Tobamovirus Infection Research Articles

A novel cellular factor of Nicotiana benthamiana susceptibility to tobamovirus infection.

Viral infection, which entails synthesis of viral proteins and active reproduction of the viral genome, effects significant changes in the functions of many intracellular systems in plants. Along with these processes, a virus has to suppress cellular defense to create favorable conditions for its successful systemic spread in a plant. The virus exploits various cellular factors of a permissive host modulating its metabolism as well as local and systemic transport of macromolecules and photoassimilates. The Nicotiana benthamiana stress-induced gene encoding Kunitz peptidase inhibitor-like protein (KPILP) has recently been shown to be involved in chloroplast retrograde signaling regulation and stimulation of intercellular transport of macromolecules. In this paper we demonstrate the key role of KPILP in the development of tobamovius infection. Systemic infection of N. benthamiana plants with tobacco mosaic virus (TMV) or the closely related crucifer-infecting tobamovirus (crTMV) induces a drastic increase in KPILP mRNA accumulation. KPILP knockdown significantly reduces the efficiency of TMV and crTMV intercellular transport and reproduction. Plants with KPILP silencing become partially resistant to tobamovirus infection. Therefore, KPILP could be regarded as a novel proviral factor in the development of TMV and crTMV infection in N. benthamiana plants.

Tobamovirus infection aggravates gray mold disease caused by Botrytis cinerea by manipulating the salicylic acid pathway in tomato.

Botrytis cinerea is the causative agent of gray mold disease, and infects more than 1400 plant species, including important crop plants. In tomato, B. cinerea causes severe damage in greenhouses and post-harvest storage and transport. Plant viruses of the Tobamovirus genus cause significant damage to various crop species. In recent years, the tobamovirus tomato brown rugose fruit virus (ToBRFV) has significantly affected the global tomato industry. Most studies of plant-microbe interactions focus on the interaction between the plant host and a single pathogen, however, in agricultural or natural environments, plants are routinely exposed to multiple pathogens. Here, we examined how preceding tobamovirus infection affects the response of tomato to subsequent infection by B. cinerea. We found that infection with the tobamoviruses tomato mosaic virus (ToMV) or ToBRFV resulted in increased susceptibility to B. cinerea. Analysis of the immune response of tobamovirus-infected plants revealed hyper-accumulation of endogenous salicylic acid (SA), upregulation of SA-responsive transcripts, and activation of SA-mediated immunity. Deficiency in SA biosynthesis decreased tobamovirus-mediated susceptibility to B. cinerea, while exogenous application of SA enhanced B. cinerea symptoms. These results suggest that tobamovirus-mediated accumulation of SA increases the plants' susceptibility to B. cinerea, and provide evidence for a new risk caused by tobamovirus infection in agriculture.

A cysteine-rich secretory protein involves in phytohormone melatonin mediated plant resistance to CGMMV

BackgroundMelatonin is considered to be a polyfunctional master regulator in animals and higher plants. Exogenous melatonin inhibits plant infection by multiple diseases; however, the role of melatonin in Cucumber green mottle mosaic virus (CGMMV) infection remains unknown.ResultsIn this study, we demonstrated that exogenous melatonin treatment can effectively control CGMMV infection. The greatest control effect was achieved by 3 days of root irrigation at a melatonin concentration of 50 μM. Exogenous melatonin showed preventive and therapeutic effects against CGMMV infection at early stage in tobacco and cucumber. We utilized RNA sequencing technology to compare the expression profiles of mock-inoculated, CGMMV-infected, and melatonin+CGMMV-infected tobacco leaves. Defense-related gene CRISP1 was specifically upregulated in response to melatonin, but not to salicylic acid (SA). Silencing CRISP1 enhanced the preventive effects of melatonin on CGMMV infection, but had no effect on CGMMV infection. We also found exogenous melatonin has preventive effects against another Tobamovirus, Pepper mild mottle virus (PMMoV) infection.ConclusionsTogether, these results indicate that exogenous melatonin controls two Tobamovirus infections and inhibition of CRISP1 enhanced melatonin control effects against CGMMV infection, which may lead to the development of a novel melatonin treatment for Tobamovirus control.

Population structure and genetic diversity of tobacco mild green mosaic virus variants in Western Anatolia of Turkey

Tobamoviruses have been pioneers in understanding the population genetic structure, host-vector interactions, and evolutionary processes of viruses. To better understand the population genetic structure and molecular evolutionary relationships of tobacco mild green mosaic virus (TMGMV), a significant species of tobamoviruses, extensive analyses were performed using bioinformatics tools in this study. 300 samples were collected from plants exhibiting viruses and virus-like symptoms from Turkey's largest tobacco cultivation areas during the plant vegetation period between 2019 and 2020. Samples were tested using conventional molecular techniques for tobamoviruses, including ToBRFV, TMV, TMGMV, ToMV, and ToMMV species. Single and double tobamovirus infections were determined in 258 of 300 samples. Single infections were 225 (75%), 10 (3.33%), and 1 (0.33%) for TMGMV, TMV, and ToMV, respectively. Double infections were 21 (7.0%) and 1 (0.33%) for TMGMV + TMV and TMV + ToMV, respectively. TMGMV-infected samples from each region were chosen, and their complete genomes were revealed. Detailed analyses were carried out from Turkey, with global variants available in GenBank. Molecular evolutionary analyses revealed three main lineages (Clades I, II, and III) at the p126, p183, MP, and complete genome levels and two main lineages at the CP gene (Clades I and II). Variants from Turkey were distributed in two different branches in other phylogenetic trees, except for the CP gene. High haplotype diversity and low nucleotide diversity were determined in each gene region, indicating consistent genetic stability. In addition, purifying selection pressures were determined in specific gene regions. With AMOVA (analysis of molecular variance), it was determined that the source of genetic variation came from within the main lineages, including various hosts and geographies. The differences in lineages were confirmed by independent test statistics. While neutrality tests revealed population expansions in the CP and MP genes, other p183 and p126 genes revealed bottlenecks or balancing selection. The fact that TMGMV was more common than TMV in Turkey strongly supported the phenomenon called “mutational melting” or “Müller latch,” which presumably causes TMV to disappear from its niche.

Editor’s Note: The Impact of Tobamovirus Infection on Root Development Involves Induction of Auxin Response Factor 10a in Tomato

Editor’s Note: The Impact of Tobamovirus Infection on Root Development Involves Induction of <i>Auxin Response Factor 10a</i> in Tomato

Knockout of SlTOM1 and SlTOM3 results in differential resistance to tobamovirus in tomato.

During tobamovirus–host coevolution, tobamoviruses developed numerous interactions with host susceptibility factors and exploited these interactions for replication and movement. The plant‐encoded TOBAMOVIRUS MULTIPLICATION (TOM) susceptibility proteins interact with the tobamovirus replicase proteins and allow the formation of the viral replication complex. Here CRISPR/Cas9‐mediated mutagenesis allowed the exploration of the roles of SlTOM1a, SlTOM1b, and SlTOM3 in systemic tobamovirus infection of tomato. Knockouts of both SlTOM1a and SlTOM3 in sltom1a/sltom3 plants resulted in an asymptomatic response to the infection with recently emerged tomato brown rugose fruit virus (ToBRFV). In addition, an accumulation of ToBRFV RNA and coat protein (CP) in sltom1a/sltom3 mutant plants was 516‐ and 25‐fold lower, respectively, than in wild‐type (WT) plants at 12 days postinoculation. In marked contrast, sltom1a/sltom3 plants were susceptible to previously known tomato viruses, tobacco mosaic virus (TMV) and tomato mosaic virus (ToMV), indicating that SlTOM1a and SlTOM3 are not essential for systemic infection of TMV and ToMV in tomato plants. Knockout of SlTOM1b alone did not contribute to ToBRFV and ToMV resistance. However, in triple mutants sltom1a/sltom3/sltom1b, ToMV accumulation was three‐fold lower than in WT plants, with no reduction in symptoms. These results indicate that SlTOM1a and SlTOM3 are essential for the replication of ToBRFV, but not for ToMV and TMV, which are associated with additional susceptibility proteins. Additionally, we showed that SlTOM1a and SlTOM3 positively regulate the tobamovirus susceptibility gene SlARL8a3. Moreover, we found that the SlTOM family is involved in the regulation of plant development.

The Impact of Tobamovirus Infection on Root Development Involves Induction of Auxin Response Factor 10a in Tomato.

Plant viruses cause systemic diseases that severely impair plant growth and development. While the accumulation of viruses in the root system has long been established, little is known as to how viruses affect root architecture. Here, we examined how the emerging tobamovirus, tomato brown rugose fruit virus (ToBRFV), alters root development in tomato. We found that ToBRFV and tobacco mosaic virus both invaded root systems during the first week of infection. ToBRFV infection of tomato plants resulted in a significant decrease in root biomass and elongation and root-to-shoot ratio and a marked suppression of root branching. Mutation in RNA-dependent RNA polymerase 6 increased the susceptibility of tomato plants to ToBRFV, resulting in severe reduction of various root growth parameters including root branching. Viral root symptoms were associated with the accumulation of auxin response factor 10a (SlARF10a) transcript, a homolog of Arabidopsis ARF10, a known suppressor of lateral root development. Interestingly, loss-of-function mutation in SlARF10a moderated the effect of ToBRFV on root branching. In contrast, downregulation of sly-miR160a, which targets SlARF10a, was associated with constitutive suppression root branching independent of viral infection. In addition, overexpression of a microRNA-insensitive mutant of SlARF10a mimicked the effect of ToBRFV on root development, suggesting a specific role for SlARF10a in ToBRFV-mediated suppression of root branching. Taken together, our results provide new insights into the impact of tobamoviruses on root development and the role of ARF10a in the suppression of root branching in tomato.

First Report of Tomato Brown Rugose Fruit Virus Infecting Tomato in Greece

During summer and autumn 2019, virus-like disease symptoms were observed in tomato (Solanum lycopersicum) grown in several greenhouses in Chania (Crete, Greece) and Kyparissia (Messenia, Peloponnese, Greece). The disease showed a particularly aggressive profile reaching in some cases 100% incidence. Symptoms included mild to severe mosaic, deformations, and blistering on leaves accompanied by brown and yellow spots on fruits. Sap inoculation of extracts from symptomatic tissues derived from seven tomato plants on Nicotiana tabacum cv. Xanthi-nc plants resulted in local necrotic lesions, indicative of a tobamovirus infection. To identify the virus, total RNAs were extracted from the seven symptomatic tomato plants and examined with a commercial tobamovirus screen reverse transcription PCR (RT-PCR) kit (LOEWE Biochemica, Germany). PCR products of 568 nucleotides were obtained from all samples examined. The seven PCR products were further subjected to sequencing. BLAST analysis of the sequences revealed the presence of tomato brown rugose fruit virus (ToBRFV) in all samples (sharing 100% nucleotide identity with each other and 99% identity with GenBank accession MK881102 from Palestine). ToBRFV, a relatively new member of the genus Tobamovirus, was originally described in Jordan (Salem et al. 2016). It is already known for its ability to overcome the so far well-established Tm-2² resistance to tobamoviruses in tomato. To further characterize the Greek isolate, total RNA from an infected tomato plant was subjected to Illumina RNA-Seq. The analysis of approximately 14,000,000, 75-bp single-end reads confirmed the presence of ToBRFV (and the absence of other viruses in the sample). BLAST analysis of the assembled sequence of the Greek isolate (ToBRFV-Gr) (6,354 nucleotides, GenBank accession no. MN815773) revealed a close relationship to the Israeli ToBRFV-IL isolate (99.8% identity, GenBank accession no. KX619418). The ToBRFV-Gr isolate was propagated through mechanical inoculation in N. tabacum ‘Turkish Samsun’, and infected tissue from the latter was used to reproduce symptoms in virus tested-negative tomato plants (cv. Rutgers). Ten days later the inoculated plants showed narrowed leaves with mild mosaic symptoms similar to those described previously by Luria et al. (2017). The presence of ToBRFV was confirmed in symptomatic leaves by RT-PCR using primer pairs ToBRFVGr-F (5′-GAATATCCGGCCTTGCAGAC-3′) and ToBRFVGr-R (5′-TCGAAATTCCACATAAGGTTGGC-3′) and by double antibody sandwich ELISA with the commercial polyclonal antibodies against ToBRFV (LOEWE Biochemica). Τhis is the first report of ToBRFV in Greece. The fact that the virus can be easily transmitted mechanically (through wounding, human activities, contact with infected plants, and insects used for pollination) and by infected seeds, in combination with the lack of any commercial resistant tomato cultivars, constitutes an emerging threat to Greek tomato and potentially pepper cultivation. ToBRFV is included in EPPO’s alert list and is regulated in the European Union since November 2019 (Commission Implementing Decision [EU] 2019/1615). Containment actions against the further spread of ToBRFV were taken at the sites of its emergence.

Biosynthesis of novel phytochemicals in tobacco plant infected with tobacco mosaic tobamovirus (TMV)

The main purpose of this work was to study the effect of TMV infection on physiology of active gradient photochemicals and protein expression in infected tobacco plants. Impact of Tobacco mosaic tobamovirus (TMV) on active gradient photochemicals quantitative and qualitative was evaluated in Nicotiana tobaccum cv. white burly. First, the TMV samples were isolated from single local lesions of infected leaves of N. glutinosa. Then, the N. tobaccum cv. white burly plants were inoculated with TMV. The infected plants showed severe systemic mosaic symptoms and reduction of leave size. We used Datura metel as a diagnostic tool-plant (indicator) for mosaic virus because of its vast exhibitory ability to show the symptoms incited by viruses. It was confirmed that these symptoms were due to the effect of TMV virus, comparing with Datura plant (as control). Analysis of TMV infected leaves by GC-mass detected biosynthesis of novel photochemicals (2-cyclopenten-1-one, Furfural, Indene, Pyrrole, Benzonitrile, Guaiacol and Oxime, methoxy-phenyl) that could not be detected in healthy plants. Furthermore, a 56.17% decreased in nicotine content was observed in infected plants compared with healthy ones. Also, increase of soluble protein contents was observed in infected leaves in response to TMV infection, compared with healthy ones. Alterations in protein patterns were observed in N. tabaccum leaves in response to TMV infection using SDS PAGE. Several secondary bioactive compounds were also found to hold important functions in infected plants. For example, flavonoids could protect against free radicals generated during photosynthesis. Terpenoids may attract pollinators or seed dispersers, or inhibit competing plants. Alkaloids usually ward-off herbivore animals or insect attacks (phytoalexins).

Altered Subcellular Localization of a Tobacco Membrane Raft-Associated Remorin Protein by Tobamovirus Infection and Transient Expression of Viral Replication and Movement Proteins.

Remorins are plant specific proteins found in plasma membrane microdomains (termed lipid or membrane rafts) and plasmodesmata. A potato remorin is reported to be involved in negatively regulating potexvirus movement and plasmodesmal permeability. In this study, we isolated cDNAs of tobacco remorins (NtREMs) and examined roles of an NtREM in infection by tomato mosaic virus (ToMV). Subcellular localization analysis using fluorescently tagged NtREM, ToMV, and viral replication and movement proteins (MPs) indicated that virus infection and transient expression of the viral proteins promoted the formation of NtREM aggregates by altering the subcellular distribution of NtREM, which was localized uniformly on the plasma membrane under normal conditions. NtREM aggregates were often observed associated closely with endoplasmic reticulum networks and bodies of the 126K replication and MPs. The bimolecular fluorescence complementation assay indicated that NtREM might interact directly with the MP on the plasma membrane and around plasmodesmata. In addition, transient overexpression of NtREM facilitated ToMV cell-to-cell movement. Based on these results, we discuss possible roles of the tobacco remorin in tobamovirus movement.

Tobamovirus spread and diversity in Anthropocene

The term “anthropocene” (Crutzen, 2002) refers to our current geological epoch and illustrates the manifold influences of human existence and actions on geology and evolution. Ornamentals are a true anthropogenic product solely manufactured to please the eye of the beholder. They are produced and traded all over the globe thus opening gateways for viruses. We investigated consequences of anthropogenic impact on virus spread and diversity in ornamentals. We focused on Tobamoviruses that are mechanically transmitted and are lacking a true natural vector. Our study spanned the period from 2000 to 2016 and identified Tobamovirus infections in host plants belonging to 15 different plant families. The ten identified virus species belonged mostly to Tobamovirus subgroup 1. In Solanaceae as well as in Cactaceae members of both Tobamovirus subgroup 1 and 3 were present. Different patterns of virus-host plant associations seem to exist when comparing members of both virus subgroups. Phylogenetic analysis of a partially sequenced Tobamovirus isolated from Hoya, Asclepiadaceae, in 2014 indicates the emergence of a new Tobamovirus species positioned separately from known viruses of the subgroup 3 cluster. Similar evidence has been reported independently from Florida (Schubert and Davison, 2012; Adkins et al., 2016) indicating manmediated global spread of this virus.

Virus-induced changes in photosynthetic parameters and peroxidase isoenzyme contents in tomato (Solanum lycopersicum L.) plants

Tomato samples were collected from the field of Absheron peninsula in Azerbaijan in order to evaluate the incidence of main Tobamoviruses. According to results of serological and molecular tests, Tomato mosaic virus (ToMV), Tobacco mosaic virus (TMV), and Pepper mild mottle virus (PMMoV) were detected as single and mixed infections (TMV + PMMoV; ToMV + PMMoV) in various tomato samples. It was found that Tobamovirus infection caused an increase in the content of malondialdehyde, alterations in the activities of peroxidase enzymes and quantitative and qualitative changes in their molecular isoforms. A comparison of thylakoid membrane polypeptides from virus-infected leaves indicated a decrease in the content of the thylakoid membrane polypeptides with molecular masses of 123, 55, 47, 33, 28-24, 17, and 15 kD. PSII efficiency and the content of chlorophylls (a and b) were significantly lower in the virus-infected leaves.

A New Israeli Tobamovirus Isolate Infects Tomato Plants Harboring Tm-22 Resistance Genes

An outbreak of a new disease infecting tomatoes occurred in October-November 2014 at the Ohad village in Southern Israel. Symptomatic plants showed a mosaic pattern on leaves accompanied occasionally by narrowing of leaves and yellow spotted fruit. The disease spread mechanically and rapidly reminiscent of tobamovirus infection. Epidemiological studies showed the spread of the disease in various growing areas, in the South and towards the Southeast and Northern parts of the country within a year. Transmission electron microscope (TEM) analysis showed a single rod-like form characteristic to the Tobamovirus genus. We confirmed Koch’s postulates for the disease followed by partial host range determination and revealed that tomato cultivars certified to harbor the Tm-22 resistance gene are susceptible to the new viral disease. We further characterized the viral source of the disease using a range of antisera for serological detection and analyzed various virus genera and families for cross-reactivity with the virus. In addition, next generation sequencing of total small RNA was performed on two cultivars grown in two different locations. In samples collected from commercial cultivars across Israel, we found a single virus that caused the disease. The complete genome sequence of the new Israeli tobamovirus showed high sequence identity to the Jordanian isolate of tomato brown rugose fruit virus.

Transgenic expression of pectin methylesterase inhibitors limits tobamovirus spread in tobacco and Arabidopsis.

Plant infection by a virus is a complex process influenced by virus-encoded factors and host components which support replication and movement. Critical factors for a successful tobamovirus infection are the viral movement protein (MP) and the host pectin methylesterase (PME), an important plant counterpart that cooperates with MP to sustain viral spread. The activity of PME is modulated by endogenous protein inhibitors (pectin methylesterase inhibitors, PMEIs). PMEIs are targeted to the extracellular matrix and typically inhibit plant PMEs by forming a specific and stable stoichiometric 1:1 complex. PMEIs counteract the action of plant PMEs and therefore may affect plant susceptibility to virus. To test this hypothesis, we overexpressed genes encoding two well-characterized PMEIs in tobacco and Arabidopsis plants. Here, we report that, in tobacco plants constitutively expressing a PMEI from Actinidia chinensis (AcPMEI), systemic movement of Tobacco mosaic virus (TMV) is limited and viral symptoms are reduced. A delayed movement of Turnip vein clearing virus (TVCV) and a reduced susceptibility to the virus were also observed in Arabidopsis plants overexpressing AtPMEI-2. Our results provide evidence that PMEIs are able to limit tobamovirus movement and to reduce plant susceptibility to the virus.

The Role of Hydrogen Peroxide and Nitric Oxide in the Induction of Plant-Encoded RNA-Dependent RNA Polymerase 1 in the Basal Defense against Tobacco Mosaic Virus

Plant RNA-dependent RNA Polymerase 1 (RDR1) is an important element of the RNA silencing pathway in the plant defense against viruses. RDR1 expression can be elicited by viral infection and salicylic acid (SA), but the mechanisms of signaling during this process remains undefined. The involvement of hydrogen peroxide (H2O2) and nitric oxide (NO) in RDR1 induction in the compatible interactions between Tobacco mosaic tobamovirus (TMV) and Nicotiana tabacum, Nicotiana benthamiana, and Arabidopsis thaliana was examined. TMV inoculation onto the lower leaves of N. tabacum induced the rapid accumulation of H2O2 and NO followed by the increased accumulation of RDR1 transcripts in the non-inoculated upper leaves. Pretreatment with exogenous H2O2 and NO on upper leaf led to increased RDR1 expression and systemic TMV resistance. Conversely, dimethylthiourea (an H2O2 scavenger) and 2-(4-carboxyphenyl)- 4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (an NO scavenger) partly blocked TMV- and SA-induced RDR1 expression and increased TMV susceptibility, whereas pretreatment with exogenous H2O2 and NO failed to diminish TMV infection in N. benthamiana plants with naturally occurring RDR1 loss-of-function. Furthermore, in N. tabacum and A. thaliana, TMV-induced H2O2 accumulation was NO-dependent, whereas NO generation was not affected by H2O2. These results suggest that, in response to TMV infection, H2O2 acts downstream of NO to mediate induction of RDR1, which plays a critical role in strengthening RNA silencing to restrict systemic viral infection.

The splice variant Ntr encoded by the tobacco resistance gene N has a role for negative regulation of antiviral defense responses

Tobamovirus infection or transient elicitor expression induces a hypersensitive response (HR) in tobacco plants carrying the N resistance gene, which produces two gene products (N and Ntr) through alternative splicing. In this study, the biological significance of Ntr was investigated. The transient overexpression of Ntr markedly inhibited the induction of HR and N-dependent antiviral defense responses in tobacco and Nicotiana benthamiana, respectively. In addition, there were no drastic changes observed in the ratio of the two alternative transcripts during early tobamovirus infection. Our data suggest that Ntr may play a role as a negative regulator of N-dependent antiviral defenses.

Induction of Resistance in Tomato Plants Against Tomato mosaic tobamovirus Using Beneficial Microbial Isolates

The possibility of making use of the phenome non of Systemic Acquired Resistance (SAR) to control viruses achieved by the soaking treatment of tomato seeds cv. Castl Rock with three growth forms to Bacillus circulans, Pseudomonas fluorescens 2 and Trichoderma harzianum against Tomato mosaic tobamovirus (ToMV) infection. All the application forms of beneficial biotic inducers were reduced the mean number of ToMV local lesions on Datura metel. P. fluorescens 2 was found to be the best treatment in three forms on reduction of local lesion number 42.2, 32.7 and 38.1 of microbial liquid culture, microbial cells or spores and microbial culture filtrate forms, respectively, while the highest mean numbers of local lesions were 51.5, 61.7 and 73.5 of microbial liquid culture, microbial cells or spores and microbial culture filtrate, respectively for T. harzianum. The microbial culture filtrate form was more effective than other microbial forms to reduce mean number of ToMV local lesions to B. circulans, P. fluorescens 2 and T. harzianum isolates, 40.7, 32.1 and 51.5, respectively. The individual microbial isolates on all three microbial forms able to vary ToMV local lesions similarity (homologous or heterologous) and morphology (size center and surrounded with halo or without halo) compared with TMV mother strain.

Specific impact of tobamovirus infection on the Arabidopsis small RNA profile.

Tobamoviruses encode a silencing suppressor that binds small RNA (sRNA) duplexes in vitro and supposedly in vivo to counteract antiviral silencing. Here, we used sRNA deep-sequencing combined with transcriptome profiling to determine the global impact of tobamovirus infection on Arabidopsis sRNAs and their mRNA targets. We found that infection of Arabidopsis plants with Oilseed rape mosaic tobamovirus causes a global size-specific enrichment of miRNAs, ta-siRNAs, and other phased siRNAs. The observed patterns of sRNA enrichment suggest that in addition to a role of the viral silencing suppressor, the stabilization of sRNAs might also occur through association with unknown host effector complexes induced upon infection. Indeed, sRNA enrichment concerns primarily 21-nucleotide RNAs with a 5′-terminal guanine. Interestingly, ORMV infection also leads to accumulation of novel miRNA-like sRNAs from miRNA precursors. Thus, in addition to canonical miRNAs and miRNA*s, miRNA precursors can encode additional sRNAs that may be functional under specific conditions like pathogen infection. Virus-induced sRNA enrichment does not correlate with defects in miRNA-dependent ta-siRNA biogenesis nor with global changes in the levels of mRNA and ta-siRNA targets suggesting that the enriched sRNAs may not be able to significantly contribute to the normal activity of pre-loaded RISC complexes. We conclude that tobamovirus infection induces the stabilization of a specific sRNA pool by yet unknown effector complexes. These complexes may sequester viral and host sRNAs to engage them in yet unknown mechanisms involved in plant:virus interactions.

Effect of Plant Extracts and Acetone Precipitated Proteins from Six Medicinal Plants Against Tobamovirus Infection*

Effect of Plant Extracts and Acetone Precipitated Proteins from Six Medicinal Plants Against Tobamovirus Infection*

Changes in antioxidant enzymes, hydrogen peroxide, salicylic acid and oxidative stress in compatible and incompatible host-tobamovirus interaction

The effect of tobamovirus (TMV and ToMV) infection on changes in antioxidant enzymes, endogenous salicylic acid, endogenous hydrogen peroxide, lipid peroxidation and protein oxidation was studied in compatible (tomato and bell pepper) and incompatible hosts (Nicotiana glutinosa and Nicotiana tabacum cv. xanthi). A time course study of antioxidant enzymes such as peroxidase and catalase was studied in both interactions. Along with these studies, a time course change of endogenous hydrogen peroxide in both compatible and incompatible hosts against tobamoviruses was carried out spectrophotometrically. Histochemical changes in hydrogen peroxide were studied by using Starch-potassium iodide (KI) method. The endogenous levels of salicylic acid were quantified at 24 hours post inoculation (hpi). The effect of tobamoviruses on the lipid peroxidation levels along with protein oxidation was also studied at 24 hpi. The results showed increased peroxidase, salicylic acid, lipid peroxidation, protein oxidation, hydrogen peroxide and decreased catalase in incompatible host-tobamovirus interaction in comparison to compatible hosts. In incompatible hosts, more numbers of cells showed hydrogen peroxide accumulation at 24 hpi. These results suggested the involvement of antioxidant enzymes, salicylic acid, hydrogen peroxide and oxidative stress in incompatible host defense against tobamoviruses. As a consequence of tobamovirus infection, an oxidative stress, indicated by the increase in lipid peroxidation was observed in incompatible interaction. Slight changes in enzymes, lipid peroxidation and hydrogen peroxide in incompatible hosts indicate that these changes have roles in susceptible hosts.