Rapid diagnosis of plant virus diseases by transmission electron microscopy

Limonene anti-TMV activity and its mode of action

The main component of orange peel essential oil is limonene. Limonene is a natural active monoterpene with multiple functions, such as antibacterial, antiseptic and antitumor activity, and has important development value in agriculture. This study found that limonene exhibited excellent anti-tobacco mosaic virus (TMV) bioactivity, with results showing that its protection activity, inactivation activity, and curative activity at 800 μg/mL were 84.93%, 59.28%, and 58.89%, respectively-significantly higher than those of chito-oligosaccharides. A direct effect of limonene on TMV particles was not observed, but limonene triggered the hypersensitive response (HR) in tobacco. Further determination of the induction activity of limonene against TMV demonstrated that it displayed good induction activity at 800 μg/mL, with a value of 60.59%. The results of physiological and biochemical experiments showed that at different treatment days, 800 μg/mL limonene induced the enhancement of defense enzymes activity in tobacco, including of SOD, CAT, POD, and PAL, which respectively increased by 3.2, 4.67, 4.12, and 2.33 times compared with the control (POD and SOD activities reached highest on the seventh day, and PAL and CAT activities reached highest on the fifth day). Limonene also enhanced the relative expression levels of pathogenesis related (PR) genes, including NPR1, PR1, and PR5, which were upregulated 3.84-fold, 1.86-fold and 1.71-fold, respectively. Limonene induced the accumulation of salicylic acid (SA), and increased the relative expression levels of genes related to SA biosynthesis (PAL) and reactive oxygen species (ROS) burst (RBOHB), which respectively increased by 2.76 times and 4.23 times higher than the control. Systemic acquired resistance (SAR) is an important plant immune defense against pathogen infection. The observed accumulation of SA, the enhancement of defense enzymes activity and the high-level expression of defense-related genes suggested that limonene may induce resistance to TMV in tobacco by activating SAR mediated by the SA signaling pathway. Furthermore, the experimental results demonstrated that the expression level of the chlorophyll biosynthesis gene POR1 was increased 1.72-fold compared to the control in tobacco treated with 800 μg/mL limonene, indicating that limonene treatment may increase chlorophyll content in tobacco. The results of pot experiment showed that 800 μg/mL limonene induced plant resistance against Sclerotinia sclerotiorum (33.33%), Phytophthora capsici (54.55%), Botrytis cinerea (50.00%). The bioassay results indicated that limonene provided broad-spectrum and long-lasting resistance to pathogen infection. Therefore, limonene has good development and utilization value, and is expected to be developed into a new botanical-derived anti-virus agent and plant immunity activator in addition to insecticides and fungicides.

First Report of the Association of Zygocactus virus X with Dragon Fruit (Hylocereus spp.) plants from Telangana, India

Dragon fruit (Hylocereus spp.) a member of the family Cactaceae, is widely cultivated throughout the world, includingspan style="font-family:'Times New Roman'; letter-spacing:0.05pt; color:#333333"> India. During 2020-2021 crop growing season, mosaic symptoms were observed on the cladodes of dragon fruit plants (Purple Pink cultivar: 1-2% disease incidence) grown at a farmer's field of Telangana, India (Fig. S1 a). The symptomatic cladodes (n= 4), observed under leaf-dip electron microscopy (Zuchmaan and Zellnig, 2009) at Indian Agricultural Research Institute, New Delhi, revealed the presence of flexuous rod- shaped virus-like particles (Fig S1 b). Virus particles were of 580 x 13 nm size, corresponding to the genus Potexvirus. For further confirmation, the total RNA isolated from symptomatic cladodes using a NucleoSpin RNA Plant Mini kit (Macherey-Nagel). Subsequently, a reverse transcription polymerase chain reaction (RT-PCR) was performed using the PrimeScript 1st strand cDNA Synthesis Kit (Takara Bio). The cDNA was further amplified with the primers specific to coat protein (CP) gene of four different species of the genus Potexvirus known to infect members of Cactaceae family. Four sets of primers were used for detection, viz., Cactus virus X (CVX) (F, 5'-ATGTCTACTACTGGAGTCCA-3'; R, 5'-CTACTCAGGGCCTGGGAGAA-3'); Pitaya virus X (PiVX) (F, 5'-ATGGCTACTCAAACAGCACAA-3'; R, 5'-CTACTCTGGGGAGGGAAG-3'); Schlumbergera virus X (SchVX) (F, 5'-ATGTCGACCACTCCATCTTC-3'; R, 5'-TTATTCAGGGGATGGTAGTA-3') and Zygocactus virus X (ZyVX) (F, 5'-ATGTCTAACACTGCAGGAGT-3'; R, TCATTC GGGACCCGGTAGGA-3') (Duarte et al., 2008; Janssen et al., 2021; Parameswari et al., 2021), by following the PCR profile (Park et al., 2018). The species-specific primers of CVX, PiVX and SchVX did not amplify any amplicon, whereas the primers specific to ZyVX at nucleotide position 5841-6521 from complete CP gene have resulted in amplification of expected size (~680 base pairs) from all the samples. The gel-purified RT-PCR products were cloned into a pDrive cloning vector (Qiagen, Germany) and sequenced bi-directionally using Sanger sequencing. The resultant sequences (681 nt) of the CP gene showed 98% (nucleotide) and 100% (amino acid) sequence similarity with the CP gene sequence (Accession No: KY581590) of ZyVX. Hence, one representative sequence was deposited to the NCBI GenBank database as ZyVX-DPC isolate (Accession number- OK415019). The Neighbour Joining Phylogenetic Tree constructed using MEGA6 software (Tamura et al. 2013) showed grouping of Indian ZyVX-DPC isolate with the previously reported ZyVX isolates from Korea, Taiwan, China and Germany (Fig. S1c). These results confirmed the association of ZyVX with the symptomatic cladodes of dragon fruit plants collected from Telangana, India. Earlier studies revealed that ZyVX is a member of the genus Potexvirus known to infect dragon fruit plants from Brazil and China (Duarte et al., 2008). In India until now, anthracnose disease (Colletotrichum siamense) and CVX from Hylocereus spp. were reported (Abirami et al., 2019; Parameswari et al., 2021). To the best of our knowledge, this is the first report of ZyVX infection on dragon fruit in India. The draon fruit, being vegetatively propagated and with increasing cultivable area in India (Abirami et al, 2019), the present study gains significance. Further studies on mode of virus transmission, estimation of crop yield losses, host range studies and finding out source of resistance are essential.

Investigation on Natural Infection of Covert Mortality Nodavirus in Farmed Giant Freshwater Prawn (Macrobrachium rosenbergii)

Simple Summary

Covert mortality nodavirus (CMNV) is a newly discovered aquatic animal virus in recent years. Here, we detected CMNV positive in farmed giant freshwater prawn (Macrobrachium rosenbergii) from Jiangsu, China by TaqMan RT-qPCR. Meanwhile, in situ hybridization and histological analysis indicated that the intestine, gill, hepatopancreas and ovary of giant freshwater prawn were the target organs of CMNV. In addition, a large number of CMNV-like particles were observed in the hepatopancreas and gill tissues under transmission electron microscopy. Overall, our study confirms that giant freshwater prawn is a susceptible host of CMNV, further expands the known host range of CMNV, and provided a new direction for further investigation and exploration of multiple pathogenic factors of giant freshwater prawn disease.

Abstract

Covert mortality nodavirus (CMNV), from the Nodaviridae family, is characterized by its unique cross-species transmission and wide epidemic distribution features. In this study, Macrobrachium rosenbergii was proved to be infected naturally by CMNV, which further expand the known host range of CMNV. Here, 61.9% (70/113) of the M. rosenbergii samples collected from Jiangsu Province were CMNV positive in the TaqMan RT-qPCR assay, which indicated the high prevalence of CMNV in M. rosenbergii. Meanwhile, the sequences of CMNV RdRp gene cloned from M. rosenbergii were highly identical to that of the original CMNV isolate from Penaeus vannamei. In situ hybridization (ISH) and histology analysis indicated that the intestine, gill, hepatopancreas and ovary were the targeted organs of CMNV infection in M. rosenbergii, and obvious histopathological damage including vacuolation and karyopyknosis were occurred in the above organs. Notably, the presence of CMNV in gonad alerted its potential risk of vertical transmission in M. rosenbergii. Additionally, numerous CMNV-like particles could be observed in tissues of hepatopancreas and gill under transmission electron microscopy. Collectively, our results call for concern of the potential negative impact of the spread and prevalence of CMNV in M. rosenbergii on its aquaculture, as well as providing a renewed orientation for further investigation and exploration of the diverse pathogenic factors causing M. rosenbergii diseases.

Photosynthetic characterization and expression profiles of sugarcane infected by Sugarcane mosaic virus (SCMV)

Sugarcane mosaic virus (SCMV), belonging to genus Potyvirus, family Potyviridae, is a severe pathogen of several agricultural important crops, mainly sugarcane. Due to complex nature of sugarcane, the effect of SCMV pathogenicity on sugarcane photosynthetic systems remains to be explored. In this study, we investigated the alterations occurring in the photosynthetic system in the sugarcane genotypes at the cytopathological, physiological and biological, transcriptome and proteome level. We generated the transcriptome assembly of two genotypes (susceptible Badila and resistant B-48) using Saccharum spontaneum L. as a reference genome. RNA-sequencing data revealed the significant upregulation of NAD(P)H, RubisCO, oxygen-evolving complex, chlorophyll a and b binding protein, Psb protein family, PSI reaction center subunit II, and IVgenes in B-48, as compared to its counterparts. Upregulated genes in B-48 are associated with various processes such as stability and assembly of photosystem, protection against photoinhibition and antiviral defense. The expression pattern of differentially abundant genes were further verified at the proteomics level. Overall, differentially expressed genes/proteins (DEGs/DEPs) showed the consistency of expression at both transcriptome and proteome level in B-48 genotype. Comprehensively, these data supported the efficiency of B-48 genotype under virus infection conditions and provided a better understanding of the expression pattern of photosynthesis-related genes in sugarcane.

Three-dimensional quantitative imaging of Tobacco mosaic virus and Zucchini yellow mosaic virus induced ultrastructural changes

Two-dimensional ultrastructural changes of Tobacco mosaic virus (TMV) and Zucchini yellow mosaic virus (ZYMV) in tobacco and pumpkin plants, respectively, are well studied. To provide 3D data, representative control and infected cells were reconstructed using serial sectioning and transmission electron microscopy. Quantitative data of 3D ultrastructural changes were then extracted from the cytosol and organelles by image analysis. While TMV induced the accumulation of an average of 40 virus inclusion bodies in the cytosol, which covered about 13% of the cell volume, ZYMV caused the accumulation of an average of 1752 cylindrical inclusions in the cytosol, which covered about 2.7% of the total volume of the cell. TMV infection significantly decreased the number and size of mitochondria (- 49 and - 20%) and peroxisomes (- 62 and - 28%) of the reconstructed cell. The reconstructed ZYMV-infected cell contained more (105%) and larger (109%) mitochondria when compared to the control cell. While the reconstructed TMV-infected cell contained larger (20%) and the ZYMV-infected smaller (19%) chloroplasts, both contained less chloroplasts (- 40% for TMV and - 23% for ZYMV). In chloroplasts, the volume of starch and plastoglobules increased (664% and 150% for TMV and 1324% and 1300% for ZYMV) when compared to the control. The latter was correlated with a decrease in the volume of thylakoids in the reconstructed ZYMV-infected cell (- 31%) indicating that degradation products from thylakoids are transported and stored in plastoglobules. Summing up, the data collected in this study give a comprehensive overview of 3D changes induced by TMV and ZYMV in plants.

Natural infection of covert mortality nodavirus affects Zebrafish (Danio rerio)

Covert mortality nodavirus (CMNV), a novel aquatic pathogen, causes viral covert mortality disease (VCMD) in shrimps and also known to infect farmed marine fish. To date, there has no report regarding the ability of this virus to infect freshwater fish. In this study, we screened and discovered CMNV-positive freshwater zebrafish individuals by reverse transcription-nested PCR (RT-nPCR). The sequence of CMNV amplicons from zebrafish was found to share 99% identity with RNA-dependent RNA polymerase (RdRp) gene of the original CMNV isolate. Histopathological examination of the CMNV-positive zebrafish samples revealed extensive vacuolation and karyopyknosis lesions in the retina of the eye and the midbrain mesencephalon. CMNV-like virus particles were visualized in these tissues under transmission electron microscope. Different degrees of pathological damages were also found in muscle, gills, thymus and ovarian tissues. Strong positive signals of CMNV probe were observed in these infected tissues by in situ hybridization. Overall, all results indicated that zebrafish, an acknowledged model organism, could be infected naturally by CMNV. Thus, it is needed to pay close attention to the possible interference of CMNV whether in assessment of toxic substances, or in studying the developmental characterization and the nerval function, when zebrafish was used as model animal.

Evolution of Plant Virus Diagnostics Used in Australian Post Entry Quarantine

As part of a special edition for MDPI on plant virology in Australia, this review provides a brief high-level overview on the evolution of diagnostic techniques used in Australian government Post-Entry Quarantine (PEQ) facilities for testing imported plants for viruses. A comprehensive range of traditional and modern diagnostic approaches have historically been employed in PEQ facilities using bioassays, serological, and molecular techniques. Whilst these techniques have been effective, they are time consuming, resource intensive and expensive. The review highlights the importance of ensuring the best available science and diagnostic developments are constantly tested, evaluated, and implemented by regulators to ensure primary producers have rapid and safe access to new genetics to remain productive, sustainable and competitive.

Co-existence of chlorosis inducing strain of Cucumber mosaic virus with tospoviruses on hot pepper (Capsicum annuum) in India

Cucumo- and tospoviruses are the most destructive viruses infecting hot pepper (chilli). A diagnostic survey was conducted to assess the prevalence of cucumo and tospoviruses in chilli growing tracts of Tamil Nadu. Infected plants showing mosaic with chlorotic and necrotic rings, veinal necrosis, mosaic mottling, leaf filiformity and malformation were collected. Molecular indexing carried out through reverse transcription polymerase chain reaction (RT-PCR) with coat protein gene specific primer of Cucumber mosaic virus (CMV) and tospovirus degenerate primer corresponding to the L segment (RdRp). Ostensibly, amplifications were observed for both CMV and tospoviruses as sole as well for mixed infections. The sequence analysis indicated that the Capsicum chlorosis virus (CaCV) and Groundnut bud necrosis virus (GBNV) to be involved with CMV in causing combined infections. The co-infection of CMV with CaCV was detected in 10.41% of the symptomatic plant samples and combined infection of CMV with GBNV was recorded in around 6.25% of the symptomatic plants surveyed. The amino acid substitution of Ser¹²⁹ over conserved Pro¹²⁹ in coat protein of CMV implies that CMV strain involved in mixed infection as chlorosis inducing strain. Further, the electron microscopy of symptomatic plant samples explicated the presence of isometric particles of CMV and quasi spherical particles of tospoviruses. This is the first molecular evidence for the natural co-existence of chlorosis inducing CMV strain with CaCV and GBNV on hot pepper in India.

First Report on Natural Infection of Nodavirus in an Echinodermata, Sea Cucumber (Apostichopus japonicas)

Cross-species transmission of emerging viruses happens occasionally due to epidemiological, biological, and ecological factors, and it has caused more concern recently. Covert mortality nodavirus (CMNV) was revealed to be a unique shrimp virus that could cross species barrier to infect vertebrate fish. In the present study, CMNV reverse transcription-nested PCR (RT-nPCR)-positive samples were identified from farmed sea cucumber (Apostichopus japonicas) in the CMNV host range investigation. The amplicons of RT-nPCR from sea cucumber were sequenced, and its sequences showed 100% identity with the RNA-dependent RNA polymerase gene of the original CMNV isolate. Histopathological analysis revealed pathologic changes, including karyopyknosis and vacuolation of the epithelial cells, in the sea cucumber intestinal tissue. The extensive positive hybridization signals with CMNV probe were shown in the damaged epithelial cells in the in situ hybridization assay. Meanwhile, transmission electron microscopy analysis revealed CMNV-like virus particles in the intestine epithelium. All the results indicated that the sea cucumber, an Echinodermata, is a new host of CMNV. This study supplied further evidence of the wide host range of CMNV and also reminded us to pay close attention to its potential risk to threaten different aquaculture animal species.

Current Developments and Challenges in Plant Viral Diagnostics: A Systematic Review

Plant viral diseases are the foremost threat to sustainable agriculture, leading to several billion dollars in losses every year. Many viruses infecting several crops have been described in the literature; however, new infectious viruses are emerging frequently through outbreaks. For the effective treatment and prevention of viral diseases, there is great demand for new techniques that can provide accurate identification on the causative agents. With the advancements in biochemical and molecular biology techniques, several diagnostic methods with improved sensitivity and specificity for the detection of prevalent and/or unknown plant viruses are being continuously developed. Currently, serological and nucleic acid methods are the most widely used for plant viral diagnosis. Nucleic acid-based techniques that amplify target DNA/RNA have been evolved with many variants. However, there is growing interest in developing techniques that can be based in real-time and thus facilitate in-field diagnosis. Next-generation sequencing (NGS)-based innovative methods have shown great potential to detect multiple viruses simultaneously; however, such techniques are in the preliminary stages in plant viral disease diagnostics. This review discusses the recent progress in the use of NGS-based techniques for the detection, diagnosis, and identification of plant viral diseases. New portable devices and technologies that could provide real-time analyses in a relatively short period of time are prime important for in-field diagnostics. Current development and application of such tools and techniques along with their potential limitations in plant virology are likewise discussed in detail.

The length of an internal poly(A) tract of hibiscus latent Singapore virus is crucial for its replication

Hibiscus latent Singapore virus (HLSV) mutants were constructed to study roles of its internal poly(A) tract (IPAT) in viral replication and coat protein (CP) expression. Shortening of the IPAT resulted in reduced HLSV RNA accumulation and its minimal length required for HLSV CP expression in plants was 24 nt. Disruption of a putative long range RNA-RNA interacting structure between 5' and 3' untranslated regions of HLSV-22A and -24A resulted in reduced viral RNA and undetectable CP accumulation in inoculated leaves. Replacement of the IPAT in HLSV with an upstream pseudoknot domain (UPD) of Tobacco mosaic virus (TMV) or insertion of the UPD to the immediate downstream of a 24 nt IPAT in HLSV resulted in drastically reduced viral RNA replication. Plants infected with a TMV mutant by replacement of the UPD with 43 nt IPAT exhibited milder mosaic symptoms without necrosis. We have proposed a model for HLSV replication.

Efficient purification and concentration of viruses from a large body of high turbidity seawater

Marine viruses are the most abundant entities in the ocean and play crucial roles in the marine ecological system. However, understanding of viral diversity on large scale depends on efficient and reliable viral purification and concentration techniques. Here, we report on developing an efficient method to purify and concentrate viruses from large body of high turbidity seawater. The developed method characterizes with high viral recovery efficiency, high concentration factor, high viral particle densities and high-throughput, and is reliable for viral concentration from high turbidity seawater. Recovered viral particles were used directly for subsequent analysis by epifluorescence microscopy, transmission electron microscopy and metagenomic sequencing. Three points are essential for this method:•

The sampled seawater (>150 L) was initially divided into two parts, water fraction and settled matter fraction, after natural sedimentation.

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Both viruses in the water fraction concentrated by tangential flow filtration (TFF) and viruses isolated from the settled matter fraction were considered as the whole viral community in high turbidity seawater.

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The viral concentrates were re-concentrated by using centrifugal filter device in order to obtain high density of viral particles.

Single virus detection by means of atomic force microscopy in combination with advanced image analysis

In the present contribution virions of five different virus species, namely Varicella-zoster virus, Porcine teschovirus, Tobacco mosaic virus, Coliphage M13 and Enterobacteria phage PsP3, are investigated using atomic force microscopy (AFM). From the resulting height images quantitative features like maximal height, area and volume of the viruses could be extracted and compared to reference values. Subsequently, these features were accompanied by image moments, which quantify the morphology of the virions. Both types of features could be utilized for an automatic discrimination of the five virus species. The accuracy of this classification model was 96.8%. Thus, a virus detection on a single-particle level using AFM images is possible. Due to the application of advanced image analysis the morphology could be quantified and used for further analysis. Here, an automatic recognition by means of a classification model could be achieved in a reliable and objective manner.

Detecting Virus-Like Particles from the Umgeni River, South Africa

It is important to consider viruses in water quality because of their incidence as causal agents for diarrhoeal disease, and due to their characteristics, which allow them to survive in changing environmental conditions indefinitely. This study assessed the viral quality of the Umgeni River in South Africa seasonally. A two-step tangential flow filtration process was setup to remove the bacteria and to concentrate the virus populations from large volume water samples. The concentrated water samples contained up to 659 and 550 pfu/mL of somatic and F-RNA coliphages, respectively. Several virus families including Adenoviridae, Herpesviridae, Orthomyxoviridae, Picornaviridae, Poxviridae and Reoviridae were found in the river based on the morphologies examined under transmission electron microscopy. All concentrated water samples produced substantial cytopathic effects on the Vero, HEK 293, Hela and A549 cell lines. These results indicate the potential of viruses in the water samples especially from the lower catchment areas of the Umgeni River to infect human hosts throughout the year. The present study highlights the importance of routine environmental surveillance of human enteric viruses in water sources. This can contribute to a better understanding of the actual burden of disease on those who might be using the water directly without treatment.

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.

Rapid immunohistochemical diagnosis of tobacco mosaic virus disease by microwave-assisted plant sample preparation

Immunoelectron microscopy is a powerful method to diagnose viral diseases and to study the distribution of the viral agent within plant cells and tissues. Nevertheless, current protocols for the immunological detection of viral diseases with transmission electron microscopy (TEM) in plants take between 3 and 6 days and are therefore not suited for rapid diagnosis of virus diseases in plants. In this study, we describe a method that allows rapid cytohistochemical detection of tobacco mosaic virus (TMV) in leaves of tobacco plants. With the help of microwave irradiation, sample preparation of the leaves was reduced to 90 min. After sample sectioning, virus particles were stained on the sections by immunogold labelling of the viral coat protein, which took 100 min. After investigation with the TEM, a clear visualization of TMV in tobacco cells was achieved altogether in about half a day. Comparison of gold particle density by image analysis revealed that samples prepared with the help of microwave irradiation yielded significantly higher gold particle density as samples prepared conventionally at room temperature. This study clearly demonstrates that microwave-assisted plant sample preparation in combination with cytohistochemical localization of viral coat protein is well suited for rapid diagnosis of plant virus diseases in altogether about half a day by TEM.

Microwaves and tea: new tools to process plant tissue for transmission electron microscopy

Optimizing sample processing, reducing the duration of the preparation of specimen, and adjusting procedures to adhere to new health and safety regulations, are the current challenges of plant electron microscopists. To address these issues, plant processing protocols for TEM, combining the use of polyphenolic compounds as substitute for uranyl acetate with microwave technology are being developed. In the present work, we optimized microwave-assisted processing of different types of plant tissue for ultrastuctural and immunocytochemical studies. We also explored Oolong tea extract as alternative for uranyl acetate for the staining of plant samples. We obtained excellent preservation of cell ultrastructure when samples were embedded in epoxy resin, and of cell antigenicity, when embedded in LR-White resin. Furthermore, Oolong tea extract successfully replaced uranyl acetate as a counterstain on ultrathin sections, and for in block staining. These novel protocols reduce the time spent at the bench, and improve safety conditions for the investigator. The preservation of the cell components when following these approaches is of high quality. Altogether, they offer significant simplification of the procedures required for electron microscopy of plant ultrastructure.

The invasion of tobacco mosaic virus RNA induces endoplasmic reticulum stress-related autophagy in HeLa cells

The ability of human cells to defend against viruses originating from distant species has long been ignored. Owing to the pressure of natural evolution and human exploration, some of these viruses may be able to invade human beings. If their ‘fresh’ host had no defences, the viruses could cause a serious pandemic, as seen with HIV, SARS (severe acute respiratory syndrome) and avian influenza virus that originated from chimpanzees, the common palm civet and birds, respectively. It is unknown whether the human immune system could tolerate invasion with a plant virus. To model such an alien virus invasion, we chose TMV (tobacco mosaic virus) and used human epithelial carcinoma cells (HeLa cells) as its ‘fresh’ host. We established a reliable system for transfecting TMV-RNA into HeLa cells and found that TMV-RNA triggered autophagy in HeLa cells as shown by the appearance of autophagic vacuoles, the conversion of LC3-I (light chain protein 3-I) to LC3-II, the up-regulated expression of Beclin1 and the accumulation of TMV protein on autophagosomal membranes. We observed suspected TMV virions in HeLa cells by TEM (transmission electron microscopy). Furthermore, we found that TMV-RNA was translated into CP (coat protein) in the ER (endoplasmic reticulum) and that TMV-positive RNA translocated from the cytoplasm to the nucleolus. Finally, we detected greatly increased expression of GRP78 (78 kDa glucose-regulated protein), a typical marker of ERS (ER stress) and found that the formation of autophagosomes was closely related to the expanded ER membrane. Taken together, our data indicate that HeLa cells used ERS and ERS-related autophagy to defend against TMV-RNA.

Microwave assisted rapid diagnosis of plant virus diseases by transmission electron microscopy

Investigations of ultrastructural changes induced by viruses are often necessary to clearly identify viral diseases in plants. With conventional sample preparation for transmission electron microscopy (TEM) such investigations can take several days and are therefore not suited for a rapid diagnosis of plant virus diseases. Microwave fixation can be used to drastically reduce sample preparation time for TEM investigations with similar ultrastructural results as observed after conventionally sample preparation. Many different custom made microwave devices are currently available which can be used for the successful fixation and embedding of biological samples for TEM investigations. In this study we demonstrate on Tobacco Mosaic Virus (TMV) infected Nicotiana tabacum plants that it is possible to diagnose ultrastructural alterations in leaves in about half a day by using microwave assisted sample preparation for TEM. We have chosen to perform this study with a commercially available microwave device as it performs sample preparation almost fully automatically in contrast to the other available devices where many steps still have to be performed manually and are therefore more time and labor consuming. As sample preparation is performed fully automatically negative staining of viral particles in the sap of the remaining TMV-infected leaves and the following examination of ultrastructure and size can be performed during fixation and embedding.

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

Background

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

Results

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

Conclusion

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

Subcellular compartmentation of glutathione in dicotyledonous plants

This study describes the subcellular distribution of glutathione in roots and leaves of different plant species (Arabidopsis, Cucurbita, and Nicotiana). Glutathione is an important antioxidant and redox buffer which is involved in many metabolic processes including plant defense. Thus information on the subcellular distribution in these model plants especially during stress situations provides a deeper insight into compartment specific defense reactions and reflects the occurrence of compartment specific oxidative stress. With immunogold cytochemistry and computer-supported transmission electron microscopy glutathione could be localized in highest contents in mitochondria, followed by nuclei, peroxisomes, the cytosol, and plastids. Within chloroplasts and mitochondria, glutathione was restricted to the stroma and matrix, respectively, and did not occur in the lumen of cristae and thylakoids. Glutathione was also found at the membrane and in the lumen of the endoplasmic reticulum. It was also associated with the trans and cis side of dictyosomes. None or only very little glutathione was detected in vacuoles and the apoplast of mesophyll and root cells. Additionally, glutathione was found in all cell compartments of phloem vessels, vascular parenchyma cells (including vacuoles) but was absent in xylem vessels. The specificity of this method was supported by the reduction of glutathione labeling in all cell compartments (up to 98%) of the glutathione-deficient Arabidopsis thaliana rml1 mutant. Additionally, we found a similar distribution of glutathione in samples after conventional fixation and rapid microwave-supported fixation. Thus, indicating that a redistribution of glutathione does not occur during sample preparation. Summing up, this study gives a detailed insight into the subcellular distribution of glutathione in plants and presents solid evidence for the accuracy and specificity of the applied method.