Identification of amino acids of the beet necrotic yellow vein virus p25 protein required for induction of the resistance response in leaves of Beta vulgaris plants

A single viral amino acid shapes the root system architecture of a plant host upon virus infection

BACKGROUND

Grapevine fanleaf virus (GFLV) is one of the most detrimental viral pathogens of grapevines worldwide but no information is available on its effect on the root system architecture (RSA) of plant hosts. We used two wildtype GFLV strains and their single amino acid mutants to assess RSA traits in infected Nicotiana benthamiana and evaluate transcriptomic changes in host root gene expression in replicated time course 3'RNA-Seq experiments. Mutations targeted the multi-functional GFLV-encoded protein 1EPol*/Sd, a putative RNA-dependent RNA polymerase and determinant of foliar symptoms in N. benthamiana plants.

RESULTS

Plant infection with wildtype GFLV strain GHu and mutant GFLV strain F13 1EPol G802K, both carrying a lysine in position 802 of protein 1EPol*/Sd, resulted in a significantly lower number of root tips (-30%), and a significantly increased average root diameter (+ 20%) at 17 days post inoculation (dpi) in comparison with roots of mock inoculated plants. In contrast, the RSA of plants infected with wildtype GFLV strain F13 and mutant GFLV strain GHu 1EPol K802G, both carrying a glycine in position 802 of protein 1EPol*/Sd, resembled that of mock inoculated plants. Modifications of RSA traits were not associated with GFLV titer. Root tissue transcriptome analysis at 17 dpi indicated dysregulation of pattern recognition receptors, plant hormones, RNA silencing, and genes related to the production of reactive oxygen species (ROS). For wildtype GFLV strain GHu, RSA modifications were correlated with an abundant accumulation of ROS in the pericycle of primary roots at 7 dpi and the duration of vein clearing symptom expression in apical leaves. Dysegulation of a hypersensitive response was an overarching gene ontology found through enrichment analyses of 3'RNA-Seq data.

CONCLUSIONS

Our findings revealed the causative role of lysine in position 802 of protein 1EPol*/Sd in a novel RSA phenotype during viral infection and documented GFLV-N. benthamiana interactions at the root level based on (i) antiviral response, (ii) receptor mediated production of ROS, and (iii) hormone regulation. A correlation between above and below ground symptoms was reported for the first time in plants infected with wildtype GFLV strain GHu. Further work is warranted to test whether the modified RSA of a plant host might impact GFLV acquisition and transmission by the ectoparasitic dagger nematode Xiphinema index.

Analysis of the relative frequencies of the multipartite BNYVV genomic RNAs in different plants and tissues

Multipartite virus genomes are composed of two or more segments, each packaged into an independent viral particle. A potential advantage of multipartitism is the regulation of gene expression through changes in the segment copy number. Soil-borne beet necrotic yellow vein virus (BNYVV) is a typical example of multipartism, given its high number of genomic positive-sense RNAs (up to five). Here we analyse the relative frequencies of the four genomic RNAs of BNYVV type B during infection of different host plants (Chenopodium quinoa, Beta macrocarpa and Spinacia oleracea) and organs (leaves and roots). By successfully validating a two-step reverse-transcriptase digital droplet PCR protocol, we show that RNA1 and -2 genomic segments always replicate at low and comparable relative frequencies. In contrast, RNA3 and -4 accumulate with variable relative frequencies, resulting in distinct RNA1 : RNA2 : RNA3 : RNA4 ratios, depending on the infected host species and organ.

The Beta vulgaris-derived resistance gene Rz2 confers broad-spectrum resistance against soilborne sugar beet-infecting viruses from different families by recognizing triple gene block protein 1

Sugar beet cultivation is dependent on an effective control of beet necrotic yellow vein virus (BNYVV, family Benyviridae), which causes tremendous economic losses in sugar production. As the virus is transmitted by a soilborne protist, the use of resistant cultivars is currently the only way to control the disease. The Rz2 gene product belongs to a family of proteins conferring resistance towards diverse pathogens in plants. These proteins contain coiled-coil and leucine-rich repeat domains. After artificial inoculation of homozygous Rz2 resistant sugar beet lines, BNYVV and beet soilborne mosaic virus (BSBMV, family Benyviridae) were not detected. Analysis of the expression of Rz2 in naturally infected plants indicated constitutive expression in the root system. In a transient assay, coexpression of Rz2 and the individual BNYVV-encoded proteins revealed that only the combination of Rz2 and triple gene block protein 1 (TGB1) resulted in a hypersensitive reaction (HR)-like response. Furthermore, HR was also triggered by the TGB1 homologues from BSBMV as well as from the more distantly related beet soilborne virus (family Virgaviridae). This is the first report of an R gene providing resistance across different plant virus families.

Pest categorisation of beet necrotic yellow vein virus

Following a request from the EU Commission, the Panel on Plant Health performed a categorisation of beet necrotic yellow vein virus (BNYVV), the causal agent of the sugar beet rhizomania disease. The virus is currently listed in Annex III as a protected zone (PZ) quarantine pest of the Commission Implementing Regulation (EU) 2019/2072. The identity of the BNYVV is well established. BNYVV is a soil-borne virus transmitted by the obligate root plasmodiophorid endoparasite Polymyxa betae. BNYVV is widely distributed in the EU, but is not reported in the following EU PZs: Ireland, France (Brittany), Portugal (Azores), Finland and Northern Ireland. The virus may enter, become established and spread in the PZs via P. betae resting spores with soil and growing media as such or attached to machinery and with roots and tubercles of species other than B. vulgaris and with plants for planting. Introduction of BNYVV would have a negative impact on sugar beet and other beet crops in PZs, because of yield and sugar content reduction. Phytosanitary measures are available to reduce the likelihood of entry and spread in the PZs. Once the virus and its plasmodiophorid vector have entered a PZ, their eradication would be difficult due to the persistence of viruliferous resting spores in the soil. The main knowledge gaps or uncertainties identified concerning the presence of BNYVV in the PZs and the incidence and distribution of BNYVV in Switzerland, a country to which a range of specific requirements do not apply. BNYVV meets all the criteria that are within the remit of EFSA to qualify as a potential protected zone union quarantine pest. Plants for planting are not considered as a main means of spread, and therefore BNYVV does not satisfy all the criteria evaluated by EFSA to qualify as potential Union regulated non-quarantine pest.

Application of a Reverse Genetic System for Beet Necrotic Yellow Vein Virus to Study Rz1 Resistance Response in Sugar Beet

Beet necrotic yellow vein virus (BNYVV) is causal agent of rhizomania disease, which is the most devastating viral disease in sugar beet production leading to a dramatic reduction in beet yield and sugar content. The virus is transmitted by the ubiquitous distributed soil-borne plasmodiophoromycete Polymyxa betae that infects the root tissue of young sugar beet plants. Rz1 is the major resistance gene widely used in most sugar beet varieties to control BNYVV. The strong selection pressure on the virus population promoted the development of strains that can overcome Rz1 resistance. Resistance-breaking has been associated with mutations in the RNA3-encoded pathogenicity factor P25 at amino acid positions 67-70 (tetrad) as well as with the presence of an additional RNA component (RNA5). However, respective studies investigating the resistance-breaking mechanism by a reverse genetic system are rather scarce. Therefore, we studied Rz1 resistance-breaking in sugar beet using a recently developed infectious clone of BNYVV A-type. A vector free infection system for the inoculation of young sugar beet seedlings was established. This assay allowed a clear separation between a susceptible and a Rz1 resistant genotype by measuring the virus content in lateral roots at 52 dpi. However, mechanical inoculation of sugar beet leaves led to the occurrence of genotype independent local lesions, suggesting that Rz1 mediates a root specific resistance toward BNYVV that is not active in leaves. Mutation analysis demonstrated that different motifs within the P25 tetrad enable increased virus replication in roots of the resistant genotype. The resistance-breaking ability was further confirmed by the visualization of BNYVV in lateral roots and leaves using a fluorescent-labeled complementary DNA clone of RNA2. Apart from that, reassortment experiments evidenced that RNA5 enables Rz1 resistance-breaking independent of the P25 tetrad motif. Finally, we could identify a new resistance-breaking mutation, which was selected by high-throughput sequencing of a clonal virus population after one host passage in a resistant genotype. Our results demonstrate the feasibility of the reverse genetic system for resistance-breaking analysis and illustrates the genome plasticity of BNYVV allowing the virus to adapt rapidly to sugar beet resistance traits.

Fluorescent labelling of Beet necrotic yellow vein virus and Beet soil-borne mosaic virus for co- and superinfection experiments in Nicotiana benthamiana

Infectious full-length clones of Beet necrotic yellow vein virus (BNYVV) and Beet soil-borne mosaic virus (BSBMV), both genus Benyvirus, were used for fluorescent labelling with the objective to study their interaction in coinfection and superinfection experiments. Fluorescent labelling was achieved by replacing a part of the RNA2 encoded coat protein read-through domain with either GFP or mRFP fluorescent marker proteins. This resulted in a translational fusion comprising the coat and the fluorescent protein. The labelled viruses were infectious and moved systemically in Nicotiana benthamiana, producing wild-type-like symptoms. Virus particles could be observed by electron microscopy, demonstrating that the viral read-through domain is dispensable for particle formation. Coinfection experiments revealed a spatial separation of differentially labelled populations of both identical and different Benyvirus species after N. benthamiana agro-inoculation. Identical observations were obtained when Tobacco rattle virus (TRV) was differentially labelled and used for coinfection. In contrast, coinfections of BSBMV with Potato virus X (PVX) or TRV resulted in many co-infected cells lacking spatial separation. Micro-projectile co-bombardment of N. benthamiana leaves revealed that two differently labelled populations of the same virus co-infected only a few cells before starting to separate. In superinfection experiments with N. benthamiana, BSBMV and BNYVV were unable to establish a secondary infection in plants that were previously infected with BNYVV or BSBMV. Taken together, this is the first work to describe the interaction between two economically important Benyviruses using fluorescence-labelled full-length clones.

Long Term Management of Rhizomania Disease-Insight Into the Changes of the Beet necrotic yellow vein virus RNA-3 Observed Under Resistant and Non-resistant Sugar Beet Fields

Rhizomania disease, caused by the Beet necrotic yellow vein virus (BNYVV), is considered as one of the major constraints for sugar beet production, worldwide. As a result of the introgression of major resistance genes (Holly, Rz2) in commercially available sugar beet varieties, the virus has endured strong selection pressure since the 90s'. Understanding the virus response and diversity to sugar beet resistance is a key factor for a sustainable management of only few resistance genes. Here we report rhizomania surveys conducted in a rhizomania hot spot, the Pithiviers area (France) during a 4-year period and complementary to the study of Schirmer et al. (2005). The study aimed at evaluating the intra- and inter-field BNYVV diversity in response to different sources of resistance and over the growing season. To follow rhizomania development over the sugar beet growing season, extensive field samplings combined with field assays were performed in this study. The evolution of the BNYVV diversity was assessed at intra- and inter-field levels, with sugar beet cultivars containing different resistance genes (Rz1, Rz1 + Heterodera schachtii resistance and Rz1Rz2). Intra-field diversity was analyzed at the beginning and the end of the growing season of each field. From more than one thousand field samples, the simultaneous presence of the different A, B and P types of BNYVV was confirmed, with 21 variants identified at positions 67-70 of the p25 tetrad. The first variant, AYHR, was found most commonly followed by SYHG. Numerous mixed infections (9.93% of the samples), mostly of B-type with P-type, have also been evidenced. Different tetrads associated with the A- or B-type were also found with a fifth RNA-genome component known to allow more aggressiveness to BNYVV on sugar beet roots. Cultivars with Rz1+Rz2 resistant genes showed few root symptoms even if the BNYVV titre was quite high according to the BNYVV type present. The virus infectious potential in the soil at the end of the growing season with such cultivars was also lower despite a wider diversity at the BNYVV RNA3 sequence level. Rz1+Rz2 cultivars also exhibited a lower presence of Beet soil-borne virus (BSBV), a P. betae-transmitted Pomovirus. Cultivars with Rz1 and nematode (N) resistance genes cultivated in field infected with nematodes showed lower BNYVV titre than those with Rz1 or Rz1+Rz2 cultivars. Overall, the population structure of BNYVV in France is shown to be different from that previously evidenced in different world areas. Implications for long-term management of the resistance to rhizomania is discussed.

Biological properties of Beet soil-borne mosaic virus and Beet necrotic yellow vein virus cDNA clones produced by isothermal in vitro recombination: Insights for reassortant appearance

Two members of the Benyviridae family and genus Benyvirus, Beet soil-borne mosaic virus (BSBMV) and Beet necrotic yellow vein virus (BNYVV), possess identical genome organization, host range and high sequence similarity; they infect Beta vulgaris with variable symptom expression. In the US, mixed infections are described with limited information about viral interactions. Vectors suitable for agroinoculation of all genome components of both viruses were constructed by isothermal in vitro recombination. All 35S promoter-driven cDNA clones allowed production of recombinant viruses competent for Nicotiana benthamiana and Beta macrocarpa systemic infection and Polymyxa betae transmission and were compared to available BNYVV B-type clone. BNYVV and BSBMV RNA1 + 2 reassortants were viable and spread long-distance in N. benthamiana with symptoms dependent on the BNYVV type. Small genomic RNAs were exchangeable and systemically infected B. macrocarpa. These infectious clones represent a powerful tool for the identification of specific molecular host-pathogen determinants.

Genetic diversity and population structure of beet necrotic yellow vein virus in China

Beet necrotic yellow vein virus (BNYVV) is a serious threat to the sugar beet industry worldwide. However, little information is available regarding the genetic diversity and population structure of BNYVV in China. Here, we analyzed multiple sequences from four genomic regions (CP, RNA3, RNA4 and RNA5) of a set of Chinese isolates. Sequence analyses revealed that several isolates were mixed infections of variants with different genotypes and/or different p25 tetrad motifs. In total, 12 distinct p25 tetrads were found in the Chinese BNYVV population, of which four tetrads were newly identified. Phylogenetic analyses based on four genes (CP, RNA3-p25, RNA4-p31 and RNA5-p26) in isolates from around the world revealed the existence of two to four groups, which mostly corresponded to previously reported phylogenetic groups. Two new subgroups and a new group were identified from the Chinese isolates in p25 and p26 trees, respectively. Selection pressure analysis indicated that there was a positive selection pressure on the p25 from the Chinese isolates, but the other three proteins were under a negative selection pressure. There was frequent gene flow between geographically distant populations, which meant that BNYVV populations from different provinces were not geographically differentiated.

Infection of Beet necrotic yellow vein virus with RNA4-encoded P31 specifically up-regulates pathogenesis-related protein 10 in Nicotiana benthamiana

BACKGROUND

Beet necrotic yellow vein virus (BNYVV) is the infectious agent of sugar beet rhizomania, which consists of four or five plus-sense RNAs. RNA4 of BNYVV is not essential for virus propagation in Nicotiana benthamiana but has a major effect on symptom expression. Early reports showed that RNA4-encoded P31 was associated with severe symptoms, such as curling and dwarfing, in N. benthamiana.

RESULTS

We discovered that the pathogenesis-related protein 10 (PR-10) gene can be up-regulated in BNYVV-infected N. benthamiana in the presence of RNA4 and that it had a close link with symptom development. Our frame-shift, deletion and substitution analysis showed that only the entire P31 could induce PR-10 up-regulation during BNYVV infection and that all the tryptophans and six cysteines (C174, C183, C186, C190, C197 and C199) in the cysteine-rich P31 had significant effects on PR-10 expression. However, P31 could not interact directly with PR-10 in yeast.

CONCLUSIONS

Our data demonstrated that only integrated P31 specifically induced PR-10 transcription, which coincided closely with the appearance of severe symptoms in BNYVV-infected N. benthamiana, although they could not interact directly with each other in yeast.

Effect of sugar beet genotype on the Beet necrotic yellow vein virus P25 pathogenicity factor and evidence for a fitness penalty in resistance-breaking strains

Beet necrotic yellow vein virus (BNYVV), vectored by Polymyxa betae, causes rhizomania in sugar beet. For disease control, the cultivation of hybrids carrying Rz1 resistance is crucial, but is compromised by resistance-breaking (RB) strains with specific mutations in the P25 protein at amino acids 67-70 (tetrad). To obtain evidence for P25 variability from soil-borne populations, where the virus persists for decades, populations with wild-type (WT) and RB properties were analysed by P25 deep sequencing. The level of P25 variation in the populations analysed did not correlate with RB properties. Remarkably, one WT population contained P25 with RB mutations at a frequency of 11%. To demonstrate selection by Rz1 and the influence of RB mutations on relative fitness, competition experiments between strains were performed. Following a mixture of strains with four RNAs, a shift in tetrad variants was observed, suggesting that strains did not mix or transreplicate. The plant genotype exerted a clear influence on the frequency of RB tetrads. In Rz1 plants, the RB variants outcompeted the WT variants, and mostly vice versa in susceptible plants, demonstrating a relative fitness penalty of RB mutations. The strong genotype effect supports the hypothesized Rz1 RB strain selection with four RNAs, suggesting that a certain tetrad needs to become dominant in a population to influence its properties. Tetrad selection was not observed when an RB strain, with an additional P26 protein encoded by a fifth RNA, competed with a WT strain, supporting its role as a second BNYVV pathogenicity factor and suggesting the reassortment of both types.

Ribosomal DNA analyses reveal greater sequence variation in Polymyxa species than previously thought and indicate the possibility of new ribotype-host-virus associations

Polymyxa species transmit viruses to many important crops. They are poorly understood obligate parasites occupying a distinct position in the Tree of Life. To better understand the potential for spread of Polymyxa-vectored diseases, ribosomal DNA was analysed from isolates covering a wide range of geographical locations, virus associations and hosts. Internal transcribed spacer 2 structure analysis indicated that Polymyxa graminis isolates could represent many species and there was more sequence variation within the known subgroups (ribotypes) than previously described. In cereal crops and soils from temperate climates Polymyxa isolates were usually ribotype I or II, but their host specificities or preferences were unclear. For the first time, there was evidence that ribotype I (in addition to ribotype II) could transmit SBWMV/SBCMV. Different ribotypes often occurred together in the same soil or plant. New hosts were identified for particular ribotypes, including the first detection of the sugar beet-infecting Polymyxa betae, in wheat. Unexpectedly, ribotype III-like sequences, usually restricted to crops in the tropics, were found in wheat from the USA. P. betae isolates showed limited variation (≤ 2%) and the recent change in susceptibility of sugar beet varieties to BNYVV in the USA is unlikely to be due to changes in P. betae.

High level resistance against rhizomania disease by simultaneously integrating two distinct defense mechanisms

With the aim of achieving durable resistance against rhizomania disease of sugar beet, the employment of different sources of resistance to Beet necrotic yellow vein virus was pursued. To this purpose, Nicotiana benthamiana transgenic plants that simultaneously produce dsRNA originating from a conserved region of the BNYVV replicase gene and the HrpZ(Psph) protein in a secreted form (SP/HrpZ(Psph)) were produced. The integration and expression of both transgenes as well as proper production of the harpin protein were verified in all primary transformants and selfed progeny (T1, T2). Transgenic resistance was assessed by BNYVV-challenge inoculation on T2 progeny by scoring disease symptoms and DAS-ELISA at 20 and 30 dpi. Transgenic lines possessing single transformation events for both transgenes as well as wild type plants were included in inoculation experiments. Transgenic plants were highly resistant to virus infection, whereas in some cases immunity was achieved. In all cases, the resistant phenotype of transgenic plants carrying both transgenes was superior in comparison with the ones carrying a single transgene. Collectively, our findings demonstrate, for a first time, that the combination of two entirely different resistance mechanisms provide high level resistance or even immunity against the virus. Such a novel approach is anticipated to prevent a rapid virus adaptation that could potentially lead to the emergence of isolates with resistance breaking properties.

The P25 pathogenicity factor of Beet necrotic yellow vein virus targets the sugar beet 26S proteasome involved in the induction of a hypersensitive resistance response via interaction with an F-box protein

P25, a Beet necrotic yellow vein virus (BNYVV) pathogenicity factor, interacts with a sugar beet protein with high homology to Arabidopsis thaliana kelch repeat containing F-box family proteins (FBK) of unknown function in yeast. FBK are members of the Skp1-Cullin-F-box (SCF) complex that mediate protein degradation. Here, we confirm this sugar beet FBK-P25 interaction in vivo and in vitro and provide evidence for in planta interaction and similar subcellular distribution in Nicotiana tabacum leaf cells. P25 even interacts with an FBK from A. thaliana, a BNYVV nonhost. FBK functional classification was possible by demonstrating the interaction with A. thaliana orthologs of Skp1-like (ASK) genes, a member of the SCF E3 ligase. By means of a yeast two-hybrid bridging assay, a direct effect of P25 on SCF-complex formation involving ASK1 protein was demonstrated. FBK transient Agrobacterium tumefaciens-mediated expression in N. benthamiana leaves induced a hypersensitive response. The full-length F-box protein consists of one F-box domain followed by two kelch repeats, which alone were unable to interact with P25 in yeast and did not lead to cell-death induction. The results support the idea that P25 is involved in virus pathogenicity in sugar beet and suggest suppression of resistance response.

Detection and characterization of spontaneous internal deletion mutants of Beet Necrotic yellow vein virus RNA3 from systemic host Nicotiana benthamiana

BACKGROUND

Beet Necrotic Yellow Vein virus (BNYVV) is a member of the genus Benyvirus causing a worldwide sugar beet disease rhizomania. BNYVV contains four or five plus-sense single stranded RNAs. In altered selective conditions, multipartite RNA viruses of plant are prone to undergoing internal deletions, thus turning into Defective RNAs (D RNAs). Although several D RNAs have been reported in BNYVV infection, the spontaneous internal deletion mutants responsible for severe symptom in systemic host Nicotiana benthamiana (N. benthamiana) are not described so far.

RESULTS

Systemic host N. benthamiana was inoculated by Chinese BNYVV isolates. RT-PCR and Northern blot showed that the D RNAs forms of BNYVV RNA3 were present in the systemic infection of the N. benthamiana. Three distinct D-RNA3s, named as D-RNA 3α, D-RNA 3β and D-RNA 3γ, were made into infectious clones. When inoculated on the N. benthamiana, the in vitro transcripts of D forms exhibited more stable than that of wild-type RNA3 in systemic movement. Among the detected mutant, the p25 protein frame-shift mutant (D-RNA3α) induced obvious necrotic lesions on Tetragonia.expansa (T. expansa) and pronounced systemic symptom on the N. benthamiana. The D-RNA3α was further mutated artificially to pre-terminate the downstream N protein, leading to the abolishment of the pathogenicity, indicating the N protein was responsible for the necrotic symptom.

CONCLUSION

Our studies demonstrated the internal deletion mutants of BNYVV-RNA3 were spontaneously generated in the systemic infection on N. benthamiana. The internal deletions didn't affect the efficient replication of D-RNA3s, instead by improving the stability and pathogenicity of RNA3 in the systemic host N. benthamiana. Besides, our results also suggested the downstream N protein of RNA3, but not the upstream p25 protein, may play an important role in the systemic infection on N. benthamiana.

The evolutionary history of Beet necrotic yellow vein virus deduced from genetic variation, geographical origin and spread, and the breaking of host resistance

Beet necrotic yellow vein virus (BNYVV) is an economically important pathogen of sugar beet and has been found worldwide, probably as the result of recent worldwide spread. The BNYVV genome consists of four or five RNA components. Here, we report analysis of sequence variation in the RNA3-p25, RNA4-p31, RNA2-CP, and RNA5-p26 genes of 73 worldwide isolates. The RNA3-p25 gene encodes virulence and avirulence factors. These four sets of gene sequences each fell into two to four groups, of which the three groups of p25 formed eight subgroups with different geographical distributions. Each of these subgroup isolates (strains) could have arisen from four original BNYVV population and their mixed infections. The genetic diversity for BNYVV was relatively small. Selection pressure varied greatly depending on the BNYVV gene and geographical location. Isolates of the Italy strain, in which p25 was subject to the strongest positive selection, were able to overcome the Rz1-host resistance gene to differing degrees, whereas other geographically limited strains could not. Resistance-breaking variants were generated by p25 amino acid changes at positions 67 and 68. Our studies suggest that BNYVV originally evolved in East Asia and has recently become a pathogen of cultivated sugar beet followed by the emergence of new resistance-breaking variants.

Constraints on evolution of virus avirulence factors predict the durability of corresponding plant resistances

SUMMARY Understanding the factors driving pathogen emergence and re-emergence is a major challenge, particularly in agriculture, where the use of resistant plant cultivars imposes strong selective pressures on plant pathogen populations and leads frequently to 'resistance breakdown'. Presently, durable resistances are only identified after a long period of large-scale cultivation of resistant cultivars. We propose a new predictor of the durability of plant resistance. Because resistance breakdown involves modifications in the avirulence factors of pathogens, we tested for correlations between the evolutionary constraints acting on avirulence factors or their diversity and the durability of the corresponding resistance genes in the case of plant-virus interactions. An analysis performed on 20 virus species-resistance gene combinations revealed that the selective constraints applied on amino acid substitutions in virus avirulence factors correlate with the observed durability of the corresponding resistance genes. On the basis of this result, a model predicting the potential durability of resistance genes as a function of the selective constraints applied on the corresponding avirulence factors is proposed to help breeders to select the most durable resistance genes.

Identification of Beet necrotic yellow vein virus P25 pathogenicity factor-interacting sugar beet proteins that represent putative virus targets or components of plant resistance

Beet necrotic yellow vein virus (BNYVV) induces the most important disease threatening sugar beet. The growth of partially resistant hybrids carrying monogenic dominant resistance genes stabilize yield but are unable to entirely prevent virus infection and replication. P25 is responsible for symptom development and previous studies have shown that recently occurring resistance-breaking isolates possess increased P25 variability. To better understand the viral pathogenicity factor's interplay with plant proteins and to possibly unravel the molecular basis of sugar beet antivirus resistance, P25 was applied in a yeast two-hybrid screen of a resistant sugar beet cDNA library. This screen identified candidate proteins recognized as orthologues from other plant species which are known to be expressed following pathogen infection and involved in plant defense response. Most of the candidates potentially related to host-pathogen interactions were involved in the ubiquitylation process and plants response to stress, and were part of cell and metabolism components. The interaction of several candidate genes with P25 was confirmed in Nicotiana benthamiana leaf cells by transient agrobacterium-mediated expression applying bimolecular fluorescence complementation assay. The putative functions of several of the candidates identified support previous findings and present first targets for understanding the BNYVV pathogenicity and antivirus resistance mechanism.

A single U/C nucleotide substitution changing alanine to valine in the beet necrotic yellow vein virus P25 protein promotes increased virus accumulation in roots of mechanically inoculated, partially resistant sugar beet seedlings

Beet necrotic yellow vein virus (BNYVV) A type isolates E12 and S8, originating from areas where resistance-breaking had or had not been observed, respectively, served as starting material for studying the influence of sequence variations in BNYVV RNA 3 on virus accumulation in partially resistant sugar beet varieties. Sub-isolates containing only RNAs 1 and 2 were obtained by serial local lesion passages; biologically active cDNA clones were prepared for RNAs 3 which differed in their coding sequences for P25 aa 67, 68 and 129. Sugar beet seedlings were mechanically inoculated with RNA 1+2/RNA 3 pseudorecombinants. The origin of RNAs 1+2 had little influence on virus accumulation in rootlets. E12 RNA 3 coding for V(67)C(68)Y(129) P25, however, enabled a much higher virus accumulation than S8 RNA 3 coding for A(67)H(68)H(129) P25. Mutants revealed that this was due only to the V(67) 'GUU' codon as opposed to the A(67) 'GCU' codon.

The first 17 amino acids of the beet necrotic yellow vein virus RNA-5-encoded p26 protein are sufficient to activate transcription in a yeast one-hybrid system

The beet necrotic yellow vein virus (BNYVV) RNA-5-encoded p26 protein is involved in the accentuation of symptoms expression of infected Chenopodium quinoa plants and is capable of transcription activation (TA) in yeast. TA was previously localized within the first 55 residues of the p26 protein. Interestingly, TA did not occur when C-terminally deleted forms of p26 were used. We used a genetic screen in the yeast one-hybrid system to select restored TA from randomly generated mutants. The TA domain was found to be located within the first 17 residues. Alanine replacement of aspartic acids 11, 16, and 17 within the full-length p26 prevented TA but did not impair subcellular localization and the symptom expression.

Progress towards the understanding and control of sugar beet rhizomania disease

Rhizomania is a soil-borne disease that occurs throughout the major sugar beet growing regions of the world, causing severe yield losses in the absence of effective control measures. It is caused by Beet necrotic yellow vein virus (BNYVV), which is transmitted by the obligate root-infecting parasite Polymyxa betae. BNYVV has a multipartite RNA genome with all natural isolates containing four RNA species, although some isolates have a fifth RNA. The larger RNA1 and RNA2 contain the housekeeping genes of the virus and are always required for infection, whereas the smaller RNAs are involved in pathogenicity and vector transmission. RNA5-containing isolates are restricted to Asia and some parts of Europe, and these isolates tend to be more aggressive. With no acceptable pesticides available to restrict the vector, the control of rhizomania is now achieved almost exclusively through the use of resistant cultivars. A single dominant resistance gene, Rz1, has been used to manage the disease worldwide in recent years, although this gene confers only partial resistance. More recently, new variants of BNYVV have evolved (both with and without RNA5) that are able to cause significant yield penalties on resistant cultivars. These isolates are not yet widespread, but their appearance has resulted in accelerated searches for new sources of resistance to both the virus and the vector. Combined virus and vector resistance, achieved either by conventional or transgenic breeding, offers the sugar beet industry a new approach in its continuing struggle against rhizomania.