Within plant distribution of Potato Virus Y in hairy nightshade (Solanum sarrachoides): an inoculum source affecting PVY aphid transmission

Tiny but Mighty: Nanoscale Materials in Plant Disease Management

Nanoscale materials are promising tools for managing plant diseases and are becoming important players in the current agritech revolution. However, adopting modern methodologies requires a broad understanding of their effectiveness in solving target problems and their effects on the environment and food chain. Furthermore, it is paramount that such technologies are mechanistically and economically feasible for growers to adopt in order to be sustainable in the long run. This Feature Article summarizes the latest findings on the role of nanoscale materials in managing agricultural plant pathogens. Herein, we discussed the benefits and limitations of using nanoscale materials in plant disease management and their potential impacts on the environment and global food security.

The Temporal and Geographical Dynamics of Potato Virus Y Diversity in Russia

Potato virus Y, an important viral pathogen of potato, has several genetic variants and geographic distributions which could be affected by environmental factors, aphid vectors, and reservoir plants. PVY is transmitted to virus-free potato plants by aphids and passed on to the next vegetative generations through tubers, but the effects of tuber transmission in PVY is largely unknown. By using high-throughput sequencing, we investigated PVY populations transmitted to potato plants by aphids in different climate zones of Russia, namely the Moscow and Astrakhan regions. We analyzed sprouts from the tubers produced by field-infected plants to investigate the impact of tuber transmission on PVY genetics. We found a significantly higher diversity of PVY isolates in the Astrakhan region, where winters are shorter and milder and summers are warmer compared to the Moscow region. While five PVY types, NTNa, NTNb, N:O, N-Wi, and SYR-I, were present in both regions, SYRI-II, SYRI-III, and 261-4 were only found in the Astrakhan region. All these recombinants were composed of the genome sections derived from PVY types O and N, but no full-length sequences of such types were present. The composition of the PVY variants in the tuber sprouts was not always the same as in their parental plants, suggesting that tuber transmission impacts PVY genetics.

Infection Dynamics of Potato Virus Y Isolate Combinations in Three Potato Cultivars

The United States potato industry has recently experienced a strain shift; recombinant potato virus Y (PVY) strains (e.g., PVY^NTN) have emerged as the predominant strains over the long dominant ordinary strain (PVY^O), yet both are often found as single infections within the same field and as mixed infections within individual plants. To understand mixed infection dynamics in potato plants and in daughter tubers, three potato varieties varying for PVY resistance, 'Red Maria', 'CalWhite', and 'Pike', were mechanically inoculated either at the pre- or postflowering stage with all possible heterologous isolate combinations of two PVY^O and two PVY^NTN isolates. Virus titer was determined from leaves collected at different positions on the plant at different times, and tuber-borne infection was determined for two successive generations. PVY^NTN accumulated to higher levels than PVY^O at nearly all sampling time points in 'Pike' potato. However, both virus strains accumulated to similar amounts in 'Red Maria' and 'CalWhite' potato early in the infection when inoculated preflowering; however, PVY^NTN dominated at later stages and in plants inoculated postflowering. Regardless of inoculation time, both virus strains were transmitted to daughter plants raised from the tubers for most isolate combinations. The relative titer of PVY^NTN and PVY^O isolates at the later stages of mother plant development was indicative of what was found in the daughter plants. Although virus titer differed among cultivars depending on their genetics and virus isolates, it did not change the strain outcome in tuber-borne infection in subsequent generations. Differential virus accumulation in these cultivars suggests isolate-specific resistance to PVY accumulation.

Freshwater macrophytes harbor viruses representing all five major phyla of the RNA viral kingdom Orthornavirae

Research on aquatic plant viruses is lagging behind that of their terrestrial counterparts. To address this knowledge gap, here we identified viruses associated with freshwater macrophytes, a taxonomically diverse group of aquatic phototrophs that are visible with the naked eye. We surveyed pooled macrophyte samples collected at four spring sites in Florida, USA through next generation sequencing of RNA extracted from purified viral particles. Sequencing efforts resulted in the detection of 156 freshwater macrophyte associated (FMA) viral contigs, 37 of which approximate complete genomes or segments. FMA viral contigs represent putative members from all five major phyla of the RNA viral kingdom Orthornavirae. Similar to viral types found in land plants, viral sequences identified in macrophytes were dominated by positive-sense RNA viruses. Over half of the FMA viral contigs were most similar to viruses reported from diverse hosts in aquatic environments, including phototrophs, invertebrates, and fungi. The detection of FMA viruses from orders dominated by plant viruses, namely Patatavirales and Tymovirales, indicate that members of these orders may thrive in aquatic hosts. PCR assays confirmed the presence of putative FMA plant viruses in asymptomatic vascular plants, indicating that viruses with persistent lifestyles are widespread in macrophytes. The detection of potato virus Y and oat blue dwarf virus in submerged macrophytes suggests that terrestrial plant viruses infect underwater plants and highlights a potential terrestrial-freshwater plant virus continuum. Defining the virome of unexplored macrophytes will improve our understanding of virus evolution in terrestrial and aquatic primary producers and reveal the potential ecological impacts of viral infection in macrophytes.

Different potato virus Y strains frequently co-localize in single epidermal leaf cells and in the aphid stylet

Single aphids can simultaneously or sequentially acquire and transmit multiple potato virus Y (PVY) strains. Multiple PVY strains are often found in the same field and occasionally within the same plant, but little is known about how PVY strains interact in plants or in aphid stylets. Immuno-staining and confocal microscopy were used to examine the spatial and temporal dynamics of PVY strain mixtures (PVYO and PVYNTN or PVYO and PVYN) in epidermal leaf cells of 'Samsun NN' tobacco and 'Goldrush' potato. Virus binding and localization was also examined in aphid stylets following acquisition. Both strains systemically infected tobacco and co-localized in cells of all leaves examined; however, the relative amounts of each virus changed over time. Early in the tobacco infection, when mosaic symptoms were observed, PVYO dominated the infection although PVYNTN was detected in some cells. As the infection progressed and vein necrosis developed, PVYNTN was prevalent. Co-localization of PVYO and PVYN was also observed in epidermal cells of potato leaves with most cells infected with both viruses. Furthermore, two strains could be detected binding to the distal end of aphid stylets following virus acquisition from a plant infected with a strain mixture. These data are in contrast with the traditional belief of spatial separation of two closely related potyviruses and suggest apparent non-antagonistic interaction between PVY strains that could help explain the multitude of emerging recombinant PVY strains discovered in potato in recent years.

The Gustavus Gene Can Regulate the Fecundity of the Green Peach Aphid, Myzus persicae (Sulzer)

Myzus persicae (Sulzer), commonly known as the green peach aphid, is a notorious pest that causes substantial losses to a range of crops and can transmit several plant viruses, including potato virus Y (PVY). Chemical insecticides provide only partial control of this pest and their use is not environmentally sustainable. In recent years, many genes related to growth, development, and reproduction have been used as targets for pest control. These include Gustavus (Gus), a highly conserved gene that has been reported to play an essential part in the genesis of germline cells and, hence, in fecundity in the model insect Drosophila melanogaster. We hypothesized that the Gustavus (Gus) gene was a potential target that could be used to regulate the M. persicae population. In this study, we report the first investigation of an ortholog of Gus in M. persicae, designated MpGus, and describe its role in the fecundity of this insect. First, we identified the MpGus mRNA sequence in the M. persicae transcriptome database, verified its identity with reverse transcription-polymerase chain reaction (RT-PCR), and then evaluated the transcription levels of MpGus in M. persicae nymphs of different instars and tissues with real-time quantitative PCR (RT-qPCR). To investigate its role in regulating the fecundity of M. persicae, we used RNA interference (RNAi) to silence the expression of MpGus in adult insects; this resulted in a significant reduction in the number of embryos (50.6%, P < 0.01) and newborn nymphs (55.7%, P < 0.01) in the treated aphids compared with controls. Interestingly, MpGus was also significantly downregulated in aphids fed on tobacco plants that had been pre-infected with PVY^N, concomitant with a significant reduction (34.1%, P < 0.01) in M. persicae fecundity. Collectively, these data highlight the important role of MpGus in regulating fecundity in M. persicae and indicate that MpGus is a promising RNAi target gene for control of this pest species.

The Evolutionary Significance of Generalist Viruses with Special Emphasis on Plant Viruses and their Hosts

The host range of a virus is defined as the number of species a virus potentially infects. The specialist virus infects one or few related species while the generalist virus infects several different species, possibly in different families. Origin of generalist viruses from their specialist nature and the expansion of the host range of the generalist virus occur with the host shift event in which the virus encounters and adapts to a new host. Host shift events have resulted in the majority of the newly emerging viral diseases. This review discusses the advantages and disadvantages of generalist over specialist viruses and the unique features of plant viruses and their hosts that result in a higher incidence of generalist viruses in plants.

Potato Virus Y Emergence and Evolution from the Andes of South America to Become a Major Destructive Pathogen of Potato and Other Solanaceous Crops Worldwide

The potato was introduced to Europe from the Andes of South America in the 16th century, and today it is grown worldwide; it is a nutritious staple food eaten by millions and underpins food security in many countries. Unknowingly, potato virus Y (PVY) was also introduced through trade in infected potato tubers, and it has become the most important viral pathogen of potato. Phylogenetic analysis has revealed the spread and emergence of strains of PVY, including strains causing economically important diseases in tobacco, tomato and pepper, and that the virus continues to evolve with the relatively recent emergence of new damaging recombinant strains. High-throughput, next-generation sequencing platforms provide powerful tools for detection, identification and surveillance of new PVY strains. Aphid vectors of PVY are expected to increase in incidence and abundance in a warmer climate, which will increase the risk of virus spread. Wider deployment of crop cultivars carrying virus resistance will be an important means of defence against infection. New cutting-edge biotechnological tools such as CRISPR and SIGS offer a means for rapid engineering of resistance in established cultivars. We conclude that in future, human activities and ingenuity should be brought to bear to control PVY and the emergence of new strains in key crops by increased focus on host resistance and factors driving virus evolution and spread.

Anthropogenic influences on emergence of vector-borne plant viruses: the persistent problem of Potato virus Y

Potato virus Y (PVY) has reemerged as a serious impediment to seed potato production, responsible for reduced yields and tuber quality, as well as the majority of seed lot rejections by certification programs due to excessive virus incidence. This has led to seed shortages, especially in cultivars highly susceptible to infection. While seed certification programs have been effective at managing many virus diseases below economic thresholds, PVY has rapidly evolved in recent decades to become a complex of strains that evade many certification and farm management practices. The evolution of PVY strains is naturally occurring, but several human influences can be linked to the rapid change in PVY populations affecting the potato crop. Here we highlight the recent history and current status of PVY in potatoes and suggest some approaches for managing the virus moving forward.

Effect of Potato virus Y Presence in Solanum tuberosum (Solanales: Solanaceae) and Chenopodium album on Aphid (Hemiptera: Aphididae) Behavior

This study establishes the effect of Potato Virus Y (PVY; Potyvirus) in potatoes, Solanum tuberosum L. (Solanales: Solanaceae) and in common-lambs' quarter Chenopodium album L. (Amaranthaceae) on Macrosiphum euphorbiae Thomas (Hemiptera: Aphididae) and Myzus persicae Sulzer (Hemiptera: Aphididae) behavior, host preference, transmission, and fitness under field and laboratory studies. In the field, several weeds, besides C. album, were collected, including: Sisymbrium altissimum L. (Brassicaceae), Erodium cicutarium L., Lactuca serriola L., Solanum sarrachoides Sendtner (Solanaceae), and S. dulcamara L. (Solanaceae). All weeds were serologically tested for the presence of PVY. From all weeds collected, 2.3 and 34% of C. album and S. sarrachoides, respectively, were PVY-positive. From those positive samples, 72% of the PVY found were PVYN; the remaining 28% was PVYO. In addition, several aphid species were collected from those weeds: Ovatus crataegarious Walker, Macrosiphum euphorbiae (Hemiptera: Aphididae), Hyalopterus pruni Geoffroy (Hemiptera: Aphididae), Rophalosiphum madis Fitch, and 'others aphid' species were collected. The highest number of aphids were collected in E. cicutarium, followed by S. dulcamara, L. serriola, S. altissimum, and C. album. In laboratory studies, PVY-infected C. album does not induce the production of aphids. Moreover, M. persicae did not appear to have a strong preference for either healthy or PVY-infected potato plant, but they did develop a preference for infected plants after prolonged feeding. M. persicae and M. euphorbiae transmitted PVY from C. album to S. tuberosum, 44 and 37.5 % of the time. Future research should seek to identify not only other sources and prevalence of PVY in the field but vector relationships. In insect-pathogen complex continues to persist in solanaceous field crops around the world.

Sequential acquisition of Potato virus Y strains by Myzus persicae favors the transmission of the emerging recombinant strains

In the past decade recombinant strains of Potato virus Y (PVY) have overtaken the ordinary strain, PVY^O, as the predominant viruses affecting the US seed potato crop. Aphids may be a contributing factor in the emergence of the recombinant strains, but studies indicate that differences in transmission efficiency of individual PVY strains either from single or mixed infections, although variable, are not generally significant. Multiple strains of PVY are present in all potato production areas and common in many potato fields. Therefore, it is likely that individual alate aphids moving through a potato field will sequentially encounter multiple strains as they "taste test" multiple potato plants while looking for a suitable host. This study examined the transmission likelihood and efficiency of three common PVY strains when acquired sequentially by individual aphids. Green peach aphids (Myzus persicae, Sulzer) were allowed a 2-3min acquisition access period (AAP) on potato leaves infected with PVY^O, PVY^N:O or PVY^NTN, followed by another 2-3min AAP on a second potato leaf infected with a different PVY strain before being transferred to healthy potato seedlings for a 24h inoculation access period. All possible combinations of the three strains were tested. Strain-specific infection of the recipient plants was determined by TAS-ELISA and RT-PCR 3-4wk post-inoculation. The recombinant strains, PVY^N:O and PVY^NTN, were transmitted more efficiently than PVY^O when they were sequentially acquired regardless of the order acquired. PVY^N:O and PVY^NTN were transmitted with similar efficiencies when they were sequentially acquired regardless of the order. The recombinant strains appear to preferentially bind to the aphid stylet over PVY^O or they may be preferentially released during inoculation. This may contribute to the increased incidence of the recombinant strains over PVY^O in fields or production regions where multiple PVY strains are detected.

Potato virus Y Transmission Efficiency from Potato Infected with Single or Multiple Virus Strains

There has been a recent shift in the prevalence of Potato virus Y (PVY) strains affecting potato with the ordinary strain PVY^O declining and the recombinant strains PVY^NTN and PVY^N:O emerging in the United States. Multiple PVY strains are commonly found in potato fields and even in individual plants. Factors contributing to the emergence of the recombinant strains are not well defined but differential aphid transmission of strains from single and mixed infections may play a role. We found that the transmission efficiencies by Myzus persicae, the green peach aphid, of PVY^NTN, PVY^N:O, and PVY^O varied depending on the potato cultivar serving as the virus source. Overall transmission efficiency was highest from sources infected with three virus strains, whereas transmission from sources infected with one or two virus strains was not significantly different. Two strains were concomitantly transmitted by individual aphids from many of the mixed-source combinations, especially if PVY^O was present. Triple-strain infections were not transmitted by any single aphid. PVY^O was transmitted most efficiently from mixed-strain infection sources. The data do not support the hypothesis that differential transmission of PVY strains by M. persicae is a major contributing factor in the emergence of recombinant PVY strains in the U.S. potato crop.

Impact of aphid alarm pheromone release on virus transmission efficiency: When pest control strategy could induce higher virus dispersion

Aphids cause serious damages to crops not only by tacking sap but also by transmitting numerous viruses. To develop biological control, the aphid alarm pheromone, namely E-β-farnesene (EβF), has been demonstrated to be efficient to repel aphids and as attract beneficials, making it a potential tool to control aphid pests. Considering aphids also as virus vectors, changes of their behavior could also interfere with the virus acquisition and transmission process. Here, a combination of two aphid species and two potato virus models were selected to test the influence of EβF release on aphid and virus dispersion under laboratory conditions. EβF release was found to significantly decrease the population of Myzus persicae and Macrosiphum euphorbiae around the infochemical releaser but simultaneously also increasing the dispersal of Potato Virus Y (PVY). At the opposite, no significant difference for Potato Leaf Roll Virus (PLRV) transmission efficiency was observed with similar aphid alarm pheromone releases for none of the aphid species. These results provide some support to carefully consider infochemical releasers not only for push-pull strategy and pest control but also to include viral disease in a the plant protection to aphids as they are also efficient virus vectors. Impact of aphid kinds and transmission mechanisms will be discussed according to the large variation found between persistent and non persistent potato viruses and interactions with aphids and related infochemicals.

Preferential acquisition and inoculation of PVYNTN over PVYO in potato by the green peach aphid Myzus persicae (Sulzer)

In the past decade, the incidence and distribution of the recombinant, tuber necrotic strain of Potato virus Y (PVYNTN) has been increasing in the US seed potato crop while the ordinary strain (PVYO) has been decreasing. The transmission efficiency of both strains was determined from two potato cultivars when acquired sequentially by the same aphid or when acquired by separate aphids and inoculated to the same plant. PVYNTN was transmitted more efficiently than PVYO and the order of acquisition or inoculation did not affect the preferential transmission of PVYNTN. When a recipient plant became infected with both strains, PVYNTN maintained higher titre than PVYO and would facilitate the acquisition of PVYNTN. Furthermore, the acquisition and transmission of PVYNTN over PVYO was enhanced in the potato cultivar that expressed a strain-specific Ny-like resistance gene that confers partial resistance to PVYO.

Interactions among potato genotypes, growth stages, virus strains, and inoculation methods in the potato virus Y and green peach aphid pathosystem

Potato virus Y (PVY) is an economically important and reemerging potato pathogen in North America. PVY infection reduces yield, and some necrotic and recombinant strains render tubers unmarketable. Although PVY(O) is the most prevalent strain in the United States, the necrotic and recombinant strains PVY(NTN) and PVY(N:O) are becoming more widespread. Infection rates in aphid-inoculated (Myzus persicae (Sulzer)) and mechanically inoculated plants were compared across two potato genotypes ('Yukon Gold' and A98345-1), three PVY strains (PVY(O), PVY(N:O), and PVY(NTN)), and two growth stages at inoculation (pre- and postflowering). Susceptibility of genotypes was measured as infection rate using a double-antibody sandwich-enzyme-linked immunosorbent assay; virus titer and tuber mass also were recorded from the infected plants. Yukon Gold generally was more susceptible than A98345-1 to all three PVY strains, especially following mechanical inoculation. Within genotypes, Yukon Gold was most susceptible to PVY(O) and A98345-1 was most susceptible to PVY(N:O). Plants exhibited age-based resistance, with both genotypes showing higher susceptibility at the pre- than postflowering stage. The overall ranking pattern of virus titer in infected plants was PVY(O) > PVY(NTN) > PVY(N:O); across all three strains, infected Yukon Gold had higher titer than infected A98345-1 plants. Yukon Gold plants had lower tuber mass than A98345-1 when infected, and there were differences between the two inoculation methods in regard to tuber mass for the three stains. The results showed differences in infection response between inoculation methods and as a function of genotype, strain, inoculation stage, and their interactions. These factors should be considered when screening genotypes for resistance.

Effect of Host Plant, Aphid Species, and Virus Infection Status on Transmission of Sweetpotato feathery mottle virus

Sweetpotato feathery mottle virus (SPFMV) is a nonpersistently transmitted virus known to infect sweetpotato (Ipomoea batatas) and wild morning glory plants. SPFMV is vectored by various aphid species, among them the green peach aphid, Myzus persicae, and the cotton aphid, Aphis gossypii. Our objective was to determine whether differences in acquisition hosts (sweetpotato and morning glory), aphid species (M. persicae and A. gossypii), and infection status (single versus mixed infection) influenced transmission of SPFMV. SPFMV transmission from I. hederacea with a natural mixed infection by A. gossypii (39%) was significantly greater than in other host-virus combinations. Successful transmissions by A. gossypii were significantly greater compared with M. persicae in all host-virus combinations. Virus titers in source leaves were significantly greater in single- and mixed-infected I. hederacea and single-infected I. cordatotriloba compared with other host-virus combinations. There was a significant positive correlation between virus titer and transmission by both aphid species. These results suggest that, under controlled conditions, SPFMV is more readily transmitted from infected morning glory plants than from sweetpotato. Additionally, mixed-infected plants are better virus sources for transmission than single-infected, and A. gossypii is a more efficient vector than M. persicae under laboratory conditions.

Strain specificity and simultaneous transmission of closely related strains of a Potyvirus by Myzus persicae

Potato virus Y (PVY), a Potyvirus, is transmitted by aphids in a nonpersistent manner. PVY severely affects potato production worldwide. Single and mixed infections of PVY strains, namely PVY(O), PVY(NTN), and PVY(N:O) are a common occurrence in potato systems. However, information available on the ability of aphids to simultaneously transmit multiple PVY strains, specificity associated with simultaneous transmission, and factors affecting specificity are limited. Aphid-mediated transmission experiments were conducted to test the ability of individual aphids to transmit multiple strains using a PVY indicator host. Preliminary results revealed that aphids can transmit at least two viral strains simultaneously. Subsequently, aphid-mediated transmission of three dual-strain combinations was tested using potato plants. Individual aphids transmitted two viral strains simultaneously for all three dual-strain combinations. In all aphid-mediated dual-strain infections involving PVY(NTN), the rate of PVY(NTN) infection was greater than the infection rates of the second strain and dual-strain combinations, indicating specificity associated with transmission of PVY strains. Results of aphid-mediated transmission experiments were compared with results obtained through mechanical transmission. In general, PVY infection rates from aphid-mediated transmission were lower than the rates obtained through mechanical transmission. Unlike aphid-mediated transmission, component strains in dual-strain inoculations were not eliminated during mechanical transmission. These results suggest that there may also be interference associated with aphid-mediated transmission of closely related PVY strains. Perhaps, the observed specificity and/or interference may explain the increase in the incidence of PVY(NTN) and other necrotic strains in recent years.