Isolation of an Arabidopsis thaliana mutant in which the multiplication of both cucumber mosaic virus and turnip crinkle virus is affected

Peculiarities of the regulation of translation initiation in plants

Protein synthesis is a fundamental process for life and, as such, plays a crucial role in the adaptation to energy, developmentaland environmental conditions. For these reasons, and despite the general conservation of the eukaryotic translational machinery, it is not surprising that organisms with different lifestyles have evolved distinct mechanisms of regulation to adapt translation initiation to their intrinsic growth and development. Plants have clear peculiarities compared with other eukaryotes that have also extended to translation control. This review describes the plant-specific mechanisms for regulation of translation initiation, with a focus on those that modulate the eIF4F complexes, central translational regulatory hubs in all eukaryotes, and highlights the latest discoveries on the signaling pathways that regulate their constituents and activity.

Identification of Cucumber mosaic resistance 2 (cmr2) That Confers Resistance to a New Cucumber mosaic virus Isolate P1 (CMV-P1) in Pepper (Capsicum spp.)

Cucumber mosaic virus (CMV) is one of the most devastating phytopathogens of Capsicum. The single dominant resistance gene, Cucumber mosaic resistant 1 (Cmr1), that confers resistance to the CMV isolate P0 has been overcome by a new isolate (CMV-P1) after being deployed in pepper (Capsicum annuum) breeding for over 20 years. A recently identified Indian C. annuum cultivar, "Lam32," displays resistance to CMV-P1. In this study, we show that the resistance in "Lam32" is controlled by a single recessive gene, CMV resistance gene 2 (cmr2). We found that cmr2 conferred resistance to CMV strains including CMV-Korean, CMV-Fny, and CMV-P1, indicating that cmr2 provides a broad-spectrum type of resistance. We utilized two molecular mapping approaches to determine the chromosomal location of cmr2. Bulked segregant analysis (BSA) using amplified fragment-length polymorphism (AFLP) (BSA-AFLP) revealed one marker, cmvAFLP, located 16 cM from cmr2. BSA using the Affymetrix pepper array (BSA-Affy) identified a single-nucleotide polymorphism (SNP) marker (Affy4) located 2.3 cM from cmr2 on chromosome 8. We further screened a pepper germplasm collection of 4,197 accessions for additional CMV-P1 resistance sources and found that some accessions contained equivalent levels of resistance to that of "Lam32." Inheritance and allelism tests demonstrated that all the resistance sources examined contained cmr2. Our result thus provide genetic and molecular evidence that cmr2 is a single recessive gene that confers to pepper an unprecedented resistance to the dangerous new isolate CMV-P1 that had overcome Cmr1.

Trans-species synthetic gene design allows resistance pyramiding and broad-spectrum engineering of virus resistance in plants

To infect plants, viruses rely heavily on their host's machinery. Plant genetic resistances based on host factor modifications can be found among existing natural variability and are widely used for some but not all crops. While biotechnology can supply for the lack of natural resistance alleles, new strategies need to be developed to increase resistance spectra and durability without impairing plant development. Here, we assess how the targeted allele modification of the Arabidopsis thaliana translation initiation factor eIF4E1 can lead to broad and efficient resistance to the major group of potyviruses. A synthetic Arabidopsis thaliana eIF4E1 allele was designed by introducing multiple amino acid changes associated with resistance to potyvirus in naturally occurring Pisum sativum alleles. This new allele encodes a functional protein while maintaining plant resistance to a potyvirus isolate that usually hijacks eIF4E1. Due to its biological functionality, this synthetic allele allows, at no developmental cost, the pyramiding of resistances to potyviruses that selectively use the two major translation initiation factors, eIF4E1 or its isoform eIFiso4E. Moreover, this combination extends the resistance spectrum to potyvirus isolates for which no efficient resistance has so far been found, including resistance-breaking isolates and an unrelated virus belonging to the Luteoviridae family. This study is a proof-of-concept for the efficiency of gene engineering combined with knowledge of natural variation to generate trans-species virus resistance at no developmental cost to the plant. This has implications for breeding of crops with broad-spectrum and high durability resistance using recent genome editing techniques.

Recessive Resistance to Plant Viruses: Potential Resistance Genes Beyond Translation Initiation Factors

The ability of plant viruses to propagate their genomes in host cells depends on many host factors. In the absence of an agrochemical that specifically targets plant viral infection cycles, one of the most effective methods for controlling viral diseases in plants is taking advantage of the host plant’s resistance machinery. Recessive resistance is conferred by a recessive gene mutation that encodes a host factor critical for viral infection. It is a branch of the resistance machinery and, as an inherited characteristic, is very durable. Moreover, recessive resistance may be acquired by a deficiency in a negative regulator of plant defense responses, possibly due to the autoactivation of defense signaling. Eukaryotic translation initiation factor (eIF) 4E and eIF4G and their isoforms are the most widely exploited recessive resistance genes in several crop species, and they are effective against a subset of viral species. However, the establishment of efficient, recessive resistance-type antiviral control strategies against a wider range of plant viral diseases requires genetic resources other than eIF4Es. In this review, we focus on recent advances related to antiviral recessive resistance genes evaluated in model plants and several crop species. We also address the roles of next-generation sequencing and genome editing technologies in improving plant genetic resources for recessive resistance-based antiviral breeding in various crop species.

3' cap-independent translation enhancers of plant viruses

In the absence of a 5' cap, plant positive-strand RNA viruses have evolved a number of different elements in their 3' untranslated region (UTR) to attract initiation factors and/or ribosomes to their templates. These 3' cap-independent translational enhancers (3' CITEs) take different forms, such as I-shaped, Y-shaped, T-shaped, or pseudoknotted structures, or radiate multiple helices from a central hub. Common features of most 3' CITEs include the ability to bind a component of the translation initiation factor eIF4F complex and to engage in an RNA-RNA kissing-loop interaction with a hairpin loop located at the 5' end of the RNA. The two T-shaped structures can bind to ribosomes and ribosomal subunits, with one structure also able to engage in a simultaneous long-distance RNA-RNA interaction. Several of these 3' CITEs are interchangeable and there is evidence that natural recombination allows exchange of modular CITE units, which may overcome genetic resistance or extend the virus's host range.

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.

Multigenic system controlling viral systemic infection determined by the interactions between Cucumber mosaic virus genes and quantitative trait loci of soybean cultivars

Soybean 'Harosoy' is resistant to Cucumber mosaic virus soybean strain C (CMV-SC) and susceptible to CMV-S strain D (CMV-SD). Using enzyme-linked immunosorbent assay and Northern hybridization, we characterized the Harosoy resistance and found that CMV-SC did not spread systemically but was restricted to the inoculated leaves in Harosoy. Harosoy resistance was not controlled by either a dominant or recessive single gene. To dissect this system controlling long-distance movement of CMV in soybean, we constructed infectious cDNA clones of CMV-SC and CMV-SD. Using these constructs and the chimeric RNAs, we demonstrated that two viral components were required for systemic infection by the virus. The region including the entire 2b gene and the 5' region of RNA3 (mainly the 5' untranslated region) together were required. By quantitative trait locus (QTL) analysis using an F(2) population and the F(3) families derived from Harosoy and susceptible 'Nemashirazu', we also showed that at least three QTLs affected systemic infection of CMV in soybean. Our study on Harosoy resistance to CMV-SC revealed an interesting mechanism, in which multiple host and viral genes coordinately controlled viral systemic infection.

Nicotiana benthamiana resistance to non-adapted Melon necrotic spot virus results from an incompatible interaction between virus RNA and translation initiation factor 4E

Nicotiana benthamiana has been described as non-host for Melon necrotic spot virus (MNSV). We investigated the basis of this resistance using the unique opportunity provided by strain MNSV-264, a recombinant virus that is able to overcome the resistance. Analysis of chimeric MNSV mutants showed that virulence in N. benthamiana is conferred by a 49 nucleotide section of the MNSV-264 3'-UTR, which acts in this host as a cap-independent translational enhancer (3'-CITE). Although the 3'-CITE of non-adapted MNSV-Mα5 is active in susceptible melon, it does not promote efficient translation in N. benthamiana, thus preventing expression of proteins required for virus replication. However, MNSV-Mα5 gains the ability to multiply in N. benthamiana cells if eIF4E from a susceptible melon variety (Cm-eIF4E-S) is supplied in trans. These data show that N. benthamiana resistance to MNSV-Mα5 results from incompatibility between the MNSV-Mα5 3'-CITE and N. benthamiana eIF4E in initiating efficient translation of the viral genome. Therefore, non-host resistance conferred by the inability of a host susceptibility factor to support viral multiplication may be a possible mechanism for this type of resistance to viruses.

Recessive resistance to plant viruses

About half of the approximately 200 known virus resistance genes in plants are recessively inherited, suggesting that this form of resistance is more common for viruses than for other plant pathogens. The use of such genes is therefore a very important tool in breeding programs to control plant diseases caused by pathogenic viruses. Over the last few years, the detailed analysis of many host/virus combinations has substantially advanced basic research on recessive resistance mechanisms in crop species. This type of resistance is preferentially expressed in protoplasts and inoculated leaves, influencing virus multiplication at the single-cell level as well as cell-to-cell movement. Importantly, a growing number of recessive resistance genes have been cloned from crop species, and further analysis has shown them all to encode translation initiation factors of the 4E (eIF4E) and 4G (eIF4G) families. However, not all of the loss-of-susceptibility mutants identified in collections of mutagenized hosts correspond to mutations in eIF4E and eIF4G. This, together with other supporting data, suggests that more extensive characterization of the natural variability of resistance genes may identify new host factors conferring recessive resistance. In this chapter, we discuss the recent work carried out to characterize loss-of-susceptibility and recessive resistance genes in crop and model species. We review actual and probable recessive resistance mechanisms, and bring the chapter to a close by summarizing the current state-of-the-art and offering perspectives on potential future developments.

Disruption of a novel gene for a NAC-domain protein in rice confers resistance to Rice dwarf virus

Rice dwarf virus (RDV) is a serious viral pest that is transmitted to rice plants (Oryza sativa L.) by leafhoppers and causes a dwarfism in infected plants. To identify host factors involved in the multiplication of RDV, we screened Tos17 insertion mutant lines of rice for mutants with reduced susceptibility to RDV. One mutant, designated rim1-1, did not show typical disease symptoms upon infection with RDV. The accumulation of RDV capsid proteins was also drastically reduced in inoculated rim1-1 mutant plants. Co-segregation and complementation analyses revealed that the rim1-1 mutation had been caused by insertion of Tos17 in an intron of a novel NAC gene. The rim1-1 mutant remained susceptible to the two other viruses tested, one of which is also transmitted by leafhoppers, suggesting that the multiplication rather than transmission of RDV is specifically impaired in this mutant. We propose that RIM1 functions as a host factor that is required for multiplication of RDV in rice.

Dissection of the oligogenic resistance to Cucumber mosaic virus in the melon accession PI 161375

Resistance to Cucumber mosaic virus (CMV) in the exotic melon accession PI 161375, cultivar "Sonwang Charmi" (SC) had previously been described as oligogenic, recessive and quantitative, with a major QTL residing in linkage group XII (LGXII). We have used a collection of near isogenic lines (NILs) with introgressions of SC into the genome of the susceptible accession Piel de Sapo (PS) to further characterise this resistance. Infection of NILs carrying introgressions on LGXII showed that only NIL SC12-1 was resistant to CMV strains P9 and P104.82, but not to strains M6 and TL. Further mapping of this region showed that the resistance, named cmv1 maps in an area of 2.2 cM, between markers CMN61_44 and CMN21_55. Moreover, cmv1 confers total resistance to strains P9 and P104.82, indicating that in these cases it is not quantitative and that cmv1 is sufficient to confer full resistance to these CMV strains. Candidate gene mapping of ten translation initiation factors in the melon genome failed to find any of them in the interval between markers CMN61_44 and CMN21_55. All these results suggest that the resistance to CMV present in SC is oligogenic, where different loci confer resistance to different CMV strains, but not necessarily quantitative, since at least one of these genes (cmv1) confers total resistance, similar to that of the parental SC, and does not need the contribution of other loci.

Mutational analysis of plant cap-binding protein eIF4E reveals key amino acids involved in biochemical functions and potyvirus infection

The eukaryotic translation initiation factor 4E (eIF4E) (the cap-binding protein) is involved in natural resistance against several potyviruses in plants. In lettuce, the recessive resistance genes mo1(1) and mo1(2) against Lettuce mosaic virus (LMV) are alleles coding for forms of eIF4E unable, or less effective, to support virus accumulation. A recombinant LMV expressing the eIF4E of a susceptible lettuce variety from its genome was able to produce symptoms in mo1(1) or mo1(2) varieties. In order to identify the eIF4E amino acid residues necessary for viral infection, we constructed recombinant LMV expressing eIF4E with point mutations affecting various amino acids and compared the abilities of these eIF4E mutants to complement LMV infection in resistant plants. Three types of mutations were produced in order to affect different biochemical functions of eIF4E: cap binding, eIF4G binding, and putative interaction with other virus or host proteins. Several mutations severely reduced the ability of eIF4E to complement LMV accumulation in a resistant host and impeded essential eIF4E functions in yeast. However, the ability of eIF4E to bind a cap analogue or to fully interact with eIF4G appeared unlinked to LMV infection. In addition to providing a functional mutational map of a plant eIF4E, this suggests that the role of eIF4E in the LMV cycle might be distinct from its physiological function in cellular mRNA translation.

A multidirectional non-cell autonomous control and a genetic interaction restricting tobacco etch virus susceptibility in Arabidopsis

Background

Viruses constitute a major class of pathogens that infect a variety of hosts. Understanding the intricacies of signaling during host-virus interactions should aid in designing disease prevention strategies and in understanding mechanistic aspects of host and pathogen signaling machinery.

Methodology/Principal Findings

An Arabidopsis mutant, B149, impaired in susceptibility to Tobacco etch virus (TEV), a positive strand RNA virus of picoRNA family, was identified using a high-throughput genetic screen and a counterselection scheme. The defects include initiation of infection foci, rate of cell-to-cell movement and long distance movement.

Conclusions/Significance

The defect in infectivity is conferred by a recessive locus. Molecular genetic analysis and complementation analysis with three alleles of a previously published mutant lsp1 (loss of susceptibility to potyviruses) indicate a genetic interaction conferring haploinsufficiency between the B149 locus and certain alleles of lsp1 resulting in impaired host susceptibility. The pattern of restriction of TEV foci on leaves at or near the boundaries of certain cell types and leaf boundaries suggest dysregulation of a multidirectional non-cell autonomous regulatory mechanism. Understanding the nature of this multidirectional signal and the molecular genetic mechanism conferring it should potentially reveal a novel arsenal in the cellular machinery.

[Host factors and their relevance to virus infection in plants]

Virus infection is established when viral proteins can interact with host factors to execute replication and/or cell-to-cell movement. Even after the virus infection has started, host resistance reactions, if trigged, would suppress further virus propagation. We would like to introduce what we understand about host factors as determinants of infection establishment and as key resistance molecules. Genome-wide information of Arabidopsis is providing us much information about such host factors involved in virus infection.

Replication of positive-strand RNA viruses in plants: contact points between plant and virus components

Positive-strand RNA viruses constitute the largest group of plant viruses and have an important impact on world agriculture. These viruses have small genomes that encode a limited number of proteins and depend on their hosts to complete the various steps of their replication cycle. In this review, the contact points between positive-strand RNA plant viruses and their hosts, which are necessary for the translation and replication of the viral genomes, are discussed. Special emphasis is placed on the description of viral replication complexes that are associated with specific membranous compartments derived from plant intracellular membranes and contain viral RNAs and proteins as well as a variety of host proteins. These complexes are assembled via an intricate network of protein–protein, protein–membrane, and protein–RNA interactions. The role of host factors in regulating the assembly, stability, and activity of viral replication complexes are also discussed.

The Arabidopsis cucumovirus multiplication 1 and 2 loci encode translation initiation factors 4E and 4G

The cum1 and cum2 mutations of Arabidopsis thaliana inhibit cucumber mosaic virus (CMV) multiplication. In cum1 and cum2 protoplasts, CMV RNA and the coat protein accumulated to wild-type levels, but the accumulation of the 3a protein of CMV, which is necessary for cell-to-cell movement of the virus, was strongly reduced compared with that in wild-type protoplasts. In cum2 protoplasts, the accumulation of turnip crinkle virus (TCV)-related RNA and proteins was also reduced. Positional cloning demonstrated that CUM1 and CUM2 encode eukaryotic translation initiation factors 4E and 4G, respectively. Unlike most cellular mRNA, the CMV RNA lacks a poly(A) tail, whereas the TCV RNA lacks both a 5'-terminal cap and a poly(A) tail. In vivo translation analyses, using chimeric luciferase mRNA carrying the terminal structures and untranslated sequences of the CMV or TCV RNA, demonstrated that these viral untranslated sequences contain elements that regulate the expression of encoded proteins positively or negatively. The cum1 and cum2 mutations had different effects on the action of these elements, suggesting that the cum1 and cum2 mutations cause inefficient production of CMV 3a protein and that the cum2 mutation affects the production of TCV-encoded proteins.

Advances in understanding recessive resistance to plant viruses

SUMMARY Recent work carried out to characterize recessive mutations which render experimental hosts non-permissive to viral infection (loss-of-susceptibility mutants) seems to be converging with new data on natural recessive resistance in crop species, and also with functional analyses of virus avirulence determinants. Perhaps the most well known examples are the studies that identified the eukaryotic translation initiation factors 4E(iso) (eIF(iso)4E) and 4E(eIF4E) as the host factors required for potyvirus multiplication within experimental and natural hosts, respectively, and the potyviral genome-linked protein (VPg) as the viral factor that directly interacts with eIF4E to promote potyvirus multiplication. The purpose of this paper is to review the available information on the characterization of loss-of-susceptibility mutants in experimental hosts, natural recessive resistances and virus avirulence factors, and also to comment on possible implications for the design of new sources of sustainable virus resistance.

Infection and RNA recombination of Brome mosaic virus in Arabidopsis thaliana

Ecotypes of Arabidopsis thaliana supported the replication and systemic spread of Brome mosaic virus (BMV) RNAs. Infection was induced either by manual inoculation with viral RNA or by BMV virions, demonstrating that virus disassembly did not prevent infection. When in vitro-transcribed BMV RNAs 1-3 were used, production of subgenomic RNA4 was observed, showing that BMV RNA replication and transcription had occurred. Furthermore, inoculations of the transgenic Arabidopsis line that expressed a suppressor of RNA interference (RNAi) pathway markedly increased the BMV RNA concentrations. Inoculations with designed BMV RNA3 recombination vectors generated both homologous and nonhomologous BMV RNA-RNA recombinants. Thus, all cellular factors essential for BMV RNA replication, transcription, and RNA recombination were shown to be present in Arabidopsis. The current scope of understanding of the model Arabidopsis plant system should facilitate the identification of these factors governing the BMV life cycle.

Cucumoviruses

Research on the molecular biology of cucumoviruses and their plant-virus interactions has been very extensive in the last decade. Cucumovirus genome structures have been analyzed, giving new insights into their genetic variability, evolution, and taxonomy. A new viral gene has been discovered, and its role in promoting virus infection has been delineated. The localization and various functions of each viral-encoded gene product have been established. The particle structures of Cucumber mosaic virus (CMV) and Tomato aspermy virus have been determined. Pathogenicity domains have been mapped, and barriers to virus infection have been localized. The movement pathways of the viruses in some hosts have been discerned, and viral mutants affecting the movement processes have been identified. Host responses to viral infection have been characterized, both temporally and spatially. Progress has been made in determining the mechanisms of replication, gene expression, and transmission of CMV. The pathogenicity determinants of various satellite RNAs have been characterized, and the importance of secondary structure in satellite RNA-mediated interactions has been recognized. Novel plant genes specifying resistance to infection by CMV have been identified. In some cases, these genes have been mapped, and one resistance gene to CMV has been isolated and characterized. Pathogen-derived resistance has been demonstrated against CMV using various segments of the CMV genome, and the mechanisms of some of these forms of resistances have been analyzed. Finally, the nature of synergistic interactions between CMV and other viruses has been characterized. This review highlights these various achievements in the context of the previous work on the biology of cucumoviruses and their interactions with plants.

Rsv1-mediated resistance against soybean mosaic virus-N is hypersensitive response-independent at inoculation site, but has the potential to initiate a hypersensitive response-like mechanism

Rsv1, a single dominant gene in soybean PI 96983, confers resistance to most strains of Soybean mosaic virus (SMV), including strain G2. The phenotypic response includes the lack of symptoms and virus recovery from mechanically inoculated leaves. To study the resistance mechanism, SMV-N (an isolate of strain G2) was introduced into PI 96983 by grafting. Hypersensitive response (HR)-like lesions occurred on the stems, petioles, and leaf veins, and virus was recovered from these lesions. The response demonstrated the cytological and histological characteristics of HR as well as elevated transcription of a soybean salicylic acid-inducible, pathogenesis-related (PR-1) protein gene. Mechanical inoculation of PI 96983 primary leaves with a high level of SMV-N virions caused no symptoms or up regulation of the PR-1 protein gene transcript. Furthermore, inoculation with infectious viral RNA did not alter the resistance phenotype. The data suggest that interaction of SMV-N with Rsv1 has the potential to induce an HR-like defense reaction. Rsv1-mediated resistance in the inoculated leaf, however, is HR-independent and operates after virion disassembly.

The multifunctional capsid proteins of plant RNA viruses

This article summarizes studies of viral coat (capsid) proteins (CPs) of RNA plant viruses. In addition, we discuss and seek to interpret the knowledge accumulated to data. CPs are named for their primary function; to encapsidate viral genomic nucleic acids. However, encapsidation is only one feature of an extremely diverse array of structural, functional, and ecological roles played during viral infection and spread. Herein, we consider the evolution of viral CPs and their multitude of interactions with factors encoded by the virus, host plant, or viral vector (biological transmission agent) that influence the infection and epidemiological facets of plant disease. In addition, applications of today's understanding of CPs in the protection of crops from viral infection and use in the manufacture of valuable compounds are considered.

Susceptibility and symptom development in Arabidopsis thaliana to Tobacco mosaic virus is influenced by virus cell-to-cell movement

To identify host factors that regulate susceptibility to Tobacco mosaic virus (TMV), 14 Arabidopsis thaliana ecotypes were screened for their ability to support TMV systemic movement. The susceptibility phenotypes observed included one ecotype that permitted rapid TMV movement accompanied by symptoms, nine ecotypes that allowed a slower intermediate rate of systemic movement without symptoms, and four ecotypes that allowed little or no systemic TMV movement. Molecular comparisons between ecotypes representing the rapid (Shahdara), intermediate (Col-1), and slow (Tsu-1) movement phenotypes revealed a positive correlation between the ability of TMV to move cell to cell and its speed of systemic movement. Additionally, protoplasts prepared from all three ecotypes supported similar levels of TMV replication, indicating that viral replication did not account for differences in systemic movement. Furthermore, induction of the pathogenesis-related genes PR-1 and PR-5 occurred only in the highly susceptible ecotype Shahdara, demonstrating that reduced local and systemic movement in Col-1 and Tsu-1 was not due to the activation of known host defense responses. Genetic analysis of F2 progeny derived from crosses made between Shahdara and Tsu-1 or Col-1 and Tsu-1 showed the faster cell-to-cell movement phenotypes of Shahdara and Col-1 segregated as single dominant genes. In addition, the Shahdara symptom phenotype segregated independently as a single recessive gene. Taken together, these findings suggest that, within Arabidopsis ecotypes, at least two genes modulate susceptibility to TMV.

Host suppressors in Arabidopsis thaliana of mutations in the movement protein gene of Cauliflower mosaic virus

A novel genetic screen was used to identify host factors in Arabidopsis thaliana that suppress mutations in the Cauliflower mosaic virus (CaMV) movement protein gene (gene I). A series of small mutations was made in gene I and the mutations were tested for their suitability in a suppressor screen. The first round of screening yielded only revertants or second-site mutations in gene I. A derivative of one of the second-site mutant viruses (N7) that was delayed in symptom production was used in a second round of screening for suppressor plants that accelerated symptom production. Two candidate suppressor plants were found that accelerated by 1 to 4 days the first appearance of symptoms caused by the mutant viruses. One of the suppressors (5-2), called asc1 (acceleration of symptoms by CaMV N7), was mapped to chromosome 1. Two additional loci that differentially affect N7 virus susceptibility in the parental Columbia and Ler ecotypes were mapped to chromosomes 3 and 4 by quantitative trait locus (QTL) analysis.

An improved protocol for the preparation of protoplasts from an established Arabidopsis thaliana cell suspension culture and infection with RNA of turnip yellow mosaic tymovirus: a simple and reliable method

An improved method for preparation of protoplasts of Arabidopsis thaliana cells grown in suspension culture is presented. This method is fast, reliable and can be used for the production of virtually an unlimited number of protoplasts at any time. These protoplasts can be transformed efficiently with RNA from turnip yellow mosaic tymovirus (TYMV) by polyethyleneglycol-mediated transfection. The simple transfection procedure has been optimized at various steps. Replication of TYMV can be monitored routinely by detection of the coat protein in as few as 2 x 10(4) infected protoplasts.

A single-stranded loop in the 5' untranslated region of cucumber mosaic virus RNA 4 contributes to competitive translational activity

The 5' untranslated region (UTR) of cucumber mosaic virus (CMV) RNA 4 confers a highly competitive translational advantage on a heterologous luciferase open reading frame. Here we investigated whether secondary structure in the 5' UTR contributes to this translational advantage. Stabilization of the 5' UTR RNA secondary structure inhibited competitive translational activity. Alteration of a potential single-stranded loop to a stem by substitution mutations greatly inhibited the competitive translational activity. Tobacco plants infected with wild type virus showed a 2.5-fold higher accumulation of maximal coat protein than did plants infected with a loop-mutant virus. Amplification of viral RNA in these plants could not explain the difference in accumulation of coat protein. Phylogenetic comparison showed that potential single-stranded loops of 12-23 nucleotides in length exist widely in subgroups of CMV.

Evidence for autoregulation of cystathionine gamma-synthase mRNA stability in Arabidopsis

Control of messenger RNA (mRNA) stability serves as an important mechanism for regulating gene expression. Analysis of Arabidopsis mutants that overaccumulate soluble methionine (Met) revealed that the gene for cystathionine gamma-synthase (CGS), the key enzyme in Met biosynthesis, is regulated at the level of mRNA stability. Transfection experiments with wild-type and mutant forms of the CGS gene suggest that an amino acid sequence encoded by the first exon of CGS acts in cis to destabilize its own mRNA in a process that is activated by Met or one of its metabolites.

Resistances to turnip mosaic potyvirus in Arabidopsis thaliana

The responses of a collection of Arabidopsis thaliana ecotypes to mechanical inoculation with turnip mosaic potyvirus were assessed. The virus induced characteristic severe symptoms of infection in systemically infected plants. Resistance was found in four ecotypes: Bay-0, Di-0, Er-0, and Or-0. Enzyme-linked immunosorbent assay results of the resistant ecotypes suggested that ecotypes Di-0, Er-0, and Or-0 actually consist of mixed genotypes with resistances acting at different levels in the virus life cycle. Another form of resistance was found in ecotype Bay-0, for which several lines of evidence indicated an interference with viral cell-to-cell movement in the inoculated leaves.