The triple gene block movement proteins of a grape virus in the genus Foveavirus confer limited cell-to-cell spread of a mutant Potato virus X

The functions of triple gene block proteins and coat protein of apple stem pitting virus in viral cell-to-cell movement

Apple stem pitting virus is a species in the genus Foveavirus in the family Betaflexiviridae. Apple stem pitting virus (ASPV) commonly infects apple and pear plants grown worldwide. In this study, by integrating bimolecular fluorescence complementation, split-ubiquitin-based membrane yeast two-hybrid, and Agrobacterium-mediated expression assays, the interaction relationships and the subcellular locations of ASPV proteins TGBp1-3 and CP in Nicotiana benthamiana leaf cells were determined. Proteins CP, TGBp1, TGBp2, and TGBp3 were self-interactable, and TGBp2 played a role in the formation of perinuclear viroplasm and enhanced the colocalization of TGBp3 with CP and TGBp1. We found that the plant microfilament and endoplasmic reticulum structures were involved in the production of TGBp3 and TGBp2 vesicles, and their disruption decreased the virus accumulation level in the systemic leaves. The TGBp3 motile vesicles functioned in delivering the viral ribonucleoprotein complexes to the plasma membrane. Two cysteine residues at sites 35 and 49 of the TGBp3 sorting signal were necessary for the diffusion of TGBp3-marked vesicles. Furthermore, our results revealed that TGBp1, TGBp2, and CP could increase plasmodesmal permeability and move to the adjacent cells. This study demonstrates an interaction network and a subcellular location map of four ASPV proteins and for the first time provides insight into the functions of these proteins in the movement of a foveavirus.

Subcellular localization and interactions among TGB proteins of cowpea mild mottle virus

Cowpea mild mottle virus (CPMMV) is a flexuous filamentous virus that belongs to the genus Carlavirus (family Betaflexiviridae). The CPMMV genome contains six open reading frames (ORFs), among which the triple gene block (TGB), encoded by ORFs 2 to 4, has been reported to encode movement proteins for different viruses. The subcellular localization of the TGB proteins of CPMMV isolate CPMMV:BR:MG:09:2 was analysed by transient expression of each protein fused to a fluorophore. Overall, the accumulation pattern and interactions among CPMMV TGB proteins (TGBp) were similar to those of their counterparts from the potex-like group. Considering these similarities, we evaluated the potential interactions between the TGB proteins of CPMMV and of potato virus X, which could complement cell-to-cell movement. The TGBp2 and TGBp3 of PVX had an effect on CPMMV TGBp1, directing it to the plasmodesmata, but the reverse was not true.

Variability, Functions and Interactions of Plant Virus Movement Proteins: What Do We Know So Far?

Of the various proteins encoded by plant viruses, one of the most interesting is the movement protein (MP). MPs are unique to plant viruses and show surprising structural and functional variability while maintaining their core function, which is to facilitate the intercellular transport of viruses or viral nucleoprotein complexes. MPs interact with components of the intercellular channels, the plasmodesmata (PD), modifying their size exclusion limits and thus allowing larger particles, including virions, to pass through. The interaction of MPs with the components of PD, the formation of transport complexes and the recruitment of host cellular components have all revealed different facets of their functions. Multitasking is an inherent property of most viral proteins, and MPs are no exception. Some MPs carry out multitasking, which includes gene silencing suppression, viral replication and modulation of host protein turnover machinery. This review brings together the current knowledge on MPs, focusing on their structural variability, various functions and interactions with host proteins.

Complete genome sequence of a novel foveavirus isolated from Allium sativum L. in China

The complete genome sequence of a novel foveavirus identified in garlic (Allium sativum L.) in China was determined using RNA-seq, reverse transcription polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends (RACE) PCR. The entire genomic RNA (GenBank accession MT981417) is 8748 nucleotides long excluding the 3'-terminal poly(A) tail and contains five open reading frames (ORFs). These ORFs encode the viral replicase, a triple gene block, and a coat protein. The virus was tentatively named "garlic yellow stripe associated virus" (GarYSaV). Pairwise comparisons of protein sequences show that GarYSaV encodes proteins that share less than 47% identity with those of other foveaviruses, suggesting that it represents a new species in the genus. Phylogenetic analysis of amino acid sequences of the replicase and CP confirm that GarYSaV is a member of the genus Foveavirus. To our knowledge, this is the first report of a foveavirus in a monocot plant.

Camellia ringspot-associated virus 4, a proposed new foveavirus from Camellia japonica

One large contig with high sequence similarity to Asian prunus virus 2 was identified by high-throughput sequencing from a camellia (Camellia japonica) tree with ringspot symptoms. The complete genome of this new virus was determined to be 8829 nucleotides long, excluding the 3' poly(A) tail. Its genome organization resembles that of known foveaviruses but contains an additional open reading frame in the 3'-terminal region. Phylogenetic analysis also places this virus with members of the genus Foveavirus in the family Betaflexiviridae in the same subgroup. The virus, which is provisionally named "camellia ringspot-associated virus 4″, shares 50-56% nucleotide sequence identity with other foveaviruses and should represent a new species in the genus.

Grapevine virus T diversity as revealed by full-length genome sequences assembled from high-throughput sequence data

RNASeq or double-stranded RNA based approaches allowed the reconstruction of a total of 9 full-length or near full-length genomes of the recently discovered grapevine virus T (GVT). In addition, datamining of publicly available grapevine RNASeq transcriptome data allowed the reconstruction of a further 14 GVT genomes from five grapevine sources. Together with four GVT sequences available in Genbank, these novel sequences were used to analyse GVT diversity. GVT shows a very limited amount of indels variation but a high level of nucleotide and aminoacid polymorphism. This level is comparable to that shown in the closely related grapevine rupestris stem pitting-associated virus (GRSPaV). Further analyses showed that GVT mostly evolves under conservative selection pressure and that recombination has contributed to its evolutionary history. Phylogenetic analyses allow to identify at least seven clearly separated groups of GVT isolates. Analysis of the only reported PCR GVT-specific detection primer pair indicates that it is likely to fail to amplify some GVT isolates. Taken together these results point at the distinctiveness of GVT but also at the many points it shares with GRSPaV. They constitute the first pan-genomic analysis of the diversity of this novel virus.

Subcellular localization and membrane association of the replicase protein of grapevine rupestris stem pitting-associated virus, family Betaflexiviridae

As a member of the newly established Betaflexiviridae family, grapevine rupestris stem pitting-associated virus (GRSPaV) has an RNA genome containing five ORFs. ORF1 encodes a putative replicase polyprotein typical of the alphavirus superfamily of positive-strand ssRNA viruses. Several viruses of this superfamily have been demonstrated to replicate in structures designated viral replication complexes associated with intracellular membranes. However, structure and cellular localization of the replicase complex have not been studied for members of Betaflexiviridae, a family of mostly woody plant viruses. As a first step towards the elucidation of the replication complex of GRSPaV, we investigated the subcellular localization of full-length and truncated versions of its replicase polyprotein via fluorescent tagging, followed by fluorescence microscopy. We found that the replicase polyprotein formed distinctive punctate bodies in both Nicotiana benthamiana leaf cells and tobacco protoplasts. We further mapped a region of 76 amino acids in the methyl-transferase domain responsible for the formation of these punctate structures. The punctate structures are distributed in close proximity to the endoplasmic reticulum network. Membrane flotation and biochemical analyses demonstrate that the N-terminal region responsible for punctate structure formation associated with cellular membrane is likely through an amphipathic α helix serving as an in-plane anchor. The identity of this membrane is yet to be determined. This is, to our knowledge, the first report on the localization and membrane association of the replicase proteins of a member of the family Betaflexiviridae.

Recombinant keratinocyte growth factor 1 in tobacco potentially promotes wound healing in diabetic rats

Keratinocyte growth factor 1 (KGF1) is a growth factor that promotes epidermal cell proliferation, migration, differentiation, and wound repair. It is expressed at low levels in a form of inclusion body in E. coli. In order to increase its expression and activity, we produced tobacco plants expressing KGF1 via Agrobacterium-mediated transformation using a potato virus X (PVX)-based vector (pgR107). The vector contained the sequence encoding the KGF1 gene fused with a green florescence protein. The recombinant plasmid was introduced into leaf cells of Nicotiana benthamiana (a wild Australian tobacco) via Agrobacterium-mediated agroinfiltration. As determined by fluorescence and Western blot of leaf extracts, the KGF1 gene was correctly translated into the tobacco plants. The recombinant KGF1 was purified from plant tissues by heparin affinity chromatography, and cell proliferation in NIH/3T3 cells was stimulated by the purified KGF1. The purified KGF1 was also applied to the wounds of type-II diabetic rats. KGF1 had accumulated to levels as high as 530  μ g/g fresh weight in the leaves of agroinfected plants. We show that plant-derived KGF1 can promote the proliferation of NIH/3T3 cells and have significant effects on the type-II diabetic rat. The present findings indicated that KGF1 from tobacco maintains its biological activity, implying prospective industrial production in a plant bioreactor.