Host Factors Involved in the Intracellular Movement of Bamboo mosaic virus

The gibberellic acid derived from the plastidial MEP pathway is involved in the accumulation of Bamboo mosaic virus

A gene upregulated in Nicotiana benthamiana after Bamboo mosaic virus (BaMV) infection was revealed as 1-deoxy-d-xylulose-5-phosphate reductoisomerase (NbDXR). DXR is the key enzyme in the 2-C-methyl-d-erythritol-4-phosphate (MEP) pathway that catalyzes the conversion of 1-deoxy-d-xylulose 5-phosphate to 2-C-methyl-d-erythritol-4-phosphate. Knockdown and overexpression of NbDXR followed by BaMV inoculation revealed that NbDXR is involved in BaMV accumulation. Treating leaves with fosmidomycin, an inhibitor of DXR function, reduced BaMV accumulation. Subcellular localization confirmed that DXR is a chloroplast-localized protein by confocal microscopy. Furthermore, knockdown of 1-hydroxy-2-methyl-2-(E)-butenyl-4-diphosphate reductase, one of the enzymes in the MEP pathway, also reduced BaMV accumulation. The accumulation of BaMV increased significantly in protoplasts treated with isopentenyl pyrophosphate. Thus, the metabolites of the MEP pathway could be involved in BaMV infection. To identify the critical components involved in BaMV accumulation, we knocked down the crucial enzyme of isoprenoid synthesis, NbGGPPS11 or NbGGPPS2. Only NbGGPPS2 was involved in BaMV infection. The geranylgeranyl pyrophosphate (GGPP) synthesized by NbGGPPS2 is known for gibberellin synthesis. We confirmed this result by supplying gibberellic acid exogenously on leaves, which increased BaMV accumulation. The de novo synthesis of gibberellic acid could assist BaMV accumulation.

The function of chloroplast ferredoxin-NADP+ oxidoreductase positively regulates the accumulation of bamboo mosaic virus in Nicotiana benthamiana

A gene down-regulated in Nicotiana benthamiana after bamboo mosaic virus (BaMV) infection had high identity to the nuclear-encoded chloroplast ferredoxin NADP+ oxidoreductase gene (NbFNR). NbFNR is a flavoenzyme involved in the photosynthesis electron transport chain, catalysing the conversion of NADP+ into NADPH. To investigate whether NbFNR is involved in BaMV infection, we used virus-induced gene silencing to reduce the expression of NbFNR in leaves and protoplasts. After BaMV inoculation, the accumulation of BaMV coat protein and RNA was significantly reduced. The transient expression of NbFNR fused with orange fluorescent protein (OFP) localized in the chloroplasts and elevated the level of BaMV coat protein. These results suggest that NbFNR could play a positive role in regulating BaMV accumulation. Expressing a mutant that failed to translocate to the chloroplast did not assist in BaMV accumulation. Another mutant with a catalytic site mutation could support BaMV accumulation to some extent, but accumulation was significantly lower than that of the wild type. In an in vitro replication assay, the replicase complex with FNR inhibitor, heparin, the RdRp activity was reduced. Furthermore, BaMV replicase was revealed to interact with NbFNR in yeast two-hybrid and co-immunoprecipitation experiments. Overall, these results suggest that NbFNR localized in the chloroplast with functional activity could efficiently assist BaMV accumulation.

A Concise Review of Dendrocalamus asper and Related Bamboos: Germplasm Conservation, Propagation and Molecular Biology

Bamboos represent an emerging forest resource of economic significance and provide an avenue for sustainable development of forest resources. The development of the commercial bamboo industry is founded upon efficient molecular and technical approaches for the selection and rapid multiplication of elite germplasm for its subsequent propagation via commercial agro-forestry business enterprises. This review will delve into the micropropagation of Dendrocalamus asper, one of the most widely cultivated commercial varieties of bamboo, and will encompass the selection of germplasm, establishment of explants in vitro and micropropagation techniques. The currently available information pertaining to molecular biology, DNA barcoding and breeding, has been included, and potential areas for future research in the area of genetic engineering and gene regulation have been highlighted. This information will be of relevance to both commercial breeders and molecular biologists who have an interest in establishing bamboo as a crop of the future.

iTRAQ-based protein analysis provides insight into heterologous superinfection exclusion with TMV-43A against CMV in tobacco (Nicotiana benthamiana) plants

Heterologous superinfection exclusion (HSE) is a phenomenon of an initial virus infection which prevents reinfection by a distantly related or unrelated challenger virus strain in the same host. Here, we demonstrate that a mild strain mutant of Tobacco mosaic virus (TMV-43A) can protect Nicotiana benthamiana plants against infection by a challenger Cucumber mosaic virus (CMV)-Fny strain. The isobaric tags for relative and absolute quantification (iTRAQ) technique was used to investigate proteome of N. benthamiana plant during HSE. Our results indicated that in superinfected plants, the PSI and PSII proteins in the photosynthetic pathway increased in abundance, providing sufficient energy to plants for survival. The fatty acid synthesis-related proteins acetyl-CoA carboxylase 1-like and fatty acid synthase were decreased in abundance, affecting the formation of virus replication complex, which in turn reduced CMV replication and lessen hijacking of basic building blocks of RNA transcription and protein synthesis required for normal host functions. This is the first analyses of host proteins that are correlated to HSE between two unrelated plant viruses TMV-43A and CMV in N. benthamiana plants. BIOLOGICAL SIGNIFICANCE: CMV is one of the most studied host-virus interaction models in plants. It infects both monocot and dicot crop plants, causing significant economic losses. Superinfection exclusion (also known as cross protection) is one of the methods to combat virus infection. However, there is lack of proteome information of heterologous superinfection exclusion between two taxonomically unrelated plant viruses (such as between CMV and TMV). An iTRAQ-based quantitative approach was used to study proteomics of superinfection, where TMV-43A acts as a protector of N. benthamiana plants against its challenger CMV. Results showed that TMV-43A protects host plants and prevents plant death from CMV infection. This study provided insights into host responses involving multiple host pathways: photosynthesis, plant defence, carbon metabolism, translation and protein processing, fatty acid metabolism and amino acid biosynthesis. The findings provide a reference database for other viruses and increase our knowledge in host proteins that are correlated to superinfection.

Hijacking of host cellular components as proviral factors by plant-infecting viruses

Plant viruses are important pathogens that cause serious crop losses worldwide. They are obligate intracellular parasites that commandeer a wide array of proteins, as well as metabolic resources, from infected host cells. In the past two decades, our knowledge of plant-virus interactions at the molecular level has exploded, which provides insights into how plant-infecting viruses co-opt host cellular machineries to accomplish their infection. Here, we review recent advances in our understanding of how plant viruses divert cellular components from their original roles to proviral functions. One emerging theme is that plant viruses have versatile strategies that integrate a host factor that is normally engaged in plant defense against invading pathogens into a viral protein complex that facilitates viral infection. We also highlight viral manipulation of cellular key regulatory systems for successful virus infection: posttranslational protein modifications for fine control of viral and cellular protein dynamics; glycolysis and fermentation pathways to usurp host resources, and ion homeostasis to create a cellular environment that is beneficial for viral genome replication. A deeper understanding of viral-infection strategies will pave the way for the development of novel antiviral strategies.

Disrupting the Homeostasis of High Mobility Group Protein Promotes the Systemic Movement of Bamboo mosaic virus

Viruses hijack various organelles and machineries for their replication and movement. Ever more lines of evidence indicate that specific nuclear factors are involved in systemic trafficking of several viruses. However, how such factors regulate viral systemic movement remains unclear. Here, we identify a novel role for Nicotiana benthamiana high mobility group nucleoprotein (NbHMG1/2a) in virus movement. Although infection of N. benthamiana with Bamboo mosaic virus (BaMV) decreased NbHMG1/2a expression levels, nuclear-localized NbHMG1/2a protein was shuttled out of the nucleus into cytoplasm upon BaMV infection. NbHMG1/2a knockdown or even overexpression did not affect BaMV accumulation in inoculated leaves, but it did enhance systemic movement of the virus. Interestingly, the positive regulator Rap-GTPase activation protein 1 was highly upregulated upon infection with BaMV, whereas the negative regulator thioredoxin h protein was greatly reduced, no matter if NbHMG1a/2a was silenced or overexpressed. Our findings indicate that NbHMG1/2a may have a role in plant defense responses. Once its homeostasis is disrupted, expression of relevant host factors may be perturbed that, in turn, facilitates BaMV systemic movement.

Nicotiana benthamiana Argonaute10 plays a pro-viral role in Bamboo mosaic virus infection

RNA silencing is a major defense mechanism against invading viruses in plants. Argonaute proteins (AGOs) are the key players in RNA silencing. The number of AGO family members involved varies depending on the plant species and they play distinct or sometimes redundant roles in antiviral defense. By using a virus-induced gene silencing technique, it was found that Nicotiana benthamiana AGO1 restricted Bamboo mosaic virus (BaMV) accumulation, but NbAGO10, the closest paralog of NbAGO1, positively regulated BaMV accumulation. Immunoprecipitation assay revealed BaMV virus-derived small interfering RNAs (vsiRNAs) in NbAGO10 complexes. Transient overexpression of NbAGO10 increased BaMV RNA accumulation, but with co-expression of NbAGO1, BaMV RNA accumulation was reduced, which suggests that NbAGO10 may have competed with NbAGO1 for sequestering BaMV vsiRNA and prevented the formation of RNA-induced silencing complexes. In addition, overexpression of NbAGO10 decreased BaMV vsiRNA accumulation. A host enzyme, small RNA degrading nuclease 1 (SDN1), also was found to interact with NbAGO10 on in vivo pull-down assay. Silencing of SDN1 elevated BaMV vsiRNA level and decreased BaMV RNA accumulation in N. benthamiana, indicating that NbAGO10 might recruit SDN1 for BaMV vsiRNA degradation. The results herein suggested that NbAGO10 plays a pro-viral role by BaMV vsiRNA sequestration and degradation.

The Coat Protein of Citrus Yellow Vein Clearing Virus Interacts with Viral Movement Proteins and Serves as an RNA Silencing Suppressor

Citrus yellow vein clearing virus is a newly accepted member of the genus Mandarivirus in the family Alphaflexiviridae. The triple gene block proteins (TGBp1, TGBp2 and TGBp3) encoded by plant viruses in this family function on facilitating virus movement. However, the protein function of citrus yellow vein clearing virus (CYVCV) have never been explored. Here, we showed in both yeast two-hybrid (Y2H) and bimolecular fluorescence (BiFC) assays that the coat protein (CP), TGBp1 and TGBp2 of CYVCV are self-interacting. Its CP also interacts with all three TGB proteins, and TGBp1 and TGBp2 interact with each other but not with TGBp3. Furthermore, the viral CP colocalizes with TGBp1 and TGBp3 at the plasmodesmata (PD) of epidermal cells of Nicotiana benthamiana leaves, and TGBp1 can translocate TGBp2 from granular-like structures embedded within ER networks to the PD. The results suggest that these proteins could coexist at the PD of epidermal cells of N. benthamiana. Using Agrobacterium infiltration-mediated RNA silencing assays, we show that CYVCV CP is a strong RNA silencing suppressor (RSS) triggered by positive-sense green fluorescent protein (GFP) RNA. The presented results provide insights for further revealing the mechanism of the viral movement and suppression of RNA silencing.