Increase in Zucchini yellow mosaic virus Symptom Severity in Tolerant Zucchini Cultivars Is Related to a Point Mutation in P3 Protein and Is Associated with a Loss of Relative Fitness on Susceptible Plants

Challenges and opportunities for plant viruses under a climate change scenario

There is an increasing societal awareness on the enormous threat that climate change may pose for human, animal and plant welfare. Although direct effects due to exposure to heat, drought or elevated greenhouse gasses seem to be progressively more obvious, indirect effects remain debatable. A relevant aspect to be clarified relates to the relationship between altered environmental conditions and pathogen-induced diseases. In the particular case of plant viruses, it is still unclear whether climate change will primarily represent an opportunity for the emergence of new infections in previously uncolonized areas and hosts, or if it will mostly be a strong constrain reducing the impact of plant virus diseases and challenging the pathogen's adaptive capacity. This review focuses on current knowledge on the relationship between climate change and the outcome plant-virus interactions. We summarize work done on how this relationship modulates plant virus pathogenicity, between-host transmission (which include the triple interaction plant-virus-vector), ecology, evolution and management of the epidemics they cause. Considering these studies, we propose avenues for future research on this subject.

Specificity of Resistance and Tolerance to Cucumber Vein Yellowing Virus in Melon Accessions and Resistance Breaking with a Single Mutation in VPg

Cucumber vein yellowing virus (CVYV) is an emerging virus on cucurbits in the Mediterranean Basin, against which few resistance sources are available, particularly in melon. The melon accession PI 164323 displays complete resistance to isolate CVYV-Esp, and accession HSD 2458 presents a tolerance, i.e., very mild symptoms despite virus accumulation in inoculated plants. The resistance is controlled by a dominant allele Cvy-1¹, while the tolerance is controlled by a recessive allele cvy-2, independent from Cvy-1¹. Before introducing the resistance or tolerance in commercial cultivars through a long breeding process, it is important to estimate their specificity and durability. Upon inoculation with eight molecularly diverse CVYV isolates, the resistance was found to be isolate-specific because many CVYV isolates induced necrosis on PI 164323, whereas the tolerance presented a broader range. A resistance-breaking isolate inducing severe mosaic on PI 164323 was obtained. This isolate differed from the parental strain by a single amino acid change in the VPg coding region. An infectious CVYV cDNA clone was obtained, and the effect of the mutation in the VPg cistron on resistance to PI 164323 was confirmed by reverse genetics. This represents the first determinant for resistance-breaking in an ipomovirus. Our results indicate that the use of the Cvy-1¹ allele alone will not provide durable resistance to CVYV and that, if used in the field, it should be combined with other control methods such as cultural practices and pyramiding of resistance genes to achieve long-lasting resistance against CVYV.

Concurrent evolution of resistance and tolerance to potato virus Y in Capsicum annuum revealed by genome-wide association

We performed a genome-wide association study of pepper (Capsicum annuum) tolerance to potato virus Y (PVY). For 254 pepper accessions, we estimated the tolerance to PVY as the coefficient of regression of the fresh weight (or height) of PVY-infected and mock-inoculated plants against within-plant virus load. Small (strongly negative) coefficients of regression indicate low tolerance because plant biomass or growth decreases sharply as virus load increases. The tolerance level varied largely, with some pepper accessions showing no symptoms or fairly mild mosaics, whereas about half (48%) of the accessions showed necrotic symptoms. We found two adjacent single-nucleotide polymorphisms (SNPs) at one extremity of chromosome 9 that were significantly associated with tolerance to PVY. Similarly, in three biparental pepper progenies, we showed that the induction of necrosis on PVY systemic infection segregated as a monogenic trait determined by a locus on chromosome 9. Our results also demonstrate the existence of a negative correlation between resistance and tolerance among the cultivated pepper accessions at both the phenotypic and genetic levels. By comparing the distributions of the tolerance-associated SNP alleles and previously identified PVY resistance-associated SNP alleles, we showed that cultivated pepper accessions possess favourable alleles for both resistance and tolerance less frequently than expected under random associations, while the minority of wild pepper accessions frequently combined resistance and tolerance alleles. This divergent evolution of PVY resistance and tolerance could be related to pepper domestication or farmer's selection.

A role of flowering genes in the tolerance of Arabidopsis thaliana to cucumber mosaic virus

The genetic basis of plant tolerance to parasites is poorly understood. We have previously shown that tolerance of Arabidopsis thaliana to its pathogen cucumber mosaic virus is achieved through changes in host life-history traits on infection that result in delaying flowering and reallocating resources from vegetative growth to reproduction. In this system we analyse here genetic determinants of tolerance using a recombinant inbred line family derived from a cross of two accessions with extreme phenotypes. Three major quantitative trait loci for tolerance were identified, which co-located with three flowering repressor genes, FLC, FRI, and HUA2. The role of these genes in tolerance was further examined in genotypes carrying functional or nonfunctional alleles. Functional alleles of FLC together with FRI and/or HUA2 were required for both tolerance and resource reallocation from growth to reproduction. Analyses of FLC alleles from wild accessions that differentially modulate flowering time showed that they ranked differently for their effects on tolerance and flowering. These results pinpoint a role of FLC in A. thaliana tolerance to cucmber mosaic virus, which is a novel major finding, as FLC has not been recognized previously to be involved in plant defence. Although tolerance is associated with a delay in flowering that allows resource reallocation, our results indicate that FLC regulates tolerance and flowering initiation by different mechanisms. Thus, we open a new avenue of research on the interplay between defence and development in plants.

Host population structure for tolerance determines the evolution of plant-virus interactions

Heterogeneity for plant defences determines both the capacity of host populations to buffer the effect of infection and the pathogen´s fitness. However, little information is known on how host population structure for tolerance, a major plant defence, impacts the evolution of plant-pathogen interactions. By performing 10 serial passages of Turnip mosaic virus (TuMV) in Arabidopsis thaliana populations with varying proportion of tolerant genotypes simulating different structures for this trait, we analysed how host heterogeneity for this defence shapes the evolution of both virus multiplication, the effect of infection on plant fecundity and mortality, and plant tolerance and resistance. Results indicated that a higher proportion of tolerant genotypes in the host population promotes virus multiplication and reduces the effect of infection on plant mortality, but not on plant fecundity. These changes resulted in more effective plant tolerance to virus infection. Conversely, a lower proportion of tolerant genotypes reduced virus multiplication, boosting plant resistance. Our work for the first time provides evidence of the main role of host population structure for tolerance on pathogen evolution and on the subsequent feedback loops on plant defences.

Adaptation and Codon-Usage Preference of Apple and Pear-Infecting Apple Stem Grooving Viruses

Apple stem grooving virus (ASGV; genus Capillovirus) is an economically important virus. It has an approx. 6.5 kb, monopartite, linear, positive-sense, single-stranded RNA genome. The present study includes identification of 24 isolates—13 isolates from apple (Pyrus malus L.) and 11 isolates from pear (Pyrus communis L.)—from different agricultural fields in South Korea. The coat protein (CP) gene of the corresponding 23 isolates were amplified, sequenced, and analyzed. The CP sequences showed phylogenetic separation based on their host species, and not on the geography, indicating host adaptation. Further analysis showed that the ASGV isolated in this study followed host adaptation influenced and preferred by the host codon-usage.

Genome characterization of zucchini yellow mosaic virus infecting cucurbits reveals the presence of a new genotype in Trinidad and Tobago in the Caribbean region

Zucchini yellow mosaic virus (ZYMV) is a member of the genus Potyvirus that is becoming a serious pathogen of pumpkin and other cucurbits in Trinidad and Tobago and the entire Caribbean region. In this study, four ZYMV isolates infecting pumpkin in Trinidad and Tobago were characterized by complete genome sequencing. Phylogenetic analysis showed 5.9-6.0% nt and 7.7-7.9% aa sequence divergence in comparison to the most closely related isolates NAT and AG from Israel and SE04T from Slovakia. Based on the variations in the complete genome sequence as well as individual gene sequences, a new genotype, designated ZYMV-Trini, is proposed for these isolates. Among the gene sequences of ZYMV-Trini isolates, the greatest variation was observed in the HC-Pro gene, with 20.8% aa sequence divergence from their closest relatives, whereas the least variation was observed in the NIb, P3, and CP genes, with 1.8-2.2% aa sequence divergence. This study also showed that transmission of ZYMV can occur through seeds, but this was less common than transmission via the aphid Aphis gossypii. The progression of ZYMV in pumpkin seedlings was quantified by RT-qPCR, which showed a rapid surge in viral load after 37 days. From recombination analysis, it could be concluded that the isolates SE04T from Slovakia, NAT from Israel, and AG from Israel have made major contributions to the genome architecture of ZYMV-Trini isolates.

Association between flower stalk elongation, an Arabidopsis developmental trait, and the subcellular location and movement dynamics of the nonstructural protein P3 of Turnip mosaic virus

Virus infections affect plant developmental traits but this aspect of the interaction has not been extensively studied so far. Two strains of Turnip mosaic virus differentially affect Arabidopsis development, especially flower stalk elongation, which allowed phenotypical, cellular, and molecular characterization of the viral determinant, the P3 protein. Transiently expressed wild-type green fluorescent protein-tagged P3 proteins of both strains and selected mutants of them revealed important differences in their behaviour as endoplasmic reticulum (ER)-associated peripheral proteins flowing along the reticulum, forming punctate accumulations. Three-dimensional (3D) model structures of all expressed P3 proteins were computationally constructed through I-TASSER protein structure predictions, which were used to compute protein surfaces and map electrostatic potentials to characterize the effect of amino acid changes on features related to protein interactions and to phenotypical and subcellular results. The amino acid at position 279 was the main determinant affecting stalk development. It also determined the speed of ER-flow of the expressed proteins and their final location. A marked change in the protein surface electrostatic potential correlated with changes in subcellular location. One single amino acid in the P3 viral protein determines all the analysed differential characteristics between strains differentially affecting flower stalk development. A model proposing a role of the protein in the intracellular movement of the viral replication complex, in association with the viral 6K2 protein, is proposed. The type of association between both viral proteins could differ between the strains.

Tolerance of Plants to Pathogens: A Unifying View

Increasing evidence indicates that tolerance is a host defense strategy against pathogens as widespread and successful as resistance. Since the concept of tolerance was proposed more than a century ago, it has been in continuous evolution. In parallel, our understanding of its mechanistic bases and its consequences for host and pathogen interactions, ecology, and evolution has grown. This review aims at summarizing the conceptual changes in the meaning of tolerance inside and outside the field of phytopathology, emphasizing difficulties in demonstrating and quantifying this trait. We also discuss evidence of tolerance and current knowledge on its genetic regulation, mechanisms, and role in host-pathogen coevolution, highlighting common patterns across hosts and pathogens. We hope that this comprehensive review attracts more plant pathologists to the study of this key plant defense response.

Detection of Plant Viruses and Disease Management: Relevance of Genetic Diversity and Evolution

Plant viruses cause considerable economic losses and are a threat for sustainable agriculture. The frequent emergence of new viral diseases is mainly due to international trade, climate change, and the ability of viruses for rapid evolution. Disease control is based on two strategies: i) immunization (genetic resistance obtained by plant breeding, plant transformation, cross-protection, or others), and ii) prophylaxis to restrain virus dispersion (using quarantine, certification, removal of infected plants, control of natural vectors, or other procedures). Disease management relies strongly on a fast and accurate identification of the causal agent. For known viruses, diagnosis consists in assigning a virus infecting a plant sample to a group of viruses sharing common characteristics, which is usually referred to as species. However, the specificity of diagnosis can also reach higher taxonomic levels, as genus or family, or lower levels, as strain or variant. Diagnostic procedures must be optimized for accuracy by detecting the maximum number of members within the group (sensitivity as the true positive rate) and distinguishing them from outgroup viruses (specificity as the true negative rate). This requires information on the genetic relationships within-group and with members of other groups. The influence of the genetic diversity of virus populations in diagnosis and disease management is well documented, but information on how to integrate the genetic diversity in the detection methods is still scarce. Here we review the techniques used for plant virus diagnosis and disease control, including characteristics such as accuracy, detection level, multiplexing, quantification, portability, and designability. The effect of genetic diversity and evolution of plant viruses in the design and performance of some detection and disease control techniques are also discussed. High-throughput or next-generation sequencing provides broad-spectrum and accurate identification of viruses enabling multiplex detection, quantification, and the discovery of new viruses. Likely, this technique will be the future standard in diagnostics as its cost will be dropping and becoming more affordable.

Soybean Cytochrome b5 Is a Restriction Factor for Soybean Mosaic Virus

Soybean mosaic virus (SMV) is one of the most destructive viral diseases in soybeans (Glycine max). In this study, an interaction between the SMV P3 protein and cytochrome b5 was detected by yeast two-hybrid assay, and bimolecular fluorescence complementation assay showed that the interaction took place at the cell periphery. Further, the interaction was confirmed by co-immunoprecipitation analysis. Quantitative real-time polymerase chain reaction analysis revealed that GmCYB5 gene was differentially expressed in resistant and susceptible soybean plants after inoculation with SMV-SC15 strain. To test the involvement of this gene in SMV resistance, the GmCYB5 was silenced using a bean pod mottle virus (BPMV)-based vector construct. Results showed that GmCYB5-1 was 83% and 99% downregulated in susceptible (NN1138-2) and resistant (RN-9) cultivars, respectively, compared to the empty vector-treated plants. Silencing of GmCYB5 gene promotes SMV replication in soybean plants. Our results suggest that during SMV infection, the host CYB5 protein targets P3 protein to inhibit its proliferation. Taken together, these results suggest that CYB5 is an important factor in SMV infection and replication in soybeans, which could help soybean breeders develop SMV resistant soybean cultivars.

Tolerance to Plant Pathogens: Theory and Experimental Evidence

The two major mechanisms of plant defense against pathogens are resistance (the host's ability to limit pathogen multiplication) and tolerance (the host's ability to reduce the effect of infection on its fitness regardless of the level of pathogen multiplication). There is abundant literature on virtually every aspect of plant resistance to pathogens. Although tolerance to plant pathogens is comparatively less understood, studies on this plant defense strategy have led to major insights into its evolution, mechanistic basis and genetic determinants. This review aims at summarizing current theories and experimental evidence on the evolutionary causes and consequences of plant tolerance to pathogens, as well as the existing knowledge on the genetic determinants and mechanisms of tolerance. Our review reveals that (i) in plant-pathogen systems, resistance and tolerance generally coexist, i.e., are not mutually exclusive; (ii) evidence of tolerance polymorphisms is abundant regardless of the pathogen considered; (iii) tolerance is an efficient strategy to reduce the damage on the infected host; and (iv) there is no evidence that tolerance results in increased pathogen multiplication. Taken together, the work discussed in this review indicates that tolerance may be as important as resistance in determining the dynamics of plant-pathogen interactions. Several aspects of plant tolerance to pathogens that still remain unclear and which should be explored in the future, are also outlined.

Complete Genome Sequence of Zucchini Yellow Mosaic Virus Strain Kurdistan, Iran

The complete genome sequence of Zucchini yellow mosaic virus strain Kurdistan (ZYMV-Kurdistan) infecting squash from Iran was determined from 13 overlapping fragments. Excluding the poly (A) tail, ZYMV-Kurdistan genome consisted of 9593 nucleotides (nt), with 138 and 211 nt at the 5' and 3' non-translated regions, respectively. It contained two open-reading frames (ORFs), the large ORF encoding a polyprotein of 3080 amino acids (aa) and the small overlapping ORF encoding a P3N-PIPO protein of 74 aa. This isolate had six unique aa differences compared to other ZYMV isolates and shared 79.6-98.8% identities with other ZYMV genome sequences at the nt level and 90.1-99% identities at the aa level. A phylogenetic tree of ZYMV complete genomic sequences showed that Iranian and Central European isolates are closely related and form a phylogenetically homogenous group. All values in the ratio of substitution rates at non-synonymous and synonymous sites (d_N/d_S) were below 1, suggestive of strong negative selection forces during ZYMV protein history. This is the first report of complete genome sequence information of the most prevalent virus in the west of Iran. This study helps our understanding of the genetic diversity of ZYMV isolates infecting cucurbit plants in Iran, virus evolution and epidemiology and can assist in designing better diagnostic tools.

Host-associated selection of a P3 mutant of zucchini yellow mosaic virus affects viral infectivity in watermelon

In this study, we found that the infectivity of zucchini yellow mosaic virus (ZYMV) in watermelon lines H1 and K6 changed from partial to complete after propagation in the susceptible watermelon line ZXG637. When using cucumber infected with strain ZYMV-CH87 as an inoculum (named ZYMV-CH87C), the mean incidences of infection in lines H1 and K6 were 6% and 11%, respectively. However, when these lines were inoculated with ZXG637 infected with ZYMV-CH87C (named ZYMV-637), 100% of the plants became infected. Sequencing of ZYMV from these different inoculums revealed two nucleotide changes in the P3 cistron in ZYMV-637, which resulted in changes in the amino acids at positions 768 and 857 of the P3 protein, compared with the original strain ZYMV-CH87. We named this variant the M₇₆₈I₈₅₇-variant. The M₇₆₈I₈₅₇-variant was detected at low levels (3.9%) in ZYMV-CH87C. When ZYMV-CH87C was passaged with ZXG637, the M₇₆₈I₈₅₇-variant was selected by the host, and the original sequence was replaced entirely after two passages. These results may be explained by host-associated selection due to an unknown host-encoded factor. Using the M₇₆₈I₈₅₇-variant as an inoculum, 100% of the H1 and K6 plants showed systemic symptoms. These results suggest that (1) changing the individual amino acids at the end of the P3 N-terminus induces resistance-breaking, and (2) the P3 N-terminus may be involved in host recognition.

Fasting alters aphid probing behaviour but does not universally increase the transmission rate of non-circulative viruses

A fasting period prior to non-circulative virus acquisition has been shown to increase the rate of transmission by aphids. However, this effect has only been studied for a few virus-vector combinations, and there are contradictory results in the literature as to the role of fasting on virus acquisition. We analysed the influence of fasting on the transmission of three non-circulative viruses, Cucumber mosaic virus, Zucchini yellow mosaic virus and Cauliflower mosaic virus, by two aphid vector species: Myzus persicae Sulzer (Hemiptera: Aphididae) and Aphis gossypii Glover (Hemiptera: Aphididae). All variables tested, including the virus species and isolate, and the species of aphid, influenced the effect of a fasting period on virus transmission efficiency. Furthermore, when aphids were subjected to an overnight feeding period on a sucrose solution, the fasting effect disappeared and the probing behaviour of these aphids was markedly different to plant-reared aphids. The electrical penetration graph (EPG) technique revealed that fasting altered the probing behaviour of M. persicae and A. gossypii, with fasted aphids beginning to feed sooner and having a significantly longer first intracellular puncture, measured as a potential drop. Significantly longer sub-phase II-3 of the potential drop and more archlets during this sub-phase were also observed for fasted aphids of both species. However, these behavioural changes were not predictive of increasing virus transmission following a fasting period. The impacts of pre-acquisition fasting on aphid probing behaviour and on the mechanisms of non-circulative virus transmission are discussed.

Molecular biology of potyviruses

Potyvirus is the largest genus of plant viruses causing significant losses in a wide range of crops. Potyviruses are aphid transmitted in a nonpersistent manner and some of them are also seed transmitted. As important pathogens, potyviruses are much more studied than other plant viruses belonging to other genera and their study covers many aspects of plant virology, such as functional characterization of viral proteins, molecular interaction with hosts and vectors, structure, taxonomy, evolution, epidemiology, and diagnosis. Biotechnological applications of potyviruses are also being explored. During this last decade, substantial advances have been made in the understanding of the molecular biology of these viruses and the functions of their various proteins. After a general presentation on the family Potyviridae and the potyviral proteins, we present an update of the knowledge on potyvirus multiplication, movement, and transmission and on potyvirus/plant compatible interactions including pathogenicity and symptom determinants. We end the review providing information on biotechnological applications of potyviruses.

Resistance genes against plant-parasitic nematodes: a durable control strategy?

Plant-parasitic nematodes are a major pest of all agricultural systems, causing extensive economic losses. Natural resistance (R) genes offer an alternative to chemical control and have been shown effectively to limit nematode damage to crops in the field. Whilst a number of resistant cultivars have conferred resistance against root-knot and cyst nematodes for many decades, an increasing number of reports of resistance-breaking nematode pathotypes are beginning to emerge. The forces affecting the emergence of virulent nematodes are complex, multifactorial and involve both the host and parasite of the plant-nematode interaction. This review provides an overview of the root-knot and cyst nematodeRgenes characterised to date, in addition to examining the evolutionary forces influencing nematode populations and the emergence of virulence. Finally, potential strategies to improveRgene durability in the field are outlined, and areas that would benefit from further research efforts are highlighted.

The movement protein (NSm) of Tomato spotted wilt virus is the avirulence determinant in the tomato Sw-5 gene-based resistance

The avirulence determinant triggering the resistance conferred by the tomato gene Sw-5 against Tomato spotted wilt virus (TSWV) is still unresolved. Sequence comparison showed two substitutions (C118Y and T120N) in the movement protein NSm present only in TSWV resistance-breaking (RB) isolates. In this work, transient expression of NSm of three TSWV isolates [RB1 (T120N), RB2 (C118Y) and non-resistance-breaking (NRB)] in Nicotiana benthamiana expressing Sw-5 showed a hypersensitive response (HR) only with NRB. Exchange of the movement protein of Alfalfa mosaic virus (AMV) with NSm supported cell-to-cell and systemic transport of the chimeric AMV RNAs into N. tabacum with or without Sw-5, except for the constructs with NBR when Sw-5 was expressed, although RB2 showed reduced cell-to-cell transport. Mutational analysis revealed that N120 was sufficient to avoid the HR, but the substitution V130I was required for systemic transport. Finally, co-inoculation of RB and NRB AMV chimeric constructs showed different prevalence of RB or NBR depending on the presence or absence of Sw-5. These results indicate that NSm is the avirulence determinant for Sw-5 resistance, and mutations C118Y and T120N are responsible for resistance breakdown and have a fitness penalty in the context of the heterologous AMV system.

Control of cucurbit viruses

More than 70 well-characterized virus species transmitted by a diversity of vectors may infect cucurbit crops worldwide. Twenty of those cause severe epidemics in major production areas, occasionally leading to complete crop failures. Cucurbit viruses' control is based on three major axes: (i) planting healthy seeds or seedlings in a clean environment, (ii) interfering with vectors activity, and (iii) using resistant cultivars. Seed disinfection and seed or seedling quality controls guarantee growers on the sanitary status of their planting material. Removal of virus or vector sources in the crop environment can significantly delay the onset of viral epidemics. Insecticide or oil application may reduce virus spread in some situations. Diverse cultural practices interfere with or prevent vector reaching the crop. Resistance can be obtained by grafting for soil-borne viruses, by cross-protection, or generally by conventional breeding or genetic engineering. The diversity of the actions that may be taken to limit virus spread in cucurbit crops and their limits will be discussed. The ultimate goal is to provide farmers with technical packages that combine these methods within an integrated disease management program and are adapted to different countries and cropping systems.

Fitness penalty in susceptible host is associated with virulence of Soybean mosaic virus on Rsv1-genotype soybean: a consequence of perturbation of HC-Pro and not P3

The multigenic Rsv1 locus in the soybean plant introduction (PI) 'PI96983' confers extreme resistance against the majority of Soybean mosaic virus (SMV) strains, including SMV-N, but not SMV-G7 and SMV-G7d. In contrast, in susceptible soybean cultivars lacking a functional Rsv1 locus, such as 'Williams82' (rsv1), SMV-N induces severe disease symptoms and accumulates to a high level, whereas both SMV-G7 and SMV-G7d induce mild symptoms and accumulate to a significantly lower level. Gain of virulence by SMV-N on Rsv1-genotype soybean requires concurrent mutations in both the helper-component proteinase (HC-Pro) and P3 cistrons. This is because of the presence of at least two resistance (R) genes, probably belonging to the nucleotide-binding leucine-rich repeat (NB-LRR) class, within the Rsv1 locus, independently mediating the recognition of HC-Pro or P3. In this study, we show that the majority of experimentally evolved mutational pathways that disrupt the avirulence functions of SMV-N on Rsv1-genotype soybean also result in mild symptoms and reduced accumulation, relative to parental SMV-N, in Williams82 (rsv1). Furthermore, the evaluation of SMV-N-derived HC-Pro and P3 chimeras, containing homologous sequences from virulent SMV-G7 or SMV-G7d strains, as well as SMV-N-derived variants containing HC-Pro or P3 point mutation(s) associated with gain of virulence, reveals a direct correlation between the perturbation of HC-Pro and a fitness penalty in Williams82 (rsv1). Collectively, these data demonstrate that gain of virulence by SMV on Rsv1-genotype soybean results in fitness loss in a previously susceptible soybean genotype, this being a consequence of mutations in HC-Pro, but not in P3.

Intra-specific variability and biological relevance of P3N-PIPO protein length in potyviruses

BACKGROUND

Pipo was recently described as a new ORF encoded within the genome of the Potyviridae family members (PNAS 105:5897-5902, 2008). It is embedded within the P3 cistron and is translated in the +2 reading frame relative to the potyviral long ORF as the P3N-PIPO fusion protein. In this work, we first collected pipo nucleotide sequences available for different isolates of 48 Potyvirus species. Second, to determine the biological implications of variation in pipo length, we measured infectivity, viral accumulation, cell-to-cell and systemic movements for two Turnip mosaic virus (TuMV) variants with pipo alleles of different length in three different susceptible host species, and tested for differences between the two variants.

RESULTS

In addition to inter-specific variation, there was high variation in the length of the PIPO protein among isolates within species (ranging from 1 to 89 amino acids). Furthermore, selection analyses on the P3 cistron did not account for the existence of stop codons in the pipo ORF, but showed that positive selection was significant in the overlapping region for Potato virus Y (PVY) and TuMV. In some cases, variability in length was associated with host species, geographic provenance and/or other strain features. We found significant empirical differences among the phenotypes associated with TuMV pipo alleles, though the magnitude and sign of the effects were host-dependent.

CONCLUSIONS

The combination of computational molecular evolution analyses and experiments stemming from these analyses provide clues about the selective pressures acting upon the different-length pipo alleles and show that variation in length may be maintained by host-driven selection.

Zucchini yellow mosaic virus (ZYMV, Potyvirus): vertical transmission, seed infection and cryptic infections

The role played by seed transmission in the evolution and epidemiology of viral crop pathogens remains unclear. We determined the seed infection and vertical transmission rates of zucchini yellow mosaic virus (ZYMV), in addition to undertaking Illumina sequencing of nine vertically transmitted ZYMV populations. We previously determined the seed-to-seedling transmission rate of ZYMV in Cucurbita pepo ssp. texana (a wild gourd) to be 1.6%, and herein observed a similar rate (1.8%) in the subsequent generation. We also observed that the seed infection rate is substantially higher (21.9%) than the seed-to-seedling transmission rate, suggesting that a major population bottleneck occurs during seed germination and seedling growth. In contrast, that two thirds of the variants present in the horizontally transmitted inoculant population were also present in the vertically transmitted populations implies that the bottleneck at vertical transmission may not be particularly severe. Strikingly, all of the vertically infected plants were symptomless in contrast to those infected horizontally, suggesting that vertical infection may be cryptic. Although no known virulence determining mutations were observed in the vertically infected samples, the 5' untranslated region was highly variable, with at least 26 different major haplotypes in this region compared to the two major haplotypes observed in the horizontally transmitted population. That the regions necessary for vector transmission are retained in the vertically infected populations, combined with the cryptic nature of vertical infection, suggests that seed transmission may be a significant contributor to the spread of ZYMV.

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.

Intrahost mechanisms governing emergence of resistance-breaking variants of Potato virus Y

The emergence of resistance breaking (RB) variants starting from the avirulent Potato virus Y NN strain (PVY(NN)) was analyzed after imposing different selective host constraints. Tobacco resistance to PVY(NN) is conferred by va in both NC745 and VAM genotypes, but VAM carries an extra resistance gene, va2. RB-variants emerged only in NC745 and unexpectedly accumulated higher in the original host, tobacco B21, than the parental PVY(NN). However, the recovery of RB-variants was interfered by PVY(NN) in mixed infections. Further analysis indicated that RB-variants also arose in tobacco VAM, but they were limited to subliminal local infections. Their inability to breakout was associated with absence of a mutational adaptation in the viral VPg gene, which implied a loss of fitness in tobacco B21. Altogether, the emergence of RB-variants was conditioned by inherited host constraints, interference by co-infecting avirulent virus genotypes, and fitness tradeoff of virus adaptations.

Viruses of cucurbit crops in the Mediterranean region: an ever-changing picture

Cucurbit crops may be affected by at least 28 different viruses in the Mediterranean basin. Some of these viruses are widely distributed and cause severe yield losses while others are restricted to limited areas or specific crops, and have only a negligible economic impact. A striking feature of cucurbit viruses in the Mediterranean basin is their always increasing diversity. Indeed, new viruses are regularly isolated and over the past 35 years one "new" cucurbit virus has been reported on average every 2 years. Among these "new" viruses some were already reported in other parts of the world, but others such as Zucchini yellow mosaic virus (ZYMV), one of the most severe cucurbit viruses and Cucurbit aphid-borne yellows virus (CABYV), one of the most prevalent cucurbit viruses, were first described in the Mediterranean area. Why this region may be a potential "hot-spot" for cucurbit virus diversity is not fully known. This could be related to the diversity of cropping practices, of cultivar types but also to the important commercial exchanges that always prevailed in this part of the world. This chapter describes the major cucurbit viruses occurring in the Mediterranean basin, discusses factors involved in their emergence and presents options for developing sustainable control strategies.

Amino acid changes in P3, and not the overlapping pipo-encoded protein, determine virulence of soybean mosaic virus on functionally immune Rsv1-genotype soybean

A small open reading frame, termed 'pipo', is embedded in the P3 cistron of potyviruses. Currently, knowledge on pipo and its role(s) in the life cycle of potyviruses is limited. The P3 and helper-component proteinase (HC-Pro) cistrons of Soybean mosaic virus (SMV) harbour determinants affecting virulence on functionally immune Rsv1-genotype soybeans. Interestingly, a key virulence determinant of SMV on Rsv1-genotype soybeans (i.e. soybeans containing the Rsv1 resistance gene) that resides at polyprotein codon 947 overlaps both P3 and a pipo-encoded codon. This raises the question of whether PIPO or P3 is the virulence factor. To answer this question, the corresponding pipo of an avirulent and two virulent strains of SMV were studied by comparative genomics, followed by syntheses and analyses of site-directed mutants. Our data demonstrate that the virulence of SMV on Rsv1-genotype soybeans is affected by P3 and not the overlapping pipo-encoded protein.

How do plant viruses induce disease? Interactions and interference with host components

Plant viruses are biotrophic pathogens that need living tissue for their multiplication and thus, in the infection-defence equilibrium, they do not normally cause plant death. In some instances virus infection may have no apparent pathological effect or may even provide a selective advantage to the host, but in many cases it causes the symptomatic phenotypes of disease. These pathological phenotypes are the result of interference and/or competition for a substantial amount of host resources, which can disrupt host physiology to cause disease. This interference/competition affects a number of genes, which seems to be greater the more severe the symptoms that they cause. Induced or repressed genes belong to a broad range of cellular processes, such as hormonal regulation, cell cycle control and endogenous transport of macromolecules, among others. In addition, recent evidence indicates the existence of interplay between plant development and antiviral defence processes, and that interference among the common points of their signalling pathways can trigger pathological manifestations. This review provides an update on the latest advances in understanding how viruses affect substantial cellular processes, and how plant antiviral defences contribute to pathological phenotypes.

Minimising losses caused by Zucchini yellow mosaic virus in vegetable cucurbit crops in tropical, sub-tropical and Mediterranean environments through cultural methods and host resistance

Between 2006 and 2009, 10 field experiments were done at Kununurra, Carnarvon or Medina in Western Australia (WA) which have tropical, sub-tropical and Mediterranean climates, respectively. These experiments investigated the effectiveness of cultural control measures in limiting ZYMV spread in pumpkin, and single-gene resistance in commercial cultivars of pumpkin, zucchini and cucumber. Melon aphids (Aphis gossypii) colonised field experiments at Kununurra; migrant green peach aphids (Myzus persicae) visited but did not colonise at Carnarvon and Medina. Cultural control measures that diminished ZYMV spread in pumpkin included manipulation of planting date to avoid exposing young plants to peak aphid vector populations, deploying tall non-host barriers (millet, Pennisetum glaucum) to protect against incoming aphid vectors and planting upwind of infection sources. Clustering of ZYMV-infected pumpkin plants was greater without a 25m wide non-host barrier between the infection source and the pumpkin plants than when one was present, and downwind compared with upwind of an infection source. Host resistance gene zym was effective against ZYMV isolate Knx-1 from Kununurra in five cultivars of cucumber. In zucchini, host resistance gene Zym delayed spread of infection (partial resistance) in 2 of 14 cultivars but otherwise did not diminish final ZYMV incidence. Zucchini cultivars carrying Zym often developed severe fruit symptoms (8/14), and only the two cultivars in which spread was delayed and one that was tolerant produced sufficiently high marketable yields to be recommended when ZYMV epidemics are anticipated. In three pumpkin cultivars with Zym, this gene was effective against isolate Cvn-1 from Carnarvon under low inoculum pressure, but not against isolate Knx-1 under high inoculum pressure, although symptoms were milder and marketable yields greater in them than in cultivars without Zym. These findings allowed additional cultural control recommendations to be added to the existing Integrated Disease Management strategy for ZYMV in vegetable cucurbits in WA, but necessitated modification of its recommendations over deployment of cultivars with resistance genes.

The hallmarks of "green" viruses: do plant viruses evolve differently from the others?

All viruses are obligatory parasites that must develop tight interactions with their hosts to complete their infectious cycle. Viruses infecting plants share many structural and functional similarities with those infecting other organisms, particularly animals and fungi. Quantitative data regarding their evolutionary mechanisms--generation of variability by mutation and recombination, changes in populations by selection and genetic drift have been obtained only recently, and appear rather similar to those measured for animal viruses.This review presents an update of our knowledge of the phylogenetic and evolutionary characteristics of plant viruses and their relation to their plant hosts, in comparison with viruses infecting other organisms.

A single-stranded conformational polymorphism (SSCP)-derived quantitative variable to monitor the virulence of a Barley yellow dwarf virus-PAV (BYDV-PAV) isolate during adaptation to the TC14 resistant wheat line

A standardized single-stranded conformational polymorphism (SSCP) procedure is proposed as an alternative to the time-consuming biological characterization of Barley yellow dwarf virus-PAV (BYDV-PAV) isolates. Using this procedure, six of 21 overlapping regions used to scan the viral genome gave patterns specific to '4E' (avirulent) or '4T' ('4E'-derived virulent) isolates. The calibration of samples and integration of SSCP patterns corresponding to the nucleotide region 1482-2023 allowed the estimation of P(T) values that reflect the proportions of a '4T'-specific band. Analysis of the biological (area under the pathogen progress curve) and molecular (P(T)) data suggested a positive linear relation between these variables. Moreover, sequence analysis of the nucleotide region 1482-2023 highlighted the presence of a nucleotide polymorphism (C/A(1835)) which can be considered as a candidate for virus-host interactions linked to the monitored virulence. According to these parameters, P(T) values associated with '4E'- and '4T'-derived populations show that: (i) long-term infection of a BYDV-PAV isolate on the 'TC14' resistant host leads to the fixation of virulent individuals in viral populations; and (ii) the introduction of susceptible hosts in successive 'TC14' infections results in the maintenance of low virulence of the populations. Thus, the presented study demonstrates that SSCP is a useful tool for monitoring viral populations during the host adaptation process. The described impact of host alternation provides new opportunities for the use of the 'TC14' resistance source in BYDV-resistant breeding programmes. This study is part of the global effort made by the scientific community to propose sustainable alternatives to the chemical control of this viral disease.

The coevolution of plants and viruses: resistance and pathogenicity

Virus infection may damage the plant, and plant defenses are effective against viruses; thus, it is currently assumed that plants and viruses coevolve. However, and despite huge advances in understanding the mechanisms of pathogenicity and virulence in viruses and the mechanisms of virus resistance in plants, evidence in support of this hypothesis is surprisingly scant, and refers almost only to the virus partner. Most evidence for coevolution derives from the study of highly virulent viruses in agricultural systems, in which humans manipulate host genetic structure, what determines genetic changes in the virus population. Studies have focused on virus responses to qualitative resistance, either dominant or recessive but, even within this restricted scenario, population genetic analyses of pathogenicity and resistance factors are still scarce. Analyses of quantitative resistance or tolerance, which could be relevant for plant-virus coevolution, lag far behind. A major limitation is the lack of information on systems in which the host might evolve in response to virus infection, that is, wild hosts in natural ecosystems. It is presently unknown if, or under which circumstances, viruses do exert a selection pressure on wild plants, if qualitative resistance is a major defense strategy to viruses in nature, or even if characterized genes determining qualitative resistance to viruses did indeed evolve in response to virus infection. Here, we review evidence supporting plant-virus coevolution and point to areas in need of attention to understand the role of viruses in plant ecosystem dynamics, and the factors that determine virus emergence in crops.

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.

Key determinants of resistance durability to plant viruses: insights from a model linking within- and between-host dynamics

The emergence of new genotypes of parasites involves several evolutionary, epidemiological and ecological processes whose individual effects and interactions are difficult to disentangle using experimental approaches. Here, a model is proposed to investigate how these processes lead to the emergence of plant viral genotypes breaking down qualitative resistance genes. At the individual plant scale, selection, drift and mutation processes shape the evolution of viral populations from a set of differential equations. The spatial segregation of virus genotypes in their hosts is also considered. At the host population scale, the epidemiological dynamics is given by an individual-based algorithm. Global sensitivity analyses allowed ranking the ten demo-genetic and epidemiological parameters of the model according to their impact on the mean and variance of the risk of breakdown of a plant resistance. Demo-genetic parameters (number and nature of mutations involved in breakdown, fitness of mutant genotypes) had the largest impact on the mean breakdown risk, whereas epidemiological parameters had more influence on its standard deviation. It is discussed how these results can be used to choose the potentially most durable resistance genes among a pool of candidates. Finally, our analyses point out the parameters which should be estimated more precisely to improve durability predictions.

Recombination analysis of Soybean mosaic virus sequences reveals evidence of RNA recombination between distinct pathotypes

RNA recombination is one of the two major factors that create RNA genome variability. Assessing its incidence in plant RNA viruses helps understand the formation of new isolates and evaluate the effectiveness of crop protection strategies. To search for recombination in Soybean mosaic virus (SMV), the causal agent of a worldwide seed-borne, aphid-transmitted viral soybean disease, we obtained all full-length genome sequences of SMV as well as partial sequences encoding the N-terminal most (P1 protease) and the C-terminal most (capsid protein; CP) viral protein. The sequences were analyzed for possible recombination events using a variety of automatic and manual recombination detection and verification approaches. Automatic scanning identified 3, 10, and 17 recombination sites in the P1, CP, and full-length sequences, respectively. Manual analyses confirmed 10 recombination sites in three full-length SMV sequences. To our knowledge, this is the first report of recombination between distinct SMV pathotypes. These data imply that different SMV pathotypes can simultaneously infect a host cell and exchange genetic materials through recombination. The high incidence of SMV recombination suggests that recombination plays an important role in SMV evolution. Obtaining additional full-length sequences will help elucidate this role.

Epidemiological models for invasion and persistence of pathogens

Motivated by questions such as "Why do some diseases take off, while others die out?" and "How can we optimize the deployment of control methods," we introduce simple epidemiological concepts for the invasion and persistence of plant pathogens. An overarching modeling framework is then presented that can be used to analyze disease invasion and persistence at a range of scales from the microscopic to the regional. Criteria for invasion and persistence are introduced, initially for simple models of epidemics, and then for models with greater biological realism. Some ways in which epidemiological models are used to identify optimal strategies for the control of disease are discussed. Particular attention is given to the spatial structure of host populations and to the role of chance events in determining invasion and persistence of plant pathogens. Finally, three brief case studies are used to illustrate the practical applications of epidemiological theory to understand invasion and persistence of plant pathogens. These comprise long-term predictions for the persistence and control of Dutch elm disease; identification of methods to manage the spread of rhizomania on sugar beet in the U.K. by matching the scale of control with the spatial and temporal scales of the disease; and analysis of evolutionary change in virus control to identify risks of inadvertent selection for damaging virus strains.

Analysis of the molecular and biological variability of zucchini yellow mosaic virus isolates from Slovakia and Czech Republic

The diversity of ZYMV isolates was analysed by the biological and molecular characterisation of 11 isolates sampled from cucumber, squash and zucchini between 2001 and 2006 in various localities of Slovakia and Czech Republic. Analysis of the molecular variability targeting three separate genomic regions of the ZYMV genome [P1, P3 and (Cter)NIb-(Nter)CP] revealed a remarkable low level of nucleotide variability between isolates, despite their temporal and spatial distinction. Phylogenetic analysis based on the 5'-terminal part of the CP gene highlighted the close relatedness of Slovak, Czech and other central European isolates. Low level of genetic diversity within central European ZYMV isolates is in contrast to the diversity observed for isolates from other geographical regions, in particular Asia. No evidence of recombination in the ZYMV genome was detected. Sequence comparison between aggressive and moderate ZYMV isolates revealed one amino acid difference in the N-terminal part of the P3 protein, potentially involved in the tolerance breaking.

Zucchini yellow mosaic virus: insect transmission and pathogenicity -the tails of two proteins

SUMMARY Taxonomy: Zucchini yellow mosaic virus (ZYMV) is a member of genus Potyvirus, family Potyviridae. ZYMV is a positive-strand RNA virus. Physical properties: Virions are flexuous filaments of 680-730 nm in length and 11-13 nm in diameter, composed of about 2000 subunits of a single 31-kDa protein (calculated). The genome RNA size is 9.6 kb covalently linked to a viral-encoded protein (the VPg) at the 5' end, and with a 3' poly A tail. The 5' end of the sequence is AU-rich (69%). Viral proteins: The genome is expressed as a polyprotein cleaved by three viral proteases and processed into ten putative mature proteins. The structural coat protein is processed from the carboxyl terminus of the polyprotein and is highly immunogenic. Host and symptoms: Natural and experimental infection has been reported mainly in the Cucurbitaceae. Experimental local lesion hosts include Chenopodium amaranticolour, C. quinoa and Gomphrena globosa. Some ZYMV strains cause symptomless infection as in Ranunculus sardous, Nicotiana benthamiana and Sesamum indicum. ZYMV causes stunting and major foliar deformation with dark green blisters and mosaics in cucurbit hosts, eventually developing a filamentous leaf phenotype. In general, symptoms are severe on cucurbit hosts and cause dramatic reductions in yields due to severe fruit deformation. The virus is present in all the plant tissues at relatively high concentrations (c. 0.1 mg/mL of purified virus per 1 g fresh leaf tissue). The most suitable species for maintenance and purification is Cucurbita pepo.

TRANSMISSION

ZYMV is efficiently transmitted by aphids in a non-persistent manner. The coat protein (CP) and the helper component-protease (HC-Pro) are required for aphid transmission, through the CP DAG motif and the HC-Pro KLSC and PTK motifs. Mechanical transmission is efficient both in the laboratory and naturally. Economic importance: ZYMV disease is a major constraint in the production of cucurbits world-wide. The virus can cause massive damage (to total loss) to cucurbit crops, and prevents the growth of some cucurbit crops in certain areas. Control of ZYMV requires the integration of conventional resistance and transgenic breeding along with cross-protection technologies.

Different mutations in the genome-linked protein VPg of potato virus Y confer virulence on the pvr2(3) resistance in pepper

Five different amino acid substitutions in the VPg of Potato virus Y were shown to be independently responsible for virulence toward pvr2(3) resistance gene of pepper. A consequence of these multiple mutations toward virulence involving single nucleotide substitutions is a particularly high frequency of resistance breaking (37% of inoculated plants from the first inoculation) and suggests a potentially low durability of pvr2(3) resistance. These five mutants were observed with significantly different frequencies, one of them being overrepresented. Genetic drift alone could not explain the observed distribution of virulent mutants. More plausible scenarios were obtained by taking into account either the relative substitution rates, the relative fitness of the mutants in pvr2(3) pepper plants, or both.

Genetic variation and covariation for resistance and tolerance to Cucumber mosaic virus in Mimulus guttatus (Phrymaceae): a test for costs and constraints

Genetic variation for resistance and tolerance to pathogens may be maintained by costs represented as genetic tradeoffs between these traits and fitness. The evolution of resistance and tolerance also may be constrained by negative genetic correlations between these defense systems. Using a complete diallel, we measured genetic variation and covariation for and among performance, resistance, and tolerance traits in Mimulus guttatus challenged with a generalist pathogen, Cucumber mosaic virus (CMV). Viral coat protein was detected by enzyme-linked immunosorbent assay (ELISA) in all inoculated plants, indicating that all plants were susceptible to infection, although the ELISA absorbance varied quantitatively across plants. Plants inoculated with CMV had significantly reduced aboveground biomass and flower production relative to controls, although date of first flower was unaffected by infection. All three of these performance traits showed moderate to high narrow-sense heritability (h2 = 0.32-0.62) in both inoculated and control plants. We found phenotypic variation for both tolerance of and resistance to our strain of CMV, but both displayed very low narrow-sense heritability (h2 < 0.03). We found no evidence of a trade-off between resistance and tolerance. We also found no evidence for a cost of resistance or tolerance. In fact, a significant genetic correlation suggested that plants that were large when healthy had the greatest tolerance when infected. Significant, positive genetic correlations found between performance of uninfected and infected plants suggested that selection would likely favor the same M. guttatus genotypes whether CMV is present or not.

The complete genome sequence of an El Amar isolate of plum pox virus (PPV) and its phylogenetic relationship to other PPV strains

The genomic sequence of an El Amar isolate of plum pox virus (PPV) from Egypt was determined by sequencing overlapping cDNA fragments. This is the first complete sequence of a member of the El Amar (EA) strain of PPV. The genome consists of 9791 nt, excluding a poly(A) tail at the 3' terminus. The complete nt sequence of PPV EA is 79-80%, 80%, 77%, and 77% homologous with isolates of strains D/M, Rec (BOR3), C, and W, respectively. The polyprotein identity ranged from 87-91%. Phylogenetic analysis using the complete genome sequence of PPV EA confirmed its strain status. No significant recombination signals were identified using PhylPro and SimPlot scans of the PPV EA sequence, however an interesting recombination signal was identified in the P1/HC-Pro region of PPV W3174.

Durable virus resistance in plants through conventional approaches: a challenge

Breeding for virus resistance is often considered the most efficient and simplest way to avoid the losses due to plant virus diseases. Resistance mechanisms are very diverse and interact with various stages of the virus cycle in the host plant. Resistances may also differ in their specificity, stability and durability. Breeding for resistance is a long and costly process, therefore to be cost effective it should provide durable protection. Three pathosystems are discussed to illustrate some of the field and laboratory approaches that can be used to assess resistance durability: Cucumber mosaic virus-specific resistance in melon, Zucchini yellow mosaic virus tolerance in zucchini squash, and extreme resistance to Potato virus X in potato. The possibility of predicting resistance durability is discussed in relation to the nature of the resistance, the genetic changes required for a virus to overcome the resistance and the effects of such changes on virus fitness.