RESUMEN
ABSTRACT Maize streak virus (MSV) is best known as the causal agent of maize streak disease. However, only a genetically uniform subset of the viruses within this diverse species is actually capable of producing severe symptoms in maize. Whereas these "maize-type" viruses all share greater than 95% sequence identity, MSV strains isolated from grasses may share as little as 79% sequence identity with the maize-type viruses. Here, we present the complete genome sequences and biological characterization of two MSV isolates from wheat that share approximately 89% sequence identity with the maize-type viruses. Clonal populations of these two isolates, named MSV-Tas and MSV-VW, were leafhopper-transmitted to Digitaria sanguinalis and a range of maize, wheat, and barley genotypes. Whereas the two viruses showed some differences in their pathogenicity in maize, they were both equally pathogenic in D. sanguinalis and the various wheat and barley genotypes tested. Phylogenetic analyses involving the genome sequences of MSV-Tas and MSV-VW, a new maize-type virus also fully sequenced in this study (MSV-VM), and all other available African streak virus sequences, indicated that MSV-Tas and MSV-VW are close relatives that together represent a distinct MSV strain. Sequence analyses revealed that MSV-VM has a recombinant genome containing MSV-Tas/VW-like sequences within its movement protein gene.
RESUMEN
Recombination between divergent virus genomes is believed to be a major mechanism for generation of novel virus genotypes. We have examined the recombination process in geminiviruses by forcing recombination between two distinct isolates of Maize streak virus (MSV), MSV-Kom and MSV-Set. Heterodimeric agroinfectious constructs containing tandemly cloned mixtures of complete or partial MSV-Set and MSV-Kom genomes were used to simulate a circular dimeric form similar to that which would be expected to occur following a single intermolecular crossing-over event between MSV-Set and MSV-Kom replicative form DNAs at the long intergenic region (LIR)-movement protein gene (MP) interface. We isolated, analysed and biologically characterized many of the recombinant MSV genomes that were generated from the constructs in planta. Apart from having the same simulated breakpoint at the LIR-MP interface, all the genomes examined had a second breakpoint that had been generated through either intramolecular homologous recombination or a replicational release mechanism. The pathogenicities of six predominantly MSV-Kom-like recombinants were tested in maize. While all were capable of producing a symptomatic infection in this host, none was more virulent than MSV-Kom and only two were more virulent than MSV-Set. The two most virulent recombinants were leafhopper transmitted to a range of differentially MSV-resistant maize, wheat and barley genotypes and both were found to have unique biological properties.
Asunto(s)
Virus de la Veta de Maíz/genética , Recombinación Genética , Secuencia de Bases , ADN Intergénico , Genoma Viral , Genotipo , Virus de la Veta de Maíz/patogenicidad , Datos de Secuencia Molecular , Enfermedades de las Plantas/virología , Proteínas de Movimiento Viral en Plantas , Alineación de Secuencia , Proteínas Virales/genética , Virulencia , Zea mays/virologíaRESUMEN
Full genomic sequences were determined for 12 Maize streak virus (MSV) isolates obtained from Zea mays and wild grass species. These and 10 other publicly available full-length sequences were used to classify a total of 66 additional MSV isolates that had been characterized by PCR-restriction fragment length polymorphism and/or partial nucleotide sequence analysis. A description is given of the host and geographical distribution of the MSV strain and subtype groupings identified. The relationship between the genotypes of 21 fully sequenced virus isolates and their virulence in differentially MSV-resistant Z. mays genotypes was examined. Within the only MSV strain grouping that produced severe symptoms in maize, highly virulent and widely distributed genotypes were identified that are likely to pose the most serious threat to maize production in Africa. Evidence is presented that certain of the isolates investigated may be the products of either intra- or interspecific recombination.
Asunto(s)
Geminiviridae/genética , Variación Genética , Zea mays/virología , Geminiviridae/clasificación , Geminiviridae/aislamiento & purificación , Geminiviridae/patogenicidad , Genoma Viral , Recombinación Genética , Análisis de Secuencia de ADN , VirulenciaRESUMEN
beta-1,3-d-Glucans are biological response modifiers with potent effects on the immune system. A number of receptors are thought to play a role in mediating these responses, including murine Dectin-1, which we recently identified as a beta-glucan receptor. In this study we describe the characterization of the human homologue of this receptor and show that it is structurally and functionally similar to the mouse receptor. The human beta-glucan receptor is a type II transmembrane receptor with a single extracellular carbohydrate recognition domain and an immunoreceptor tyrosine activation motif in its cytoplasmic tail. The human beta-glucan receptor is widely expressed and functions as a pattern recognition receptor, recognizing a variety of beta-1,3- and/or beta-1,6-linked glucans as well as intact yeast. In contrast to the murine receptor, the human receptor mRNA is alternatively spliced, resulting in two major (A and B) and six minor isoforms. The two major isoforms differ by the presence of a stalk region separating the carbohydrate recognition domain from the transmembrane region and are the only isoforms that are functional for beta-glucan binding. The human receptor also binds T-lymphocytes at a site distinct from the beta-glucan binding site, indicating that this receptor can recognize both endogenous and exogenous ligands.
Asunto(s)
Empalme Alternativo , Receptores Inmunológicos/metabolismo , Animales , Secuencia de Bases , Candida/metabolismo , Línea Celular , Clonación Molecular , Cartilla de ADN , Humanos , Ratones , Datos de Secuencia Molecular , Unión Proteica , Receptores Inmunológicos/química , Receptores Inmunológicos/genética , Reacción en Cadena de la Polimerasa de Transcriptasa Inversa , Linfocitos T/metabolismoRESUMEN
The genomic nucleotide sequences of the cloned agroinfectious genomes of three South African mastreviruses obtained from Zea mays, a Setaria sp., and Panicum maximum (designated MSV-Kom, MSV-Set, and PanSV-Kar respectively), were determined. Additionally, their relative infectivities and virulence were analysed in a range of differentially susceptible wheat, maize, and barley genotypes. MSV-Kom produced moderate to severe streak symptoms in all maize genotypes tested, but only moderate to very mild symptoms in the wheat and barley genotypes. MSV-Set infected only the susceptible to tolerant maize genotypes, but was generally more severe in the barley and wheat genotypes than MSV-Kom. PanSV-Kar was incapable of infecting any of the wheat and barley genotypes and only produced very mild symptoms on the three most sensitive maize genotypes. Genomic characteristics in common with related mastreviruses were identified. Phylogenetic analysis indicated that while MSV-Kom was closely related to previously sequenced MSV isolates, MSV-Set and PanSV-Kar represented distinctly novel strains of MSV and PanSV respectively. In the case of MSV-Set, this is the most distantly related MSV strain yet characterised.
Asunto(s)
Geminiviridae/genética , Geminiviridae/patogenicidad , Genoma Viral , Animales , Secuencia de Bases , ADN Intergénico/genética , ADN Viral/genética , Geminiviridae/aislamiento & purificación , Hemípteros/virología , Hordeum/virología , Datos de Secuencia Molecular , Panicum/virología , Filogenia , Enfermedades de las Plantas/virología , Sudáfrica , Especificidad de la Especie , Triticum/virología , Virulencia , Zea mays/virologíaRESUMEN
Maize streak virus (MSV) is the most economically significant member of a diverse group of African grass-infecting Mastrevirus species in the family Geminiviridae. We designed a single set of degenerate primers which enables the PCR amplification of an approximately 1300 bp DNA fragment spanning both conserved (the RepA gene) and variable (the long intergenic region and MP gene) portions of these viruses' genomes. Using restriction fragment length polymorphism (RFLP) analysis of PCR products obtained from 39 MSV, one SSV, and two PanSV isolates, it was possible to both identify the different virus species, which differ in nucleotide sequence by up to 40%, and to differentiate between MSV isolates sharing up to 99% sequence identity. The reliability of the RFLP data for typing the MSV isolates was verified by the phylogenetic analysis of the partial genomic nucleotide sequences of a representative subset of the MSV isolates. Based on both the RFLP and sequence data, the MSV isolates could be clearly differentiated into the four groups: these were a group of predominantly maize-infecting isolates, and three groups containing grass/wheat-infecting isolates. RFLP analysis also revealed a number of mixed virus infections in which, in certain instances, it was possible to identify individual population members.
Asunto(s)
Geminiviridae/clasificación , Genoma Viral , África , Secuencia de Bases , Clonación Molecular , Cartilla de ADN , Geminiviridae/genética , Geminiviridae/aislamiento & purificación , Enfermedades de las Plantas/virología , Poaceae/virología , Polimorfismo de Longitud del Fragmento de Restricción , Homología de Secuencia , Zea mays/virologíaRESUMEN
ABSTRACT We devised a rapid technique for the objective and precise assessment of both the pathogenicity of maize streak virus (MSV) isolates and the MSV resistance of maize genotypes. The technique involves the use of agroinoculation to infect maize seedlings and the objective symptom evaluation by quantification of infection rates, stunting, and chlorotic leaf areas. In assessing the MSV resistance of 19 maize genotypes, we describe how the use of differentially virulent virus isolates enables the analysis of MSV resistance phenotypes, ranging from extremely susceptible to completely immune. We further demonstrate how quantification of chlorotic leaf areas by image analysis permits differentiation between degrees of MSV resistance that are indistinguishable from one another using currently employed symptom assessment approaches. Using chlorotic area measurements, we quantify the virulence of a diverse group of 10 MSV isolates and, through agroinoculation of differentially susceptible maize genotypes, we demonstrate the use of our technique in evaluating the pathogenicity of these isolates.