In vitro propagation of plant virus using different forms of plant tissue culture and modes of culture operation

Plant virus accumulation was investigated in vitro using three different forms of plant tissue culture. Suspended cells, hairy roots and shooty teratomas of Nicotiana benthamiana were infected with tobacco mosaic virus (TMV) using the same initial virus:biomass ratio. Viral infection did not affect tissue growth or morphology in any of the three culture systems. Average maximum virus concentrations in hairy roots and shooty teratomas were similar and about an order of magnitude higher than in suspended cells. Hairy roots were considered the preferred host because of their morphological stability in liquid medium and relative ease of culture. The average maximum virus concentration in the hairy roots was 0.82+/-0.14 mg g(-1) dry weight; viral coat protein represented a maximum of approximately 6% of total soluble protein in the biomass. Virus accumulation in hairy roots was investigated further using different modes of semi-continuous culture operation aimed at prolonging the root growth phase and providing nutrient supplementation; however, virus concentrations in the roots were not enhanced compared with simple batch culture. The relative infectivity of virus in the biomass declined by 80-90% during all the cultures tested, irrespective of the form of plant tissue used or mode of culture operation. Hairy root cultures inoculated with a transgenic TMV-based vector in batch culture accumulated green fluorescent protein (GFP); however, maximum GFP concentrations in the biomass were relatively low at 39 microg g(-1) dry weight, probably due to genetic instability of the vector. This work highlights the advantages of using hairy roots for in vitro propagation of TMV compared with shooty teratomas and suspended plant cells, and demonstrates that batch root culture is more effective than semi-continuous operations for accumulation of high virus concentrations in the biomass.

Functions of oligochitosan induced protein kinase in tobacco mosaic virus resistance and pathogenesis related proteins in tobacco

Oligochitosan (OC) can regulate plant defense responses in many aspects, but the basic signal transduction pathway is still unclear. In this study, we used transgenic (TG) tobacco (Nicotiana Tabacum var. Samsun NN) as plant material whose oligochitosan induced protein kinase (OIPK) gene was inhibited by antisense transformation, to study the role of OIPK in tobacco defense reactions. The results showed that OIPK could increase tobacco resistance against tobacco mosaic virus (TMV), in that wild-type (WT) tobacco showed longer lesion appearance time, higher lesion inhibition ratio, smaller average final lesion diameter and lower average final lesion area percent to whole leaf area. It led us to analyze some pathogenesis related (PR) enzymes' activities and mRNA level, which played roles in tobacco resistance against TMV. We found that phenylalanine ammonia-lyase (PAL) and peroxidase (POD) activities were positively related to OIPK, but not polyphenol oxidase (PPO). It was also demonstrated that OIPK mRNA could be induced by OC, wound and TMV infection. In addition, OIPK could up-regulated three PR genes, PAL, chitinase (CHI) and beta-1, 3-glucanase (GLU) mRNA level to different extent. Taken together, these results implied that OIPK could function in tobacco resistance against both biotic and abiotic stress, possibly via various PR proteins.

Isolation and characteristics of the CN gene, a tobacco mosaic virus resistance N gene homolog, from tobacco

Nicotiana rustica L. HZNH, a native Chinese tobacco germplasm, displays a hypersensitive response (HR) and systemic acquired resistance following infection with tobacco mosaic virus (TMV). A resistance gene, CN, cloned from HZNH plants, was homologous to the N and NH genes identified in other Nicotiana species. The CN coding region (3423 bp) shares 93.63% and 86.50% nucleotide identity with N and NH, respectively. Whereas the five CN exon sequences are highly homologous with those of N and NH, the four introns differ significantly in length and sequence. Sequence analysis revealed that CN belongs to the TIR/NBS/LRR gene class. Expression of CN was up-regulated after TMV infection and was temperature sensitive. Organ-specific expression analysis suggested that CN transcripts accumulated at high levels in leaves, low levels in stems, and minimal levels in roots. When CN was inserted into TMV-susceptible N. tabacum cv. K326 plants by Agrobacterium-mediated transformation, the transgenic plants displayed HR and systemic HR due to uninhibited movement of the virus.

Small RNA deep sequencing reveals role for Arabidopsis thaliana RNA-dependent RNA polymerases in viral siRNA biogenesis

RNA silencing functions as an important antiviral defense mechanism in a broad range of eukaryotes. In plants, biogenesis of several classes of endogenous small interfering RNAs (siRNAs) requires RNA-dependent RNA Polymerase (RDR) activities. Members of the RDR family proteins, including RDR1and RDR6, have also been implicated in antiviral defense, although a direct role for RDRs in viral siRNA biogenesis has yet to be demonstrated. Using a crucifer-infecting strain of Tobacco Mosaic Virus (TMV-Cg) and Arabidopsis thaliana as a model system, we analyzed the viral small RNA profile in wild-type plants as well as rdr mutants by applying small RNA deep sequencing technology. Over 100,000 TMV-Cg-specific small RNA reads, mostly of 21- (78.4%) and 22-nucleotide (12.9%) in size and originating predominately (79.9%) from the genomic sense RNA strand, were captured at an early infection stage, yielding the first high-resolution small RNA map for a plant virus. The TMV-Cg genome harbored multiple, highly reproducible small RNA-generating hot spots that corresponded to regions with no apparent local hairpin-forming capacity. Significantly, both the rdr1 and rdr6 mutants exhibited globally reduced levels of viral small RNA production as well as reduced strand bias in viral small RNA population, revealing an important role for these host RDRs in viral siRNA biogenesis. In addition, an informatics analysis showed that a large set of host genes could be potentially targeted by TMV-Cg-derived siRNAs for posttranscriptional silencing. Two of such predicted host targets, which encode a cleavage and polyadenylation specificity factor (CPSF30) and an unknown protein similar to translocon-associated protein alpha (TRAP α), respectively, yielded a positive result in cleavage validation by 5′RACE assays. Our data raised the interesting possibility for viral siRNA-mediated virus-host interactions that may contribute to viral pathogenicity and host specificity.

Elicitor-induced transcription factors for metabolic reprogramming of secondary metabolism in Medicago truncatula

BACKGROUND

Exposure of Medicago truncatula cell suspension cultures to pathogen or wound signals leads to accumulation of various classes of flavonoid and/or triterpene defense molecules, orchestrated via a complex signalling network in which transcription factors (TFs) are essential components.

RESULTS

In this study, we analyzed TFs responding to yeast elicitor (YE) or methyl jasmonate (MJ). From 502 differentially expressed TFs, WRKY and AP2/EREBP gene families were over-represented among YE-induced genes whereas Basic Helix-Loop-Helix (bHLH) family members were more over-represented among the MJ-induced genes. Jasmonate ZIM-domain (JAZ) transcriptional regulators were highly induced by MJ treatment. To investigate potential involvement of WRKY TFs in signalling, we expressed four Medicago WRKY genes in tobacco. Levels of soluble and wall bound phenolic compounds and lignin were increased in all cases. WRKY W109669 also induced tobacco endo-1,3-beta-glucanase (NtPR2) and enhanced the systemic defense response to tobacco mosaic virus in transgenic tobacco plants.

CONCLUSION

These results confirm that Medicago WRKY TFs have broad roles in orchestrating metabolic responses to biotic stress, and that they also represent potentially valuable reagents for engineering metabolic changes that impact pathogen resistance.

Infiltration with Agrobacterium tumefaciens induces host defense and development-dependent responses in the infiltrated zone

Despite the widespread use of Agrobacterium tumefaciens to transfer genes into plant systems, host responses to this plant pathogen are not well understood. The present study shows that disarmed strains of Agrobacterium induce distinct host responses when infiltrated into leaves of Nicotiana tabacum. The responses are limited to the infiltrated zone and consist of i) induction of pathogenesis-related (PR) gene PR-1 expression and resistance to subsequent infection with tobacco mosaic virus, ii) chlorosis and loss of chloroplast rRNAs, and iii) inhibition of leaf expansion. Induction of the latter two sets of responses depends on the age of the leaf and is most apparent in young leaves. Strains with or without binary vectors induce all the responses, showing that DNA transfer is neither required nor inhibitory. A. tumefaciens cured of the tumor-inducing (Ti) plasmid is slightly defective for induction of the three responses, showing that Ti plasmid-encoded factors produced by the disarmed strains contribute only slightly. However, T-DNA-encoded factors alter at least one of the host responses, because infiltration with the oncogenic strain C58 induced more pronounced chlorosis than the disarmed control. Auxin is one of the T-DNA products responsible for disease induction by oncogenic A. tumefaciens. We found that C58-infiltrated zones-but not those infiltrated with the disarmed control-have increased levels of miR393, a microRNA that represses auxin signaling and contributes to antibacterial resistance.

Allopurinol, an inhibitor of purine catabolism, enhances susceptibility of tobacco to Tobacco mosaic virus

Tobacco (cv. Xanthi nn) plants were watered with allopurinol [4-hydroxypyrazolo (3,4-d) pyrimidine, HPP], a xanthine oxidoreductase (XOR) inhibitor, to investigate its effects on infection by Tobacco mosaic virus engineered to express the green fluorescent protein (TMV.GFP). TMV.GFP infection was monitored by examination of inoculated leaves under UV light, by confocal scanning laser microscopy and by epifluorescence microscopy. Susceptibility to TMV.GFP was enhanced in HPP-treated plants. This was seen as a statistically significant increase in numbers of infection sites per leaf and in the number of infected cells per infection site. Two hypotheses are discussed to explain the enhanced susceptibility. The inhibition exerted by HPP against XOR activity could provoke either (i) an increased adenine and guanine nucleotide pool, which could facilitate viral RNA synthesis or (ii) it could cause changes in IAA/auxin levels, which has been proposed to influence TMV susceptibility in tobacco.

Influence of cytoplasmic heat shock protein 70 on viral infection of Nicotiana benthamiana

The accumulation of heat shock protein 70 (Hsp70) generally occurs in plants infected with viruses. However, the effect of Hsp70 accumulation on plant viral infection and pathogenesis remains elusive. In this study, the expression of six Hsp70 genes was found to be induced by the four diverse RNA viruses, Tobacco mosaic virus, Potato virus X (PVX), Cucumber mosaic virus and Watermelon mosaic virus, in Nicotiana benthamiana. Heat treatment enhanced the accumulation and systemic infection of these viruses. Similar results were obtained for viral infection in plants heterologously expressing an Arabidopsis cytoplasmic Hsp70 through either a PVX vector or Agrobacterium infiltration. In contrast, viral infection was compromised in cytoplasmic NbHsp70c-1 gene-silenced plants. These data demonstrate that the cytoplasmic Hsp70s can enhance the infection of N. benthamiana by diverse viruses.

A role for inositol hexakisphosphate in the maintenance of basal resistance to plant pathogens

Phytic acid (myo-inositol hexakisphosphate, InsP6) is an important phosphate store and signal molecule in plants. However, low-phytate plants are being developed to minimize the negative health effects of dietary InsP6 and pollution caused by undigested InsP6 in animal waste. InsP6 levels were diminished in transgenic potato plants constitutively expressing an antisense gene sequence for myo-inositol 3-phosphate synthase (IPS, catalysing the first step in InsP6 biosynthesis) or Escherichia coli polyphosphate kinase. These plants were less resistant to the avirulent pathogen potato virus Y and the virulent pathogen tobacco mosaic virus (TMV). In Arabidopsis thaliana, mutation of the gene for the enzyme catalysing the final step of InsP6 biosynthesis (InsP5 2-kinase) also diminished InsP6 levels and enhanced susceptibility to TMV and to virulent and avirulent strains of the bacterial pathogen Pseudomonas syringae. Arabidopsis thaliana has three IPS genes (AtIPS1-3). Mutant atips2 plants were depleted in InsP6 and were hypersusceptible to TMV, turnip mosaic virus, cucumber mosaic virus and cauliflower mosaic virus as well as to the fungus Botrytis cinerea and to P. syringae. Mutant atips2 and atipk1 plants were as hypersusceptible to infection as plants unable to accumulate salicylic acid (SA) but their increased susceptibility was not due to reduced levels of SA. In contrast, mutant atips1 plants, which were also depleted in InsP6, were not compromised in resistance to pathogens, suggesting that a specific pool of InsP6 regulates defence against phytopathogens.

Mammalian pro-apoptotic bax gene enhances tobacco resistance to pathogens

Emerging evidence suggests that plants and animals may share certain biochemical commonalities in apoptosis, or programmed cell death (PCD) pathways, though plants lack key animal apoptosis related genes. In plants, PCD has many important functions including a role in immunity and resistance to pathogen infection. In this study, a rice phenylalanine ammonia-lyase promoter is used to regulate the expression of the mouse pro-apoptotic bax gene in transgenic tobacco plants. Ectopic expression of the bax negatively affects the growth of transgenic plants. Nonetheless, results show that the bax transgene is induced upon infection by plant pathogens and accumulation of Bax is observed by Western blot analysis. By estimating and measuring the extent of cell death, release of active oxygen species, and accumulation defense-associated gene transcripts, it is shown that bax transgenic plants mount a more robust cell death response compared to control plants. The bax transgenic tobacco plants are also more resistant to infection by Phytophthora parasitica and Ralstonia solanacearum, but have no obvious resistance to tobacco mosaic virus. These results substantiate past studies and illustrate the powerful effects mammalian bax genes may have on plant development and disease resistance.

[Influence of kappa/beta-carrageenan from red alga Tichocarpus crinitus on development of local infection induced by tobacco mosaic virus in Xanthi-nc tobacco leaves]

The influence of kappa/beta-carrageenan from red marine alga Tichocarpus crinitus on the development of tobacco mosaic virus (TMV) infection in Xanthi-nc tobacco leaves was studied. It was shown that the number of necrotic lesions on the leaves inoculated with the mixture of TMV (2 microg/ml) and carrageenan (1 mg/ml) was reduced by 87%, compared to the leaves inoculated with the virus only. The suppression of virus infection was also observed when leaves were treated with carrageenan 24 h before or 24 h after leaf inoculation with TMV; however, in these cases, suppression was less evident than after inoculation with the virus-polysaccharide mixture. It is supposed that the antiviral activity of carrageenan applied together with TMV may be explained by its action not only on the plant but also on the virus itself. The inhibitory effect of carrageenan pretreatment can be explained by its favorable effect on tissue resistance to infection. The suppression of this resistance by actinomycin D indicates that carrageenan functions via its action on the cell genome.

Suppression of tobacco mosaic virus-induced hypersensitive-type necrotization in tobacco at high temperature is associated with downregulation of NADPH oxidase and superoxide and stimulation of dehydroascorbate reductase

Tissue necroses and resistance during the hypersensitive response (HR) of tobacco to tobacco mosaic virus (TMV) are overcome at temperatures above 28 degrees C and the virus multiplies to high levels in the originally resistant N-gene expressing plants. We have demonstrated that chemical compounds that generate reactive oxygen species (ROS) or directly applied hydrogen peroxide (H(2)O(2)) are able to induce HR-type necroses in TMV-inoculated Xanthi-nc tobacco even at high temperatures (e.g. 30 degrees C). The amount of superoxide (O(2)(*-)) decreased, while H(2)O(2) slightly increased in TMV- and mock-inoculated leaves at 30 degrees C, as compared with 20 degrees C. Activity of NADPH oxidase and mRNA levels of genes that encode NADPH oxidase and an alternative oxidase, respectively, were significantly lower, while activity of dehydroascorbate reductase was significantly higher at 30 degrees C, as compared with 20 degrees C. It was possible to reverse or suppress the chemically induced HR-type necrotization at 30 degrees C by the application of antioxidants, such as superoxide dismutase and catalase, demonstrating that the development of HR-type necroses indeed depends on a certain level of superoxide and other ROS. Importantly, high TMV levels at 30 degrees C were similar in infected plants, whether the HR-type necrotization developed or not. Suppression of virus multiplication in resistant, HR-producing tobacco at lower temperatures seems to be independent of the appearance of necroses but is associated with temperatures below 28 degrees C.

Tobacco mosaic virus (TMV) replicase and movement protein function synergistically in facilitating TMV spread by lateral diffusion in the plasmodesmal desmotubule of Nicotiana benthamiana

Virus spread through plasmodesmata (Pd) is mediated by virus-encoded movement proteins (MPs) that modify Pd structure and function. The MP of Tobacco mosaic virus ((TMV)MP) is an endoplasmic reticulum (ER) integral membrane protein that binds viral RNA (vRNA), forming a vRNA:MP:ER complex. It has been hypothesized that (TMV)MP causes Pd to dilate, thus potentiating a cytoskeletal mediated sliding of the vRNA:MP:ER complex through Pd; in the absence of MP, by contrast, the ER cannot move through Pd. An alternate model proposes that cell-to-cell spread takes place by diffusion of the MP:vRNA complex in the ER membranes which traverse Pd. To test these models, we measured the effect of (TMV)MP and replicase expression on cell-to-cell spread of several green fluorescent protein-fused probes: a soluble cytoplasmic protein, two ER lumen proteins, and two ER membrane-bound proteins. Our data support the diffusion model in which a complex that includes ER-embedded MP, vRNA, and other components diffuses in the ER membrane within the Pd driven by the concentration gradient between an infected cell and adjacent noninfected cells. The data also suggest that the virus replicase and MP function together in altering Pd conductivity.

Structure-function relationship between the tobamovirus TMV-Cg coat protein and the HR-like response

The tobamovirus TMV-Cg induces an HR-like response in Nicotiana tabacum cv. Xanthi nn sensitive plants lacking the N or N' resistance genes. This response has been characterized by the appearance of necrotic lesions in the inoculated leaf and viral systemic spread, although the defence pathways are activated in the plant. A previous study demonstrated that the coat protein (CP) of TMV-Cg (CPCg) was the elicitor of this HR-like response. We examined the influence of four specific amino acid substitutions on the structure of CPCg, as well as on the development of the host response. To gain insights into the structural implications of these substitutions, a set of molecular dynamic experiments was performed using comparative models of wild-type and mutant CPCg as well as the CP of the U1 strain of TMV (CPU1), which is not recognized by the plants. A P21L mutation produces severe changes in the three-dimensional structure of CPCg and is more unstable when this subunit is laterally associated in silico. This result may explain the observed incapacity of this mutant to assemble virions. Two other CPCg mutations (R46G and S54K) overcome recognition by the plant and do not induce an HR-like response. A double CPCg mutant P21L-S54K recovered its capacity to form virions and to induce an HR-like response. Our results suggest that the structural integrity of the CP proteins is important for triggering the HR-like response.

Construction of infectious clones for RNA viruses: TMV

The generation of infectious clones is routinely the first step for reverse genetic studies of RNA plant virus gene and sequence function. The procedure given here, details the creation of cDNA clones of tobacco mosaic virus, from which infectious transcripts can be generated in vitro with T7 RNA polymerase. The procedure describes methods for virion purification, viral RNA extraction, reverse transcription, PCR amplification of genomic cDNA fragments, generation of a full-length cDNA clone under the control of a T7 promoter, in vitro transcription, and infectivity testing.

High-throughput in planta expression screening identifies an ADP-ribosylation factor (ARF1) involved in non-host resistance and R gene-mediated resistance

To identify positive regulators of cell death in plants, we performed a high-throughput screening, employing potato virus X-based overexpression in planta of a cDNA library derived from paraquat-treated Nicotiana benthamiana leaves. The screening of 30,000 cDNA clones enabled the identification of an ADP-ribosylation factor 1 (ARF1) that induces cell death when overexpressed in N. benthamiana. Overexpression of the guanosine diphosphate (GDP)-locked mutant of ARF1 did not trigger cell death, suggesting that ARF1 guanosine triphosphatase (GTPase) activity is necessary for the observed cell death-inducing activity. The ARF1 transcript level increased strongly following treatment with Phytophthora infestans elicitor INF1, as well as inoculation with a non-host pathogen Pseudomonas cichorii in N. benthamiana. In addition, ARF1 was induced in the interaction between the N gene and tobacco mosaic virus (TMV) in Nicotiana tabacum. By contrast, inoculation with the virulent pathogen Pseudomonas syringae pv. tabaci did not affect ARF1 expression in N. benthamiana. Virus-induced gene silencing of ARF1 in N. benthamiana resulted in a stunted phenotype, and severely hampered non-host resistance towards P. cichorii. In addition, ARF1 silencing partially compromised resistance towards TMV in N. benthamiana containing the N resistance gene. By contrast, and in accordance with the ARF1 gene expression profile, silencing of ARF1 transcription did not alter the susceptibility of N. benthamiana towards the pathogen P. syringae pv. tabaci. These results strongly implicate ARF1 in the non-host resistance to bacteria and N gene-mediated resistance in N. benthamiana.

A novel role for the TIR domain in association with pathogen-derived elicitors

Plant innate immunity is mediated by Resistance (R) proteins, which bear a striking resemblance to animal molecules of similar function. Tobacco N is a TIR-NB-LRR R gene that confers resistance to Tobacco mosaic virus, specifically the p50 helicase domain. An intriguing question is how plant R proteins recognize the presence of pathogen-derived Avirulence (Avr) elicitor proteins. We have used biochemical cell fraction and immunoprecipitation in addition to confocal fluorescence microscopy of living tissue to examine the association between N and p50. Surprisingly, both N and p50 are cytoplasmic and nuclear proteins, and N's nuclear localization is required for its function. We also demonstrate an in planta association between N and p50. Further, we show that N's TIR domain is critical for this association, and indeed, it alone can associate with p50. Our results differ from current models for plant innate immunity that propose detection is mediated solely through the LRR domains of these molecules. The data we present support an intricate process of pathogen elicitor recognition by R proteins involving multiple subcellular compartments and the formation of multiple protein complexes.

Each year, up to 10% of world agricultural production is lost to pests and diseases caused by a variety of pathogens including bacteria, fungi, nematodes, and viruses. Scientists have understood for nearly a century that plants carry their own immune system that actively engages pathogens and prevents many infections. One aspect of the plant immune system is defined by the gene-for-gene hypothesis: a plant Resistance (R) gene encodes a protein that specifically recognizes and protects against one pathogen or strain of a pathogen carrying a corresponding Avirulence (Avr) gene. In tobacco and its relatives, the N resistance protein confers resistance to infection by the Tobacco mosaic virus (TMV).

We have used N, and the TMV elicitor, p50, to investigate the mechanism of gene-for-gene resistance. We show that N and p50 associate in the cytoplasm and nucleus of plant cells and that this association is mediated by N's TIR domain, which is structurally similar to animal innate immunity molecules. Our findings provide novel insight into how R proteins recognize pathogen Avr proteins, and should help in long-term efforts to enhance crop yield.

How do plant resistance gene recognize the cognate pathogen? These authors dissect the interaction of a resistance gene and a viral pathogen.

Coat protein-mediated resistance to TMV infection of Nicotiana tabacum involves multiple modes of interference by coat protein

Expression of tobacco mosaic virus (TMV) coat protein (CP) restricts virus disassembly and alters the accumulation of the movement protein (MP). To characterize the role of structure of transgenic CP in regulating virus disassembly and production of MP, we generated CPs with mutations at residues Glu50 and Asp77, located in the interface between juxtaposed CP subunits. In transgenic Nicotiana tabacum and BY-2 cells, three categories of coat protein-mediated resistance (CP-MR) levels were identified: wild-type CP-MR; elevated CP-MR; and no CP-MR. Mutant CPs that interfered with the accumulation of virus replication complexes conferred very high levels of protection to TMV, except by CP(E50D) which provided no protection in the systemic host (Xanthi-nn) but high CP-MR in the local lesion host (Xanthi-NN). In transgenic BY-2 cells CP(E50D) strongly reduced accumulation of MP:GFP. In general, there was a strong correlation between the capacity for CP to assemble to pseudovirions and CP-MR, while there was not strong correlation with packaging viral RNA and CP-MR. The data demonstrate that interference with one or more steps in virus infection and replication by wild type and mutant CP determine the degree of CP-MR.

Pathogen-induced calmodulin isoforms in basal resistance against bacterial and fungal pathogens in tobacco

Thirteen tobacco calmodulin (CaM) genes fall into three distinct amino acid homology types. Wound-inducible type I isoforms NtCaM1 and 2 were moderately induced by tobacco mosaic virus (TMV)-mediated hypersensitive reaction, and the type III isoform NtCaM13 was highly induced, while the type II isoforms NtCaM3-NtCaM12 showed little response. Type I and III knockdown tobacco lines were generated using inverted repeat sequences from NtCaM1 and 13, respectively, to evaluate the contribution of pathogen-induced calmodulins (CaMs) to disease resistance. After specific reduction of type I and III CaM gene expression was confirmed in both transgenic lines, we analyzed the response to TMV infection, and found that TMV susceptibility was slightly enhanced in type III CaM knockdown lines compared with the control line. Resistance to a compatible strain of the bacterial pathogen Ralstonia solanacearum, and fungal pathogens Rhizoctonia solani and Pythium aphanidermatum was significantly lower in type III but not in type I CaM knockdown plants. Expression of jasmonic acid (JA)- and/or ethylene-inducible basic PR genes was not affected in these lines, suggesting that type III CaM isoforms are probably involved in basal defense against necrotrophic pathogens in a manner that is independent of JA and ethylene signaling.

MAP kinases function downstream of HSP90 and upstream of mitochondria in TMV resistance gene N-mediated hypersensitive cell death

Although the involvement of heat shock protein 90 (HSP90), mitogen-activated protein kinase (MAPK) cascades and organelle dysfunction in plant hypersensitive cell death has been suggested, the mutual relationship among them has not been elucidated. Here, we show the molecular network of HSP90, the wound-induced protein kinase (WIPK)/salicylic acid-induced protein kinase (SIPK)-mediated MAPK cascade and mitochondrial dysfunction in tobacco mosaic virus (TMV) resistance gene N-dependent cell death. p50, the Avr component for N, NtMEK2(DD), a constitutively active form of a MAPK kinase of WIPK/SIPK, and a mammalian pro-apoptotic factor Bax were used for cell death induction. Suppression of HSP90 and treatment with geldanamycin, a specific inhibitor of HSP90, compromised p50- but not NtMEK2(DD)- or Bax-mediated cell death accompanying the reduction of NtMEK2, WIPK and SIPK activation. In WIPK/SIPK-double knockdown plants, p50- and NtMEK2(DD)- but not Bax-mediated cell death was suppressed. All three types of cell death induced mitochondrial dysfunction, but they were similarly suppressed by Bcl-xL, which is a mammalian anti-apoptotic factor, and prevents mitochondrial dysfunction in plants as it does in animals in the cell death signal pathway. Taken together with the expression profile of hypersensitive reaction marker genes, it was indicated that the MAPK cascade functions downstream of HSP90 and transduces the cell death signal to mitochondria for N gene-dependent cell death. Furthermore, we found that WIPK and SIPK are functionally redundant in cell death signaling using WIPK/SIPK single or double knockdown plants.

Evidence for plant viruses in the region of Argentina Islands, Antarctica

This work focused on the assessment of plant virus occurrence among primitive and higher plants in the Antarctic region. Sampling occurred during two seasons (2004/5 and 2005/6) at the Ukrainian Antarctic Station 'Academician Vernadskiy' positioned on Argentina Islands. Collected plant samples of four moss genera (Polytrichum, Plagiatecium, Sanionia and Barbilophozia) and one higher monocot plant species, Deschampsia antarctica, were further subjected to enzyme-linked immunosorbent assay to test for the presence of common plant viruses. Surprisingly, samples of Barbilophozia and Polytrichum mosses were found to contain antigens of viruses from the genus Tobamovirus, Tobacco mosaic virus and Cucumber green mottle mosaic virus, which normally parasitize angiosperms. By contrast, samples of the monocot Deschampsia antarctica were positive for viruses typically infecting dicots: Cucumber green mottle mosaic virus, Cucumber mosaic virus and Tomato spotted wilt virus. Serological data for Deschampsia antarctica were supported in part by transmission electron microscopy observations and bioassay results. The results demonstrate comparatively high diversity of plant viruses detected in Antarctica; the results also raise questions of virus specificity and host susceptibility, as the detected viruses normally infect dicotyledonous plants. However, the means of plant virus emergence in the region remain elusive and are discussed.

[Hormonal status of tobacco variety Samsun NN exposed to synthetic coenzyme Q10 (ubiquinone 50) and TMV infection]

Hormonal system status has been analyzed in leaf disks of hypersensitive tobacco Nicotiana tabacum L. variety Samsun NN during the development of resistance to tobacco mosaic virus (TMV) induced by synthetic coenzyme Q10 (ubiquinone 50). The absolute and relative content of abscisic acid (ABA), indoleacetic acid (IAA), and cytokinins (CKs) was determined after the exposure of leaves to Q10 solution and the subsequent TMV infection. In plants not treated with Q10, CK content increased about 2.5 times 1 day after TMV infection, while a significant increase in the ABA level and a decrease in the IAA level were observed only after 2 days. In the dynamics, Q10 treatment had a protective antiviral effect, significantly decreased the ABA level, and increased the IAA level in sensitized plants compared to nonsensitized ones.

Tobacco mosaic virus movement protein functions as a structural microtubule-associated protein

The cell-to-cell spread of Tobacco mosaic virus infection depends on virus-encoded movement protein (MP), which is believed to form a ribonucleoprotein complex with viral RNA (vRNA) and to participate in the intercellular spread of infectious particles through plasmodesmata. Previous studies in our laboratory have provided evidence that the vRNA movement process is correlated with the ability of the MP to interact with microtubules, although the exact role of this interaction during infection is not known. Here, we have used a variety of in vivo and in vitro assays to determine that the MP functions as a genuine microtubule-associated protein that binds microtubules directly and modulates microtubule stability. We demonstrate that, unlike MP in whole-cell extract, microtubule-associated MP is not ubiquitinated, which strongly argues against the hypothesis that microtubules target the MP for degradation. In addition, we found that MP interferes with kinesin motor activity in vitro, suggesting that microtubule-associated MP may interfere with kinesin-driven transport processes during infection.

Disruption of microtubule organization and centrosome function by expression of tobacco mosaic virus movement protein

The movement protein (MP) of Tobacco mosaic virus mediates the cell-to-cell transport of viral RNA through plasmodesmata, cytoplasmic cell wall channels for direct cell-to-cell communication between adjacent cells. Previous in vivo studies demonstrated that the RNA transport function of the protein correlates with its association with microtubules, although the exact role of microtubules in the movement process remains unknown. Since the binding of MP to microtubules is conserved in transfected mammalian cells, we took advantage of available mammalian cell biology reagents and tools to further address the interaction in flat-growing and transparent COS-7 cells. We demonstrate that neither actin, nor endoplasmic reticulum (ER), nor dynein motor complexes are involved in the apparent alignment of MP with microtubules. Together with results of in vitro coprecipitation experiments, these findings indicate that MP binds microtubules directly. Unlike microtubules associated with neuronal MAP2c, MP-associated microtubules are resistant to disruption by microtubule-disrupting agents or cold, suggesting that MP is a specialized microtubule binding protein that forms unusually stable complexes with microtubules. MP-associated microtubules accumulate ER membranes, which is consistent with a proposed role for MP in the recruitment of membranes in infected plant cells and may suggest that microtubules are involved in this process. The ability of MP to interfere with centrosomal gamma-tubulin is independent of microtubule association with MP, does not involve the removal of other tested centrosomal markers, and correlates with inhibition of centrosomal microtubule nucleation activity. These observations suggest that the function of MP in viral movement may involve interaction with the microtubule-nucleating machinery.

The Tobacco mosaic virus replicase protein disrupts the localization and function of interacting Aux/IAA proteins

Previously, we identified a correlation between the interaction of the Tobacco mosaic virus (TMV) 126/183-kDa replicase with the auxin response regulator indole acetic acid (IAA)26/PAP1 and the development of disease symptoms. In this study, the TMV replicase protein is shown to colocalize with IAA26 in the cytoplasm and prevent its accumulation within the nucleus. Furthermore, two additional auxin (Aux)/IAA family members, IAA27 and IAA18, were found to interact with the TMV replicase and displayed alterations in their cellular localization or accumulation that corresponded with their ability to interact with the TMV replicase. In contrast, the localization and accumulation of noninteracting Aux/IAA proteins were unaffected by the presence of the viral replicase. To investigate the effects of the replicase interaction on Aux/IAA function, transgenic plants expressing a proteolysis-resistant IAA26-P108L-green fluorescent protein (GFP) protein were created. Transgenic plants accumulating IAA26-P108L-GFP displayed an abnormal developmental phenotype that included severe stunting and leaf epinasty. However, TMV infection blocked the nuclear localization of IAA26-P108L-GFP and attenuated the developmental phenotype displayed by the transgenic plants. Combined, these findings suggest that TMV-induced disease symptoms can be attributed, in part, to the ability of the viral replicase protein to disrupt the localization and subsequent function of interacting Aux/IAA proteins.