Characterization of the cassava geminivirus transcription activation protein putative nuclear localization signal

The Past, Present, and Future of Wheat Dwarf Virus Management-A Review

Wheat dwarf disease (WDD) is an important disease of monocotyledonous species, including economically important cereals. The causative pathogen, wheat dwarf virus (WDV), is persistently transmitted mainly by the leafhopper Psammotettix alienus and can lead to high yield losses. Due to climate change, the periods of vector activity increased, and the vectors have spread to new habitats, leading to an increased importance of WDV in large parts of Europe. In the light of integrated pest management, cultivation practices and the use of resistant/tolerant host plants are currently the only effective methods to control WDV. However, knowledge of the pathosystem and epidemiology of WDD is limited, and the few known sources of genetic tolerance indicate that further research is needed. Considering the economic importance of WDD and its likely increasing relevance in the coming decades, this study provides a comprehensive compilation of knowledge on the most important aspects with information on the causal virus, its vector, symptoms, host range, and control strategies. In addition, the current status of genetic and breeding efforts to control and manage this disease in wheat will be discussed, as this is crucial to effectively manage the disease under changing environmental conditions and minimize impending yield losses.

Regulation of Nicotiana benthamiana cell death induced by citrus chlorotic dwarf-associated virus-RepA protein by WRKY 1

Citrus chlorotic dwarf-associated virus (CCDaV) is a Citlodavirus species in the Geminiviridae family that causes tremendous economic loss to the citrus industry in China. Some proteins encoded by geminiviruses are crucial for the interaction between the virus and its host plant. However, the exact functions of CCDaV-encoded proteins such as CCDaV-RepA have not been investigated. This study presents evidence that CCDaV-RepA elicits a hypersensitive response (HR)-like cell death in Nicotiana benthamiana that was accompanied by the production of H2O2 and ion leakage, which suggested that CCDaV-RepA is a potential recognition target for inducing host defense responses. Furthermore, the rolling-circle replication motifs of CCDaV-RepA are associated with triggering HR-like cell death in N. benthamiana. Confocal microscopy and deletion mutagenesis assays showed that CCDaV-RepA was located in the nucleus, while the first eight amino acids (aa) at the N terminus and two regions located between aa residues 122-263 and 220-264 of RepA were not associated with nuclear localization. Tobacco rattle virus-induced gene silencing of the key signaling cascade components revealed that HR-like cell death induced by RepA was inhibited in WRKY1-silenced N. benthamiana. Moreover, WRKY1 expression was upregulated in RepA-GFP infiltrated Overall, the results suggest that NbWRKY1 positively regulated CCDaV-RepA -induced cell death in N. benthamiana. These findings provide novel information for further research on the interactions between CCDaV and the host plant.

Geminivirus Resistance: A Minireview

A continuing challenge to crop production worldwide is the spectrum of diseases caused by geminiviruses, a large family of small circular single-stranded DNA viruses. These viruses are quite diverse, some containing mono- or bi-partite genomes, and infecting a multitude of monocot and dicot plants. There are currently many efforts directed at controlling these diseases. While some of the methods include controlling the insect vector using pesticides or genetic insect resistance (Rodríguez-López et al., 2011), this review will focus on the generation of plants that are resistant to geminiviruses themselves. Genetic resistance was traditionally found by surveying the wild relatives of modern crops for resistance loci; this method is still widely used and successful. However, the quick rate of virus evolution demands a rapid turnover of resistance genes. With better information about virus-host interactions, scientists are now able to target early stages of geminivirus infection in the host, preventing symptom development and viral DNA accumulation.

Infectivity of cloned begomoviral DNAs: an appraisal

Infectivity of cloned begomoviral DNAs is an important criterion to establish the etiology of the disease it causes, to study viral gene functions and host-virus interactions. Three main methods have been employed to study infectivity; mechanical inoculation with cloned viral DNA using abrasives, Agrobacterium-mediated inoculation (agroinoculation) of cloned viral DNA and bombardment using microprojectiles coated with cloned viral DNA (biolistics). Each method has its own advantages and disadvantages and the adoption of one over the other for demonstrating infectivity depends on various factors. This review compares the various features associated with the above three methods.

The C2 protein of tomato leaf curl Taiwan virus is a pathogenicity determinant that interferes with expression of host genes encoding chromomethylases

Tomato (Solanum lycopersicum) is one of the most important crops worldwide and is severely affected by geminiviruses. Tomato leaf curl Taiwan virus (ToLCTWV), belonging to the geminiviruses, was isolated in Taiwan and causes tremendous crop loss. The geminivirus-encoded C2 proteins are crucial for a successful interaction between the virus and host plants. However, the exact functions of the viral C2 protein of ToLCTWV have not been investigated. We analyzed the molecular function(s) of the C2 protein by transient or stable expression in tomato cv. Micro-Tom and Nicotiana benthamiana. Severe stunting of tomato and N. benthamiana plants infected with ToLCTWV was observed. Expression of ToLCTWV C2-green fluorescent protein (GFP) fusion protein was predominately located in the nucleus and contributed to activation of a coat protein promoter. Notably, the C2-GFP fluorescence was distributed in nuclear aggregates. Tomato and N. benthamiana plants inoculated with potato virus X (PVX)-C2 displayed chlorotic lesions and stunted growth. PVX-C2 elicited hypersensitive responses accompanied by production of reactive oxygen species in N. benthamiana plants, which suggests that the viral C2 was a potential recognition target to induce host-defense responses. In tomato and N. benthamiana, ToLCTWV C2 was found to interfere with expression of genes encoding chromomethylases. N. benthamiana plants with suppressed NbCMT3-2 expression were more susceptible to ToLCTWV infection. Transgenic N. benthamiana plants expressing the C2 protein showed decreased expression of the NbCMT3-2 gene and pNbCMT3-2::GUS (β-glucuronidase) promoter activity. C2 protein is an important pathogenicity determinant of ToLCTWV and interferes with host components involved in DNA methylation.

RepA Protein Encoded by Oat dwarf virus Elicits a Temperature-Sensitive Hypersensitive Response-Type Cell Death That Involves Jasmonic Acid-Dependent Signaling

The hypersensitive response (HR) is a component of disease resistance that is often induced by pathogen infection, but essentially no information is available for members of the destructive mastreviruses. We have investigated an HR-type response elicited in Nicotiana species by Oat dwarf virus (ODV) and have found that expression of the ODV RepA protein but not other ODV-encoded proteins elicits the HR-type cell death associated with a burst of H2O2. Deletion mutagenesis indicates that the first nine amino acids (aa) at the N terminus of RepA and the two regions located between aa residues 173 and 195 and between aa residues 241 and 260 near the C terminus are essential for HR-type cell-death elicitation. Confocal and electron microscopy showed that the RepA protein is localized in the nuclei of plant cells and might contain bipartite nuclear localization signals. The HR-like lesions mediated by RepA were inhibited by temperatures above 30°C and involvement of jasmonic acid (JA) in HR was identified by gain- and loss-of-function experiments. To our knowledge, this is the first report of an elicitor of HR-type cell death from mastreviruses.

Geminivirus protein structure and function

Geminiviruses are a family of plant viruses that cause economically important plant diseases worldwide. These viruses have circular single-stranded DNA genomes and four to eight genes that are expressed from both strands of the double-stranded DNA replicative intermediate. The transcription of these genes occurs under the control of two bidirectional promoters and one monodirectional promoter. The viral proteins function to facilitate virus replication, virus movement, the assembly of virus-specific nucleoprotein particles, vector transmission and to counteract plant host defence responses. Recent research findings have provided new insights into the structure and function of these proteins and have identified numerous host interacting partners. Most of the viral proteins have been shown to be multifunctional, participating in multiple events during the infection cycle and have, indeed, evolved coordinated interactions with host proteins to ensure a successful infection. Here, an up-to-date review of viral protein structure and function is presented, and some areas requiring further research are identified.

Mapping of functional region conferring nuclear localization and karyopherin α-binding activity of the C2 protein of bhendi yellow vein mosaic virus

Bhendi yellow vein mosaic disease is caused by a complex consisting of a monopartite begomovirus associated with a β-satellite. The C2 protein of bhendi yellow vein mosaic virus (BYVMV) is a suppressor of post-transcriptional gene silencing and also functions as a transcriptional activator. To explore the molecular mechanisms of its nuclear trafficking and self-interaction, fusion proteins of fluorescent proteins with wild-type or mutated constructs of BYVMV C2 were expressed in tobacco protoplasts. Analyses revealed that the BYVMV C2 nuclear localization signal (NLS) was located in the N terminus of the protein, comprising aa 17-31 of C2. NLSs are recognized by a class of soluble transport receptors termed karyopherins α and β. The BYVMV C2 NLS was found to be necessary for this protein's interaction with its nuclear import mediator, karyopherin α, ensuring its nuclear localization. Nevertheless, when deleted, C2 was found in both the cytoplasm and the nucleus, suggesting NLS-independent nuclear import of this protein. Homotypic interaction of BYVMV C2 was also found, which correlates with the nuclear localization needed for efficient activation of transcription.