cells have evolved an interconnected network amongst various subcellular compartments, regulating rapid signal transduction and the exchange of biomaterial. Many proteins that regulate membrane connections have recently been identified in plants, and this is the first step in elucidating both the mechanism and function of these connections. Amongst all organelles, the endoplasmic reticulum is the key structure, which likely links most of the different subcellular compartments through membrane contact sites (MCS) and the ER-PM contact sites (EPCS) have been the most intensely studied in plants. However, the molecular composition and function of plant MCS are being found to be different from other eukaryotic systems. In this article, we will summarise the most recent advances in this field and discuss the mechanism and biological relevance of these essential links in plants.

Tobacco mosaic virus movement protein complements a Potato spindle tuber viroid RNA mutant impaired for mesophyll entry but not mutants unable to enter the phloem

Tobacco mosaic virus movement protein (TMV MP) is essential for virus spread between cells. To accomplish its task, TMV MP binds viral RNA, interacts with components of the cytoskeleton, and increases the size exclusion limit (SEL) of plasmodesmata. Plasmodesmata are gated intercellular channels that allow passage of small molecules and macromolecules, including RNA and protein, between plant cells. Moreover, plasmodesmata are diverse and those connecting different cell types appear to have unique mechanisms to regulate macromolecular trafficking, which likely contributes to the establishment of distinct cell boundaries. Consequently, TMV MP might be competent to mediate RNA transport through some but not all plasmodesmal gates. Due to a lack of viral mutants defective for movement between specific cell types, the ability of TMV MP in this regard is incompletely understood. In contrast, a number of trafficking impaired Potato spindle tuber viroid (PSTVd) mutants have been identified. PSTVd is a systemically infectious non-coding RNA that nevertheless can perform all functions required for replication as well as cell-to-cell and systemic spread. Previous studies have shown that PSTVd employs different structure and sequence elements to move between diverse cell types in host plants, and mutants defective for transport between specific cell types have been identified. Therefore, PSTVd may serve as a tool to analyze the functions of MPs of viral and cellular origin. To probe the RNA transport activity of TMV MP, transgenic plants expressing the protein were inoculated with PSTVd mutants. Remarkably, TMV MP complemented a PSTVd mutant defective for mesophyll entry but could not support two mutants impaired for phloem entry, suggesting it fails to productively interface with plasmodesmata at the phloem boundary and that additional viral and host factors may be required. Consistent with this idea, TMV co-infection, but not the combination of MP and coat protein (CP) expression, was able to complement one of the phloem entry mutants. These observations suggest that phloem loading is a critical impediment to establishing systemic infection that could involve the entire ensemble of TMV proteins. They also demonstrate a novel strategy for analysis of MPs.

Acibenzolar-S-methyl-mediated restriction of loading of plantago asiatica mosaic virus into vascular tissues of Nicotiana benthamiana

Long-distance movement via vascular tissues is an essential step for systemic infection by plant viruses. We previously reported that pre-treatment of Nicotiana benthamiana with acibenzolar-S-methyl (ASM) both suppressed the accumulation of plantago asiatica mosaic virus (PlAMV) in inoculated leaves and delayed the long-distance movement to uninoculated upper leaves. These two effects occurred independently of each other. However, it remained unclear where and when the viral long-distance movement is inhibited upon ASM treatment. In this study, we found that ASM treatment restricted the loading of GFP-expressing PlAMV (PlAMV-GFP) into vascular tissues in the inoculated leaves. This led to delays in viral translocation to the petiole and the main stem, and to untreated upper leaves. We used cryohistological fluorescence imaging to show that ASM treatment affected the viral localization and reduced its accumulation in the phloem, xylem, and mesophyll tissues. A stem girdling experiment, which blocked viral movement downward through phloem tissues, demonstrated that ASM treatment could inhibit viral systemic infection to upper leaves, which occurred even with viral downward movement restricted. Taken together, our results showed that ASM treatment affects the loading of PlAMV-GFP into the vascular system in the inoculated leaf, and that this plays a key role in the ASM-mediated delay of viral long-distance movement.

The m6A RNA Demethylase ALKBH9B Plays a Critical Role for Vascular Movement of Alfalfa Mosaic Virus in Arabidopsis

The N 6-methyladenosine (m6A) pathway has been widely described as a viral regulatory mechanism in animals. We previously reported that the capsid protein (CP) of alfalfa mosaic virus (AMV) interacts with the Arabidopsis m6A demethylase ALKBH9B regulating m6A abundance on viral RNAs (vRNAs) and systemic invasion of floral stems. Here, we analyze the involvement of other ALKBH9 proteins in AMV infection and we carry out a detailed evaluation of the infection restraint observed in alkbh9b mutant plants. Thus, via viral titer quantification experiments and in situ hybridization assays, we define the viral cycle steps that are altered by the absence of the m6A demethylase ALKBH9B in Arabidopsis. We found that ALKBH9A and ALKBH9C do not regulate the AMV cycle, so ALKBH9B activity seems to be highly specific. We also define that not only systemic movement is affected by the absence of the demethylase, but also early stages of viral infection. Moreover, our findings suggest that viral upload into the phloem could be blocked in alkbh9b plants. Overall, our results point to ALKBH9B as a possible new component of phloem transport, at least for AMV, and as a potential target to obtain virus resistance crops.

Carbon-based nanomaterials suppress tobacco mosaic virus (TMV) infection and induce resistance in Nicotiana benthamiana

Although nanomaterials (NMs) may inhibit viral pathogens, the mechanisms governing plant-virus-nanomaterial interactions remain unknown. Nicotiana benthamiana plants were treated with nanoscale titanium dioxide (TiO2) and silver (Ag), C60 fullerenes, and carbon nanotubes (CNTs) at 100, 200 and 500 mg L-1 for a 21-day foliar exposure before inoculation with GFP-tagged tobacco mosaic virus (TMV). Plants treated with CNTs and C60 (200 mg L-1) exhibited normal phenotype and viral symptomology was not evident at 5 days post-infection. TiO2 and Ag failed to suppress viral infection. RT-qPCR analysis revealed that viral coat protein transcript abundance and GFP mRNA expression were reduced 74-81% upon CNTs and C60 treatment. TEM revealed that the chloroplast ultrastructure in carbon NM-treated plants was unaffected by TMV infection. Fluorescence measurement of CNTs and C60 (200 mg L-1) treated plants indicated photosynthesis equivalent to healthy controls. CNTs and C60 induced upregulation of the defense-related phytohormones abscisic acid and salicylic acid by 33-52%; the transcription of genes responsible for phytohormone biosynthesis was elevated by 94-104% in treated plants. Our findings demonstrate the protective role of carbon-based NMs, with suppression of TMV symptoms via hindered physical movement and viral replication. Given the lack of viral phytopathogen treatment options, this work represents a novel area of nano-enabled agriculture.

Conformational behavior of coat protein in plants and association with coat protein-mediated resistance against TMV

Tobacco mosaic virus (TMV) coat protein (CP) self assembles in viral RNA deprived transgenic plants to form aggregates based on the physical conditions of the environment. Transgenic plants in which these aggregates are developed show resistance toward infection by TMV referred to as CP-MR. This phenomenon has been extensively used to protect transgenic plants against viral diseases. The mutants T42W and E50Q CP confer enhanced CP-MR as compared to the WT CP. The aggregates, when examined, show the presence of helical discs in the case of WT CP; on the other hand, mutants show the presence of highly stable non-helical long rods. These aggregates interfere with the accumulation of MP as well as with the disassembly of TMV in plant cells. Here, we explored an atomic level insight to the process of CP-MR through MD simulations. The subunit-subunit interactions were assessed with the help of MM-PBSA calculations. Moreover, classification of secondary structure elements of the protein also provided unambiguous information about the conformational changes occurring in the two chains, which indicated toward increased flexibility of the mutant protein and seconded the other results of simulations. Our finding indicates the essential structural changes caused by the mutation in CP subunits, which are critically responsible for CP-MR and provides an in silico insight into the effects of these transitions over CP-MR. These results could further be utilized to design TMV-CP-based small peptides that would be able to provide appropriate protection against TMV infection.

Developmentally regulated Arabidopsis thaliana susceptibility to tomato spotted wilt virus infection

Tomato spotted wilt virus (TSWV) is one of the most devastating plant viruses and often causes severe crop losses worldwide. Generally, mature plants become more resistant to pathogens, known as adult plant resistance. In this study, we demonstrated a new phenomenon involving developmentally regulated susceptibility of Arabidopsis thaliana to TSWV. We found that Arabidopsis plants become more susceptible to TSWV as plants mature. Most young 3-week-old Arabidopsis were not infected by TSWV. Infection of TSWV in 4-, 5-, and 6-week-old Arabidopsis increased from 9%, 21%, and 25%, respectively, to 100% in 7- to 8-week-old Arabidopsis plants. Different isolates of TSWV and different tospoviruses show a low rate of infection in young Arabidopsis but a high rate in mature plants. When Arabidopsis dcl2/3/4 or rdr1/2/6 mutant plants were inoculated with TSWV, similar results as observed for the wild-type Arabidopsis plants were obtained. A cell-to-cell movement assay showed that the intercellular movement efficiency of TSWV NSm:GFP fusion was significantly higher in 8-week-old Arabidopsis leaves compared with 4-week-old Arabidopsis leaves. Moreover, the expression levels of pectin methylesterase and β-1,3-glucanase, which play critical roles in macromolecule cell-to-cell trafficking, were significantly up-regulated in 8-week-old Arabidopsis leaves compared with 4-week-old Arabidopsis leaves during TSWV infection. To date, this mature plant susceptibility to pathogen infections has rarely been investigated. Thus, the findings presented here should advance our knowledge on the developmentally regulated mature host susceptibility to plant virus infection.

Viral Hacks of the Plant Vasculature: The Role of Phloem Alterations in Systemic Virus Infection

For plant viruses, the ability to load into the vascular phloem and spread systemically within a host is an essential step in establishing a successful infection. However, access to the vascular phloem is highly regulated, representing a significant obstacle to virus loading, movement, and subsequent unloading into distal uninfected tissues. Recent studies indicate that during virus infection, phloem tissues are a source of significant transcriptional and translational alterations, with the number of virus-induced differentially expressed genes being four- to sixfold greater in phloem tissues than in surrounding nonphloem tissues. In addition, viruses target phloem-specific components as a means to promote their own systemic movement and disrupt host defense processes. Combined, these studies provide evidence that the vascular phloem plays a significant role in the mediation and control of host responses during infection and as such is a site of considerable modulation by the infecting virus. This review outlines the phloem responses and directed reprograming mechanisms that viruses employ to promote their movement through the vasculature.

Key checkpoints in the movement of plant viruses through the host

Plant viruses cannot exploit any of the membrane fusion-based routes of entry described for animal viruses. In addition, one of the distinctive structures of plant cells, the cell wall, acts as the first barrier against the invasion of pathogens. To overcome the rigidity of the cell wall, plant viruses normally take advantage of the way of life of different biological vectors. Alternatively, the physical damage caused by environmental stresses can facilitate virus entry. Once inside the cell and taking advantage of the characteristic symplastic continuity of plant cells, viruses need to remodel and/or modify the restricted pore size of the plasmodesmata (channels that connect plant cells). In a successful interaction for the virus, it can reach the vascular tissue to systematically invade the plant. The connections between the different cell types in this path are not designed to allow the passage of molecules with the complexity of viruses. During this process, viruses face different cell barriers that must be overcome to reach the distal parts of the plant. In this review, we highlight the current knowledge about how plant RNA viruses enter plant cells, move between them to reach vascular cells and overcome the different physical and cellular barriers that the phloem imposes. Finally, we update the current research on cellular organelles as key regulator checkpoints in the long-distance movement of plant viruses.

Viral sequences required for efficient viral infection differ between two Chinese pepper mild mottle virus isolates

Pepper mild mottle virus (PMMoV) causes mosaic symptoms and malformation on both leaf and fruit of pepper, reduces considerable economical yields and poses threats to human health. In this study, infectious clone of PMMoV Huludao (HLD) isolate (pCB-PMMoV-HLD) was constructed and its infectious ablility in Nicotiana benthamiana was confirmed by virions observation and Northern blot analysis. The mutant PMMoV (HLD-fsCP) that cannot express coat protein (CP) showed reduced viral accumulation but can systemically infect N. benthamiana. We constructed several chimeric mutant viruses (ZA-HB-HC, HA-ZB-HC, HA-HB-ZC and HA-ZB-ZC) by sequences substitution between PMMoV-HLD and PMMoV Zhejiang isolates (PMMoV-ZJ) and analyzed their infectious abilities in N. benthamiana and Capsicum annuum. The results showed that the chimera virus expressed by pCB-ZA-HB-HC, pCB-HA-HB-ZC and pCB-HA-ZB-ZC, but not by pCB-HA-ZB-HC, exhibited reduced infectious ability compared with wild-type PMMoV-ZJ and PMMoV-HLD, which indicated that RNA sequences required for efficient infection of PMMoV differ between the two virus isolates. The differential requirement of viral RNA sequences for efficient PMMoV infection provided theoretical value to further understand the infection and pathogenesis of PMMoV.

Ins and Outs of Multipartite Positive-Strand RNA Plant Viruses: Packaging versus Systemic Spread

Viruses possessing a non-segmented genome require a specific recognition of their nucleic acid to ensure its protection in a capsid. A similar feature exists for viruses having a segmented genome, usually consisting of viral genomic segments joined together into one viral entity. While this appears as a rule for animal viruses, the majority of segmented plant viruses package their genomic segments individually. To ensure a productive infection, all viral particles and thereby all segments have to be present in the same cell. Progression of the virus within the plant requires as well a concerted genome preservation to avoid loss of function. In this review, we will discuss the "life aspects" of chosen phytoviruses and argue for the existence of RNA-RNA interactions that drive the preservation of viral genome integrity while the virus progresses in the plant.

Tobacco mosaic virus-directed reprogramming of auxin/indole acetic acid protein transcriptional responses enhances virus phloem loading

Vascular phloem loading has long been recognized as an essential step in the establishment of a systemic virus infection. In this study, an interaction between the replication protein of tobacco mosaic virus (TMV) and phloem-specific auxin/indole acetic acid (Aux/IAA) transcriptional regulators was found to modulate virus phloem loading in an age-dependent manner. Promoter expression studies show that in mature tissues TMV 126/183-kDa-interacting Aux/IAAs predominantly express and accumulate within the nuclei of phloem companion cells (CCs). Furthermore, CC Aux/IAA nuclear localization is disrupted upon infection with an interacting virus. In situ analysis of virus spread shows that the inability to disrupt Aux/IAA CC nuclear localization correlates with a reduced ability to load into the vascular tissue. Subsequent systemic movement assays also demonstrate that a virus capable of disrupting Aux/IAA localization is significantly more competitive at moving out of older plant tissues than a noninteracting virus. Similarly, CC expression and overaccumulation of a degradation-resistant Aux/IAA-interacting protein was found to inhibit TMV accumulation and phloem loading selectively in flowering plants. Transcriptional expression studies demonstrate a role for Aux/IAA-interacting proteins in the regulation of salicylic and jasmonic acid host defense responses as well as virus-specific movement factors, including pectin methylesterase, that are involved in regulating plasmodesmata size-exclusion limits and promoting virus cell-to-cell movement. Combined, these findings indicate that TMV directs the reprogramming of auxin-regulated gene expression within the vascular phloem of mature tissues as a means to enhance phloem loading and systemic spread.

Hibiscus chlorotic ringspot virus coat protein is essential for cell-to-cell and long-distance movement but not for viral RNA replication

Hibiscus chlorotic ringspot virus (HCRSV) is a member of the genus Carmovirus in the family Tombusviridae. In order to study its coat protein (CP) functions on virus replication and movement in kenaf (Hibiscus cannabinus L.), two HCRSV mutants, designated as p2590 (A to G) in which the first start codon ATG was replaced with GTG and p2776 (C to G) in which proline 63 was replaced with alanine, were constructed. In vitro transcripts of p2590 (A to G) were able to replicate to a similar level as wild type without CP expression in kenaf protoplasts. However, its cell-to-cell movement was not detected in the inoculated kenaf cotyledons. Structurally the proline 63 in subunit C acts as a kink for β-annulus formation during virion assembly. Progeny of transcripts derived from p2776 (C to G) was able to move from cell-to-cell in inoculated cotyledons but its long-distance movement was not detected. Virions were not observed in partially purified mutant virus samples isolated from 2776 (C to G) inoculated cotyledons. Removal of the N-terminal 77 amino acids of HCRSV CP by trypsin digestion of purified wild type HCRSV virions resulted in only T = 1 empty virus-like particles. Taken together, HCRSV CP is dispensable for viral RNA replication but essential for cell-to-cell movement, and virion is required for the virus systemic movement. The proline 63 is crucial for HCRSV virion assembly in kenaf plants and the N-terminal 77 amino acids including the β-annulus domain is required in T = 3 assembly in vitro.

The photosystem II oxygen-evolving complex protein PsbP interacts with the coat protein of Alfalfa mosaic virus and inhibits virus replication

Alfalfa mosaic virus (AMV) coat protein (CP) is essential for many steps in virus replication from early infection to encapsidation. However, the identity and functional relevance of cellular factors that interact with CP remain unknown. In an unbiased yeast two-hybrid screen for CP-interacting Arabidopsis proteins, we identified several novel protein interactions that could potentially modulate AMV replication. In this report, we focus on one of the novel CP-binding partners, the Arabidopsis PsbP protein, which is a nuclear-encoded component of the oxygen-evolving complex of photosystem II. We validated the protein interaction in vitro with pull-down assays, in planta with bimolecular fluorescence complementation assays, and during virus infection by co-immunoprecipitations. CP interacted with the chloroplast-targeted PsbP in the cytosol and mutations that prevented the dimerization of CP abolished this interaction. Importantly, PsbP overexpression markedly reduced virus accumulation in infected leaves. Taken together, our findings demonstrate that AMV CP dimers interact with the chloroplast protein PsbP, suggesting a potential sequestration strategy that may preempt the generation of any PsbP-mediated antiviral state.

Factors involved in the systemic transport of plant RNA viruses: the emerging role of the nucleus

Compatible virus-host interactions depend on a suitable milieu in the host cells permitting viral gene expression, replication, and spread. During pathogenesis, viruses hijack the plant cellular machinery to access molecules, subcellular structures, and host transport pathways needed for infection. Vascular trafficking of virus transport forms (VTF) within the phloem is a crucial step in setting-up virus infection within the entire plant. Moreover, vascular trafficking is an essential step for the further transmission of the viruses by their natural vectors as movement of the viruses to the distant parts of the plant from the initial site of infection guarantees accessibility of the virus particle for vector transmission. With the recent advances in the field of plant virology several emerging themes of viral systemic movement occur linking the role of virus-mediated transcriptional reprogramming and nuclear factors in vascular trafficking. Recent studies have uncovered host factors involved in virus vascular trafficking. Surprisingly, it appears that the role of the nucleus and nuclear factors in virus movement is still under-appreciated. This review describes how these new themes started to emerge by using two contrasting modes of virus vascular trafficking. It is argued that the translocation of viral movement proteins into the nuclei is, in many cases, an essential step in promoting virus systemic infection.

Dynamics of the establishment of systemic Potyvirus infection: independent yet cumulative action of primary infection sites

In the clinic, farm, or field, for many viruses there is a high prevalence of mixed-genotype infections, indicating that multiple virions have initiated infection and that there can be multiple sites of primary infection within the same host. The dynamic process by which multiple primary infection sites interact with each other and the host is poorly understood, undoubtedly due to its high complexity. In this study, we attempted to unravel the basic interactions underlying this process using a plant RNA virus, as removing the inoculated leaf can instantly and rigorously eliminate all primary infection sites. Effective population size in the inoculated leaf and time of removal of the inoculated leaf were varied in experiments, and it was found that both factors positively influenced if the plant became systemically infected and what proportion of cells in the systemic tissue were infected, as measured by flow cytometry. Fitting of probabilistic models of infection to our data demonstrated that a null model in which the action of each focus is independent of the presence of other foci was better supported than a dependent-action model. The cumulative effect of independently acting foci therefore determined when plants became infected and how many individual cells were infected. There was no evidence for interference between primary infection sites, which is surprising given the planar structure of leaves. By showing that a simple null model is supported, we experimentally confirmed--to our knowledge for the first time--the minimal components that dictate interactions of a conspecific virus population establishing systemic infection.

Analysis of protein transport in the Brassica oleracea vasculature reveals protein-specific destinations

We investigated the vascular transport properties of exogenously applied proteins to Brassica oleracea plants and compared their delivery to various aerial parts of the plant with carboxy fluorescein (CF) dye. We identified unique properties for each protein. Alexafluor-tagged bovine serum albumin (Alexa-BSA) and Alexafluor-tagged Histone H1 (Alexa-Histone) moved slower than CF dye throughout the plant. Interestingly, Alexa-Histone was retained in the phloem and phloem parenchyma while Alexa-BSA moved into the apoplast. One possibility is that Alexa-Histone sufficiently resembles plant endogenous proteins and is retained in the vascular stream, while Alexa-BSA is exported from the cell as a foreign protein. Both proteins diffuse from the leaf veins into the leaf lamina. Alexa-BSA accumulated in the leaf epidermis while Alexa-Histone accumulated mainly in the mesophyll layers. Fluorescein-tagged hepatitis C virus core protein (fluorescein-HCV) was also delivered to B. oleracea plants and is larger than Alexa-BSA. This protein moves more rapidly than BSA through the plant and was restricted to the leaf veins. Fluorescein-HCV failed to unload to the leaf lamina. These combined data suggest that there is not a single default pathway for the vascular transfer of exogenous proteins in B. oleracea plants. Specific protein properties appear to determine their destination and transport properties within the phloem.

The nucleocapsid protein of an enveloped plant virus, Tomato spotted wilt virus, facilitates long-distance movement of Tobacco mosaic virus hybrids

To investigate the potential role(s) of the nucleocapsid (N) protein of Tomato spotted wilt virus (TSWV), the open reading frame for the N protein was expressed from a Tobacco mosaic virus (TMV)-based vector encoding only the TMV replicase proteins. In the absence of other TSWV-encoded proteins, the transiently expressed N protein facilitated long-distance movement of the TMV-based hybrids in transgenic Nicotiana benthamiana [NB-MP(+)] expressing movement protein of TMV, thus providing the functional demonstration of the N protein in long-distance RNA movement. Removal of the N-terminal 39 amino acids (N-NΔ39), the C-terminal 26 amino acids (N-CΔ26) or both of them (N-NΔ39CΔ26) abolished the long-distance movement function, indicating the essential role of both N- and C-terminus. In contrast, alanine substitution of the phenylalanines at positions 242 and 246 (N242/262A), two crucial amino acids for homotypic interaction of the N protein, had little effect, suggesting that the N protein could function in long-distance movement in the form of monomers. In addition, both the wild type N and the alanine mutant N242/262A hardly induced local symptoms in NB-MP(+) plants and TMV-MP transgenic N. tabacum cv. Xanthi. The deletion mutants N-NΔ39, N-CΔ26 and N-NΔ39CΔ26, however, induced apparent symptoms of necrotic ringspots, necrosis or chlorotic spots in all inoculated leaves. On the basis of these findings, the potential role of N during the TSWV infection was discussed. To our knowledge, this is the first report that the N protein of an enveloped plant virus functioned in long-distance movement.

Short distance movement of genomic negative strands in a host and nonhost for Sugarcane mosaic virus (SCMV)

Background

In order to obtain an initial and preliminary understanding of host and nonhost resistance in the initial step of potyvirus replication, both positive and negative Sugarcane mosaic virus (SCMV) strands where traced in inoculated and systemic leaves in host and nonhost resistant maize and sugarcane for one Mexican potyviral isolate (SCMV-VER1). Intermediary replication forms, such as the negative viral strand, seem to only move a short distance as surveyed by RT-PCR analysis and ELISA in different leaves. Virus purification was also done in leaves and stems.

Results

Susceptible maize plants allowed for viral SCMV replication, cell-to-cell, and long distance movement, as indicated by the presence of the coat protein along the plant. In the host resistant maize plants for the SCMV-VER1 isolate, the virus was able to establish the disease though the initial steps of virus replication, as detected by the presence of negative strands, in the basal area of the inoculated leaves at six and twelve days post inoculation. The nonhost sugarcane for SCMV-VER1 and the host sugarcane for SCMV-CAM6 also allowed the initial steps of viral replication for the VER1 isolate in the local inoculated leaf. SCMV-VER1 virions could be extracted from stems of susceptible maize with higher titers than leaves.

Conclusion

Nonhost and host resistance allow the initial steps of potyvirus SCMV replication, as shown by the negative strands' presence. Furthermore, both hosts allow the negative viral strands' local movement, but not their systemic spread through the stem. The presence of larger amounts of extractable virions from the stem (as compared to the leaves) in susceptible maize lines suggests their long distance movement as assembled particles. This will be the first report suggesting the long distance movement of a monocot potyvirus as a virion.

Mutation of a chloroplast-targeting signal in Alternanthera mosaic virus TGB3 impairs cell-to-cell movement and eliminates long-distance virus movement

Cell-to-cell movement of potexviruses requires coordinated action of the coat protein and triple gene block (TGB) proteins. The structural properties of Alternanthera mosaic virus (AltMV) TGB3 were examined by methods differentiating between signal peptides and transmembrane domains, and its subcellular localization was studied by Agrobacterium-mediated transient expression and confocal microscopy. Unlike potato virus X (PVX) TGB3, AltMV TGB3 was not associated with the endoplasmic reticulum, and accumulated preferentially in mesophyll cells. Deletion and site-specific mutagenesis revealed an internal signal VL(17,18) of TGB3 essential for chloroplast localization, and either deletion of the TGB3 start codon or alteration of the chloroplast-localization signal limited cell-to-cell movement to the epidermis, yielding a virus that was unable to move into the mesophyll layer. Overexpression of AltMV TGB3 from either AltMV or PVX infectious clones resulted in veinal necrosis and vesiculation at the chloroplast membrane, a cytopathology not observed in wild-type infections. The distinctive mesophyll and chloroplast localization of AltMV TGB3 highlights the critical role played by mesophyll targeting in virus long-distance movement within plants.

Long-distance transport of macromolecules through the phloem

Long-distance phloem transport of small metabolites has long been the subject of many different studies concentrating on resource allocation and signalling between plant organs. Also, phloem movement of viruses has long been examined as the route for systemic infection of the plant. Only recently, the transport of macromolecules, such as proteins and nucleic acids, has received increasing attention because they are regarded as being a new class of potential information-transmitter. A set of recent publications allows the first insights into the important roles that phloem-mobile macromolecules might play in the regulation of development and the responses to stress. Furthermore, they start to shed light on the mechanisms involved in systemic macromolecule transport.

Immunolocalization of solanaceous SUT1 proteins in companion cells and xylem parenchyma: new perspectives for phloem loading and transport

Leaf sucrose (Suc) transporters are essential for phloem loading and long-distance partitioning of assimilates in plants that load their phloem from the apoplast. Suc loading into the phloem is indispensable for the generation of the osmotic potential difference that drives phloem bulk flow and is central for the long-distance movement of phloem sap compounds, including hormones and signaling molecules. In previous analyses, solanaceous SUT1 Suc transporters from tobacco (Nicotiana tabacum), potato (Solanum tuberosum), and tomato (Solanum lycopersicum) were immunolocalized in plasma membranes of enucleate sieve elements. Here, we present data that identify solanaceous SUT1 proteins with high specificity in phloem companion cells. Moreover, comparisons of SUT1 localization in the abaxial and adaxial phloem revealed higher levels of SUT1 protein in the abaxial phloem of all three solanaceous species, suggesting different physiological roles for these two types of phloem. Finally, SUT1 proteins were identified in files of xylem parenchyma cells, mainly in the bicollateral veins. Together, our data provide new insight into the role of SUT1 proteins in solanaceous species.

Distribution and pathway for phloem-dependent movement of Melon necrotic spot virus in melon plants

The translocation of Melon necrotic spot virus (MNSV) within tissues of inoculated and systemically infected Cucumis melo L. 'Galia' was studied by tissue-printing and in situ hybridization techniques. The results were compatible with the phloem vascular components being used to spread MNSV systemically by the same assimilate transport route that runs from source to sink organs. Virus RNAs were shown to move from the inoculated cotyledon toward the hypocotyl and root system via the external phloem, whereas the upward spread through the stem to the young tissues took place via the internal phloem. Virus infection was absent from non-inoculated source tissues as well as from both shoot and root apical meristems, but active sink tissues such as the young leaves and root system were highly infected. Finally, our results suggest that the MNSV invasion of roots is due to virus replication although a destination-selective process is probably necessary to explain the high levels of virus accumulation in roots. This efficient invasion of the root system is discussed in terms of natural transmission of MNSV by the soil-borne fungal vector.

Tobacco mosaic virus defective RNAs expressing C-terminal methyltransferase domain sequences are severely impaired in long-distance movement in Nicotiana benthamiana

Tobamovirus replicase proteins, which function in replication and gene expression, are also implicated in viral cell-to-cell and long-distance movement. The role(s) of Tobacco mosaic virus (TMV) 126-/183-kDa replicase protein in the complex movement process are not understood due to lack of systems that can separate the multiple steps involved. We previously developed a bipartite TMV-defective RNA (dRNA) system to dissect the role of the N-terminal methyltransferase (MT) domain in accumulation and cell-to-cell movement of dRNAs [Knapp, E., Danyluk, G.M., Achor, D., Lewandowski, D.J., 2005. A bipartite Tobacco mosaic virus-defective RNA (dRNA) system to study the role of the N-terminal methyltransferase domain in cell-to-cell movement of dRNAs. Virology 341, 47-58]. In the current study we analyzed long-distance movement of dRNAs in the presence of helper virus in Nicotiana benthamiana. dRNAs expressing approximately 50% of the MT domain (DeltaHinc151) moved long-distances in more than half of the plants. dRNAs expressing approximately 90% of the MT domain sequences (DeltaCla151) predominantly failed to accumulate in upper leaves. The helper virus moved systemically when inoculated alone or with a dRNA. In inoculated leaves, more DeltaHinc151-induced infection foci spread adjacent to class V veins compared to those of DeltaCla151. Consequently, DeltaHinc151 infected more class V veins than DeltaCla151. DeltaCla151 was only detected in bundle sheath cells, whereas DeltaHinc151 could accumulate in bundle sheath and phloem parenchyma cells of class V veins. However, the latter accumulation pattern did not always result in systemic accumulation of DeltaHinc151, suggesting that factors in addition to those affecting cell-to-cell movement played a role in long-distance movement.

Viral infection enables phloem loading of GFP and long-distance trafficking of the protein

It is generally accepted that viral systemic infection follows the source-to-sink symplastic pathway of sugar translocation. In plants that are classified as apoplastic loaders, the boundary between the companion cell-sieve element (CC-SE) complex and neighboring cells is symplastically restricted, and the potential passage of macromolecules between the two domains has yet to be explored. Transgenic tobacco plants expressing green fluorescence protein (GFP) and cucumber mosaic virus (CMV)-encoded proteins fused to GFP under the control of the fructose-1,6-bisphosphatase (FBPase) promoter were produced in order to localize the encoded proteins in mesophyll and bundle sheath cells and to explore the influence of viral infection on the functioning of plasmodesmata interconnecting the two domains. GFP produced outside the vascular tissue could overcome the symplastic barrier between the CC-SE complex and the surrounding cells to enter the vasculature in CMV-infected plants. Grafting of control (non-transgenic) tobacco scions to CMV-infected FBPase-GFP-expressing root stocks confirmed that GFP could move long distances in the phloem. No movement of the gfp mRNA was noticeable in this set of experiments. The ability of GFP to enter the vasculature and move long distances was also evident upon infection of the grafting plants with other viruses. These results provide experimental evidence for alteration of the functioning of plasmodesmata interconnecting the CC-SE complex and neighboring cells by viral infection to enable non-selective trafficking of macromolecules from the mesophyll into the sieve tube.

The role of the C-terminal region of olive latent virus 1 coat protein in host systemic infection

A full-length cDNA clone of olive latent virus 1 (OLV-1), a member of the genus Necrovirus, family Tombusviridae, was subjected to site-directed mutagenesis, and coat protein gene mutants were constructed. A mutant clone, denoted Delta3297, was obtained by deleting the nucleotide in position 3297, thus inducing a frameshift and replacing the last 49 amino acids of the viral coat protein (CP) by a shorter sequence of 39 amino acids. This mutant was viable, stable, able to synthesize a smaller CP, and able to give rise to the formation of apparently intact virus particles. Cell-to-cell movement of Delta3297 in Nicotiana benthamiana leaves was not affected, but, contrary to wild type OLV-1, it failed to spread systemically. These results indicate that virion formation is necessary but not sufficient for long-distance movement for OLV-1 and highlights the role of the CP carboxy-terminal domain in systemic infection.

Functional analysis of the 5' untranslated region of potexvirus RNA reveals a role in viral replication and cell-to-cell movement

Cell-to-cell movement of potexviruses requires cognate recognition between the viral RNA, the triple gene block proteins (TGBp1-3) and the coat protein (CP). cis-acting motifs required for recognition and translocation of viral RNA were identified using an artificial potexvirus defective RNA encoding a green fluorescent protein (GFP) reporter transcriptionally fused to the terminal viral sequences. Analysis of GFP fluorescence produced in vivo from these defective RNA constructs, referred to as chimeric RNA reporters, was used to identify viral cis-acting motifs required for RNA trafficking. Mapping experiments localized the cis-acting element to nucleotides 1-107 of the Potato virus X (PVX) genome. This sequence forms an RNA secondary structural element that has also been implicated in viral plus-strand accumulation [Miller, E.D., Plante, C.A., Kim, K.-H., Brown, J.W. and Hemenway, C. (1998) J. Mol. Biol. 284, 591-608]. While replication and movement functions associated with this region have not been separated, these results are consistent with sequence-specific recognition of RNA by the viral movement protein(s). This situation is unusual among viral movement proteins that typically function to translocate RNA between cells in a non-sequence-specific manner. These data support the concept of cis-acting elements specifying intercellular potexvirus RNA movement and thus provide a basis for dissection of RNA-mediated intercellular communication in plants.

Potential involvement of a cucumber homolog of phloem protein 1 in the long-distance movement of Cucumber mosaic virus particles

The systemic movement of Cucumber mosaic virus (CMV) in cucumber plants was analyzed. The structure that is translocated and its putative interactions with phloem components were analyzed in phloem exudate (PE) samples, which reflect sieve tubes stream composition. Rate zonal centrifugation and electron-microscopy analyses of PE from CMV-infected plants showed that CMV moves through sieve tubes as virus particles. Gel overlay assays revealed that CMV particles interact with a PE protein, p48. The amino-acid sequence of several tryptic peptides of p48 was determined. Partial amino-acid sequence of p48 showed it was a cucumber homolog of phloem protein 1 (PP1) from pumpkin, with which p48 also shares several chemical properties. PP1 from pumpkin has plasmodesmata-gating ability and translocates in sieve tubes. Encapsidated CMV RNA in PE samples from infected plants was less accessible to digestion by RNase A than RNA in purified CMV particles, a property that was reconstituted by the in vitro interaction of purified CMV particles and protein p48. These results indicate that the interaction with p48 modifies CMV particle structure and suggest that CMV particles interact with the cucumber homolog of PP1 during translocation in the sieve tubes.

Interference of Long-Distance Movement of Grapevine berry inner necrosis virus in Transgenic Plants Expressing a Defective Movement Protein of Apple chlorotic leaf spot virus

ABSTRACT Transgenic Nicotiana occidentalis plants expressing a movement protein (P50) and partially functional deletion mutants (DeltaA and DeltaC) of the Apple chlorotic leaf spot virus (ACLSV) showed resistance to Grapevine berry inner necrosis virus (GINV). The resistance is highly effective and GINV was below the level of detection in both inoculated and uninoculated upper leaves. In contrast, GINV accumulated in inoculated and uninoculated leaves of nontransgenic (NT) plants and transgenic plants expressing a dysfunctional mutant (DeltaG). On the other hand, in some plants of a transgenic plant line expressing a deletion mutant (DeltaA', deletion of the C-terminal 42 amino acids), GINV could spread in inoculated leaves, but not move into uninoculated leaves. In a tissue blot hybridization analysis of DeltaA'-plants inoculated with GINV, virus could be detected in leaf blade, midribs, and petiole of inoculated leaves, but neither in stems immediately above inoculated leaves nor in any tissues of uninoculated leaves. Immunohistochemical analysis of GINV-inoculated leaves of DeltaA'-plants showed that GINV could invade into phloem parenchyma cells through bundle sheath of minor veins, suggesting that the long-distance transport of GINV might be inhibited between the phloem cells and sieve element (and/or within sieve element) rather than bundle sheath-phloem interfaces. Immunogold electron microscopy using an anti-P50 antiserum showed that P50 accumulated on the parietal layer of sieve elements and on sieve plates. The results suggested that resistance in P50-transgenic plants to GINV is due to the interference of both long-distance and cell-to-cell movement of the virus.

Integrative plant biology: role of phloem long-distance macromolecular trafficking

Recent studies have revealed the operation of a long-distance communication network operating within the vascular system of higher plants. The evolutionary development of this network reflects the need to communicate environmental inputs, sensed by mature organs, to meristematic regions of the plant. One consequence of such a long-distance signaling system is that newly forming organs can develop properties optimized for the environment into which they will emerge, mature, and function. The phloem translocation stream of the angiosperms contains, in addition to photosynthate and other small molecules, a variety of macromolecules, including mRNA, small RNA, and proteins. This review highlights recent progress in the characterization of phloem-mediated transport of macromolecules as components of an integrated long-distance signaling network. Attention is focused on the role played by these proteins and RNA species in coordination of developmental programs and the plant's response to both environmental cues and pathogen challenge. Finally, the importance of developing phloem transcriptome and proteomic databases is discussed within the context of advances in plant systems biology.

Identification of a tobacco protein interacting with tomato mosaic virus coat protein and facilitating long-distance movement of virus

The protein-protein interaction of virus and host is essential for virus infection and host defense. The coat protein (CP) of tomato mosaic virus (ToMV) has been proved to be involved in cell-to-cell and long-distance movements of viruses that are presumably related with the protein-protein interactions. However, the host proteins that interact with the ToMV coat protein (ToCP) are largely unknown. In this study, we isolated a cDNA from a tobacco library through yeast two-hybrid system, which encodes a protein designated the ToMV CP-interacting protein-L (IP-L) that interacted with ToCP in vitro and in vivo. Sequencing analysis revealed that the putative coding region of IP-L gene was identical to that of an 'elicitor responsive protein' gene from N. tabacum (Genbank: #AB040409). A homology was also found between the cDNA sequence of IP-L and two senescence-related cDNAs (SENU1: Z75523 and AY479987) isolated from tomato and pepper. Northern blotting analysis showed that the mRNA level of IP-L was elevated after infection of ToMV. Then, we investigated the in vivo function of IP-L using virus-induced gene silencing (VIGS) and virus challenging assay. Semi-quantitative RT-PCR and Northern blotting results showed that the endogenous mRNA of IP-L in N. benthamiana plant was silenced at 10 days post inoculation with the in vitro transcripts of PVX-IP-L that were produced from the potato virus X (PVX)-based gene silencing plasmid pPC2S.IP-L. The IP-L silent plant developed a delayed systemic symptom at 7 days post challenging with ToMV, indicating that a high expression of IP-L was necessary for the interaction with ToCP to assist the viral transportation. Together, our data suggested that IP-L is a novel plant factor that interacts with the coat protein of ToMV and facilitates the long-distance movement of virus, which may provide a valuable clue for us to further investigate the mechanisms of plant virus infection and to control plant virus diseases.

Analysis of the systemic colonization of cucumber plants by Cucumber green mottle mosaic virus

Systemic movement of Cucumber green mottle mosaic virus (CGMMV) in cucumber plants was shown to be from photoassimilate source to sink, thus indicating phloem transport. Nevertheless, CGMMV was not detected by immunocytochemical procedures in the intermediary cell-sieve element complex in inoculated cotyledons, where photoassimilate loading occurs. In stem internodes, CGMMV was first localized in the companion cells of the external phloem and subsequently in all tissues except the medulla, therefore suggesting leakage of the virus from, and reloading into, the transport phloem during systemic movement. In systemically infected sink leaves, CGMMV was simultaneously detected in the xylem and phloem. Interestingly, CGMMV accumulated to high levels in the differentiating tracheids of young leaves implying that the xylem could be involved in the systemic movement of CGMMV. This possibility was tested using plants in which cell death was induced in a portion of the stem by steam treatment. At 24 degrees C, steam treatment effectively prevented the systemic movement of CGMMV, even though viral RNA was detected in washes of the xylem above the steamed internode suggesting that xylem circulation occurred. At 29 degrees C, CGMMV systemically infected steam-treated cucumber plants, indicating that CGMMV can move systemically via the xylem. Xylem transport of CGMMV was, however, less efficient than phloem transport in terms of the time required for systemic infection and the percentage of plants infected.

Cucumber mosaic virus Establishes Systemic Infection at Increased Temperature Following Viral Entrance Into the Phloem Pathway of Tetragonia expansa

ABSTRACT A potential regulatory site for Cucumber mosaic virus (CMV, pepo strain) movement necessary to establish systemic infection was identified through immunological and hybridization studies on Tetragonia expansa, which was systemically infected by CMV at 36 degrees C but not at 24 degrees C. In inoculated leaves, cell-to-cell movement of CMV was enhanced at 36 degrees C compared with that observed at 24 degrees C. CMV was distributed in the phloem cells of minor veins as well as epidermal and mesophyll cells at both 36 and 24 degrees C. CMV was detected in the petioles of inoculated leaves, stems, and petioles of uninoculated upper leaves at 36 degrees C, whereas CMV was detected only in the petioles of inoculated leaves and in stems at 24 degrees C. CMV moved into the phloem and was transported to the stem within 24 h postinoculation (hpi) at 36 degrees C. However, it did not accumulate in the petioles of the upper leaves until 36 hpi. In petioles of inoculated leaves at 24 degrees C, CMV was detected in the external phloem but not in the internal phloem. From these results, we conclude that systemic infection is established after viral entrance into the phloem pathway in T. expansa at 36 degrees C.

Host-dependent requirement for the Potato leafroll virus 17-kda protein in virus movement

The requirement for the 17-kDa protein (P17) of Potato leafroll virus (PLRV) in virus movement was investigated in four plant species: potato (Solanum tuberosum), Physalis floridana, Nicotiana benthamiana, and N. clevelandii. Two PLRV P17 mutants were characterized, one that does not translate the P17 and another that expresses a P17 missing the first four amino acids. The P17 mutants were able to replicate and accumulate in agroinoculated leaves of potato and P. floridana, but they were unable to move into vascular tissues and initiate a systemic infection in these plants. In contrast, the P17 mutants were able to spread systemically from inoculated leaves in both Nicotiana spp., although the efficiency of infection was reduced relative to wild-type PLRV. Examination of virus distribution in N. benthamiana plants using tissue immunoblotting techniques revealed that the wild-type PLRV and P17 mutants followed a similar movement pathway out of the inoculated leaves. Virus first moved upward to the apical tissues and then downward. The P17 mutants, however, infected fewer phloem-associated cells, were slower than wild-type PLRV in moving out of the inoculated tissue and into apical tissues, and were unable to infect any mature leaves present on the plant at the time of inoculation.

Characterization of mutant tobacco mosaic virus coat protein that interferes with virus cell-to-cell movement

Expression of tobacco mosaic virus (TMV) coat protein (CP) in plants confers resistance to infection by TMV and related tobamoviruses. Certain mutants of the CP (CP(T42W)) provide much greater levels of resistance than wild-type (wt) CP. In the present work, infection induced by RNA transcripts of TMV clones that contain wt CP or mutant CP(T42W) fused to the green fluorescent protein (GFP) (TMV-CP:GFP, TMV-CP(T42W):GFP) and clones harboring TMV movement protein (MP):GFP were followed in nontransgenic and transgenic tobacco BY-2 protoplasts and Nicotiana tabaccum Xanthi-nn plants that express wt CP or CP(T42W). On nontransgenic and wt CP transgenic plants, TMV-CP:GFP produced expanding, highly fluorescent disk-shaped areas. On plants expressing CP(T42W), infection by TMV-CP:GFP or TMV-MP:GFP-CP produced infection sites of smaller size that were characterized by low fluorescence, reflecting reduced levels of virus spread and reduced accumulation of both CP:GFP and MP:GFP. TMV-CP(T42W):GFP failed to produce visible infection sites on nontransgenic plants, yet produced normal infection sites on MP-transgenic plants that produce MP. TMV infection of transgenic BY-CP(T42W) protoplasts resulted in very low levels of MP accumulation, whereas on BY-CP protoplasts (containing wt CP), infection produced higher levels of MP than in nontransgenic BY-2 cells. The results suggest that wt CP has a positive effect on the production of MP, whereas the CP(T42W) has a negative effect on MP accumulation and/or function. This effect results in very high levels of resistance to TMV infection in plants containing CP(T42W). This report shows that the CP of a plant virus regulates production of the MP, and that a mutant CP interferes with MP accumulation and cell-to-cell movement of infection.

Umbravirus-encoded proteins both stabilize heterologous viral RNA and mediate its systemic movement in some plant species

The proteins encoded by open reading frame 3 (ORF3) of the umbraviruses pea enation mosaic virus-2 and tobacco mottle virus, like that of groundnut rosette virus, mediated the movement of viral RNA through the phloem of infected Nicotiana benthamiana or N. clevelandii plants when they were expressed from chimeric tobacco mosaic virus in place of the coat protein. However, these chimeras did not move systemically in N. tabacum. In lysates of N. benthamiana or N. tabacum protoplasts, the chimeric RNAs were more stable than was RNA of tobacco mosaic virus lacking the coat protein gene. The chimeric viruses also protected the latter in trans, suggesting that the ORF3 proteins can increase the stability of heterologous viral RNA. Umbraviral ORF3 proteins contain a conserved arginine-rich domain, and the possible roles of this motif in the functions of the proteins are discussed.

The multifunctional capsid proteins of plant RNA viruses

This article summarizes studies of viral coat (capsid) proteins (CPs) of RNA plant viruses. In addition, we discuss and seek to interpret the knowledge accumulated to data. CPs are named for their primary function; to encapsidate viral genomic nucleic acids. However, encapsidation is only one feature of an extremely diverse array of structural, functional, and ecological roles played during viral infection and spread. Herein, we consider the evolution of viral CPs and their multitude of interactions with factors encoded by the virus, host plant, or viral vector (biological transmission agent) that influence the infection and epidemiological facets of plant disease. In addition, applications of today's understanding of CPs in the protection of crops from viral infection and use in the manufacture of valuable compounds are considered.

Assembly of double-shelled, virus-like particles in transgenic rice plants expressing two major structural proteins of rice dwarf virus

Rice dwarf virus (RDV) is a double-shelled particle that contains a major capsid protein (P8), a major core protein (P3), several minor core proteins, and viral genomic double-stranded RNA. Coexpression of P8 and P3 in transgenic rice plants resulted in formation of double-shelled, virus-like particles (VLPs) similar to the authentic RDV particles. The VLPs were not detected in transgenic rice plant cells expressing P8 alone. This in vivo result suggests that P8 interacted with P3 and that these two proteins provide the structural integrity required for the formation of VLPs in rice cells independently of other structural proteins, nonstructural proteins, or viral genomic double-stranded RNAs.

Vascular invasion routes and systemic accumulation patterns of tobacco mosaic virus in Nicotiana benthamiana

Plant viruses must enter the host vascular system in order to invade the young growing parts of the plant rapidly. Functional entry sites into the leaf vascular system for rapid systemic infection have not been determined for any plant/virus system. Tobacco mosaic virus (TMV) entry into minor, major and transport veins from non-vascular cells of Nicotiana benthamiana in source tissue and its exit from veins in sink tissue was studied using a modified virus expressing green fluorescent protein (GFP). Using a surgical procedure that isolated specific leaf and stem tissues from complicating vascular tissues, we determined that TMV could enter minor, major or transport veins directly from non-vascular cells to produce a systemic infection. TMV first accumulated in abaxial or external phloem-associated cells in major veins and petioles of the inoculated leaf and stems below the inoculated leaf. It also initially accumulated exclusively in internal or adaxial phloem-associated cells in stems above the inoculated leaf and petioles or major veins of sink leaves. This work shows the functional equivalence of vein classes in source leaves for entry of TMV, and the lack of equivalence of vein classes in sink leaves for exit of TMV. Thus, the specialization of major veins for transport rather than loading of photoassimilates in source tissue does not preclude virus entry. During transport, the virus initially accumulates in specific vascular-associated cells, indicating that virus accumulation in this tissue is highly regulated. These findings have important implications for studies on the identification of symplasmic domains and host macromolecule vascular transport.

Probing plant cell structure and function with viral movement proteins

Virus-encoded movement proteins are the principal strategy by which all plant viruses counter the primary physical defense of the plant to infection - the cell wall - to produce systemic infection and disease. Our understanding of how these proteins act at the molecular and cellular level has increased enormously in the past decade and ushered in an exciting new era of plant virology as an approach to investigating plant cell structure and function. The earliest studies focused on how movement proteins interacted with plasmodesmata, and were an important element in demonstrating the dynamic nature of these intercellular channels. Current efforts are focused on the role of movement proteins in coordinating the replication of viral genomes and the vectorial movement of the progeny genomes through the infected cell, as well as into adjacent cells. Movement proteins are thus providing unique approaches to unravel the fundamental mechanisms by which macromolecular transport is directed and integrated within and between plant cells.

Replication in the phloem is not necessary for efficient vascular transport of tobacco mosaic tobamovirus

Plant viruses move systemically from one leaf to another via phloem. However, the viral functions needed for systemic movement are not fully elucidated. An experimental system was designed to study the effects of low temperature on the vascular transport of the tobacco mosaic tobamovirus (TMV). Vascular transport of TMV from lower inoculated leaves to upper non-inoculated leaves via a stem segment kept at low temperature (4 degrees C) was not affected. On the other hand, several experiments were performed on tobacco leaves to demonstrate that virus replication did not occur at the same temperature. The data suggest that replication of TMV in the phloem of wild-type tobacco plants is not necessary for the vascular transport of TMV, and that the virus moves with photoassimilates as suggested previously.

A plant virus-encoded protein facilitates long-distance movement of heterologous viral RNA

Transport of plant viruses from cell to cell typically involves one or more viral proteins that supply specific cell-to-cell movement functions. Long-distance transport of viruses through the vascular system is a less well understood process with requirements different from those of cell-to-cell movement. Usually viral coat protein (CP) is required for long-distance movement, but groundnut rosette umbravirus (GRV) does not code for a CP. However, this virus moves efficiently from cell to cell and long distance. We demonstrate here that the protein encoded by ORF3 of GRV can functionally replace the CP of tobacco mosaic virus (TMV) for long-distance movement. In spite of low levels of virus RNA accumulation in infected cells, chimeric TMV with a replacement of the CP gene by GRV ORF3 was able to move rapidly through the phloem. Moreover, this chimeric virus complemented long-distance movement of another CP-deficient TMV derivative expressing the gene encoding the green fluorescent protein. Thus, the GRV ORF3-encoded protein represents a class of trans-acting long-distance movement factors that can facilitate trafficking of an unrelated viral RNA.

Virus particles of cucumber green mottle mosaic tobamovirus move systemically in the phloem of infected cucumber plants

Systemic movement through the phloem of infected host plants is a key process in the life cycle of plant viruses, knowledge of which is scant. A main point to be elucidated is the structural form in which virus infection moves within the phloem. Indirect evidence suggests that virions might be the viral structure that moves in the phloem, but data from direct analysis in phloem sap have not been reported. We have done such analysis in the system cucumber (from which phloem exudate can be collected)/cucumber green mottle mosaic tobamovirus (CGMMV). CGMMV has structurally well-characterized particles. Both CGMMV coat protein and RNA were found in phloem exudate from infected cucumbers. Analysis of the accessibility of CGMMV RNA in phloem exudate to RNase A indicates that it is protected within a ribonucleoprotein structure. The accessibility to RNase A of the RNA in these structures was as in virus particles. Centrifugation analyses showed that the ribonucleoprotein structures in the phloem exudate have the same mass and isopycnic density as virions. Virus particles indistinguishable from purified virions were detected by electron microscopy in phloem exudate. No evidence of free RNA or other CGMMV-related structure was found in phloem exudate of infected plants. These results indicate that CGMMV movement in the phloem occurs mainly, if not exclusively, in the form of virus particles.

Characterization of a pine multigene family containing elicitor-responsive stilbene synthase genes

Young pine seedlings respond to environmental stress by induced synthesis of pinosylvin, a stilbene phytoalexin. Heartwood of pine trees is characterized by a high content of pinosylvin. The formation of pinosylvin from cinnamoyl-CoA and three molecules malonyl-CoA catalysed by pinosylvin synthase is typical of the genus Pinus. Its enzyme activity not detectable in unstressed seedlings is substantially increased upon application of stimuli like UV-light or infection with the phytopathogenic fungus Botrytis cinerea. A genomic DNA library was screened with pinosylvin synthase cDNA pSP-54 as a probe. Ten clones were isolated and grouped into five subclasses according to the size of their introns. After subcloning into plasmid T7T3, four different members of the five gene subclasses were characterized by sequencing. Emphasis was put on isolating various promoters and analyzing and comparing their responsiveness. The amino acid sequences deduced from genes PST-1, PST-2, PST-3 and PST-5 shared an overall identity of more than 95%. In gene PST-5, the putative translation start site ATG was replaced by CTG. While promoter regions near the TATAA box were almost identical PST-1, PST-2 and PST-3, further upstream sequences differed substantially. Differences in promoter strength were analysed both in transgenic tobacco plants and by transient expression in tobacco protoplasts. Constructs used contained the bacterial beta-glucuronidase under the control of the promoters of pine genes PST-1, PST-2 and PST-3. Upon treatment with UV light or fungal elicitor, the promoter of PST-1 showed highest responsiveness and led to tissue-specific expression in vascular bundles. The data suggest that in pine the gene product of PST-1 is responsible for both the stress response in seedlings and pinosylvin formation in the heartwood.

Intercellular protein trafficking through plasmodesmata

During plant morphogenesis, groups of cells differentiate to form specialized tissues possessing distinct structures and functions. Cell specialization is a result of specific gene expression at the individual cell level. Coordination of differential gene expression among cells requires that cells communicate with one another. Plasmodesmata provide a cytoplasmic pathway for direct intercellular communication. Recent discoveries that macromolecules such as transcription factors, viral proteins, and plant defense-related proteins can traffic through plasmodesmata suggest that intercellular protein trafficking is potentially an important means to regulate plant developmental processes, physiological functions, plant-pathogen interactions, and plant defense reactions. Thus, elucidating the specific functions and mechanisms of intercellular protein trafficking has broad implications in understanding how a plant develops and functions at the molecular level. This review is to provide an update on this rapidly developing area of plant biology, with emphasis on the discussion of possible mechanisms underlying intercellular protein trafficking.

Identification of genes involved in replication and movement of peanut clump virus

The genome of peanut clump pecluvirus (PCV) consists of two messenger RNA components which contain, respectively, three and five open reading frames (ORFs). Inoculation of transcripts from full-length cDNA clones derived from the PCV RNAs showed that RNA-1 is able to replicate in the absence of RNA-2 in protoplasts, but both RNAs are necessary for plant infection. To investigate the role of different gene products in viral RNA replication and movement, transcripts from mutant cDNA clones were inoculated to protoplasts and to Chenopodium quinoa or Nicotiana benthamiana plants, and progeny RNA was detected by Northern blot analysis. The protein P15, encoded by the third ORF of RNA-1, is essential for efficient replication of the viral genome. The three proteins, P51, P14, and P17, of the triple gene block contained in RNA-2 are involved in localized movement of the viral genome, whereas the coat protein (P23) is also required for vascular movement. Insertion of the beta-glucuronidase reporter gene (GUS) in place of the P23 or P39 genes (the first and the second genes of RNA-2) allows visualization of the virus infection in inoculated leaves. Although the presence of the GUS gene resulted in a lower accumulation of progeny RNA and, despite instability of the construct in planta, histochemical detection of PCV multiplication was more sensitive than Northern blot detection.

Cell-to-cell and phloem-mediated transport of potato virus X. The role of virions

Movement-deficient potato virus X (PVX) mutants tagged with the green fluorescent protein were used to investigate the role of the coat protein (CP) and triple gene block (TGB) proteins in virus movement. Mutants lacking either a functional CP or TGB were restricted to single epidermal cells. Microinjection of dextran probes into cells infected with the mutants showed that an increase in the plasmodesmal size exclusion limit was dependent on one or more of the TGB proteins and was independent of CP. Fluorescently labeled CP that was injected into epidermal cells was confined to the injected cells, showing that the CP lacks an intrinsic transport function. In additional experiments, transgenic plants expressing the PVX CP were used as rootstocks and grafted with nontransformed scions. Inoculation of the PVX CP mutants to the transgenic rootstocks resulted in cell-to-cell and systemic movement within the transgenic tissue. Translocation of the CP mutants into sink leaves of the nontransgenic scions was also observed, but infection was restricted to cells close to major veins. These results indicate that the PVX CP is transported through the phloem, unloads into the vascular tissue, and subsequently is transported between cells during the course of infection. Evidence is presented that PVX uses a novel strategy for cell-to-cell movement involving the transport of filamentous virions through plasmodesmata.