Isolate fitness and tissue-tropism determine superinfection success

Transcriptome analysis of the synergistic mechanisms between two strains of potato virus Y in Solanum tuberosum L

Many viruses employ a process known as superinfection exclusion (SIE) to block subsequent entry or replication of the same or closely related viruses in the cells they occupy. SIE is also referred to as Cross-protection refers to the situation where a host plant infected by a mild strain of a virus or viroid gains immunity against a more severe strain closely related to the initial infectant. The mechanisms underlying cross-protection are not fully understood. In this study, we performed a comparative transcriptomic analysis of potato (Solanum tuberosum L.) leaves. The strains PVYN-Wi-HLJ-BDH-2 and PVYNTN-NW-INM-W-369-12 are henceforth designated as BDH and 369, respectively. In total, 806 differentially expressed genes (DEGs) were detected between the Control and JZ (preinfected with BDH and challenge with 369) treatment. Gene Ontology (GO) analysis showed that the response to external biological stimulation, signal transduction, kinase, immunity, redox pathways were significantly enriched. Among these pathways, we identified numerous differentially expressed metabolites related to virus infection. Moreover, our data also identified a small set of genes that likely play important roles in the establishment of cross-protection. Specifically, we observed significant differential expression of the A1-II gamma-like gene, elongation factor 1-alpha-like gene, and subtilisin-like protease StSBT1.7 gene, with StSBT1.7 being the most significant in our transcriptome data. These genes can stimulate the expression of defense plant genes, induce plant chemical defense, and participate in the induction of trauma and pathogenic bacteria. Our findings provided insights into the mechanisms underlying the ability of mild viruses to protect host plants against subsequent closely related virus infection in Solanum tuberosum L.

Minor Variants of Orf1a, p33, and p23 Genes of VT Strain Citrus Tristeza Virus Isolates Show Symptomless Reactions on Sour Orange and Prevent Superinfection of Severe VT Isolates

The control of tristeza quick decline (QD) of citrus is based on the use of rootstocks that are tolerant or resistant to the Citrus tristeza virus (CTV), but some of them show bio-agronomic limits. The application of cross-protection (CP) has been insufficiently explored. The present study examined the possibility of QD control by cross-protection (CP) following reports showing the dependence of the CP strategy on the close genetic relationships between the protective and challenging CTV isolates. Taking advantage of deep sequencing technologies, we located six naturally infected trees harboring no-seedling yellow (no-SY) and no QD decline (mild) VT isolates and used these for challenge inoculation with three QD VT isolates. Symptom monitoring showed that all six Sicilian mild no-SY isolates, based on their genomic relatedness and mild symptoms reactions, provide effective protection against the three severe local VT isolates. The differences between the six mild and three severe isolates were confined to just a few nucleotide variations conserved in eight positions of three CTV genes (p23, p33, and Orf1a). These results confirm that the superinfection exclusion (SIE mechanism) depends on close genetic relatedness between the protective and challenging severe VT strain isolates. Ten years of investigation suggest that CP could turn into an efficient strategy to contain CTV QD infections of sweet orange trees on SO rootstock.

Transmission of Diverse Variants of Strawberry Viruses Is Governed by a Vector Species

Advances in high-throughput sequencing methods have boosted the discovery of multistrain viral infections in diverse plant systems. This phenomenon appears to be pervasive for certain viral species. However, our knowledge of the transmission aspects leading to the establishment of such mixed infections is limited. Recently, we reported a mixed infection of a single strawberry plant with strawberry mottle virus (SMoV), strawberry crinkle virus (SCV) and strawberry virus 1 (StrV-1). While SCV and StrV-1 are represented by two and three molecular variants, respectively, SmoV has three different RNA1 and RNA2 segments. In this study, we focus on virus acquisition by individual adult aphids of the Aphis gossypii, Aphis forbesi and Chaetosiphon fragaefolii species. Single-aphid transmission trials are performed under experimental conditions. Both different viruses and individual virus strains show varying performances in single aphid acquisition. The obtained data suggests that numerous individual transmission events lead to the establishment of multistrain infections. These data will be important for the development of epidemiological models in plant virology.

Potato Virus Y Strain N-Wi Offers Cross-Protection in Potato Against Strain NTN-NW by Superior Competition

Potato virus Y (PVY) is one of the most economically important pathogens of potato. PVY exhibits different phenotypes in dissimilar potato cultivars. Previously, we observed that two recombinant isolates, PVY^N-Wi-HLJ-BDH-2 (BDH) and PVY^NTN-NW(SYR-II)-INM-W-369-12 (369), exhibited different virulence levels in potato cultivar Kexin 13 despite high genome sequence identity. Indeed, 369 induced severe necrosis and plant death in severe cases in Kexin 13 and severe mosaic in cultivar Yanshu 8, whereas BDH caused mainly mosaic symptoms on the plants of both cultivars. We hypothesized that preinfection of plants with BDH could cross-protect them from 369 infection, and not vice versa. Challenge inoculation, either by mechanical wounding or through grafting, with 369 on plants that were preinfected with BDH did not augment the symptom expression in both cultivars. Reverse transcription quantitative PCR analysis showed that, after challenge inoculation with 369, the titer of the isolate on BDH-preinfected plants remained at a low level (about 3 × 10⁴ copy/µl) during the tested time course (0 h to 30 days). In contrast, in plants that were preinoculated with buffer (mock) and challenge inoculated with 369, the titer of 369 increased continuously until reaching its highest level of about 2 × 10⁷ (Yanshu 8) and about 4 × 10⁸ (Kexin 13) during the time course. Surprisingly, in plants that were preinfected with 369 and challenge inoculated with BDH, the accumulation of BDH reached nearly the same level as that in plants that were preinoculated with buffer and challenge inoculated with BDH. Taken together, these results suggest that PVY^N-Wi mediated cross-protection against PVY^NTN-NW(SYR-II) by superior competition and better fitness.

Biological and Molecular Characterization of Two Closely Related Arepaviruses and Their Antagonistic Interaction in Nicotiana benthamiana

Previously, our group characterized two closely related viruses from Areca catechu, areca palm necrotic ringspot virus (ANRSV) and areca palm necrotic spindle-spot virus (ANSSV). These two viruses share a distinct genomic organization of leader proteases and represent the only two species of the newly established genus Arepavirus of the family Potyviridae. The biological features of the two viruses are largely unknown. In this study, we investigated the pathological properties, functional compatibility of viral elements, and interspecies interactions in the model plant, Nicotiana benthamiana. Using a newly obtained infectious clone of ANRSV, we showed that this virus induces more severe symptoms compared with ANSSV and that this is related to a rapid virus multiplication in planta. A series of hybrid viruses were constructed via the substitution of multiple elements in the ANRSV infectious clone with the counterparts of ANSSV. The replacement of either 5′-UTR-HCPro1–HCPro2 or CI effectively supported replication and systemic infection of ANRSV, whereas individual substitution of P3-7K, 9K-NIa, and NIb-CP-3′-UTR abolished viral infectivity. Finally, we demonstrated that ANRSV confers effective exclusion of ANSSV both in coinfection and super-infection assays. These results advance our understanding of fundamental aspects of these two distinct but closely related arepaviruses.

A Double Mutation in the Conserved Motifs of the Helper Component Protease of Papaya Leaf Distortion Mosaic Virus for the Generation of a Cross-Protective Attenuated Strain

Potyviral helper component protease (HC-Pro), as a major determinant of symptom expression in susceptible plants, is a likely target candidate in the production of attenuated strains for cross-protection. In this study, single or double mutations of Lys (K) to Glu (E) in the Lys-Ile-Thr-Cys motif and Arg (R) to Ile (I) in the Phe-Arg-Asn-Lys motif of the HC-Pro from the severe papaya leaf distortion mosaic virus strain DF (PLDMV-DF) reduced symptom expression and virus accumulation in infected papaya (Carica papaya) plants. The papaya plants infected with the attenuated double mutant of PLDMV-EI presented as symptomless. PLDMV-EI provided effective protection against PLDMV-DF infection in three papaya cultivars and had no effect on plant growth and development. Our result showed that PLDMV-EI is a promising mild strain for the practical use of cross-protection in the field.

A New Era for Mild Strain Cross-Protection

Societal and environmental pressures demand high-quality and resilient cropping plants and plant-based foods grown with the use of low or no synthetic chemical inputs. Mild strain cross-protection (MSCP), the pre-immunization of a plant using a mild strain of a virus to protect against subsequent infection by a severe strain of the virus, fits with future-proofing of production systems. New examples of MSCP use have occurred recently. New technologies are converging to support the discovery and mechanism(s) of action of MSCP strains thereby accelerating the popularity of their use.

Bottlenecks and complementation in the aphid transmission of citrus tristeza virus populations

Aphid transmission is a major factor in the formation of citrus tristeza virus (CTV) populations. Here, we examined the effect of population interaction on aphid transmissibility of different CTV genotypes. We found that there was no correlation between the proportion of viral genotypes in the source population and what was transmitted. We next examined the transmission of a poorly transmitted infectious cDNA clone (T36) in mixture with other CTV genotypes. T36 transmission increased from 0.5% alone, to up to 35.7%, depending on the coinfecting genotype. These results suggest that interaction between CTV genotypes affects the transmission of this virus.