G. Marchoux

Hypersensitive resistance to tomato spotted wilt virus in three Capsicum chinense accessions is controlled by a single gene and is overcome by virulent strains

The resistant reaction to tomato spotted wilt virus (TSWV) was found to be determined by a single dominant gene in three Capsicum chinense Jacq. accessions (‘PI 152225’, ‘PI 159236’, ‘7204’). Allelism studies indicated that all C. chinense lines bear the same allele located at the Tsw locus. All the inoculated plants in the allelism tests displayed a resistant hypersensitive phenotype characterized by necrotic local lesions followed by abscission of the inoculated organ. However, a small proportion of them showed late systemic infection. Nine TSWV isolates obtained from these individual plants with systemic symptoms were backinoculated to the three resistant parents. All isolates were able to infect systemically all the resistant accessions without inducing local necrotic lesions. Serological analysis confirmed that these nine viral isolates belong to the TSWV species (serogroup I). Consequently, the susceptible plants in the allelism tests could not be interpreted as possessing a recombinant genotype because of the virulence change in the viral strain. Hobbs et al. (1994) already reported the existence of TSWV pathotypes overcoming the resistance of C. chinense resistant accessions. Practical consequences for pepper breeding associated with the emergence of these resistance-breaking isolates are discussed.

Genetic analysis of broad spectrum resistance to potyviruses using doubled haploid lines of pepper (Capsicum annuum L.)

SummaryGenetic analysis of resistance to PVY in androgenetic doubled haploid lines, F1, F2 and backcross progenies of the Mexican pepper line, CM 334 (Capsicum annuum L.), was performed. Three reaction types were observed when seedlings were inoculated with several PVY strains of different pathotypes and with an American PeMV strain. Resistant genotypes never showed systemic symptoms although some individuals sporadically developed necrotic local lesions on inoculated cotyledons. Susceptible genotypes exhibited either a typical systemic mosaic or a systemic necrosis that caused the death of the inoculated seedlings. Segregation analyses indicated that resistance to pepper potyviruses in CM 334 is conferred by two genes. The first one, tentatively named Pr4, is dominant and confers the resistance to all now known pathotypes of PVY and to PeMV. The second one, tentatively named pr5, is recessive; it confers only the resistance to common strains of PVY. The systemic necrotic response is conferred by an independent dominant gene, tentatively named Pn1.

High temperature effects on hypersensitive resistance to tomato spotted wilt Tospovirus (TSWV) in pepper (Capsicum chinense Jacq.).

The tomato spotted wilt virus resistance in pepper conferred by the Tsw gene is less stable at 32 °C continuous temperature. Continuous high temperatures for at least nine days lead to systemic spread and necrotic symptoms in plants that are totally resistant at a lower temperature (22 °C). We show that continuous high temperatures destabilize this resistance in young plants, but older inoculated plants rarely develop systemic symptoms. Populations segregating for the Tsw gene showed that heterozygosity at the Tsw locus increased the chance of inoculated seedlings to develop systemic necrotic symptoms. Genetic background was less significantly involved in the thermosensitivity of the resistant response. As a consequence, it would be preferable to grow cultivars homozygous at the Tsw locus in high temperature conditions.

Serological, Molecular, and Pathotype Diversity of Pepper veinal mottle virus and Chili veinal mottle virus

ABSTRACT Variability within the pepper-infecting potyviruses Pepper veinal mottle virus (PVMV) and Chili veinal mottle virus (ChiVMV) in Africa and Asia was investigated. Coat protein (CP) gene sequence diversity revealed three clades that corresponded to three geographic locations and there was no evidence of presence of the ChiVMV/Asian group in western or central Africa. These clades included closely related isolates that potentially belong to two viral species, which is consistent with current nomenclature. These clades could not be unambiguously identified with polyclonal antisera; however, reverse transcription-polymerase chain reactions allowed differentiation of the isolates into two species based on a large indel in the CP gene. PVMV and ChiVMV isolates were classified into three and two pathotypes, respectively, in relation to pepper genotypes carrying different resistance factors. Specificity of resistance only partially corresponded to molecular diversity of the isolates. Only one isolate of PVMV could infect pepper genotypes carrying the two recessive genes pvr6 and pvr2 (2); however, these genotypes were not infected by PVMV in field trials in Senegal, despite a high prevalence of PVMV in the surrounding pepper plants.

Evaluation and inheritance of the Lycopersicon hirsutum resistance against potato virus Y

SummaryPotato virus Y (PVY) infects most Solanaceous crops grown in Mediterranean countries in open fields and in greenhouses. Necrogenic strains, which have been isolated from diseased tomatoes in France since the 1980s, seriously cause yield and quality loss of tomato fruits. Lycopersicon hirsutum PI 247087 was found to be resistant to PVY. Virus could not be detected in inoculated leaves by ELISA and/or by back-inoculation on susceptible plants. This resistance was efficient against the 16 tested isolates or strains. Temperature and inoculum concentration did not affect its expression. All the F1 plants of (Momor × PI 247087), (PI 134417 × PI 247087) and (PI 247087 × PI 134417) had symptom scores and ELISA values similar to those of the susceptible parents. The mechanism of resistance could be immunity-like or inhibition of virus migration from cell to cell. The resistance of L. hirsutum PI 247087 appeared to be governed by two independent recessive genes. In a few F2 plants of (PI 134417 × PI 247087) and F2 (Momor × PI 247087), virus was able to multiply in the inoculated leaves but could not establish a systemic infection. This finding may suggest a mechanism which interfers with the long distance migration of the virus in the plant.

Identification and characterization of resistance to tobacco etch virus in Lycopersicon species

Tomato and wild related species with or without described resistance to potyviruses were screened for resistance to tobacco etch virus (TEV). Lycopersicon pennellii LA 716 and L. pimpinellifolium LA 1478 were found to be tolerant to TEV. L. hirsutum PI 247087, previously reported resistant to potato virus Y (PVY), was found resistant to TEV. The virus was not detected by enzyme-linked immunosorbent assay (ELISA) in the inoculated leaves of L hirsutum PI 247087 but could be recovered by back-inoculations to tobacco plants. TEV was not detected in the uninoculated leaves of L. hirsutum PI 247087. Virus multiplication and/or virus migration from cell to cell appeared to be impaired, preventing systemic spread of TEV in L. hirsutum PI 247087. The resistance of L. hirsutum PI 247087 to TEV is expressed at the cotyledonary stage and is efficient against four different geographical isolates. Inheritance of the resistance appears to be controlled by one recessive gene.

Occurrence of Chilli veinal mottle virus in Solanum aethiopicum in Tanzania

African eggplant, or garden egg (Solanum aethiopicum) is an important vegetable in most sub-Saharan African countries. Since June 1997, viral symptoms, including mosaic, vein clearing, and stunting, have been observed on several crops of African eggplant cv. Tengeru White at a number of sites in the Arusha region of northern Tanzania. Field inspections revealed disease incidence ranging from 50 to 90%. During the same period, high populations of the green peach aphid Myzus persicae were observed in affected crops of African eggplant. These aphids were also found to reproduce in African eggplants. Flexuous, rodshaped virus-like particles, approximately 750 nm long and 12 nm wide, were found in electron microscope leaf dips from field samples of naturally affected African eggplants. The particle size suggested a species of Potyviridae. Thus, 20 field-infected samples of S. aethiopicum (randomly collected from four farms) were assayed in double antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA) for the presence of Potato virus Y (PVY) and Pepper veinal mottle virus (PVMV), known to infect tomato and other solanaceous crops in the region (2). However, all samples gave negative results. Further DAS-ELISA were performed with the same extracts from naturally infected plants of S. aethiopicum with antisera directed against Tobacco etch virus, Tobacco vein mottling virus, Pepper mottle virus, and Chilli veinal mottle virus (ChiVMV). All 20 samples were positive only for ChiVMV. ChiVMV, a single-stranded RNA virus transmitted in a nonpersistent manner by several aphid species, is one of the most important viruses of pepper in Asia (1). To confirm DAS-ELISA results, an isolate of ChiVMV from African eggplant was transmitted by mechanical inoculations, resulting in disease on tobacco (Nicotiana tobacco cv. Xanthi nc), pepper (Capsicum annuum cv. Yolo Wonder), tomato (Lycopersicon esculentum cv. Tengeru 97), and African eggplant (S. aethiopicum cv. Tengeru White). Extracts from the inoculated plants tested positive for the presence of ChiVMV in DAS-ELISA. This mechanically transmitted isolate did not infect melon (Cucumis melo), cucumber (C. sativus), or cowpea (Vigna unguiculata), which are nonhosts of ChiVMV. To our knowledge, this is the first report of the natural occurrence of ChiVMV in African eggplant. References: (1) S. K. Green et al. PETRIA 9:332, 1999. (2) R. Nono-Womdim et al. J. S. Afr. Soc. Hort. Sci. 6:41-44, 1996.

Biological and molecular characterization of two tomato strains of potato virus Y (PVY)

Two PVY tomato strains (LYE 84 and LYE 84.2), arising from the same natural isolate, and a strain originating from a wild Solanaceous host, Solanum nigrum (SON 41.2), were compared for host range and symptomatology. All strains induced mosaic without necrosis on tobacco as PVYO strains. The two tomato strains behaved similarly on pepper, infecting only susceptible pepper cultivars (pathotype 0), whereas SON 41.2 was able to overcome the two alleles of the recessive resistance gene pvr2 (pathotype 1.2). On the other hand, only LYE 84.2 was virulent on tomato and broke the resistance of the wild genitor Lycopersicon hirsutum PI 247087. Sequence determination of the capsid gene and the 3′ non-coding region of LYE 84 and LYE 84.2 showed a total homology at both nucleic acid and amino acid levels. This suggests that LYE 84.2 has probably derived from LYE 84, that both strains have very similar sequences and that the capsid protein does not play a direct role in the resistance-breaking capacity of LYE 84.2.

Molecular tagging of the Am gene from Lycopersicon hirsutum f. glabratum PI 134417 using AFLP markers.

Recently important epidemics caused by Alfalfa Mosaic Virus (AMV) have been reported in field grown canning tomatoes in southern Italy. The Am resistance gene identified in Lycopersicon hirsutum PI 134417 confers a complete resistance to several isolates of AMV with different geographical and botanical origins. In order to identify linked molecular markers, a segregant back-cross population of 120 individuals was obtained from the intraspecific cross between the resistant L. hirsutum accession PI 134417 and the susceptible accession L. hirsutum PI 247087. Using bulked segregant analysis combined with AFLP technology, four AFLP markers were found to be linked to the Am resistance gene. This gene will be assigned to a tomato chromosome using L. pennellii introgression lines.