Aphid transmission of a Potexvirus, Foxtail mosaic virus, in the presence of the Potyvirus helper component proteinase

To establish successful infections, plant viruses compete with the host plants for limited resources and thus alter the physiological state of the plants. After successful infection, insect vectors are required for the transmission of some plant viruses to the next host plant. One of the largest groups of plant viruses, the potyvirus, can be transmitted by aphids. During transmission, the potyvirus protein helper component proteinase (HC-Pro) binds to the yet-to-be-defined aphid receptor on the stylet, as well as to the virus particles through the Asp-Ala-Gly (DAG) motif of the viral coat protein. Previously it was determined that a naturally occurring DAG motif in the non-aphid transmissible potexvirus, Potato aucuba mosaic potexvirus (PAMV), is functional when the HC-Pro is provided through co-infection with a potyvirus. Further, the DAG motif of PAMV can be successfully transferred to another non-aphid transmissible potexvirus, Potato virus X (PVX), to convey aphid transmission capabilities. We expand on this previous work by demonstrating, the DAG motif from two different potyviruses, Sugarcane mosaic virus and Turnip mosaic virus, as well as the DAG motif from the previous potexvirus PAMV, can be added to another non-aphid transmissible potexvirus, Foxtail mosaic virus (FoMV), to make it aphid transmissible. Transmission efficiency varied from less than 10% to over 80% depending on the DAG motif and host plant used in transmission, suggesting not all DAG motifs are equal for engineering aphid transmission. The underlying mechanisms mediating this variation still need to be explored.

The P1 Protein of Watermelon mosaic virus Compromises the Activity as RNA Silencing Suppressor of the P25 Protein of Cucurbit yellow stunting disorder virus

Mixed viral infections in plants involving a potyvirus and other unrelated virus often result in synergistic effects, with significant increases in accumulation of the non-potyvirus partner, as in the case of melon plants infected by the potyvirus Watermelon mosaic virus (WMV) and the crinivirus Cucurbit yellow stunting disorder virus (CYSDV). To further explore the synergistic interaction between these two viruses, the activity of RNA silencing suppressors (RSSs) was addressed in transiently co-expressed combinations of heterologous viral products in Nicotiana benthamiana leaves. While the strong RSS activity of WMV Helper Component Proteinase (HCPro) was unaltered, including no evident additive effects observed when co-expressed with the weaker CYSDV P25, an unexpected negative effect of WMV P1 was found on the RSS activity of P25. Analysis of protein expression during the assays showed that the amount of P25 was not reduced when co-expressed with P1. The detrimental action of P1 on the activity of P25 was dose-dependent, and the subcellular localization of fluorescently labeled variants of P1 and P25 when transiently co-expressed showed coincidences both in nucleus and cytoplasm. Also, immunoprecipitation experiments showed interaction of tagged versions of the two proteins. This novel interaction, not previously described in other combinations of potyviruses and criniviruses, might play a role in modulating the complexities of the response to multiple viral infections in susceptible plants.

Genetic variability and evolutionary dynamics of atypical Papaya ringspot virus infecting Papaya.

The mutation prone RNA genome of Papaya ringspot virus could be a driving force behind its geographical spread and dissemination. Here we present the molecular investigations on atypical PRSV–P strain identified from Pakistan with genome sequence phylogenetic and recombination analysis. The PRSV-P, Pakistan outbreak strain showing a geographic spread across India and Bangladesh in phylogenetic lineage. In major recombination events, it has acquired genome variation in amino terminal of PRSV coat protein gene, whereas the gene for helper component-proteinase (HC–Pro), a nonstructural coding region of multi-domain provenance, also evolves at nucleotide and amino acid levels. The phylogenetic analysis of another highly variable P1 region showed evolutionary dynamics with respect to other geographical strains, particularly the Indian isolate from North East region (Meghalaya). PRSV–PK holds high levels of genetic divergence in comparison to American, Australian and Asian isolates. The genetic and phylogeographic analyses indicate that a spatial recombination has occurred from first PRSV, however temporal evolution is within the region of occurrence. The genetic variations and evolutionary dynamics of this virus may challenge the resistance developed in papaya against PRSV and give rise to virus lineage because of its atypical emergence where geographic spread is already occurring.

Chilli veinal mottle virus HCPro interacts with catalase to facilitate virus infection in Nicotiana tabacum

Plant symptoms are derived from specific interactions between virus and host components. However, little is known about viral or host factors that participate in the establishment of systemic necrosis. Here, we showed that helper component proteinase (HCPro), encoded by Chilli veinal mottle virus (ChiVMV), could directly interact with catalase 1 (CAT1) and catalase 3 (CAT3) in the cytoplasm of tobacco (Nicotiana tabacum) plants to facilitate viral infection. In vitro, the activities of CAT1 and CAT3 were inhibited by the interaction between HCPro and CATs. The C-terminus of HCPro was essential for their interaction and was also required for the decrease of enzyme activities. Interestingly, the mRNA and protein level of CATs were up-regulated in tobacco plants in response to ChiVMV infection. Nicotiana tabacum plants with HCPro overexpression or CAT1 knockout were more susceptible to ChiVMV infection, which was similar to the case of H2O2-pre-treated plants, and the overexpression of CAT1 inhibited ChiVMV accumulation. Also, neither CAT1 nor CAT3 could affect the RNA silencing suppression (RSS) activity of HCPro. Our results showed that the interaction between HCPro and CATs promoted the development of plant systemic necrosis, revealing a novel role for HCPro in virus infection and pathogenicity.

Precise Exchange of the Helper-Component Proteinase Cistron Between Soybean mosaic virus and Clover yellow vein virus: Impact on Virus Viability and Host Range Specificity

Soybean mosaic virus and Clover yellow vein virus are two definite species of the genus Potyvirus within the family Potyviridae. Soybean mosaic virus-N (SMV-N) is well adapted to cultivated soybean (Glycine max) genotypes and wild soybean (G. soja), whereas it remains undetectable in inoculated broad bean (Vicia faba). In contrast, clover yellow vein virus No. 30 (ClYVV-No. 30) is capable of systemic infection in broad bean and wild soybean; however, it infects cultivated soybean genotypes only locally. In this study, SMV-N was shown to also infect broad bean locally; hence, broad bean is a host for SMV-N. Based on these observations, it was hypothesized that lack of systemic infection by SMV-N in broad bean and by ClYVV-No. 30 in cultivated soybean is attributable to the incompatibility of multifunctional helper-component proteinase (HC-Pro) in these hosts. The logic of selecting the HC-Pro cistron as a target is based on its established function in systemic movement and being a relevant factor in host range specificity of potyviruses. To test this hypothesis, chimeras were constructed with precise exchanges of HC-Pro cistrons between SMV-N and ClYVV-No. 30. Upon inoculation, both chimeras were viable in infection, but host range specificity of the recombinant viruses did not differ from those of the parental viruses. These observations suggest that (i) HC-Pro cistrons from SMV-N and ClYVV-No. 30 are functionally compatible in infection despite 55.6 and 48.9% nucleotide and amino acid sequence identity, respectively, and (ii) HC-Pro cistrons from SMV-N and ClYVV-No. 30 are not the determinants of host specificity on cultivated soybean or broad beans, respectively.

Genetic Analyses of the FRNK Motif Function of Turnip mosaic virus Uncover Multiple and Potentially Interactive Pathways of Cross-Protection

Cross-protection triggered by a mild strain of virus acts as a prophylaxis to prevent subsequent infections by related viruses in plants; however, the underling mechanisms are not fully understood. Through mutagenesis, we isolated a mutant strain of Turnip mosaic virus (TuMV), named Tu-GK, that contains an Arg182Lys substitution in helper component-proteinase (HC-ProK) that confers complete cross-protection against infection by a severe strain of TuMV in Nicotiana benthamiana, Arabidopsis thaliana Col-0, and the Arabidopsis dcl2-4/dcl4-1 double mutant defective in DICER-like ribonuclease (DCL)2/DCL4-mediated silencing. Our analyses showed that HC-ProK loses the ability to interfere with microRNA pathways, although it retains a partial capability for RNA silencing suppression triggered by DCL. We further showed that Tu-GK infection triggers strong salicylic acid (SA)-dependent and SA-independent innate immunity responses. Our data suggest that DCL2/4-dependent and –independent RNA silencing pathways are involved, and may crosstalk with basal innate immunity pathways, in host defense and in cross-protection.