The cylindrical inclusion gene of Turnip mosaic virus encodes a pathogenic determinant to the Brassica resistance gene TuRB01

A developmentally linked, dramatic, and transient loss of virus from roots of Arabidopsis thaliana plants infected by either of two RNA viruses

Possible effects of host developmental stage on the amount of virus present in systemically infected plant tissues hitherto have received little attention. In this study, the pattern of virus accumulation over the plant lifespan has been examined in systemically invaded tissues of Arabidopsis thaliana infected by either of two distinct (+)RNA viruses: Turnip mosaic virus, a member of Potyvirus, and Oilseed rape mosaic virus, a member of Tobamovirus. Quantitative analyses of virus coat protein and virus genomic RNA in roots versus aerial plant parts revealed generally sinusoidal temporal patterns of virus accumulation. In noninoculated leaves, a time period was found during which no virus accumulation was detected. This period was coincident with the approximately 7 days of inflorescence bud formation and differentiation. In roots, virion content reached high levels a few days after inoculation, dropping dramatically during the period of bud formation and quickly recovering after it. These results, together with electron microscopy observations, are consistent with loss of virions due to disassembly. Fluorescence observations of green fluorescent protein-tagged virus-infected root tissue also were consistent with a net loss of virus-specified proteins. Inoculations performed after the emergence of the inflorescence and on A. thaliana flowering-time mutants support the temporal link between observed changes in virus content and inflorescence bud formation. Different host-involving biochemical processes can be invoked to provide mechanistic clues, but no one of them alone seems sufficient to explain the complex patterns of tight temporal regulation of virus accumulation observed in these experiments.

Identification of new isolates of Turnip mosaic virus that cluster with less common viral strains

Turnip mosaic virus (TuMV) was found infecting cultivated brassicas and wild and cultivated ornamental Brassicaceae plants in different regions of Spain. Five new TuMV isolates, originating from different host plant species (Brassica cretica, Brassica juncea, Brassica napus, Eruca vesicaria subsp. sativa and Sisymbrium orientale), have been identified. The nucleotide sequences of the coat protein (CP) genes of the five isolates were determined. Phylogenetic analysis of the CP sequences showed that the five isolates grouped into two different clusters. The three isolates from the central region of Spain clustered with a previously reported Pisum sativum isolate from southeastern Spain, whereas the other two isolates from the eastern region clustered with two Italian and two Greek isolates. Both clusters were genetically distinct and belonged to the multi-lineage group OBR. The OBR group contains mainly TuMV isolates from hosts other than Brassica spp. and Raphanus sativus and mostly originating from Mediterranean countries. These new sequences provide further phylogenetic resolution of the OBR group. Although new TuMV isolates have been found in Spain, they were not associated with any serious disease outbreaks.

Coordinated and selective recruitment of eIF4E and eIF4G factors for potyvirus infection in Arabidopsis thaliana

The translation initiation factors eIF4E and eIF(iso)4E play a key role during virus infection in plants. During mRNA translation, eIF4E provides the cap-binding function and is associated with the protein eIF4G to form the eIF4F complex. Susceptibility analyses of Arabidopsis mutants knocked-out for At-eIF4G genes showed that eIF4G factors are indispensable for potyvirus infection. The colonization pattern by a viral recombinant carrying GFP indicated that eIF4G is involved at a very early infection step. Like eIF4E, eIF4G isoforms are selectively recruited for infection. Moreover, the eIF4G selective involvement parallels eIF4E recruitment. This is the first report of a coordinated and selective recruitment of eIF4E and eIF4G factors, suggesting the whole eIF4F recruitment.

Plant signal transduction and defense against viral pathogens

Viral infection of plants is a complex process whereby the virus parasitizes the host and utilizes its cellular machinery to multiply and spread. In turn, plants have evolved signaling mechanisms that ultimately limit the ingress and spread of viral pathogens, resulting in resistance. By dissecting the interaction between host and virus, knowledge of signaling pathways that are deployed for resistance against these pathogens has been gained. Advances in this area have shown that resistance signaling against viruses does not follow a prototypic pathway but rather different host factors may play a role in resistance to different viral pathogens. Some components of viral resistance signaling pathways also appear to be conserved with those functioning in signaling pathways operational against other nonviral pathogens, however, these pathways may or may not overlap. This review aims to document the advances that have improved our understanding of plant resistance to viruses.

Identification of a plum pox virus CI-interacting protein from chloroplast that has a negative effect in virus infection

The cylindrical inclusion (CI) protein of potyviruses is involved in virus replication and cell-to-cell movement. These two processes should rely on multiple plant-virus interactions; however, little is known about the host factors that are involved in, or that may interfere with, CI functions. By using a yeast two-hybrid system, the CI protein from Plum pox virus (PPV) was found to interact with the photosystem I PSI-K protein, the product of the gene psaK, of Nicotiana benthamiana. Coexpression of PPV CI was shown to cause a decrease in the accumulation level of PSI-K transiently expressed in N. benthamiana leaves. To test the biological relevance of this interaction, we have analyzed the infection of PPV in N. benthamiana plants in which psaK gene expression has been silenced by RNA interference, as well as in Arabidopsis thaliana psaK knockout plants. Our results show that downregulation of the psaK gene leads to higher PPV accumulation, suggesting a role for the CI-PSI-K interaction in PPV infection.

Strain-specific P3 of Soybean mosaic virus elicits Rsv1-mediated extreme resistance, but absence of P3 elicitor function alone is insufficient for virulence on Rsv1-genotype soybean

When challenged by mechanical inoculation, the Rsv1 gene of soybean invokes extreme resistance (ER) against Soybean mosaic virus (SMV) strain N, but not SMV-G7 and its experimentally evolved variant, SMV-G7d. SMV-G7 provokes a lethal systemic hypersensitive response (LSHR), whereas SMV-G7d induces systemic mosaic. Thus, for Rsv1-genotype soybean, SMV-G7 and SMV-G7d are both virulent virus strains. The elicitor function of SMV-G7 provoking Rsv1-mediated LSHR was recently mapped to P3, and the influence of amino acids 823, 953, and 1112 of the precursor polypeptide of SMV-G7d on evasion of Rsv1-mediated recognition provoking LSHR was demonstrated. We have now extended this study to SMV-N. Initially, amino acids corresponding to those of SMV-G7d at these positions were substituted, individually or in combinations. All the mutants remained replication competent on rsv1-genotype soybean; however, none lost the elicitor function provoking Rsv1-mediated ER. Subsequently, P3 of SMV-N was precisely replaced with P3 of SMV-G7 or SMV-G7d and vice versa. All the chimeras were replication competent on rsv1-genotype soybean, but surprisingly SMV-N/G7P3 and SMV-N/G7dP3 failed to gain virulence on Rsv1-genotype soybeans. However, SMV-G7/NP3 and SMV-G7d/NP3 lost virulence, and this loss of virulence function was mapped to the N-terminus domain of SMV-N P3. The data indicate that SMV strain-specific P3 provokes Rsv1-mediated ER; however, virulence on Rsv1-genotype soybean is not solely a consequence of the absence of the P3 elicitor functions provoking Rsv1-mediated ER and LSHR.

Plant virus HC-Pro is a determinant of eriophyid mite transmission

The eriophyid mite transmitted Wheat streak mosaic virus (WSMV; genus Tritimovirus, family Potyviridae) shares a common genome organization with aphid transmitted species of the genus Potyvirus. Although both tritimoviruses and potyviruses encode helper component-proteinase (HC-Pro) homologues (required for nonpersistent aphid transmission of potyviruses), sequence conservation is low (amino acid identity, approximately 16%), and a role for HC-Pro in semipersistent transmission of WSMV by the wheat curl mite (Aceria tosichella [Keifer]) has not been investigated. Wheat curl mite transmissibility was abolished by replacement of WSMV HC-Pro with homologues of an aphid transmitted potyvirus (Turnip mosaic virus), a rymovirus (Agropyron mosaic virus) vectored by a different eriophyid mite, or a closely related tritimovirus (Oat necrotic mottle virus; ONMV) with no known vector. In contrast, both WSMV-Sidney 81 and a chimeric WSMV genome bearing HC-Pro of a divergent strain (WSMV-El Batán 3; 86% amino acid sequence identity) were efficiently transmitted by A. tosichella. Replacing portions of WSMV-Sidney 81 HC-Pro with the corresponding regions from ONMV showed that determinants of wheat curl mite transmission map to the 5'-proximal half of HC-Pro. WSMV genomes bearing HC-Pro of heterologous species retained the ability to form virions, indicating that loss of vector transmissibility was not a result of failure to encapsidate. Although titer in systemically infected leaves was reduced for all chimeric genomes relative to WSMV-Sidney 81, titer was not correlated with loss of vector transmissibility. Collectively, these results demonstrate for the first time that HC-Pro is required for virus transmission by a vector other than aphids.

Genetics of plant virus resistance

Genetic resistance to plant viruses has been used for at least 80 years to control agricultural losses to viral diseases. To date, hundreds of naturally occurring genes for resistance to plant viruses have been reported from studies of both monocot and dicot crops, their wild relatives, and the plant model, Arabidopsis. The isolation and characterization of a few of these genes in the past decade have resulted in detailed knowledge of some of the molecules that are critical in determining the outcome of plant viral infection. In this chapter, we have catalogued genes for resistance to plant viruses and have summarized current knowledge regarding their identity and inheritance. Insofar as information is available, the genetic context, genomic organization, mechanisms of resistance and agricultural deployment of plant virus resistance genes are also discussed.

Fine genetic mapping of the TuNI locus causing systemic veinal necrosis by turnip mosaic virus infection in Arabidopsis thaliana

In the pathosystem of turnip mosaic virus (TuMV) and Arabidopsis thaliana, two distinct symptoms (mosaic symptom and veinal necrosis) were observed that were dependent upon the combination of the TuMV isolate and the Arabidopsis ecotype. The Col-0 ecotype developed mosaic symptoms after infection with the TuMV isolate Azu while the Ler ecotype developed veinal necrosis after infection with the same TuMV isolate. The Ler phenotype is controlled by a single dominant gene TuNI (TuMV necrosis inducer) which is located on chromosome 1. The TuNI gene was precisely mapped to the ~105 kb interval between the two markers of mXF41 and mRF28 by using several types of DNA polymorphism markers. Within this region, which included largely duplicated sequences, a total of 19 putative genes were predicted and 15 of these were classified into five gene families. The genes belonging to the gene families At1g58480 and At1g58602 may function in response to infection by pathogens. The gene family At1g58480 encodes lipase-like proteins, which might be involved in the induction of defence responses that are mediated by salicylic acid. The gene family At1g58602 encodes the CC-NBS-LRR (CNL) proteins, which are known to function as one of the plant resistance (R) proteins against pathogens. In the present study, the possibility that TuNI might function as an R gene was discussed.

An important determinant of the ability of Turnip mosaic virus to infect Brassica spp. and/or Raphanus sativus is in its P3 protein

Turnip mosaic virus (TuMV, genus Potyvirus, family Potyviridae) infects mainly cruciferous plants. Isolates Tu-3 and Tu-2R1 of TuMV exhibit different infection phenotypes in cabbage (Brassica oleracea L.) and Japanese radish (Raphanus sativus L.). Infectious full-length cDNA clones, pTuC and pTuR1, were constructed from isolates Tu-3 and Tu-2R1, respectively. Progeny virus derived from infections with pTuC induced systemic chlorotic and ringspot symptoms in infected cabbage, but no systemic infection in radish. Virus derived from plants infected with pTuR1 induced a mild chlorotic mottle in cabbage and infected radish systemically to induce mosaic symptoms. By exchanging genome fragments between the two virus isolates, the P3-coding region was shown to be responsible for systemic infection by TuMV and the symptoms it induces in cabbage and radish. Moreover, exchanges of smaller parts of the P3 region resulted in recombinants that induced complex infection phenotypes, especially the combination of pTuC-derived N-terminal sequence and pTuR1-derived C-terminal sequence. Analysis by tissue immunoblotting of the inoculated leaves showed that the distributions of P3-chimeric viruses differed from those of the parents, and that the origin of the P3 components affected not only virus accumulation, but also long-distance movement. These results suggest that the P3 protein is an important factor in the infection cycle of TuMV and in determining the host range of this and perhaps other potyviruses.

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.

Mutations in potato virus Y genome-linked protein determine virulence toward recessive resistances in Capsicum annuum and Lycopersicon hirsutum

The recessive resistance genes pot-1 and pvr2 in Lycopersicon hirsutum and Capsicum annuum, respectively, control Potato virus Y (PVY) accumulation in the inoculated leaves. Infectious cDNA molecules from two PVY isolates differing in their virulence toward these resistances were obtained using two different strategies. Chimeras constructed with these cDNA clones showed that a single nucleotide change corresponding to an amino acid substitution (Arg119His) in the central part of the viral protein genome-linked (VPg) was involved in virulence toward the pot-1 resistance. On the other hand, 15 nucleotide changes corresponding to five putative amino acid differences in the same region of the VPg affected virulence toward the pvr2(1) and pvr2(2) resistances. Substitution models identified six and five codons within the central and C terminal parts of the VPg for PVY and for the related potyvirus Potato virus A, respectively, which undergo positive selection. This suggests that the role of the VPg-encoding region is determined by the protein and not by the viral RNA apart from its protein-encoding capacity.

Genetic mapping of the novel Turnip mosaic virus resistance gene TuRB03 in Brassica napus

A new source of resistance to the pathotype 4 isolate of Turnip mosaic virus (TuMV) CDN 1 has been identified in Brassica napus (oilseed rape). Analysis of segregation of resistance to TuMV isolate CDN 1 in a backcross generation following a cross between a resistant and a susceptible B. napus line showed that the resistance was dominant and monogenic. Molecular markers linked to this dominant resistance were identified using amplified fragment length polymorphism (AFLP) and microsatellite bulk segregant analysis. Bulks consisted of individuals from a BC(1) population with the resistant or the susceptible phenotype following challenge with CDN 1. One AFLP and six microsatellite markers were associated with the resistance locus, named TuRB03, and these mapped to the same region on chromosome N6 as a previously mapped TuMV resistance gene TuRB01. Further testing of TuRB03 with other TuMV isolates showed that it was not effective against all pathotype 4 isolates. It was effective against some, but not all pathotype 3 isolates tested. It provided further resolution of TuMV pathotypes by sub-dividing pathotypes 3 and 4. TuRB03 also provides a new source of resistance for combining with other resistances in our attempts to generate durable resistance to this virus.

The dual role of the potyvirus P3 protein of Turnip mosaic virus as a symptom and avirulence determinant in brassicas

Two isolates of the potyvirus Turnip mosaic virus (TuMV), UK 1 and CDN 1, differ both in their general symptoms on the susceptible propagation host Brassica juncea and in their ability to infect B. napus lines possessing a variety of dominant resistance genes. The isolate CDN 1 produces a more extreme mosaic in infected brassica leaves than UK 1 and is able to overcome the resistance genes TuRB01, TuRB04, and TuRB05. The resistance gene TuRB03, in the B. napus line 22S, is effective against CDN 1 but not UK 1. The nucleic acid sequences of the UK 1 and CDN 1 isolates were 90% identical. The C-terminal half of the P3 protein was identified as being responsible for the differences in symptoms in B. juncea. A single amino acid in the P3 protein was found to be the avirulence determinant for TuRB03. Previous work already has identified the P3 as an avirulence determinant for TuRB04. Our results increase the understanding of the basis of plant-virus recognition, show the importance of the potyviral P3 gene as a symptom determinant, and provide a role in planta for the poorly understood P3 protein in a normal infection cycle.

Strains of Turnip mosaic potyvirus as defined by the molecular analysis of the coat protein gene of the virus

Turnip mosaic virus (TuMV) is a member of the potyvirus genus with a wide host range and highly variable in its biological characteristics. Analysis of the CP gene sequences from databases, combined with the experimental analysis of the CP gene of further isolates, using data derived from sequence or restriction analysis, has allowed the genetic classification of 60 TuMV isolates or sequences. Two main genetic clusters MB (mostly Brassica isolates) and MR (mostly Radish isolates) were found, together with several apparently independent lineages. Isolates in the latter could be grouped as Intermediate between Brassica and Radish clusters (IBR) or outside Brassica and Radish clusters (OBR), according to their genetic distance to the main clusters. The genetic diversity of TuMV isolates deposited in the databases was increased with the sequences of the CP gene of seven selected isolates, mainly belonging to IBR or OBR groups. There was a correlation between the MR genetic cluster and JPN 1 serotype.

Turnip mosaic virus and the quest for durable resistance

Summary Taxonomy: Turnip mosaic virus (TuMV) is a member of the genus Potyvirus (type species Potato virus Y) in the family Potyviridae. To date, TuMV is the only potyvirus known to infect brassicas. There are potyvirus isolates that appear serologically similar to TuMV when tested with polyclonal antisera that do not readily infect brassicas (Lesemann and Vetten, 1985). Physical properties: Virions are approximately 720 x 15-20 nm flexuous rods (Fig. 1) and are composed of 95% coat protein (CP) and 5% RNA. Hosts: TuMV has been isolated from a wide range of crop and weed plant species. It is known to infect at least 318 species in over 43 dicot families, including Cruciferae, Compositae, Chenopodiaceae, Leguminosae and Caryophyllaceae and is also known to infect monocots. It has the broadest known host range in terms of plant genera and families of any potyvirus.

TRANSMISSION

Aphid transmitted in the non-persistent manner, by at least 89 species, including Myzus persicae and Brevicoryne brassicae. Useful website: http://www.ncbi.nlm.nih.gov/ICTVdb/ICTVdB/57010072.htm.

Mutations in Turnip mosaic virus P3 and cylindrical inclusion proteins are separately required to overcome two Brassica napus resistance genes

The Brassica napus differential line 165 is resistant to infection by Turnip mosaic virus (TuMV) isolates belonging to pathotypes 1 and 3. Nucleotide sequences of resistance-breaking mutants of pathotype 1 (UK 1), pathotype 3 (CHN 12), and wild-type isolates have been determined. When the mutations identified were introduced into an infectious clone of UK 1, a single mutation in the viral P3 protein induced a hypersensitive (necrotic) response in inoculated leaves of line 165 plants. Full systemic nonnecrotic infection was only possible when another mutation (in the cylindrical inclusion protein) was introduced. Tests on segregating populations derived from line 165 indicated that the two viral genes were pathogenicity determinants for two different resistance genes in line 165. One gene responsible for an extreme form of resistance (no symptoms seen) was epistatic to a second responsible for the hypersensitive reaction. These results help to explain the relative stability of the resistance in line 165 and to further define the genetic basis of the TuMV pathotyping system.

A fitness cost for Turnip mosaic virus to overcome host resistance

The relative fitness of the Turnip mosaic virus (TuMV) isolate UK 1 was compared with that of two other wildtype isolates CZE 1 and CDN 1. The isolates CZE 1 and CDN 1 are able to overcome the effect of the resistance gene TuRB01 and at least three other resistance sources that are effective against UK 1. Comparisons were also made between the fitness of UK 1 and a recombinant virus with a single nucleotide change (v35Tunos +5570 A>G) conferring the ability to overcome TuRB01 resistance. Co-inoculation experiments were carried out where pairs of isolates were serially passaged over 5 months in a plant line possessing no known resistance genes in order to examine the relative fitness of the isolates. In each case, UK 1 dominated the mixture in the susceptible host background. It out-competed CZE 1 and v35Tunos +5570 A>G within four passages, and CDN 1 after one passage. The greater fitness of UK 1 suggests that there may be a fitness cost to TuMV overcoming resistance genes of brassica crops. This may shed some light on the frequency of naturally occurring isolates, in that pathotype 1 isolates are found much more frequently than isolates of other pathotypes. Implications for the deployment of TuRB01 are discussed.

The Potato virus Y M(S)N(R) NIb-replicase is the elicitor of a veinal necrosis-hypersensitive response in root knot nematode resistant tobacco

Summary A root knot nematode resistance gene in Nicotiana tabacum, Rk, providing resistance to the nematode parasite Meloidogyne incognita is tightly linked to, or is a pleiotropic gene with a veinal necrosis systemic hypersensitive response to infection by Potato virus Y (PVY) M(s)N(r). The single PVY M(s)N(r) open reading frame was sequenced and found to have 89% protein identity to PVY N. Individual PVY M(s)N(r) polypeptides were deduced and the corresponding cDNA were cloned into a Potato virus X (PVX) based expression vector and used as templates for in vitro transcriptions. Infected plant sap, from N. benthamiana inoculated with infectious RNA, was used to inoculate both root knot nematode (RKN) resistant and susceptible tobacco lines. Lines were then evaluated for the induction of the hypersensitive response. The PVY M(s)N(r) NIb-replicase protein was found to induce a hypersensitive response 10 days post inoculation in nematode resistant tobacco. None of the other PVX/PVY M(s)N(r) constructs induced a hypersensitive response. The NIb-replicase of PVY N, which shares 93% identity to PVY M(s)N(r), did not induce a hypersensitive response when expressed from the PVX vector. This confirmed that the PVY M(s)N(r) NIb-replicase is the elicitor of PVY M(s)N(r) veinal necrosis on RKN plants and thus the first report of a Potyvirus replicase functioning as an avirulence factor.

Characterization of typical pepper-isolates of PVY reveals multiple pathotypes within a single genetic strain

Potato virus Y (PVY) isolates originally coming from infected pepper plants, were biologically and genetically characterized, especially in comparison with PVY potato-isolates. Pepper PVY isolates could be differentiated from potato isolates in their host range, aphid transmission efficiencies, Mab serology, and genetic status. The genetic distances estimated for PVY pepper-isolates, based on their restrictotypes with five restriction enzymes and on their coat protein gene sequences, indicated that they form a single genetic strain with different pathotypic properties. This situation is essentially different to that of PVY potato-isolates.