Rootstock-induced scion resistance against tobacco mosaic virus is associated with the induction of defence-related transcripts and graft-transmissible mRNAs

Grafting is a common horticultural practice used to confer desirable traits between rootstock and scion, including disease resistance. To investigate graft-conferred resistance against viral diseases a novel heterografting system was developed using Nicotiana benthamiana scions grafted onto different tomato rootstocks. N. benthamiana is normally highly susceptible to tobacco mosaic virus (TMV) infection. However, specific tomato rootstock varieties were found to confer a range of resistance levels to N. benthamiana scions inoculated with TMV. Conferred resistance was associated with delays in virus accumulation and the reduction in virus spread. RNA sequencing analysis showed the enrichment of transcripts associated with disease resistance and plant stress in N. benthamiana scions grafted onto resistance-inducing tomato rootstocks. Genome sequencing of resistance- and nonresistance-conferring rootstocks was used to identify mobile tomato transcripts within N. benthamiana scions. Within resistance-induced N. benthamiana scions, enriched mobile tomato transcripts were predominantly associated with defence, stress, and abscisic acid signalling when compared to similar scions grafted onto nonresistance-inducing rootstock. Combining these findings suggests that graft-induced resistance is modulated by rootstock scion transcriptional responses and rootstock-specific mobile transcripts.

Pathogen-triggered metabolic adjustments to potato virus Y infection in potato

Potato (Solanum tuberosum L) is affected by several viral pathogens with the most economically damaging being potato virus Y (PVY). At least nine biologically distinct variants of PVY are known to attack potato, with necrotic types named PVY^NTN and PVY^N-Wi being the most recent additions to the list. So far, the molecular plant-virus interactions underlying this pathogenicity are not fully understood. In this study, gas chromatography coupled with mass spectroscopy (GC-MS) was used for an untargeted investigation of the changes in leaf metabolomes of PVY-resistant cultivar Premier Russet, and a susceptible cultivar, Russet Burbank, following inoculation with three PVY strains, PVY^NTN, PVY^N-Wi, and PVY^O. Analysis of the resulting GC-MS spectra with the online software Metaboanalyst (version 5.0) uncovered several common and strain-specific metabolites that are induced by PVY inoculation. In Premier Russet, the major overlap in differential accumulation was found between PVY^N-Wi and PVY^O. However, the 14 significant pathways occurred solely due to PVY^N-Wi. In contrast, the main overlap in differential metabolite profiles and pathways in Russet Burbank was between PVY^NTN and PVY^O. Overall, limited overlap was observed between PVY^NTN and PVY^N-Wi. As a result, PVY^N-Wi-induced necrosis may be mechanistically distinguishable from that of PVY^NTN. Furthermore, 10 common and seven cultivar-specific metabolites as potential indicators of PVY infection and susceptibility/resistance were identified by using PLS-DA and ANOVA. In Russet Burbank, glucose-6-phosphate and fructose-6-phosphate were particularly affected by strain-time interaction. This highlights the relevance of the regulation of carbohydrate metabolism for defense against PVY. Some strain- and cultivar-dependent metabolite changes were also observed, reflecting the known genetic resistance-susceptibility dichotomy between the two cultivars. Consequently, engineering broad-spectrum resistance may be the most effective breeding strategy for managing these necrotic strains of PVY.

Reprogramming Virus Coat Protein Carboxylate Interactions for the Patterned Assembly of Hierarchical Nanorods

The self-assembly system of the rod-shaped tobacco mosaic virus (TMV) has been studied extensively for nanoscale applications. TMV coat protein assembly is modulated by intersubunit carboxylate groups whose electrostatic repulsion limits the assembly of virus rods without incorporating genomic RNA. To engineer assembly control into this system, we reprogrammed intersubunit carboxylate interactions to produce self-assembling coat proteins in the absence of RNA and in response to unique pH and ionic environmental conditions. Specifically, engineering a charge attraction at the intersubunit E50-D77 carboxylate group through a D77K substitution stabilized the coat proteins assembly into virus-like rods. In contrast, the reciprocal E50K modification alone did not confer virus-like rod assembly. However, a combination of R46G/E50K/E97G substitutions enabled virus-like rod assembly. Interestingly, the D77K substitution displays a unique pH-dependent assembly-disassembly profile, while the R46G/E50K/E97G substitutions confer a novel salt concentration dependency for assembly control. In addition, these unique environmentally controlled coat proteins allow for the directed assembly and disassembly of chimeric virus-like rods both in solution and on substrate-attached seed rods. Combined, these findings provide a controllable means to assemble functionally discrete virus-like rods for use in nanotechnology applications.

Dynamic changes impact the plum pox virus population structure during leaf and bud development

Plum pox virus (PPV) is a worldwide threat to stone fruit production. Its woody perennial hosts provide a dynamic environment for virus evolution over multiple growing seasons. To investigate the impact seasonal host development plays in PPV population structure, next generation sequencing of ribosome associated viral genomes, termed translatome, was used to assess PPV variants derived from phloem or whole leaf tissues over a range of plum leaf and bud developmental stages. Results show that translatome PPV variants occur at proportionately higher levels in bud and newly developing leaf tissues that have low infection levels while more mature tissues with high infection levels display proportionately lower numbers of viral variants. Additional variant analysis identified distinct groups based on population frequency as well as sets of phloem and whole tissue specific variants. Combined, these results indicate PPV population dynamics are impacted by the tissue type and developmental stage of their host.

Viral Hacks of the Plant Vasculature: The Role of Phloem Alterations in Systemic Virus Infection

For plant viruses, the ability to load into the vascular phloem and spread systemically within a host is an essential step in establishing a successful infection. However, access to the vascular phloem is highly regulated, representing a significant obstacle to virus loading, movement, and subsequent unloading into distal uninfected tissues. Recent studies indicate that during virus infection, phloem tissues are a source of significant transcriptional and translational alterations, with the number of virus-induced differentially expressed genes being four- to sixfold greater in phloem tissues than in surrounding nonphloem tissues. In addition, viruses target phloem-specific components as a means to promote their own systemic movement and disrupt host defense processes. Combined, these studies provide evidence that the vascular phloem plays a significant role in the mediation and control of host responses during infection and as such is a site of considerable modulation by the infecting virus. This review outlines the phloem responses and directed reprograming mechanisms that viruses employ to promote their movement through the vasculature.

Hop stunt viroid: Effect on Host (Humulus lupulus) Transcriptome and Its Interactions With Hop Powdery Mildew (Podospheara macularis)

Viroids are the smallest known plant pathogens that exploit host systems for their replication and cause diseases in many hosts. In this study, the host response of hop plants to Hop stunt viroid (HSVd) infection was studied through transcriptome analysis. RNA sequence analysis of hop leaves infected with HSVd revealed dynamic changes in hop gene expression. Defense-related genes and genes involved in lipid and terpenoid metabolism are the major categories that showed differential expression due to HSVd infection. Additionally, the effect of HSVd on development of hop powdery mildew (Podospheara macularis) (HPM) was studied. Transcriptome analysis followed by quantitative reverse transcription-polymerase chain reaction analysis showed that transcript levels of pathogenesis-related (PR) genes such as PR protein 1, chitinase, and thaumatin-like protein genes are induced in leaves infected with HPM alone. The response in these genes to HPM is significantly down-regulated in leaves with HSVd-HPM mixed infection. These results confirm that HSVd alters host metabolism, physiology, and plant defense responses. Nevertheless, in detached leaf assays, HPM consistently expanded faster on HSVd-negative leaves relative to HSVd-positive leaves. Although HSVd infection suppresses elements associated with the host immunity response, infection by HSVd is antagonistic to HPM infection of hops.

The effects of potato virus Y-derived virus small interfering RNAs of three biologically distinct strains on potato (Solanum tuberosum) transcriptome

BACKGROUND

Potato virus Y (PVY) is one of the most economically important pathogen of potato that is present as biologically distinct strains. The virus-derived small interfering RNAs (vsiRNAs) from potato cv. Russet Burbank individually infected with PVY-N, PVY-NTN and PVY-O strains were recently characterized. Plant defense RNA-silencing mechanisms deployed against viruses produce vsiRNAs to degrade homologous viral transcripts. Based on sequence complementarity, the vsiRNAs can potentially degrade host RNA transcripts raising the prospect of vsiRNAs as pathogenicity determinants in virus-host interactions. This study investigated the global effects of PVY vsiRNAs on the host potato transcriptome.

METHODS

The strain-specific vsiRNAs of PVY, expressed in high copy number, were analyzed in silico for their proclivity to target potato coding and non-coding RNAs using psRobot and psRNATarget algorithms. Functional annotation of target coding transcripts was carried out to predict physiological effects of the vsiRNAs on the potato cv. Russet Burbank. The downregulation of selected target coding transcripts was further validated using qRT-PCR.

RESULTS

The vsiRNAs derived from biologically distinct strains of PVY displayed diversity in terms of absolute number, copy number and hotspots for siRNAs on their respective genomes. The vsiRNAs populations were derived with a high frequency from 6 K1, P1 and Hc-Pro for PVY-N, P1, Hc-Pro and P3 for PVY-NTN, and P1, 3' UTR and NIa for PVY-O genomic regions. The number of vsiRNAs that displayed interaction with potato coding transcripts and number of putative coding target transcripts were comparable between PVY-N and PVY-O, and were relatively higher for PVY-NTN. The most abundant target non-coding RNA transcripts for the strain specific PVY-derived vsiRNAs were found to be MIR821, 28S rRNA,18S rRNA, snoR71, tRNA-Met and U5. Functional annotation and qRT-PCR validation suggested that the vsiRNAs target genes involved in plant hormone signaling, genetic information processing, plant-pathogen interactions, plant defense and stress response processes in potato.

CONCLUSIONS

The findings suggested that the PVY-derived vsiRNAs could act as a pathogenicity determinant and as a counter-defense strategy to host RNA silencing in PVY-potato interactions. The broad range of host genes targeted by PVY vsiRNAs in infected potato suggests a diverse role for vsiRNAs that includes suppression of host stress responses and developmental processes. The interactome scenario is the first report on the interaction between one of the most important Potyvirus genome-derived siRNAs and the potato transcripts.

Transcriptome-wide identification of host genes targeted by tomato spotted wilt virus-derived small interfering RNAs

RNA silencing mechanism functions as a major defense against invading viruses. The caveat in the RNA silencing mechanism is that the effector small interfering RNAs (siRNAs) act on any RNA transcripts with sequence complementarity irrespective of target's origin. A subset of highly expressed viral small interfering RNAs (vsiRNAs) derived from the tomato spotted wilt virus (TSWV; Tospovirus: Bunyaviridae) genome was analyzed for their propensity to downregulate the tomato transcriptome. A total of 11898 putative target sites on tomato transcripts were found to exhibit a propensity for down regulation by TSWV-derived vsiRNAs. In total, 2450 unique vsiRNAs were found to have potential cross-reacting capability with the tomato transcriptome. VsiRNAs were found to potentially target a gamut of host genes involved in basal cellular activities including enzymes, transcription factors, membrane transporters, and cytoskeletal proteins. KEGG pathway annotation of targets revealed that the vsiRNAs were mapped to secondary metabolite biosynthesis, amino acids, starch and sucrose metabolism, and carbon and purine metabolism. Transcripts for protein processing, hormone signalling, and plant-pathogen interactions were the most likely targets from the genetic, environmental information processing, and organismal systems, respectively. qRT-PCR validation of target gene expression showed that none of the selected transcripts from tomato cv. Marglobe showed up regulation, and all were down regulated even upto 20 folds (high affinity glucose transporter). However, the expression levels of transcripts from cv. Red Defender revealed differential regulation as three among the target transcripts showed up regulation (Cc-nbs-lrr, resistance protein, AP2-like ethylene-responsive transcription factor, and heat stress transcription factor A3). Accumulation of tomato target mRNAs of corresponding length was proved in both tomato cultivars using 5' RACE analysis. The TSWV-tomato interaction at the sRNA interface points to the ability of tomato cultivars to overcome vsiRNA-mediated targeting of NBS-LRR class R genes. These results suggest the prevalence of vsiRNA-induced RNA silencing of host transcriptome, and the interactome scenario is the first report on the interaction between tospovirus genome-derived siRNAs and tomato transcripts, and provide a deeper understanding of the role of vsiRNAs in pathogenicity and in perturbing host machinery.

A rapid isothermal assay for the detection of Hop stunt viroid in hop plants (Humulus lupulus), and its application in disease surveys

Hop stunt disease caused by Hop stunt viroid (HSVd) is a growing threat to hop cultivation globally. HSVd spreads mainly by use of contaminated planting material and by mechanical means. Thorough testing of hop yards and removal of infected bines are critical components of efforts to control the spread of the disease. Reverse transcription-polymerase chain reaction (RT-PCR) has become the primary technique used for HSVd detection; however, sample handling and analysis are technically challenging. In this study, a robust reverse transcription-recombinase polymerase amplification (RT-RPA) assay was developed to facilitate analysis of multiple samples. The assay was optimized with all major variants of HSVd from other host species in addition to hop variants. Used in conjunction with sample collection cards, RT-RPA accommodates large sample numbers. Greenhouse and farm samples tested with RT-RPA were also tested with RT-PCR and a 100% correlation between the two techniques was found.

Hop Stunt Viroid: Effects on Vegetative Growth and Yield of Hop Cultivars, and Its Distribution in Central Washington State

Hop stunt viroid (HSVd) is an economically important pathogen that reduces growth and yield of hops. Visual symptoms of infected hop are highly dependent on cultivar. A study was conducted using six cultivars of hop to determine the impact on yield. Average dry cone yields of infected 'Glacier', 'Cascade', and 'Willamette' were reduced by 62, 14, and 34%, respectively, relative to noninoculated healthy plants. No significant yield reduction was observed for 'Nugget', 'Columbus', and 'Galena'. The α-acid and β-acid contents showed a parallel pattern. Horticultural parameters of Willamette and Nugget were measured in the final year of the study. Internode length, shoot length, and side-arm length were reduced by 29, 26, and 73%, respectively, for infected Willamette bines relative to noninfected bines; no effects were observed resulting from infection of Nugget. To understand the current potential impact of HSVd, a survey was conducted to determine its distribution in central Washington. The survey revealed that 17% of hop plants tested are infected. Hop yield and hop plant longevity will be significantly affected by this level of infection. Copyright © 2017 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .