Barley Yellow Dwarf Virus Influences Its Vector's Endosymbionts but Not Its Thermotolerance

The barley yellow dwarf virus (BYDV) of cereals is thought to substantially increase the high-temperature tolerance of its aphid vector, Rhopalosiphum padi, which may enhance its transmission efficiency. This is based on experiments with North American strains of BYDV and R. padi. Here, we independently test these by measuring the temperature tolerance, via Critical Thermal Maximum (CTmax) and knockdown time, of Australian R. padi infected with a local BYDV isolate. We further consider the interaction between BYDV transmission, the primary endosymbiont of R. padi (Buchnera aphidicola), and a transinfected secondary endosymbiont (Rickettsiella viridis) which reduces the thermotolerance of other aphid species. We failed to find an increase in tolerance to high temperatures in BYDV-infected aphids or an impact of Rickettsiella on thermotolerance. However, BYDV interacted with R. padi endosymbionts in unexpected ways, suppressing the density of Buchnera and Rickettsiella. BYDV density was also fourfold higher in Rickettsiella-infected aphids. Our findings indicate that BYDV does not necessarily increase the temperature tolerance of the aphid transmission vector to increase its transmission potential, at least for the genotype combinations tested here. The interactions between BYDV and Rickettsiella suggest new ways in which aphid endosymbionts may influence how BYDV spreads, which needs further testing in a field context.

F-actin dynamics in midgut cells enables virus persistence in vector insects

Hemipteran insects that transmit plant viruses in a persistent circulative manner acquire, retain and transmit viruses for their entire life. The mechanism enabling this persistence has remained unclear for many years. Here, we determined how wheat dwarf virus (WDV) persists in its leafhopper vector Psammotettix alienus. We found that WDV caused the up-regulation of actin-depolymerizing factor (ADF) at the mRNA and protein levels in the midgut cells of leafhoppers after experiencing a WDV acquisition access period (AAP) of 6, 12 or 24 h. Experimental inhibition of F-actin depolymerization by jasplakinolide and dsRNA injection led to lower virus accumulation levels and transmission efficiencies, suggesting that depolymerization of F-actin regulated by ADF is essential for WDV invasion of midgut cells. Exogenous viral capsid protein (CP) inhibited ADF depolymerization of actin filaments in vitro and in Spodoptera frugiperda 9 (Sf9) cells because the CP competed with actin to bind ADF and then blocked actin filament disassembly. Interestingly, virions colocalized with ADF after a 24-h AAP, just as actin polymerization occurred, indicating that the binding of CP with ADF affects the ability of ADF to depolymerize F-actin, inhibiting WDV entry. Similarly, the luteovirus barley yellow dwarf virus also induced F-actin depolymerization and then polymerization in the gut cells of its vector Schizaphis graminum. Thus, F-actin dynamics are altered by nonpropagative viruses in midgut cells to enable virus persistence in vector insects.

Alfalfa leaf curl virus is efficiently acquired by its aphid vector Aphis craccivora but inefficiently transmitted

Alfalfa leaf curl virus (ALCV) is the first geminivirus for which aphid transmission was reported. Transmission by Aphis craccivora was determined previously to be highly specific and circulative. Using various complementary techniques, the transmission journey of ALCV was monitored from its uptake from infected plant tissues up to the head of its vector. ALCV was shown to be restricted to phloem tissues using fluorescence in situ hybridization (FISH) and electropenetrography (EPG) monitoring of virus acquisition. Furthermore, the virus is heterogeneously distributed in phloem tissues, as revealed by FISH and quantitative PCR of viral DNA acquired by EPG-monitored aphids. Despite the efficient ingestion of viral DNA, about 106 viral DNA copies per insect in a 15 h feeding period on ALCV-infected plants, the individual maximum transmission rate was 12 %. Transmission success was related to a critical viral accumulation, around 1.6×107 viral DNA copies per insect, a threshold that generally needed more than 48 h to be reached. Moreover, whereas the amount of acquired virus did not decrease over time in the whole aphid body, it declined in the haemolymph and heads. ALCV was not detected in progenies of viruliferous aphids and did not affect aphid fitness. Compared to geminiviruses transmitted by whiteflies or leafhoppers, or to luteoviruses transmitted by aphids, the transmission efficiency of ALCV by A. craccivora is low. This result is discussed in relation to the aphid vector of this geminivirus and the agroecological features of alfalfa, a hardy perennial host plant.

Aphid transmission of nanoviruses

The genus Nanovirus consists of plant viruses that predominantly infect legumes leading to devastating crop losses. Nanoviruses are transmitted by various aphid species. The transmission occurs in a circulative nonpropagative manner. It was long suspected that a virus-encoded helper factor would be needed for successful transmission by aphids. Recently, a helper factor was identified as the nanovirus-encoded nuclear shuttle protein (NSP). The mode of action of NSP is currently unknown in contrast to helper factors from other plant viruses that, for example, facilitate binding of virus particles to receptors within the aphids' stylets. In this review, we are summarizing the current knowledge about nanovirus-aphid vector interactions.

Combining Transient Expression and Cryo-EM to Obtain High-Resolution Structures of Luteovirid Particles

The Luteoviridae are pathogenic plant viruses responsible for significant crop losses worldwide. They infect a wide range of food crops, including cereals, legumes, cucurbits, sugar beet, sugarcane, and potato and, as such, are a major threat to global food security. Viral replication is strictly limited to the plant vasculature, and this phloem limitation, coupled with the need for aphid transmission of virus particles, has made it difficult to generate virus in the quantities needed for high-resolution structural studies. Here, we exploit recent advances in heterologous expression in plants to produce sufficient quantities of virus-like particles for structural studies. We have determined their structures to high resolution by cryoelectron microscopy, providing the molecular-level insight required to rationally interrogate luteovirid capsid formation and aphid transmission, thereby providing a platform for the development of preventive agrochemicals for this important family of plant viruses.

Resistance to Cucurbit aphid-borne yellows virus in Melon Accession TGR-1551

The genetic control of resistance to Cucurbit aphid-borne yellows virus (CABYV; genus Polerovirus, family Luteoviridae) in the TGR-1551 melon accession was studied through agroinoculation of a genetic family obtained from the cross between this accession and the susceptible Spanish cultivar 'Bola de Oro'. Segregation analyses were consistent with the hypothesis that one dominant gene and at least two more modifier genes confer resistance; one of these additional genes is likely present in the susceptible parent 'Bola de Oro'. Local and systemic accumulation of the virus was analyzed in a time course experiment, showing that TGR-1551 resistance was expressed systemically as a significant reduction of virus accumulation compared with susceptible controls, but not locally in agroinoculated cotyledons. In aphid transmission experiments, CABYV inoculation by aphids was significantly reduced in TGR-1551 plants, although the virus was acquired at a similar rate from TGR-1551 as from susceptible plants. Results of feeding behavior studies using the DC electrical penetration graph technique suggested that viruliferous aphids can salivate and feed from the phloem of TGR-1551 plants and that the observed reduction in virus transmission efficiency is not related to reduced salivation by Aphis gossypii in phloem sieve elements. Since the virus is able to accumulate to normal levels in agroinoculated tissues, our results suggest that resistance of TGR-1551 plants to CABYV is related to impairment of virus movement or translocation after it reaches the phloem sieve elements.

Cross-linking measurements of the Potato leafroll virus reveal protein interaction topologies required for virion stability, aphid transmission, and virus-plant interactions

Protein interactions are critical determinants of insect transmission for viruses in the family Luteoviridae. Two luteovirid structural proteins, the capsid protein (CP) and the readthrough protein (RTP), contain multiple functional domains that regulate virus transmission. There is no structural information available for these economically important viruses. We used Protein Interaction Reporter (PIR) technology, a strategy that uses chemical cross-linking and high resolution mass spectrometry, to discover topological features of the Potato leafroll virus (PLRV) CP and RTP that are required for the diverse biological functions of PLRV virions. Four cross-linked sites were repeatedly detected, one linking CP monomers, two within the RTP, and one linking the RTP and CP. Virus mutants with triple amino acid deletions immediately adjacent to or encompassing the cross-linked sites were defective in virion stability, RTP incorporation into the capsid, and aphid transmission. Plants infected with a new, infectious PLRV mutant lacking 26 amino acids encompassing a cross-linked site in the RTP exhibited a delay in the appearance of systemic infection symptoms. PIR technology provided the first structural insights into luteoviruses which are crucially lacking and are involved in vector-virus and plant-virus interactions. These are the first cross-linking measurements on any infectious, insect-transmitted virus.

Temporal Increase and Spatial Distribution of Sugarcane Yellow Leaf and Infestations of the Aphid Vector, Melanaphis sacchari

Yellow leaf, caused by Sugarcane yellow leaf virus (ScYLV), is a potentially important disease of sugarcane first found in Louisiana during 1996. A survey during 2002 determined that ScYLV infection was present in all sugarcane-production areas of Louisiana. Virus was detected in 48% of 42 fields, and incidence averaged 15% in these fields. Disease progress curves determined in four fields during two growing seasons indicated that the greatest temporal increase of virus infection occurred during late spring and early summer and coincided with the initial infestation and increase of the virus vector, the sugarcane aphid (Melanaphis sacchari). Aphid infestations in the experimental fields during 2002 and 2003 ranged from 1.2 to 33.0 and 1.0 to 4.2 aphids per leaf, respectively. Final disease incidences of 2.9, 5.2, and 5.2% were recorded in three fields planted with virus-free seed-cane. Distribution of ScYLV infections and aphids evaluated with spatial autocorrelation analysis indicated that ScYLV and its aphid vector both exhibited a predominantly random spatial distribution, with occasional aggregation. The low incidence and rates of disease increase observed, despite the widespread occurrence of potential vectors, suggest that inoculum pressure remains low in Louisiana. Therefore, it may be possible to keep yellow leaf at low levels by planting virus-free seed-cane.

Transmission of two viruses that cause Barley Yellow Dwarf is controlled by different loci in the aphid, Schizaphis graminum

Clonal populations of the aphid, Schizaphis graminum, have been separated into biotypes based on host preference and their ability to overcome resistance genes in wheat. Recently, several biotypes were found to differ in their ability to transmit one or more of the viruses that cause barley yellow dwarf disease in grain crops, and vector competence was linked to host preference. The genetics of host preference has been studied in S. graminum, but how this may relate to the transmission of plant viruses is unknown. Sexual morphs of a vector and nonvector S. graminum genotype were induced from parthenogenetic females and reciprocal crosses made. Eighty-nine hybrids were generated and maintained by parthenogenesis. Each hybrid was evaluated for its ability to transmit Barley yellow dwarf virus-PAV and Cereal yellow dwarf virus-RPV, and for its ability to colonize two wheat genotypes each expressing a different gene that confers resistance to S. graminum. The F1 genotypes were genetically variable for their ability to transmit virus and to colonize the aphid resistant wheat, but these traits were not genetically correlated. Individual F1 genotypes ranged in transmission efficiency from 0-100% for both viruses, although the overall mean transmission efficiency was similar to the transmission competent parent, indicating directional dominance. The direction of the cross did not significantly affect the vector competency for either virus, suggesting that maternally inherited cytoplasmic factors, or bacterial endosymbionts, did not contribute significantly to the inheritance of vector competency in S. graminum. Importantly, there was no genetic correlation between the ability to transmit Barley yellow dwarf virus and Cereal yellow dwarf virus-RPV in the F1 genotypes. These results taken together indicate that multiple loci are involved in the circulative transmission, and that the successful transmission of these closely related viruses is regulated by different sets of aphid genes.

Factors Affecting the Transmission and Spread of Sugarcane yellow leaf virus

Sugarcane, Saccharum spp. hybrid, is widely infected in the United States and many other countries with a yellowing and stunting disease called sugarcane yellow leaf syndrome. The causal agent, Sugarcane yellow leaf virus (ScYLV), is a Polerovirus of the Luteoviridae family. In this study, it was transmitted by the sugarcane aphid, Melanaphis sacchari, and also by the corn leaf aphid, Rhopalosiphum maidis, and the rice root aphid, R. rufiabdominalis. Two other aphids that infest sugarcane in Hawaii did not transmit the virus. Some Hawaiian sugarcane cultivars are susceptible to ScYLV, while others remain virus-free in the field. The latter were not infected when inoculated with viruliferous M. sacchari. Virus-free plants of susceptible cultivars were produced through apical meristem culture and were readily reinfected by viruliferous M. sacchari. They were also quickly reinfected when planted in a field in proximity to other infected sugarcane naturally infested with M. sacchari. Sugarcane cultivars are hybrids of several Saccharum species. In a field-grown collection of Saccharum and related species, 11 to 71% of the clones of four of the species were infected with ScYLV. None of the related genus Erianthus plants were infected, but four clones were infected experimentally by aphid inoculation. A low to moderate percentage of corn, rice, and sorghum seedlings became infected when inoculated with ScYLV, but barley, oats, and wheat proved to be very susceptible. None of seven weeds common in sugarcane fields were infected with ScYLV.

Reaction of Sweet Corn to Inoculation with Barley Yellow Dwarf Virus RMV-IL

The reaction of five sweet corn hybrids to barley yellow dwarf virus (BYDV-RMV-IL) was determined in 1992 and 1993. In 1992, symptoms were observed in three of the five hybrids planted 20 May and four of the five hybrids planted 20 June. No symptoms were observed in hybrids planted June or July 1993. The mean virus incidences of RMV-IL determined by enzyme-linked immunosorbent assay (ELISA) in all plots for the May and June 1992 planting dates were 3.5 and 21%, respectively. The mean virus incidence for the inoculated plots for the June 1992 planting date was significantly higher than incidence for the control plots (29 versus 13%). Ear weights were significantly lower for inoculated plots than for the control plots (1.2 kg versus 1.4 k/10 ears). The mean virus incidences of RMV-IL in all plots for the June and July 1993 planting dates were 31 and 23%, respectively. The mean virus incidence for the inoculated plots for the June 1993 planting date was significantly higher than incidence for the control plots (49 versus 14%). Plots inoculated in June 1993 also had a significantly lower ear weight than the control plots (1.8 kg versus 2.0 kg/10 ears). A range of symptoms, infection levels, and yield responses of sweet corn hybrids to BYD-RMV-IL was demonstrated in this study. Hybrid susceptibility to this strain of BYDV should be considered when planting sweet corn from middle to late June in order to limit potential yield losses.

Comparison of Techniques for Detection of Barley Yellow Dwarf Virus-PAV-IL

Detection of barley yellow dwarf virus (BYDV)-PAV-IL by an improved nucleic acid hybridization technique, using a nonradioactive probe with chromogenic and chemiluminescent substrates, was compared with detection by polymerase chain reaction (PCR), double antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA) with polyclonal antibodies, and triple antibody sandwich ELISA with polyclonal and monoclonal antibodies. Each method was used to detect purified virus and virus in sap extracts from infected oat leaves. The detection limits for both ELISA procedures were 1 ng of purified BYDV-PAV-IL and the equivalent of 78 ng of infected tissue. Nucleic acid hybridization with either chemiluminescent or chromogenic substrates also detected as little as 1 ng of purified BYDV-PAV-IL, but it was slightly more sensitive at detecting virus in tissue extracts (25 ng of infected tissue). The most sensitive detection technique was PCR amplification, which could detect as little as 0.1 pg of RNA extracted from purified virus and detected viral RNA in the equivalent of 0.5 pg of infected leaf tissue.

Translation in plants--rules and exceptions

Translation processes in plants are very similar to those in other eukaryotic organisms and can in general be explained with the scanning model. Particularly among plant viruses, unconventional mRNAs are frequent, which use modulated translation processes for their expression: leaky scanning, translational stop codon readthrough or frameshifting, and transactivation by virus-encoded proteins are used to translate polycistronic mRNAs; leader and trailer sequences confer (cap-independent) efficient ribosome binding, usually in an end-dependent mechanism, but true internal ribosome entry may occur as well; in a ribosome shunt, sequences within an RNA can be bypassed by scanning ribosomes. Translation in plant cells is regulated under conditions of stress and during development, but the underlying molecular mechanisms have not yet been determined. Only a small number of plant mRNAs, whose structure suggests that they might require some unusual translation mechanisms, have been described.