Pathway for Phloem-Dependent Movement of Pepper Mottle Potyvirus in the Stem of Capsicum annuum

Blaze a New Trail: Plant Virus Xylem Exploitation

Viruses are trailblazers in hijacking host systems for their own needs. Plant viruses have been shown to exploit alternative avenues of translocation within a host, including a challenging route through the xylem, to expand their niche and establish systemic spread, despite apparent host-imposed obstacles. Recent findings indicate that plant viruses from many families could successfully hack xylem cells in a broad range of plant hosts, including herbaceous and perennial woody plants. Similar to virus-related structures present in the phloem, virus particles and membrane-containing viral replication complexes are often observed in the xylem. Except for a few single-stranded DNA viruses in the family Geminiviridae and a negative-sense single-stranded RNA rhabdovirus, Lettuce necrotic yellows virus, the majority of the viruses that were detected in the xylem belong to the group of positive-sense RNA viruses. The diversity of the genome organization and virion morphology of those viruses indicates that xylem exploitation appears to be a widely adapted strategy for plant viruses. This review outlines the examples of the xylem-associated viruses and discusses factors that regulate virus inhabitation of the xylem as well as possible strategies of virus introduction into the xylem. In some cases, plant disease symptoms have been shown to be closely related to virus colonization of the xylem. Inhibiting viral xylem invasion could raise potential attractive approaches to manage virus diseases. Therefore, the identification of the host genes mediating virus interaction with the plant xylem tissue and understanding the underlying mechanisms call for more attention.

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.

Differential Tropism in Roots and Shoots of Resistant and Susceptible Cassava (Manihot esculenta Crantz) Infected by Cassava Brown Streak Viruses

Cassava brown streak disease (CBSD) is a destructive disease of cassava in Eastern and Central Africa. Because there was no source of resistance in African varieties to provide complete protection against the viruses causing the disease, we searched in South American germplasm and identified cassava lines that did not become infected with the cassava brown streak viruses. These findings motivated further investigations into the mechanism of virus resistance. We used RNAscope^® in situ hybridization to localize cassava brown streak virus in cassava germplasm lines that were highly resistant (DSC 167, immune) or that restricted virus infections to stems and roots only (DSC 260). We show that the resistance in those lines is not a restriction of long-distance movement but due to preventing virus unloading from the phloem into parenchyma cells for replication, thus restricting the virus to the phloem cells only. When DSC 167 and DSC 260 were compared for virus invasion, only a low CBSV signal was found in phloem tissue of DSC 167, indicating that there is no replication in this host, while the presence of intense hybridization signals in the phloem of DSC 260 provided evidence for virus replication in companion cells. In neither of the two lines studied was there evidence of virus replication outside the phloem tissues. Thus, we conclude that in resistant cassava lines, CBSV is confined to the phloem tissues only, in which virus replication can still take place or is arrested.

Three Strains of Tobacco etch virus Distinctly Alter the Transcriptome of Apical Stem Tissue in Capsicum annuum during Infection

Tobacco etch virus (TEV; genus Potyvirus) is flexuous rod shaped with a single molecule of single-stranded RNA and causes serious yield losses in species in the Solanaceae. Three TEV strains (HAT, Mex21, and N) are genetically distinct and cause different disease symptoms in plants. Here, a transcriptomic RNA sequencing approach was taken for each TEV strain to evaluate gene expression of the apical stem segment of pepper plants during two stages of disease development. Distinct profiles of Differentially Expressed Genes (DEGs) were identified for each TEV strain. DEG numbers increased with degree of symptom severity: 24 from HAT, 1190 from Mex21, and 4010 from N. At 7 days post-inoculation (dpi), when systemic symptoms were similar, there were few DEGs for HAT- and Mex21-infected plants, whereas N-infected plants had 2516 DEGs. DEG patterns from 7 to 14 dpi corresponded to severity of disease symptoms: milder disease with smaller DEG changes for HAT and Mex21 and severe disease with larger DEG changes for N. Strikingly, in each of these comparisons, there are very few overlapping DEGs among the TEV strains, including no overlapping DEGs between all three strains at 7 or 14 dpi.

The Role of Grafting in the Resistance of Tomato to Viruses

Grafting is routinely implemented in modern agriculture to manage soilborne pathogens such as fungi, oomycetes, bacteria, and viruses of solanaceous crops in a sustainable and environmentally friendly approach. Some rootstock/scion combinations use specific genetic resistance mechanisms to impact also some foliar and airborne pathogens, including arthropod or contact-transmitted viruses. These approaches resulted in poor efficiency in the management of plant viruses with superior virulence such as the strains of tomato spotted wilt virus breaking the Sw5 resistance, strains of cucumber mosaic virus carrying necrogenic satellite RNAs, and necrogenic strains of potato virus Y. Three different studies from our lab documented that suitable levels of resistance/tolerance can be obtained by grafting commercial tomato varieties onto the tomato ecotype Manduria (Ma) rescued in the framework of an Apulian (southern Italy) regional program on biodiversity. Here we review the main approaches, methods, and results of the three case studies and propose some mechanisms leading to the tolerance/resistance observed in susceptible tomato varieties grafted onto Ma as well as in self-grafted plants. The proposed mechanisms include virus movement in plants, RNA interference, genes involved in graft wound response, resilience, and tolerance to virus infection.

Comparative Evaluation of Disease Induced by Three Strains of Tobacco etch virus in Capsicum annuum L

Tobacco etch virus (TEV; genus Potyvirus) strains HAT, Mex21, and N were evaluated comparatively for their pathogenicity and effects on growth of Capsicum annuum L. 'Calwonder'. Each TEV strain induced an initial systemic symptom of vein-clearing but subsequent disease symptoms ranged from mild (HAT) to moderate (Mex21) to severe (N). Effects on plant growth parameters closely reflected disease symptoms induced by each TEV strain. HAT-infected Calwonder plants did not differ from the healthy control for plant height, internode lengths, and aboveground fresh weight of shoots. Root dry weight, however, was less for HAT-infected plants than for the healthy control. Mex21 affected plants more severely, with significantly shorter plant height (at 20, 30, and 40 days postinoculation), reduced root dry weight, and shortened internodes compared with HAT and healthy control treatments. Aboveground fresh weight of Mex21-infected plants was significantly less than for the healthy control. N induced significant negative effects relative to each of the other treatments for plant height, aboveground shoot fresh weight, root dry weight, and internode lengths. The effects on Calwonder fruit production mimicked disease severity and effects on plant growth for the respective TEV strains.

Two and three dimensional characterization of Zucchini Yellow Mosaic Virus induced structural alterations in Cucurbita pepo L. plants

Infection of plants by Zucchini Yellow Mosaic Virus (ZYMV) induces severe ultrastructural changes. The aim of this study was to investigate ultrastructural changes during ZYMV-infection in Cucurbita pepo L. plants on the two and three dimensional (2D and 3D) level and to correlate these changes with the spread of ZYMV throughout the plant by transmission electron microscopy (TEM) and image analysis.

This study revealed that after inoculation of the cotyledons ZYMV moved into roots [3 days post inoculation (dpi)], then moved upwards into the stem and apical meristem (5 dpi), then into the first true leaf (7 dpi) and could finally be found in all plant parts (9 dpi). ZYMV-infected cells contained viral inclusion bodies in the form of cylindrical inclusions (CIs). These CIs occurred in four different forms throughout the cytosol of roots and leaves: scrolls and pinwheels when cut transversely and long tubular structures and bundles of filaments when cut longitudinally. 3D reconstruction of ZYMV-infected cells containing scrolls revealed that they form long tubes throughout the cytosol. The majority has a preferred orientation and an average length and width of 3 μm and 120 nm, respectively. Image analysis revealed an increased size of cells and vacuoles (107% and 447%, respectively) in younger ZYMV-infected leaves leading to a similar ratio of cytoplasm to vacuole (about 1:1) in older and younger ZYMV-infected leaves which indicates advanced cell growth in younger tissues. The collected data advances the current knowledge about ZYMV-induced ultrastructural changes in Cucurbita pepo.

Viral and cellular factors involved in Phloem transport of plant viruses

Phloem transport of plant viruses is an essential step in the setting-up of a complete infection of a host plant. After an initial replication step in the first cells, viruses spread from cell-to-cell through mesophyll cells, until they reach the vasculature where they rapidly move to distant sites in order to establish the infection of the whole plant. This last step is referred to as systemic transport, or long-distance movement, and involves virus crossings through several cellular barriers: bundle sheath, vascular parenchyma, and companion cells for virus loading into sieve elements (SE). Viruses are then passively transported within the source-to-sink flow of photoassimilates and are unloaded from SE into sink tissues. However, the molecular mechanisms governing virus long-distance movement are far from being understood. While most viruses seem to move systemically as virus particles, some viruses are transported in SE as viral ribonucleoprotein complexes (RNP). The nature of the cellular and viral factors constituting these RNPs is still poorly known. The topic of this review will mainly focus on the host and viral factors that facilitate or restrict virus long-distance movement.

Cucumber mosaic virus 2b-Deficient Mutant Causes Limited, Asymptomatic Infection of Bell Pepper

Cucumber mosaic virus Fast New York strain (CMV-Fny) containing a mutated 2b protein (CMV-FnyΔ2b) was evaluated for the ability to infect 'Calwonder' bell pepper (Capsicum annuum) plants in comparative tests with the parent virus, CMV-Fny. Plants inoculated with CMV-FnyΔ2b did not develop local or systemic symptoms of infection, whereas CMV-Fny-infected plants developed systemic chlorosis by 7 days post inoculation (dpi), followed by mosaic and leaf deformation. Virus accumulation, determined by enzyme-linked immunosorbent assay (ELISA), revealed that CMV-FnyΔ2b accumulated in inoculated Calwonder leaves and inconsistently infected some noninoculated leaves at a low titer but was not detected in the youngest, noninoculated leaves. Immuno-tissue blot tests did not detect CMV-FnyΔ2b in the stems of infected plants, whereas CMV-Fny accumulated throughout the length of the stems of inoculated plants. In two experiments, protoplasts were isolated from Calwonder leaves, inoculated with viral RNAs of CMV-Fny or CMV-FnyΔ2b, and tested by ELISA for infection. In both experiments, less CMV-FnyΔ2b than CMV-Fny accumulated in protoplasts. These results suggest that the CMV 2b protein is needed for systemic infection of Calwonder pepper plants and for accumulation of the virus in inoculated protoplasts.

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.

Potide-G derived from potato (Solanum tuberosum L.) is active against potato virus YO (PVYO) infection

A PVYO virus-resistant potato (Solanum tuberosum L. cv. Golden Valley) was identified, and further, from its tubers, a small (5.57 kDa) antiviral peptide potide-G was isolated. Application of potide-G on virus susceptible potato (cv. Winter valley) expressed robust resistance to PVYO infection and showed no virus infected morphology. We found that PVYO infection spreads up completely within 3 days post inoculation (dpi) in susceptible cultivar. PVYO was more accumulated toward the basal leaves, when infection occurred longer. Combined results of morphology of PVYO infection, ELISA, RT-PCR, and real-time PCR showed the resistance to the PVYO infection depends on the expression of Ry gene. Indeed, the real-time PCR result showed that the Ry gene up-regulated to 3 times higher in PVYO infected cv. Golden valley. Golden crude protein was found to be active against PVYO infection in the in vivo test. In addition, application of potide-G in a virus susceptible potato potently reduced the viral infection actively with 50 times lower concentration than that of the Golden protein. Further identification of a host-specific resistant gene in a plant and the peptide derived from it offers new opportunities for the development of novel bio-pesticides against plant virus.

Synergistic Disease in Pepper Caused by the Mixed Infection of Cucumber mosaic virus and Pepper mottle virus

ABSTRACT The occurrence of more than one virus species in a single plant is not uncommon in cultivated and native plant species. A mixed virus infection may lead to greater disease severity than individual viral components and this is sometimes referred to as a synergistic disease. Although, in some cases, synergism has been demonstrated for various plant growth parameters such as plant height, weight, and yield, proof of synergy typically has not been demonstrated for symptom severity when the mixed virus infection was not lethal. We demonstrated synergy in bell pepper plants co-infected with Cucumber mosaic virus (CMV) and Pepper mottle virus (PepMoV) relative to each virus alone for stem height (two of three trials) and aboveground fresh weight (one of three trials) using factorial analysis and Abbott's equation for synergy. This approach allowed affirmation of the type of response (i.e., synergistic rather than antagonistic) and statistical proof of synergy. A detailed evaluation of symptom severity for each viral treatment revealed three phases associated with host plant developmental stages. A numerical symptom severity rating scale was developed and used in each of two equations to demonstrate statistical proof for synergy based on symptom severity for co-infected plants. Virus accumulation in noninoculated leaves was determined by enzyme-linked immunosorbent assay. In singly infected plants, CMV titers declined in mildly symptomatic leaves representing later stages of plant development, but titers increased in similar leaves of co-infected plants. In contrast, PepMoV titers did not differ in singly or co-infected plants.

Cucumber mosaic virus 2b protein compensates for restricted systemic spread of Potato virus Y in doubly infected tobacco

Tobacco plants (Nicotiana tabacum cv. Xanthi-nc) inoculated with a necrotic strain of Potato virus Y (PVY, T01 isolate) developed necrotic symptoms in some systemically infected leaves, but not in younger leaves. However, PVY expressed distinct symptoms not only in the older leaves, but also in the younger leaves, of plants that had been doubly inoculated with PVY and with Cucumber mosaic virus (CMV, strain Pepo). A tissue blot immunoassay of tissues from various positions of the stem detected PVY weakly in each stem, but not in the shoot apex, of singly infected plants, whereas PVY was detected at high levels in almost all sections of doubly infected plants. CMV was also detected at high levels in sections of singly and doubly infected plants. Immunohistochemistry of stem tissues showed that in singly infected plants, PVY was confined to external phloem cells and was not detected in internal phloem cells. However, in doubly infected plants, PVY was distributed uniformly throughout whole tissues, including the external phloem, xylem parenchyma and internal phloem cells. In plants that were doubly infected with PVY and Pepo Delta 2b, a modified CMV that cannot translate the 2b protein, the spread of PVY was restricted as in singly infected plants. These results suggested that the plant host has a counterdefence mechanism that restricts systemic spread of PVY T01, and that the 2b protein of CMV strain Pepo negates this restriction.

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.

Cucumoviruses

Research on the molecular biology of cucumoviruses and their plant-virus interactions has been very extensive in the last decade. Cucumovirus genome structures have been analyzed, giving new insights into their genetic variability, evolution, and taxonomy. A new viral gene has been discovered, and its role in promoting virus infection has been delineated. The localization and various functions of each viral-encoded gene product have been established. The particle structures of Cucumber mosaic virus (CMV) and Tomato aspermy virus have been determined. Pathogenicity domains have been mapped, and barriers to virus infection have been localized. The movement pathways of the viruses in some hosts have been discerned, and viral mutants affecting the movement processes have been identified. Host responses to viral infection have been characterized, both temporally and spatially. Progress has been made in determining the mechanisms of replication, gene expression, and transmission of CMV. The pathogenicity determinants of various satellite RNAs have been characterized, and the importance of secondary structure in satellite RNA-mediated interactions has been recognized. Novel plant genes specifying resistance to infection by CMV have been identified. In some cases, these genes have been mapped, and one resistance gene to CMV has been isolated and characterized. Pathogen-derived resistance has been demonstrated against CMV using various segments of the CMV genome, and the mechanisms of some of these forms of resistances have been analyzed. Finally, the nature of synergistic interactions between CMV and other viruses has been characterized. This review highlights these various achievements in the context of the previous work on the biology of cucumoviruses and their interactions with plants.

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.

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.

Resistance of Capsicum annuum 'Avelar' to pepper mottle potyvirus and alleviation of this resistance by co-infection with cucumber mosaic cucumovirus are associated with virus movement

Capsicum annuum cv. Avelar plants resist systemic infection by the Florida isolate of pepper mottle potyvirus (PepMoV-FL). Immuno-tissue blot analysis for detection of PepMoV-FL infection in selected stem segments revealed that virus moved down the stem in external phloem, and, over time, accumulated to detectable levels throughout stem sections (appearing to accumulate in external and internal phloem) taken from below the inoculated leaf. At 21 days post-inoculation, PepMoV-FL was detected in stem segments one or two internodes above the inoculated leaf; however, no virus was observed in internal phloem in stem segments beyond these internodes. In contrast to these observations, PepMoV-FL was detected in the internal phloem of all internodes of the stem located above the inoculated leaf, with subsequent movement into non-inoculated leaves, in Avelar plants co-infected with PepMoV-FL and cucumber mosaic cucumovirus (CMV-KM). No apparent enhancement of PepMoV-FL accumulation occurred in protoplasts inoculated with PepMoV-FL alone versus a mixed inoculum of PepMoV-FL and CMV-KM. These findings confirm earlier observations that potyvirus movement up the stem of Capsicum species occurs via internal phloem. It is also shown that PepMoV-FL does not accumulate to detectable levels in internal phloem in the stem of Avelar plants, thereby limiting its movement to within the inoculated leaf and lower portions of the stem; however, co-infection of Avelar plants with CMV-KM alleviates this restricted movement, allowing PepMoV-FL to invade young tissues systemically.