Biological properties and molecular characterization of beet chlorosis virus (BChV)

Virus Yellows and Syndrome "Basses Richesses" in Western Switzerland: A Dramatic 2020 Season Calls for Urgent Control Measures

Massive outbreaks of virus yellows (VY) and syndrome “basses richesses” (SBR) are thought to be responsible for the major loss of sugar beet yields in 2020 in western cantons of Switzerland. Typical yellowing symptoms were visible during field inspections, and control measures were reportedly ineffective or even absent. Both diseases induce yellowing but have distinct etiologies; while VY is caused by aphid-transmitted RNA viruses, SBR is caused by the cixiid-transmitted γ-proteobacterium Candidatus Arsenophonus phytopathogenicus. To clarify the situation, samples from diseased plants across the country were screened for the causal agents of VY and SBR at the end of the season. Beet yellows virus (BYV) and Beet chlorosis virus (BChV) showed high incidence nationwide, and were frequently found together in SBR-infected fields in the West. Beet mild yellowing virus (BMYV) was detected in two sites in the West, while there was no detection of Beet western yellows virus or Beet mosaic virus. The nucleotide diversity of the detected viruses was then investigated using classic and high-throughput sequencing. For both diseases, outbreaks were analyzed in light of monitoring of the respective vectors, and symptoms were reproduced in greenhouse conditions by means of insect-mediated inoculations. Novel quantification tools were designed for BYV, BChV and Ca. A. phytopathogenicus, leading to the identification of specific tissues tropism for these pathogens.

Efficiency and Persistence of Movento® Treatment against Myzus persicae and the Transmission of Aphid-Borne Viruses

Neonicotinoids are widely used to protect fields against aphid-borne viral diseases. The recent ban of these chemical compounds in the European Union has strongly impacted rapeseed and sugar beet growing practices. The poor sustainability of other insecticide families and the low efficiency of prophylactic methods to control aphid populations and pathogen introduction strengthen the need to characterize the efficiency of new plant protection products targeting aphids. In this study, the impact of Movento^® (Bayer S.A.S., Leverkusen, Germany), a tetrameric acid derivative of spirotetramat, on Myzus persicae and on viral transmission was analyzed under different growing temperatures. The results show (i) the high efficiency of Movento^® to protect rapeseed and sugar beet plants against the establishment of aphid colonies, (ii) the impact of temperature on the persistence of the Movento^® aphicid properties and (iii) a decrease of approximately 10% of the viral transmission on treated plants. These observations suggest a beneficial effect of Movento^® on the sanitary quality of treated crops by directly reducing primary infections and indirectly altering, through aphid mortality, secondary infections on which the spread of disease within field depends. These data constitute important elements for the future development of management strategies to protect crops against aphid-transmitted viruses.

Production of a Beet chlorosis virus full-length cDNA clone by means of Gibson assembly and analysis of biological properties

Beet chlorosis virus (genus Polerovirus, family Luteoviridae), which is persistently transmitted by the aphid Myzus persicae, is part of virus yellows in sugar beet and causes interveinal yellowing as well as significant yield loss in Beta vulgaris. To allow reverse genetic studies and replace vector transmission, an infectious cDNA clone under cauliflower mosaic virus 35S control in a binary vector for agrobacterium-mediated infection was constructed using Gibson assembly. Following agroinoculation, the BChV full-length clone was able to induce a systemic infection of the cultivated B. vulgaris. The engineered virus was successfully aphid-transmitted when acquired from infected B. vulgaris and displayed the same host plant spectrum as wild-type virus. This new polerovirus infectious clone is a valuable tool to identify the viral determinants involved in host range and study BChV protein function, and can be used to screen sugar beet for BChV resistance.

Divergence of host range and biological properties between natural isolate and full-length infectious cDNA clone of the Beet mild yellowing virus 2ITB

Plant infection by poleroviruses is restricted to phloem tissues, preventing any classical leaf rub inoculation with viral RNA or virions. Efficient virus inoculation to plants is achieved by viruliferous aphids that acquire the virus by feeding on infected plants. The use of promoter-driven infectious cDNA is an alternative means to infect plants and allows reverse genetic studies to be performed. Using Beet mild yellowing virus isolate 2ITB (BMYV-2ITB), we produced a full-length infectious cDNA clone of the virus (named BMYV-EK) placed under the control of the T7 RNA polymerase and the Cauliflower mosaic virus 35S promoters. Infectivity of the engineered BMYV-EK virus was assayed in different plant species and compared with that of the original virus. We showed that in vitro- or in planta-derived transcripts were infectious in protoplasts and in whole plants. Importantly, the natural aphid vector Myzus persicae efficiently transmitted the viral progeny produced in infected plants. By comparing agroinoculation and aphid infection in a host range assay, we showed that the engineered BMYV-EK virus displayed a similar host range to BMYV-2ITB, except for Nicotiana benthamiana, which proved to be resistant to systemic infection with BMYV-EK. Finally, both the BMYV-EK P0 and the full-length clone were able to strongly interfere with post-transcriptional gene silencing.

Genetic diversity and potential vectors and reservoirs of Cucurbit aphid-borne yellows virus in southeastern Spain

The genetic variability of a Cucurbit aphid-borne yellows virus (CABYV) (genus Polerovirus, family Luteoviridae) population was evaluated by determining the nucleotide sequences of two genomic regions of CABYV isolates collected in open-field melon and squash crops during three consecutive years in Murcia (southeastern Spain). A phylogenetic analysis showed the existence of two major clades. The sequences did not cluster according to host, year, or locality of collection, and nucleotide similarities among isolates were 97 to 100 and 94 to 97% within and between clades, respectively. The ratio of nonsynonymous to synonymous nucleotide substitutions reflected that all open reading frames have been under purifying selection. Estimates of the population's genetic diversity were of the same magnitude as those previously reported for other plant virus populations sampled at larger spatial and temporal scales, suggesting either the presence of CABYV in the surveyed area long before it was first described, multiple introductions, or a particularly rapid diversification. We also determined the full-length sequences of three isolates, identifying the occurrence and location of recombination events along the CABYV genome. Furthermore, our field surveys indicated that Aphis gossypii was the major vector species of CABYV and the most abundant aphid species colonizing melon fields in the Murcia (Spain) region. Our surveys also suggested the importance of the weed species Ecballium elaterium as an alternative host and potential virus reservoir.

P0 proteins of European beet-infecting poleroviruses display variable RNA silencing suppression activity

Post-transcriptional gene silencing (PTGS), or RNA silencing, is one of the key mechanisms of antiviral defence used by plants. To counter this defence response, viruses produce suppressor proteins that are able to inhibit the PTGS pathway or to interfere with some of its function. The aim of this study was to evaluate the RNA silencing suppressor (RSS) activity of P0 proteins from selected European isolates of the beet-infecting poleroviruses beet chlorosis virus (BChV) and beet mild yellowing virus (BMYV) using two different experimental systems: (i) agro-infiltration of Nicotiana benthamiana green fluorescent protein-positive plants and (ii) mechanical inoculation of Chenopodium quinoa using a beet necrotic yellow vein virus (BNYVV, genus Benyvirus) RNA3-based replicon. The results demonstrated that P0 of most BMYV isolates exhibited RSS activity, although at various efficiencies among isolates. Conversely, P0 of BChV isolates displayed no RSS activity in either of the two systems under the experimental conditions used. These results are the first reported evidence that P0 proteins of two closely related beet poleroviruses show strain-specific differences in their effects on RNA silencing.

Molecular characterization of two Chinese isolates of Beet western yellows virus infecting sugar beet

Beet western yellows virus (BWYV) has previously been reported as an agent of sugar beet yellowing disease in China. In this article, the complete genomic RNA sequences of two Chinese BWYV isolates infecting beet from Inner Mongolia (BWYV-IM) and Gansu (BWYV-GS) were determined and compared with three beet poleroviruses (BMYV, BChV and BWYV-US) and other non-beet-infecting poleroviruses. The genomes of the two isolates were 5,668 nt in length, and had almost the same genomic organization and characteristics as BWYV-US. The full length of BWYV-IM shared nucleotide sequence identities of 97.4, 86.6, 64.4 and 70.8% with BWYV-GS, BWYV-US, BChV and BMYV, respectively. Further sequence analysis indicated that the Chinese BWYV isolates were more closely related to BWYV-US; however, the identity of any gene product between the Chinese isolates and BWYV-US was <90%. Therefore, on the basis of genome sequence, we propose that these Chinese isolates are a distinct strain of BWYV that infect sugar beet. In addition, recombinant detection analysis revealed that BWYV-IM might be a recombinant virus.

Biological and Molecular Characterization of an American Sugar Beet-Infecting Beet western yellows virus Isolate

Three aphid-transmitted viruses belonging to the Polerovirus genus, Beet mild yellowing virus (BMYV), Beet chlorosis virus (BChV), and Beet western yellows virus (BWYV), have been described as pathogens of sugar beet. We present the complete biological, serological, and molecular characterization of an American isolate of Beet western yellows virus (BWYV-USA), collected from yellow beet leaves. The biological data suggested that BWYV-USA displayed a host range similar to that of BMYV, but distinct from those of BChV and the lettuce and rape isolates of Turnip yellows virus. The complete genomic RNA sequence of BWYV-USA showed a genetic organization and expression typical of other Polerovirus members. Comparisons of deduced amino acid sequences showed that P0 and the putative replicase complex (P1-P2) of BWYV-USA are more closely related to Cucurbit aphid-borne yellows virus (CABYV) than to BMYV, whereas alignments of P3, P4, and P5 showed the highest homology with BMYV. Intraspecific and interspecific phylogenetic analyses have suggested that the BWYV-USA genome may be the result of recombination events between a CABYV-like ancestor contributing open reading frame (ORF) 0, ORF 1, and ORF 2, and a beet Polerovirus progenitor providing the 3' ORFs, with a similar mechanism of speciation occurring for BMYV in Europe. Results demonstrate that BWYV-USA is a distinct species in the Polerovirus genus, clarifying the nomenclature of this important group of viruses.

Development of a multiplex immunocapture-RT-PCR for simultaneous detection of BMYV and BChV in plants and single aphids

A multiplex immunocapture-reverse transcription-polymerase chain reaction protocol (mIC-RT-PCR) was successfully developed to improve the detection of Beet mild yellowing virus (BMYV) and Beet chlorosis virus (BChV) in plants and aphids in single and mixed infections. Viral particles from plant and aphid extracts were enriched by antibody-capture and lysed by heating to release the viral RNA. During the RT-PCR step, 5' end sequences specific to each virus were amplified and the products analysed by gel electrophoresis; the PCR products corresponding to BMYV and BChV were 440 and 348bp respectively. The test was evaluated on single aphids carrying BMYV, BChV or both viruses and the results demonstrated that the mIC-RT-PCR is specific and sensitive. Its sensitivity was found to be 10(5) times higher than the TAS-ELISA routinely used for detecting BMYV and BChV and 10(4) times better than RT-PCR when both viruses were present. Eliminating the antibody-capture step to simplify the technique did not affect the sensitivity of the test and a procedure using microtitre plates was developed to allow simultaneous processing of large numbers of samples.

Yield-limiting potential of Beet western yellows virus in Brassica napus

Losses in seed yield and quality caused by infection with Beet western yellows virus (BWYV) alone or in combination with direct feeding damage by Myzus persicae (green peach aphid) were quantified in field experiments with Brassica napus (canola, oilseed rape) in the ‘grainbelt’ region of south-western Australia. Plants infected with BWYV and infested with M. persicae were introduced into plots early to provide infection sources and spread BWYV to B. napus plants. Insecticides were applied as seed dressings and/or foliar applications to generate a wide range of BWYV incidences in plots. Colonisation by vector aphids and spread of BWYV infection were recorded in the plots of the different treatments. At sites A (Medina) and B (Badgingarra) in 2001, foliar insecticide applications were applied differentially at first, but, later, ‘blanket’ insecticide sprays were applied to all plots to exclude any direct feeding damage by aphids. When BWYV infection at sites A and B reached 96% and 100% of plants, it decreased seed yield by up to 46% and 37%, respectively. Also, variation in BWYV incidence explained 95% (site A) and 96% (site B) of the variation in yield gaps, where for each 1% increase in virus incidence there was a yield decrease of 12 (site A) and 6 (site B) kg/ha. At both sites, this yield decline was entirely because fewer seeds formed on infected plants. At site B, BWYV infection significantly diminished oil content of seeds (up to 3%), but significantly increased individual seed weight (up to 11%) and erucic acid content (up to 44%); significant increases in seed protein content (up to 6–11%) were recorded at both sites. In field experiments at sites B and C (Avondale) in 2002, insecticides were applied as seed dressings or foliar sprays. At site B, when BWYV incidence reached 98%, the overall yield loss caused by BWYV and direct M. persicae feeding damage combined was 50%. At site C, when BWYV incidence reached 97%, the overall combined yield decline caused by BWYV and direct feeding damage was 46%. This research under Australian conditions shows that, when aphids spread it to B. napus plantings such that many plants become infected at an early growth stage, BWYV has substantial yield-limiting potential in B. napus crops. Although the results represent a worst case scenario, the losses were greater than those reported previously in Europe and are cause for concern for the Australian B. napus industry. When applied at 525 g a.i./100 kg of seed, imidacloprid seed dressing controlled insecticide-resistant M. persicae and effectively suppressed spread of BWYV for 2.5 months and increased seed yield by 84% at site B and 88% at site C. Therefore, provided that mixing the insecticide with seed is sufficiently thorough, dressing seed with imidacloprid before sowing provides good prospects for control of BWYV and M. persicae in B. napus crops.

Production of a full-length infectious GFP-tagged cDNA clone of Beet mild yellowing virus for the study of plant–polerovirus interactions

The full-length cDNA of Beet mild yellowing virus (Broom's Barn isolate) was sequenced and cloned into the vector pLitmus 29 (pBMYV-BBfl). The sequence of BMYV-BBfl (5721 bases) shared 96% and 98% nucleotide identity with the other complete sequences of BMYV (BMYV-2ITB, France and BMYV-IPP, Germany respectively). Full-length capped RNA transcripts of pBMYV-BBfl were synthesised and found to be biologically active in Arabidopsis thaliana protoplasts following electroporation or PEG inoculation when the protoplasts were subsequently analysed using serological and molecular methods. The BMYV sequence was modified by inserting DNA that encoded the jellyfish green fluorescent protein (GFP) into the P5 gene close to its 3' end. A. thaliana protoplasts electroporated with these RNA transcripts were biologically active and up to 2% of transfected protoplasts showed GFP-specific fluorescence. The exploitation of these cDNA clones for the study of the biology of beet poleroviruses is discussed.

Chickpea chlorotic stunt virus: A New Polerovirus Infecting Cool-Season Food Legumes in Ethiopia

ABSTRACT Serological analysis of diseased chickpea and faba bean plantings with yellowing and stunting symptoms suggested the occurrence of an unknown or uncommon member of the family Luteoviridae in Ethiopia. Degenerate primers were used for reverse transcriptase-polymerase chain reaction amplification of the viral coat protein (CP) coding region from both chickpea and faba bean samples. Cloning and sequencing of the amplicons yielded nearly identical (96%) nucleotide sequences of a previously unrecognized species of the family Luteoviridae, with a CP amino acid sequence most closely related (identity of approximately 78%) to that of Groundnut rosette assistor virus. The complete genome (5,900 nts) of a faba bean isolate comprised six major open reading frames characteristic of polero-viruses. Of the four aphid species tested, only Aphis craccivora transmitted the virus in a persistent manner. The host range of the virus was confined to a few species of the family Fabaceae. A rabbit antiserum raised against virion preparations cross-reacted unexpectedly with Beet western yellows virus-like viruses. This necessitated the production of murine monoclonal antibodies which, in combination with the polyclonal antiserum, permitted both sensitive and specific detection of the virus in field samples by triple-antibody sandwich, enzyme-linked immunosorbent assay. Because of the characteristic field and greenhouse symptoms in chickpea, the name Chickpea chlorotic stunt virus is proposed for this new member of the genus Polerovirus (family Luteoviridae).

Biological properties of Beet mild yellowing virus derived from a full-length cDNA clone

A German isolate of Beet mild yellowing virus (BMYV-IPP) was used for RT-PCR-based construction of the first infectious full-length cDNA clone of the virus (BMYV(fl)). The complete genomic sequence was determined and displayed high similarity to the French isolate BMYV-2ITB. The host range of BMYV(fl) was examined by agroinoculation and aphid transmission. Both methods lead to systemic infections in Beta vulgaris, Nicotiana benthamiana, N. clevelandii, N. hesperis, Capsella bursa-pastoris and Lamium purpureum. Immunological investigation by tissue-print immunoassay (TPIA) of agroinoculated plant tissues revealed only local infections restricted to the agroinoculated mesophyll tissues in some plant species. In Nicotiana glutinosa and N. edwardsonii, BMYV was not found in either the agroinoculated tissue or distant tissues by TPIA. So far, BMYV(fl) agroinoculation did not extend or confine the BMYV host range known from aphid transmission experiments but it did describe new local hosts for BMYV.

Beet poleroviruses: close friends or distant relatives?

SUMMARY Taxonomy: There are three members of the genus Polerovirus (family Luteoviridae) that induce yellowing of sugar beet: Beet mild yellowing virus (BMYV), Beet chlorosis virus (BChV) and Beet western yellows virus-USA (BWYV-USA, Fig. 1). Non-beet-infecting isolates of BWYV found particularly within Europe have now been re-named Turnip yellows virus (TuYV). Species-specific antibodies are unavailable, but the viruses can be distinguished by RT-PCR using primers specifically designed to the 5' end of their respective genomes. Physical properties: The isometric virus particles are approximately 26 nm in diameter and the genome consists of a single strand of positive sense RNA that utilizes almost all known plant virus gene expression strategies (initiation bypass, translational frameshifting and readthrough, synthesis of subgenomic RNA and proteolytic processing).

HOST RANGE

Many members of the Chenopodiaceae are susceptible, including commercial crops of sugar beet (Beta vulgaris), red beet and spinach. Experimental hosts include Montia perfoliata, Nicotiana benthamiana and Arabidopsis thaliana.

SYMPTOMS

Sugar beet infected with beet poleroviruses show patches of chlorosis on the older leaves 4-6 weeks post-infection; these areas expand until the whole leaf becomes yellow and older leaves then tend to thicken and become brittle.

TRANSMISSION

Beet poleroviruses are transmitted in a persistent (circulative, non-propagative) manner by several different aphid species, Myzus persicae being the most important vector.