Claude Bragard

Unusual Features of Pomoviral RNA Movement

Potato mop-top pomovirus (PMTV) is one of a few viruses that can move systemically in plants in the absence of the capsid protein (CP). Pomoviruses encode the triple gene block genetic module of movement proteins (TGB 1, 2, and 3) and recent research suggests that PMTV RNA is transported either as ribonucleoprotein (RNP) complexes containing TGB1 or encapsidated in virions containing TGB1. Furthermore, there are different requirements for local or systemic (long-distance) movement. Research suggests that nucleolar passage of TGB1 may be important for the long-distance movement of both RNP and virions. Moreover, and uniquely, the long-distance movement of the CP-encoding RNA requires expression of both major and minor CP subunits and is inhibited when only the major CP sub unit is expressed. This paper reviews pomovirus research and presents a current model for RNA movement.

Multiplex Reverse Transcription-PCR for Simultaneous Detection of Beet Necrotic Yellow Vein Virus, Beet Soilborne Virus, and Beet Virus Q and Their Vector Polymyxa betae KESKIN on Sugar Beet

ABSTRACT Three soilborne viruses transmitted by Polymyxa betae KESKIN in sugar beet have been described: Beet necrotic yellow vein virus (BNYVV), the agent of rhizomania, Beet soilborne virus (BSBV), and Beet virus Q (BVQ). A multiplex reverse transcription-PCR technique was developed to simultaneously detect BNYVV, BSBV, and BVQ, together with their vector, P. betae. The detection threshold of the test was up to 128 times greater than that of an enzyme-linked immunosorbent assay. Systematic association of BNYVV with one or two different pomoviruses was observed. BVQ was detected in samples from Belgium, Bulgaria, France, Germany, Hungary, Italy, Sweden, and The Netherlands but not in samples from Turkey.

Status and Prospects of Plant Virus Control Through Interference with Vector Transmission

Most plant viruses rely on vector organisms for their plant-to-plant spread. Although there are many different natural vectors, few plant virus-vector systems have been well studied. This review describes our current understanding of virus transmission by aphids, thrips, whiteflies, leafhoppers, planthoppers, treehoppers, mites, nematodes, and zoosporic endoparasites. Strategies for control of vectors by host resistance, chemicals, and integrated pest management are reviewed. Many gaps in the knowledge of the transmission mechanisms and a lack of available host resistance to vectors are evident. Advances in genome sequencing and molecular technologies will help to address these problems and will allow innovative control methods through interference with vector transmission. Improved knowledge of factors affecting pest and disease spread in different ecosystems for predictive modeling is also needed. Innovative control measures are urgently required because of the increased risks from vector-borne infections that arise from environmental change.

Genetic characterization of Pepino mosaic virus isolates from Belgian greenhouse tomatoes reveals genetic recombination

Over a period of a few years, Pepino mosaic virus (PepMV) has become one of the most important viral diseases in tomato production worldwide. Infection by PepMV can cause a broad range of symptoms on tomato plants, often leading to significant financial losses. At present, five PepMV genotypes (EU, LP, CH2, US1 and US2) have been described, three of which (EU, LP and US2) have been reported in Europe. Thus far, no correlation has been found between different PepMV genotypes and the symptoms expressed in infected plants. In this paper, the genetic diversity of the PepMV population in Belgian greenhouses is studied and related to symptom development in tomato crops. A novel assay based on restriction fragment length polymorphism (RFLP) was developed to discriminate between the different PepMV genotypes. Both RFLP and sequence analysis revealed the occurrence of two genotypes, the EU genotype and the CH2 genotype, within tomato production in Belgium. Whereas no differences were observed in symptom expression between plants infected by one of the two genotypes, co-infection with both genotypes resulted in more severe PepMV symptoms. Furthermore, our study revealed that PepMV recombinants frequently occur in mixed infections under natural conditions. This may possibly result in the generation of viral variants with increased aggressiveness.

Xanthomonas translucens from small grains: Diversity and phytopathological relevance

ABSTRACT Sixty-eight presumptive Xanthomonas translucens strains isolated from 15 small grains or grass species were studied by pathogenicity tests on barley, bread wheat, oat, and bromegrass species, and also by AFLP, analysis of fatty acid methyl esters (FAME), and sodium dodecyl sulfate-polyacrylamide gel electrophoresis of protein extracts. The X. translucens strains were divided into three pathogenicity types based on differences observed on barley and bread wheat. Two unspeciated strains producing atypical symptoms formed a fourth pathogenicity type. Pathogenicity on oat and bromegrass species varied within these types. Clusterings observed by AFLP analysis and, to a lesser extent, by FAME analysis were consistent with these pathogenicity groupings. The current results, as well as those of previous restriction fragment length polymorphism analyses of the same strains, support the recent reclassification of X. translucens pv. translucens and X. translucens pv. hordei as true synonyms. X. translucens pv. cerealis, X. translucens pv. translucens, and X. translucens pv. undulosa cluster in different groups by AFLP and FAME analyses. Even though distinction by simple biochemical tests is not clear-cut, the data indicate that the pathovars cerealis, translucens, and undulosa correspond to true biological entities.

Dual infection by cassava begomoviruses in two leguminous species (Fabaceae) in Yangambi, Northeastern Democratic Republic of Congo.

A study on cassava mosaic begomoviruses was conducted around Yangambi (DR Congo) by sampling 10 different leguminous species with or without symptoms similar to cassava mosaic disease. DNA was isolated to amplify CMBs using primers targeting AC2 and AC4 genes for virus detection by PCR. The results showed a dual infection by ACMV and EACMV in two weed species, Centrosema pubescens and Pueraria javanica, associated with mosaic symptoms. The DNA-A genome component of ACMV and EACMV from the infested weeds was sequenced. Seven ACMV and four EACMV isolates are reported. The major ACMV strains were closely related to ACMV-NGogo, ACMV-IC and ACMV-UGMld, whereas all EACMV strains were closely related to a Uganda variant, the most prevalent virus. This study shows that whiteflies may transmit CMBs to non-cassava plants under high epidemic pressure.

Identification and Quantification of Polymyxa graminis f.sp. temperata and P. graminis f.sp. tepida on Barley and Wheat

Polymyxa graminis f. sp. temperata and P. graminis f. sp. tepida are distinguished on the basis of their specific ribosomal DNA sequences. In order to evaluate whether or not host specialization is associated with the special form, the occurrence of infection of both forms on barley and wheat was studied. P. graminis inocula were obtained from soils collected in Belgium and France. Their ribotypes were characterized using molecular tools specific to P. graminis f. sp. temperata or P. graminis f. sp. tepida such as restriction fragment length polymorphism (RFLP) analysis of polymerase chain reaction (PCR)-amplified rDNA, nested and multiplex PCR. Both special forms were found in each country and coexisted in some soils. The host specificity of P. graminis special forms for barley and wheat was studied from two soils collected at Gembloux (Belgium) and Chambon-sur-Cisse (France), each infested by bymo- and furoviruses. P. graminis f. sp. temperata is more frequent on barley and P. graminis f. sp. tepida on wheat. Furthermore, the quantification of each form on barley and wheat by two separated real-time quantitative PCR assays confirms the observations on the vector specialization. These results suggest a certain but not exclusive host specificity of P. graminis special forms.

Iranian beet necrotic yellow vein virus (BNYVV): pronounced diversity of the p25 coding region in A-type BNYVV and identification of P-type BNYVV lacking a fifth RNA species.

Beet necrotic yellow vein virus (BNYVV) was detected in 288 of the 392 samples collected in Iran. A-type BNYVV was detected most frequently. The p25 coding region on BNYVV RNA-3 was amplified by RT-PCR and sequenced. Nine different variants of the highly variable amino acid tetrad at positions 67–70 of p25 were identified, i.e. ACHG, AHHG, AYHG, ALHG, AFHR, AFHG, AHYG, VLHG and VHHG. These are more different tetrad variants than have been reported from any other country. The first three variants were found most commonly. In 23 out of the 288 BNYVV-positive samples, we detected P-type BNYVV that had previously been identified only in France, Kazakhstan and recently in the UK. Surprisingly, none of these samples contained the fifth RNA species usually associated with P-type BNYVV in other countries. As in other BNYVV P-type sources, the p25 amino acid tetrad in positions 67–70 of the Iranian P-type consists of SYHG.

Comparison of immunofluorescence and two assays for detection of Xanthomonas campestris pv. undulosa in seeds of small grains

A monoclonal antibody specific for Xanthomonas campestris pathovars in the translucens group, including X. c. pv. undulosa, the causal agent of bacterial leaf streak or black chaff in wheat and triticale, was used in immunofluorescence and a dot-immunobinding assay to detect pathogens in aqueous seed extracts. These techniques were compared to dilution plating of seed wash water as potential routine seed-indexing methods. Twenty-six cereal seed lots were harvested from plants with different levels of natural disease incidence grown in three locations in Mexico. Among the 26 lots analyzed, pathogen-free seed came from clean fields principally in Ciudad Obregon, where conditions are less favorable for its development. Pathogen populations detected with dilution plate ranged from 1.3 X 10(4) to 5.3 X 10(6) cfu/g of air-dry weight. Populations were recovered from seed lots that had been stored for 3 yr. Some discrepancies in pathogen detection occurred among the different methods used. Seed lots with high pathogen populations were consistently identified with all three methods. Differences in detection thresholds were found with seed wash water as compared to reference strain from pure culture. Apparently a substantial number of dead cells were present in wash water from some seed lots. Immunofluorescence was more sensitive than dot-immunobinding assay and produced more reproducible results. It is proposed as a standard for indexing germ plasm. Because of the high variation in data, direct counts of fluorescent bacterial cells under UV microscopy will probably not be necessary for identifying infected seed. It is recommended that immunofluorescence-positive seed lots should not be considered for sowing in areas favorable for disease development.

Seed, soil and vegetative transmission contribute to the spread of pecluviruses in Western Africa and the Indian sub-continent

Peanut clump and sugarcane red leaf mottle diseases are caused by viruses of the genus Pecluvirus. Indian peanut clump virus occurs in the Indian sub-continent and Peanut clump virus in West Africa. A feature of these viruses is that they are both seed and soil transmitted. Both modes of transmission contribute to long-term persistence and field spread. Data on seed transmission in pearl millet, virus movement within the plant and virus diversity based on RNA-1 partial sequences are presented. This study emphasizes that pecluviruses are also viruses of cereals infecting sorghum and pearl millet, and highlights a correlation between the countries cultivating these two crops and the virus distribution. Ways of controlling pecluviruses and their vector, Polymyxa graminis, taking into account the virus dissemination routes, are proposed.