In Situ Localization of Barley Yellow Dwarf Virus-PAV 17-kDa Protein and Nucleic Acids in Oats

Appropriate reduction of importin-α gene expression enhances yellow dwarf disease resistance in common wheat

Barley yellow dwarf viruses (BYDVs) cause widespread damage to global cereal crops. Here we report a novel strategy for elevating resistance to BYDV infection. The 17K protein, a potent virulence factor conserved in BYDVs, interacted with barley IMP-α1 and -α2 proteins that are nuclear transport receptors. Consistently, a nuclear localization signal was predicted in 17K, which was found essential for 17K to be transported into the nucleus and to interact with IMP-α1 and -α2. Reducing HvIMP-α1 and -α2 expression by gene silencing attenuated BYDV-elicited dwarfism, accompanied by a lowered nuclear accumulation of 17K. Among the eight common wheat CRISPR mutants with two to four TaIMP-α1 and -α2 genes mutated, the triple mutant α1aaBBDD /α2AAbbdd and the tetra-mutant α1aabbdd /α2AAbbDD displayed strong BYDV resistance without negative effects on plant growth under field conditions. The BYDV resistance exhibited by α1aaBBDD /α2AAbbdd and α1aabbdd /α2AAbbDD was correlated with decreased nuclear accumulation of 17K and lowered viral proliferation in infected plants. Our work uncovers the function of host IMP-α proteins in BYDV pathogenesis and generates the germplasm valuable for breeding BYDV-resistant wheat. Appropriate reduction of IMP-α gene expression may be broadly useful for enhancing antiviral resistance in agricultural crops and other economically important organisms.

Yellow Dwarf Viruses of Cereals: Taxonomy and Molecular Mechanisms

Yellow dwarf viruses are the most economically important and widespread viruses of cereal crops. Although they share common biological properties such as phloem limitation and obligate aphid transmission, the replication machinery and associated cis-acting signals of these viruses fall into two unrelated taxa represented by Barley yellow dwarf virus and Cereal yellow dwarf virus. Here, we explain the reclassification of these viruses based on their very different genomes. We also provide an overview of viral protein functions and their interactions with the host and vector, replication mechanisms of viral and satellite RNAs, and the complex gene expression strategies. Throughout, we point out key unanswered questions in virus evolution, structural biology, and genome function and replication that, when answered, may ultimately provide new tools for virus management.

Barley GRIK1-SnRK1 kinases subvert a viral virulence protein to upregulate antiviral RNAi and inhibit infection

Viruses often usurp host machineries for their amplification, but it remains unclear if hosts may subvert virus proteins to regulate viral proliferation. Here, we show that the 17K protein, an important virulence factor conserved in barley yellow dwarf viruses (BYDVs) and related poleroviruses, is phosphorylated by host GRIK1‐SnRK1 kinases, with the phosphorylated 17K (P17K) capable of enhancing the abundance of virus‐derived small interfering RNAs (vsiRNAs) and thus antiviral RNAi. Furthermore, P17K interacts with barley small RNA‐degrading nuclease 1 (HvSDN1) and impedes HvSDN1‐catalyzed vsiRNA degradation. Additionally, P17K weakens the HvSDN1‐HvAGO1 interaction, thus hindering HvSDN1 from accessing and degrading HvAGO1‐carried vsiRNAs. Importantly, transgenic expression of 17K phosphomimetics (17K^5D), or genome editing of SDN1, generates stable resistance to BYDV through elevating vsiRNA abundance. These data validate a novel mechanism that enhances antiviral RNAi through host subversion of a viral virulence protein to inhibit SDN1‐catalyzed vsiRNA degradation and suggest new ways for engineering BYDV‐resistant crops.

A distinct class of plant and animal viral proteins that disrupt mitosis by directly interrupting the mitotic entry switch Wee1-Cdc25-Cdk1

Many animal viral proteins, e.g., Vpr of HIV-1, disrupt host mitosis by directly interrupting the mitotic entry switch Wee1-Cdc25-Cdk1. However, it is unknown whether plant viruses may use this mechanism in their pathogenesis. Here, we report that the 17K protein, encoded by barley yellow dwarf viruses and related poleroviruses, delays G2/M transition and disrupts mitosis in both host (barley) and nonhost (fission yeast, Arabidopsis thaliana, and tobacco) cells through interrupting the function of Wee1-Cdc25-CDKA/Cdc2 via direct protein-protein interactions and alteration of CDKA/Cdc2 phosphorylation. When ectopically expressed, 17K disrupts the mitosis of cultured human cells, and HIV-1 Vpr inhibits plant cell growth. Furthermore, 17K and Vpr share similar secondary structural feature and common amino acid residues required for interacting with plant CDKA. Thus, our work reveals a distinct class of mitosis regulators that are conserved between plant and animal viruses and play active roles in viral pathogenesis.

Disruption of Chloroplast Function Through Downregulation of Phytoene Desaturase Enhances the Systemic Accumulation of an Aphid-Borne, Phloem-Restricted Virus

Chloroplasts play a central role in pathogen defense in plants. However, most studies explaining the relationship between pathogens and chloroplasts have focused on pathogens that infect mesophyll cells. In contrast, the family Luteoviridae includes RNA viruses that replicate and traffic exclusively in the phloem. Recently, our lab has shown that Potato leafroll virus (PLRV), the type species in the genus Polerovirus, forms an extensive interaction network with chloroplast-localized proteins that is partially dependent on the PLRV capsid readthrough domain (RTD). In this study, we used virus-induced gene silencing to disrupt chloroplast function and assess the effects on PLRV accumulation in two host species. Silencing of phytoene desaturase (PDS), a key enzyme in carotenoid, chlorophyll, and gibberellic acid (GA) biosynthesis, resulted in a substantial increase in the systemic accumulation of PLRV. This increased accumulation was attenuated when plants were infected with a viral mutant that does not express the RTD. Application of GA partially suppressed the increase in virus accumulation in PDS-silenced plants, suggesting that GA signaling also plays a role in limiting PLRV infection. In addition, the fecundity of the aphid vector of PLRV was increased when fed on PDS-silenced plants relative to PLRV-infected plants.

Barley yellow dwarf virus Infection Leads to Higher Chemical Defense Signals and Lower Electrophysiological Reactions in Susceptible Compared to Tolerant Barley Genotypes

Barley yellow dwarf virus (BYDV) is a phloem limited virus that is persistently transmitted by aphids. Due to huge yield losses in agriculture, the virus is of high economic relevance. Since the control of the virus itself is not possible, tolerant barley genotypes are considered as the most effective approach to avoid yield losses. Although several genes and quantitative trait loci are known and used in barley breeding for virus tolerance, little is known about molecular and physiological backgrounds of this trait. Therefore, we compared the anatomy and early defense responses of a virus susceptible to those of a virus-tolerant cultivar. One of the very early defense responses is the transmission of electrophysiological reactions. Electrophysiological reactions to BYDV infection might differ between susceptible and tolerant cultivars, since BYDV causes disintegration of sieve elements in susceptible cultivars. The structure of vascular bundles, xylem vessels and sieve elements was examined using microscopy. All three were significantly decreased in size in infected susceptible plants where the virus causes disintegration of sieve elements. This could be associated with an uncontrolled ion exchange between the sieve-element lumen and apoplast. Further, a reduced electrophysiological isolation would negatively affect the propagation of electrophysiological reactions. To test the influence of BYDV infection on electrophysiological reactions, electropotential waves (EPWs) induced by leaf-tip burning were recorded using aphids as bioelectrodes. EPWs in infected susceptible plants disappeared already after 10 cm in contrast to those in healthy susceptible or infected tolerant or healthy tolerant plants. Another early plant defense reaction is an increase in reactive oxygen species (ROS). Using a fluorescent dye, we found a significant increase in ROS content in infected susceptible plants but not in infected tolerant plants. Similar results were found for the phytohormones abscisic acid and three jasmonates. Salicylic acid levels were generally higher after BYDV infection compared to uninfected plants. Heat stimulation caused an increase in jasmonates. By shedding light on the plant defense mechanisms against BYDV, this study, provides further knowledge for breeding virus tolerant plants.

Localization of Barley yellow dwarf virus Movement Protein Modulating Programmed Cell Death in Nicotiana benthamiana

Barley yellow dwarf virus (BYDV) belongs to Luteovirus and is limited only at phloem related tissues. An open reading frame (ORF) 4 of BYDV codes for the movement protein (MP) of BYDV gating plasmodesmata (PD) to facilitate virus movement. Like other Luteoviruses, ORF 4 of BYDV is embedded in the ORF3 but expressed from the different reading frame in leaky scanning manner. Although MP is a very important protein for systemic infection of BYDV, there was a little information. In this study, MP was characterized in terms of subcellular localization and programmed cell death (PCD). Gene of MP or its mutant (ΔMP) was expressed by Agroinfiltration method. MP was clearly localized at the nucleus and the PD, but ΔMP which was deleted distal N-terminus of MP showed no localization to PD exhibited the different target with original MP. In addition to PD localization, MP appeared associated with small granules in cytoplasm whereas ΔMP did not. MP associated with PD and small granules induced PCD, but ΔMP showed no association with PD and small granules did not exhibit PCD. Based on this study, the distal N-terminal region within MP is seemingly responsible for the localization of PD and the induction small granules and PCD induction. These results suggest that subcellular localization of BYDV MP may modulate the PCD in Nicotiana benthamiana.

Viral pathogen production in a wild grass host driven by host growth and soil nitrogen

Nutrient limitation is a basic ecological constraint that has received little attention in studies on virus production and disease dynamics. Nutrient availability could directly limit the production of viral nucleic acids and proteins, or alternatively limit host growth and thus indirectly limit metabolic pathways necessary for viral replication. In order to compare direct and indirect effects of nutrient limitation on virus production within hosts, we manipulated soil nitrogen (N) and phosphorus (P) availability in a glasshouse for the wild grass host Bromus hordeaceus and the viral pathogen Barley yellow dwarf virus-PAV. We found that soil N additions increased viral concentrations within host tissues, and the effect was mediated by host growth. Specifically, in statistical models evaluating the roles of host biomass production, leaf N and leaf P, viral production depended most strongly on host biomass, rather than the concentration of either nutrient. Furthermore, at low soil N, larger plants supported greater viral concentrations than smaller ones, whereas at high N, smaller plants supported greater viral concentrations. Our results suggest that enhanced viral productivity under N enrichment is an indirect consequence of nutrient stimulation to host growth rate. Heightened pathogen production in plants has important implications for a world facing increasing rates of nutrient deposition.

Fluorescently Tagged Potato virus Y: A Versatile Tool for Functional Analysis of Plant-Virus Interactions

Potato virus Y (PVY) is an economically important plant virus that infects Solanaceous crops such as tobacco and potato. To date, studies into the localization and movement of PVY in plants have been limited to detection of viral RNA or proteins ex vivo. Here, a PVY N605 isolate was tagged with green fluorescent protein (GFP), characterized and used for in vivo tracking. In Nicotiana tabacum cv. Xanthi, PVY N605-GFP was biologically comparable to nontagged PVY N605, stable through three plant-to-plant passages and persisted for four months in infected plants. GFP was detected before symptoms and fluorescence intensity correlated with PVY RNA concentrations. PVY N605-GFP provided in vivo tracking of long-distance movement, allowing estimation of the cell-to-cell movement rate of PVY in N. tabacum cv. Xanthi (7.1 ± 1.5 cells per hour). PVY N605-GFP was adequately stable in Solanum tuberosum cvs. Désirée and NahG-Désirée and able to infect S. tuberosum cvs. Bintje and Bea, Nicotiana benthamiana, and wild potato relatives. PVY N605-GFP is therefore a powerful tool for future studies of PVY-host interactions, such as functional analysis of viral and plant genes involved in viral movement.

Distribution of Potato virus Y in potato plant organs, tissues, and cells

The distribution of Potato virus Y (PVY) in the systemically infected potato (Solanum tuberosum) plants of the highly susceptible cultivar Igor was investigated. Virus presence and accumulation was analyzed in different plant organs and tissues using real-time polymerase chain reaction and transmission electron microscopy (TEM) negative staining methods. To get a complete insight into the location of viral RNA within the tissue, in situ hybridization was developed and optimized for the detection of PVY RNA at the cellular level. PVY was shown to accumulate in all studied leaf and stem tissues, in shoot tips, roots, and tubers; however, the level of virus accumulation was specific for each organ or tissue. The highest amounts of viral RNA and viral particles were found in symptomatic leaves and stem. By observing cell ultrastructure with TEM, viral cytoplasmic inclusion bodies were localized in close vicinity to the epidermis and in trichomes. Our results show that viral RNA, viral particles, and cytoplasmic inclusion bodies colocalize within the same type of cells or in close vicinity.

A potential nuclear envelope-targeting domain and an arginine-rich RNA binding element identified in the putative movement protein of the GAV strain of Barley yellow dwarf virus

In this study, the structural elements in the putative movement protein (MP) of the GAV strain of Barley yellow dwarf virus (BYDV-GAV) were investigated. The GFP fusion protein of BYDV-GAV MP was found to be associated with the nuclear envelope (NE) in transgenic Arabidopsis thaliana (L.) cells. Serial deletion mapping demonstrated that the predicted α-helical domain located at the N-terminus of BYDV-GAV MP was required and sufficient for NE targeting in onion epidermal cells. This α-helical domain does not contain any sequence elements similar to known nuclear localisation signals or bear any significant resemblance to previously characterised NE-targeting structure, indicating that it may represent a novel NE-targeting domain in plant cells. Deletion mutagenesis showed that the C-terminal end of BYDV-GAV MP possessed an element required for its RNA binding activity in vitro. Further analysis revealed that the arginine amino acids within the last 11 residues of the C-terminal end were crucial for the binding of BYDV-GAV MP to RNA. This C-terminal element enriched in basic residues was also present in the MPs of other BYDV strains and the polerovirus Potato leaf roll virus (PLRV), suggesting the conservation of a RNA binding element in the MPs from both luteoviruses and poleroviruses. The data in this work present an initial characterisation of a novel plant NE-targeting domain and a RNA binding element on BYDV-GAV MP. Further studies are underway to investigate the function of these elements in the biology of natural BYDV-GAV infection.

Characterization of the N-terminal segment used by the barley yellow dwarf virus movement protein to promote interaction with the nuclear membrane of host plant cells

The barley yellow dwarf virus movement protein (BYDV-MP) requires its N-terminal sequence to promote the transport of viral RNA into the nuclear compartment of host plant cells. Here, graphical analysis predicts that this sequence would form a membrane interactive amphiphilic alpha-helix. Confirming this prediction, NT1, a peptide homologue of the BYDV-MP N-terminal sequence, was found to be alpha-helical (65%) in the presence of vesicles mimics of the nuclear membrane. The peptide increased the fluidity of these nuclear membrane mimics (rise in wavenumber of circa 0.5-1.0 cm(-1)) and induced surface pressure changes of 2 mN m(-1) in lipid monolayers with corresponding compositions. Taken with isotherm analysis these results suggest that BYDV-MP forms an N-terminal amphiphilic alpha-helix, which partitions into the nuclear membrane primarily through thermodynamically stable associations with the membrane lipid headgroup region. We speculate that these associations may play a role in targeting of the nuclear membrane by BYDM-MP.

Nucleolar localization of potato leafroll virus capsid proteins

Potato leafroll virus (PLRV) encodes two capsid proteins, major protein (CP) and minor protein (P5), an extended version of the CP produced by occasional translational 'readthrough' of the CP gene. Immunogold electron microscopy showed that PLRV CP is located in the cytoplasm and also localized in the nucleus, preferentially targeting the nucleolus. The nucleolar localization of PLRV CP was also confirmed when it was expressed as a fusion with green fluorescent protein (GFP) via an Agrobacterium vector. Mutational analysis identified a particular sequence within PLRV CP involved in nucleolar targeting [the nucleolar localization signal (NoLS)]. Minor protein P5 also contains the same NoLS, and was targeted to the nucleolus when it was expressed as a fusion with GFP from Agrobacterium. However, P5-GFP lost its nucleolar localization in the presence of replicating PLRV.

Interaction between the movement protein of barley yellow dwarf virus and the cell nuclear envelope: role of a putative amphiphilic alpha-helix at the N-terminus of the movement protein

The open reading frame 4 (ORF 4) gene product of barley yellow dwarf virus (BYDV) may act as a movement protein (MP) by assisting the transport of viral genomic RNA across the nuclear envelope (NE) of host plant cells. To investigate interactions between BYDV MP and the NE, wild-type and mutant open reading frame (ORF 4)-green fluorescent protein (GFP) fusion cistrons were expressed in insect cells. A fusion protein expressed by the wild-type ORF 4-GFP cistron associated with the NE and caused protrusions from its surface. The fusion protein expressed by the mutant ORF 4-GFP cistron lacked a putative amphiphilic alpha-helix at its N-terminus and although associating with the NE, showed decreased levels of protrusions. A peptide homologue of this putative alpha-helix induced an increase of 7 degrees C in the phase transition temperature of dimyrystoyl phosphatidylserine (DMPS) membranes, accompanied by a decrease in membrane fluidity, but exhibited no significant interaction with either dimyristoyl phosphatidylcholine (DMPC) or dimyristoyl phosphatidylethanolamine (DMPE) membranes. These results strongly support the view that BYDV MP may interact with the NE to help transport viral genomic RNA into the nuclear compartment. This function of BYDV MP appears to involve protrusions on the surface of the NE and may require the presence of an N-terminal amphiphilic alpha-helix, which is speculated to destabilize membranes, thereby assisting the entry of BYDV-GAV into the nuclear compartment.