Programmed responses to virus replication in plants

Melon Genome Regions Associated with TGR-1551-Derived Resistance to Cucurbit yellow stunting disorder virus

Cucurbit yellow stunting disorder virus (CYSDV) is one of the main limiting factors of melon cultivation worldwide. To date, no commercial melon cultivars resistant to CYSDV are available. The African accession TGR-1551 is resistant to CYSDV. Two major quantitative trait loci (QTLs) have been previously reported, both located near each other in chromosome 5. With the objective of further mapping the gene or genes responsible of the resistance, a recombinant inbred line (RIL) population derived from the cross between TGR-1551 and the susceptible cultivar ‘Bola de Oro’ was evaluated for resistance to CYSDV in five different assays and genotyped in a genotyping by sequencing (GBS) analysis. The major effect of one of the two QTLs located on chromosome 5 was confirmed in the multienvironment RIL assay and additionally verified through the analysis of three segregating BC₁S₁ populations derived from three resistant RILs. Furthermore, progeny test using the offspring of selected BC₃ plants allowed the narrowing of the candidate interval to a 700 kb region. The SNP markers identified in this work will be useful in marker-assisted selection in the context of introgression of CYSDV resistance in elite cultivars.

Prosystemin overexpression induces transcriptional modifications of defense-related and receptor-like kinase genes and reduces the susceptibility to Cucumber mosaic virus and its satellite RNAs in transgenic tomato plants

Systemin is a plant signal peptide hormone involved in the responses to wounding and insect damage in the Solanaceae family. It works in the same signaling pathway of jasmonic acid (JA) and enhances the expression of proteinase inhibitors. With the aim of studying a role for systemin in plant antiviral responses, a tomato (Solanum lycopersicum) transgenic line overexpressing the prosystemin cDNA, i.e. the systemin precursor, was inoculated with Cucumber mosaic virus (CMV) strain Fny supporting either a necrogenic or a non-necrogenic satellite RNA (satRNA) variant. Transgenic plants showed reduced susceptibility to both CMV/satRNA combinations. While symptoms of the non-necrogenic inoculum were completely suppressed, a delayed onset of lethal disease occurred in about half of plants challenged with the necrogenic inoculum. RT-qPCR analysis showed a correlation between the systemin-mediated reduced susceptibility and the JA biosynthetic and signaling pathways (e.g. transcriptional alteration of lipoxygenase D and proteinase inhibitor II). Moreover, transgenically overexpressed systemin modulated the expression of a selected set of receptor-like protein kinase (RLK) genes, including some playing a known role in plant innate immunity. A significant correlation was found between the expression profiles of some RLKs and the systemin-mediated reduced susceptibility to CMV/satRNA. These results show that systemin can increase plant defenses against CMV/satRNA through transcriptional reprogramming of diverse signaling pathways.

The coat protein of Alfalfa mosaic virus interacts and interferes with the transcriptional activity of the bHLH transcription factor ILR3 promoting salicylic acid-dependent defence signalling response

During virus infection, specific viral component-host factor interaction elicits the transcriptional reprogramming of diverse cellular pathways. Alfalfa mosaic virus (AMV) can establish a compatible interaction in tobacco and Arabidopsis hosts. We show that the coat protein (CP) of AMV interacts directly with transcription factor (TF) ILR3 of both species. ILR3 is a basic helix-loop-helix (bHLH) family member of TFs, previously proposed to participate in diverse metabolic pathways. ILR3 has been shown to regulate NEET in Arabidopsis, a critical protein in plant development, senescence, iron metabolism and reactive oxygen species (ROS) homeostasis. We show that the AMV CP-ILR3 interaction causes a fraction of this TF to relocate from the nucleus to the nucleolus. ROS, pathogenesis-related protein 1 (PR1) mRNAs, salicylic acid (SA) and jasmonic acid (JA) contents are increased in healthy Arabidopsis loss-of-function ILR3 mutant (ilr3.2) plants, which implicates ILR3 in the regulation of plant defence responses. In AMV-infected wild-type (wt) plants, NEET expression is reduced slightly, but is induced significantly in ilr3.2 mutant plants. Furthermore, the accumulation of SA and JA is induced in Arabidopsis wt-infected plants. AMV infection in ilr3.2 plants increases JA by over 10-fold, and SA is reduced significantly, indicating an antagonist crosstalk effect. The accumulation levels of viral RNAs are decreased significantly in ilr3.2 mutants, but the virus can still systemically invade the plant. The AMV CP-ILR3 interaction may down-regulate a host factor, NEET, leading to the activation of plant hormone responses to obtain a hormonal equilibrium state, where infection remains at a level that does not affect plant viability.

Molecular biology of potyviruses

Potyvirus is the largest genus of plant viruses causing significant losses in a wide range of crops. Potyviruses are aphid transmitted in a nonpersistent manner and some of them are also seed transmitted. As important pathogens, potyviruses are much more studied than other plant viruses belonging to other genera and their study covers many aspects of plant virology, such as functional characterization of viral proteins, molecular interaction with hosts and vectors, structure, taxonomy, evolution, epidemiology, and diagnosis. Biotechnological applications of potyviruses are also being explored. During this last decade, substantial advances have been made in the understanding of the molecular biology of these viruses and the functions of their various proteins. After a general presentation on the family Potyviridae and the potyviral proteins, we present an update of the knowledge on potyvirus multiplication, movement, and transmission and on potyvirus/plant compatible interactions including pathogenicity and symptom determinants. We end the review providing information on biotechnological applications of potyviruses.

A plastid-targeted heat shock cognate 70kDa protein interacts with the Abutilon mosaic virus movement protein

The movement protein (MP) of bipartite geminiviruses facilitates cell-to-cell as well as long-distance transport within plants and influences viral pathogenicity. Yeast two-hybrid assays identified a chaperone, the nuclear-encoded and plastid-targeted heat shock cognate 70kDa protein (cpHSC70-1) of Arabidopsis thaliana, as a potential binding partner for the Abutilon mosaic virus (AbMV) MP. In planta, bimolecular fluorescence complementation (BiFC) analysis showed cpHSC70-1/MP complexes and MP homooligomers at the cell periphery and co-localized with chloroplasts. BiFC revealed cpHSC70-1 oligomers associated with chloroplasts, but also distributed at the cellular margin and in filaments arising from plastids reminiscent of stromules. Silencing the cpHSC70 gene of Nicotiana benthamiana using an AbMV DNA A-derived gene silencing vector induced minute white leaf areas, which indicate an effect on chloroplast stability. Although AbMV DNA accumulated within chlorotic spots, a spatial restriction of these occurred, suggesting a functional relevance of the MP-chaperone interaction for viral transport and symptom induction.

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.

Altered gene expression changes in Arabidopsis leaf tissues and protoplasts in response to Plum pox virus infection

Background

Virus infection induces the activation and suppression of global gene expression in the host. Profiling gene expression changes in the host may provide insights into the molecular mechanisms that underlie host physiological and phenotypic responses to virus infection. In this study, the Arabidopsis Affymetrix ATH1 array was used to assess global gene expression changes in Arabidopsis thaliana plants infected with Plum pox virus (PPV). To identify early genes in response to PPV infection, an Arabidopsis synchronized single-cell transformation system was developed. Arabidopsis protoplasts were transfected with a PPV infectious clone and global gene expression changes in the transfected protoplasts were profiled.

Results

Microarray analysis of PPV-infected Arabidopsis leaf tissues identified 2013 and 1457 genes that were significantly (Q ≤ 0.05) up- (≥ 2.5 fold) and downregulated (≤ -2.5 fold), respectively. Genes associated with soluble sugar, starch and amino acid, intracellular membrane/membrane-bound organelles, chloroplast, and protein fate were upregulated, while genes related to development/storage proteins, protein synthesis and translation, and cell wall-associated components were downregulated. These gene expression changes were associated with PPV infection and symptom development. Further transcriptional profiling of protoplasts transfected with a PPV infectious clone revealed the upregulation of defence and cellular signalling genes as early as 6 hours post transfection. A cross sequence comparison analysis of genes differentially regulated by PPV-infected Arabidopsis leaves against uniEST sequences derived from PPV-infected leaves of Prunus persica, a natural host of PPV, identified orthologs related to defence, metabolism and protein synthesis. The cross comparison of genes differentially regulated by PPV infection and by the infections of other positive sense RNA viruses revealed a common set of 416 genes. These identified genes, particularly the early responsive genes, may be critical in virus infection.

Conclusion

Gene expression changes in PPV-infected Arabidopsis are the molecular basis of stress and defence-like responses, PPV pathogenesis and symptom development. The differentially regulated genes, particularly the early responsive genes, and a common set of genes regulated by infections of PPV and other positive sense RNA viruses identified in this study are candidates suitable for further functional characterization to shed lights on molecular virus-host interactions.

Association of the transcriptional response of soybean plants with soybean mosaic virus systemic infection

Compatible virus infection induces and suppresses host gene expression at the global level. These gene-expression changes are the molecular basis of symptom development and general stress and defence-like responses of the host. To assess transcriptional changes in soybean plants infected with soybean mosaic virus (SMV), the first soybean trifoliate leaf, immediately above the SMV-inoculated unifoliate leaf, was sampled at 7, 14 and 21 days post-inoculation (p.i.) and subjected to microarray analysis. The identified changes in gene expression in soybean leaves with SMV infection at different time points were associated with the observed symptom development. By using stringent selection criteria (>or=2- or Collapse

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MESH Headings

• Gene Expression
• Gene Expression Profiling
• Genes, Plant
• Mosaic Viruses
• Plant Diseases/virology
• Plant Leaves/metabolism
• Plant Proteins/genetics
• Plant Proteins/metabolism
• Glycine max/metabolism
• Glycine max/virology
• Time Factors

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Affiliation(s)

Mohan Babu Southern Crop Protection and Food Research Centre, Agriculture and Agri-Food Canada (AAFC), 1391 Sandford St, London, ON N5V 4T3, Canada
Alla G Gagarinova Department of Biology, The University of Western Ontario, Biological and Geological Building, 1151 Richmond St, London, ON N6A 5B7, Canada Southern Crop Protection and Food Research Centre, Agriculture and Agri-Food Canada (AAFC), 1391 Sandford St, London, ON N5V 4T3, Canada
James E Brandle Southern Crop Protection and Food Research Centre, Agriculture and Agri-Food Canada (AAFC), 1391 Sandford St, London, ON N5V 4T3, Canada
Aiming Wang Department of Biology, The University of Western Ontario, Biological and Geological Building, 1151 Richmond St, London, ON N6A 5B7, Canada Southern Crop Protection and Food Research Centre, Agriculture and Agri-Food Canada (AAFC), 1391 Sandford St, London, ON N5V 4T3, Canada

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Plant NB-LRR immune receptors: from recognition to transcriptional reprogramming

Both plants and animals contain nucleotide-binding domain and leucine-rich repeat (NB-LRR)-type immune receptors that function during defense against pathogens. Unlike animal NB-LRRs that recognize general pathogen or microbe-associated molecular patterns (PAMPs or MAMPs), plant NB-LRR immune receptors have evolved the ability to specifically recognize a wide range of effector proteins from different pathogens. Recent research has revealed that plant NB-LRRs are incredibly adaptive in their ways of pathogen recognition and defense initiation. This review focuses on the remarkable variety of functions, recognition mechanisms, subcellular localizations, and host factors associated with plant NB-LRR immune receptors.

Virus-induced disease: altering host physiology one interaction at a time

Virus infections are the cause of numerous plant disease syndromes that are generally characterized by the induction of disease symptoms such as developmental abnormalities, chlorosis, and necrosis. How viruses induce these disease symptoms represents a long-standing question in plant pathology. Recent studies indicate that symptoms are derived from specific interactions between virus and host components. Many of these interactions have been found to contribute to the successful completion of the virus life-cycle, although the role of other interactions in the infection process is not yet known. However, all share the potential to disrupt host physiology. From this information we are beginning to decipher the progression of events that lead from specific virus-host interactions to the establishment of disease symptoms. This review highlights our progress in understanding the mechanisms through which virus-host interactions affect host physiology. The emerging picture is one of complexity involving the individual effects of multiple virus-host interactions.

The potyvirus recessive resistance gene, sbm1, identifies a novel role for translation initiation factor eIF4E in cell-to-cell trafficking

From the characterization of the recessive resistance gene, sbm1, in pea we have identified the eukaryotic translation initiation factor, eIF4E, as a susceptibility factor required for infection with the Potyvirus, Pea seed-borne mosaic virus. A functional analysis of the mode of action of the product of the dominant allele revealed a novel function for eIF4E in its support for virus movement from cell-to-cell, in addition to its probable support for viral RNA translation, and hence replication. Different resistance specificities in two independent pea lines were explained by different mutations in eIF4E. On the modelled structure of eIF4E the coding changes were in both cases lying in and around the structural pocket involved in binding the 5'-m7G cap of eukaryotic mRNAs. Protein expression and cap-binding analysis showed that eIF4E encoded by a resistant plant could not bind to m7G-Sepharose, a result which may point to functional redundancy between eIF4E and the paralogous eIF(iso)4E in resistant peas. These observations, together with related findings for other potyvirus recessive resistances, provide a more complete picture of the potyvirus life cycle.

Escape of a plant virus from amplicon-mediated RNA silencing is associated with biotic or abiotic stress

Strong RNA silencing was induced in plants transformed with an amplicon consisting of full-length cDNA of potato leafroll virus (PLRV) expressing green fluorescent protein (GFP), as shown by low levels of PLRV-GFP accumulation, lack of symptoms and accumulation of amplicon-specific short interfering RNAs (siRNAs). Inoculation of these plants with various viruses known to encode silencing suppressor proteins induced a striking synergistic effect leading to the enhanced accumulation of PLRV-GFP, suggesting that it had escaped from silencing. However, PLRV-GFP escape also occurred following inoculation with viruses that do not encode known silencing suppressors and treatment of silenced plants with biotic or abiotic stress agents. We propose that viruses can evade host RNA-silencing defences by a previously unrecognized mechanism that may be associated with a host response to some types of abiotic stress such as heat shock.

Melon Resistance to Cucurbit yellow stunting disorder virus Is Characterized by Reduced Virus Accumulation

ABSTRACT The pattern of accumulation of Cucurbit yellow stunting disorder virus (CYSDV; genus Crinivirus, family Closteroviridae) RNA has been analyzed in several cucurbit accessions. In susceptible accessions of melon (Cucumis melo), cucumber (Cucumis sativus), marrow (Cucurbita maxima), and squash (Cucurbita pepo), CYSDV RNA accumulation peaked during the first to second week postinoculation in the first to third leaf above the inoculated one; younger leaves showed very low or undetectable levels of CYSDV. Three melon accessions previously shown to remain asymptomatic after CYSDV inoculation under natural conditions were also assayed for their susceptibility to CYSDV. Hybridization and reverse transcription-polymerase chain reaction (RT-PCR) analysis of noninoculated leaves showed that only one of these, C-105, remained virus-free for up to 6 weeks after whitefly inoculation. In this accession, very low CYSDV levels were detected by RT-PCR in whitefly-inoculated leaves, and therefore, multiplication or spread of CYSDV in C-105 plants appeared to remain restricted to the inoculated leaves. When C-105 plants were graft inoculated, CYSDV RNA could be detected in phloem tissues, but the systemic colonization of C-105 by CYSDV upon graft inoculation seemed to be seriously impeded. Additionally, in situ hybridization experiments showed that, after C-105 graft inoculation, only a portion of the vascular bundles in petioles and stems were colonized by CYSDV and virus could not be found in leaf veins. RT-PCR experiments using primers to specifically detect negative-sense CYSDV RNA were carried out and showed that CYSDV replication took place in graft-inoculated C-105 scions. Therefore, the resistance mechanism may involve a restriction of the virus movement in the vascular system of the plants and/or prevention of high levels of virus accumulation.

The multifunctional capsid proteins of plant RNA viruses

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

A comparison between virus replication and abiotic stress (heat) as modifiers of host gene expression in pea

Abstract Pea embryonic tissues respond to active replication of pea seed-borne mosaic potyvirus (PSbMV) by the down-regulation of a range of genes and the induction of others. Both of these responses can be seen when tissues are subjected to abiotic stress, particularly heat. We have compared the effects of the two inducers to assess whether the host alterations following virus replication represent generic responses to stress, or more specific effects. Five classes of response were identified: (i) genes induced by both stresses (e.g. heat shock protein 70, hsp70); (ii) genes induced by virus replication but unaffected by heat (e.g. glutathione reductase 2, gor2); (iii) genes induced by heat but unaffected by virus replication (e.g. heat shock factor, hsf); (iv) genes down-regulated by virus replication and unaffected by heat (e.g. vicilin, vic); and (v) genes unaffected by both inducers (e.g. actin, act and beta-tubulin, tub). A change in the appearance and organization of the endoplasmic reticulum (ER) was also seen in cells actively replicating PSbMV RNA. Heat treatment of pea embryonic tissues also produced altered ER, although the changes were different from those seen following virus infection. Collectively, these data show that, while there are some common features of the responses to virus infection and heat, there are also substantial differences. Hence, it appears that the host response to virus replication is not a general stress response.