Wild Lycopersicon spp. have gradually increasing their importance on tomato crop improvement as a source of genetic variability. Several tomato breeding programs are looking for sources of new pest and disease resistance genes. In the case of the cultivated tomato, the majority of the genes for TSWV resistance, that were incorporated into commercial cultivars, have been introgressed from its wild relatives. The main objective of this work was to detect new sources of resistance genes to TSWV in several accessions of wild Lycopersicon species.
Screening trials were conducted under greenhouse conditions at CNPH/EMBRAPA in Brasilia (DF), Brazil. Sixteen accessions and lines of wild tomatoes belonged to seven different species were used in this experiment. The F1 hybrid 'Toyofuku' was used as susceptible control (Table 1). The lines were inoculated with four TSWV isolates (DF-T, GO-P, SC-P, and SP-T) obtained, respectively, from naturally infected tomato, pepper, sweet pepper and tomato plants from four different regions of Brazil [Distrito Federal (DF), Goias (GO, Santa Catarina (SC), and Sao Paulo (SP) States]. Six to twelve plants of each introduction were used for each isolate. Inocula were prepared from a pool of leaves of artificially inoculated Nicotiana tabacum 'Xanthi nc', N. restica, N. benthamiana, N. clevelandii, tomato 'Rutgers' and pea (Pisum sativum) plants. Inoculation was made by griding leaf extracts in 0.05M phosphate buffer (pH 7.0) containing 0.1% sodium sulfite and rubbing extracts on leaves of Lycoprsicon spp. plants 20-30 days after planting. The test plants were kept in darkness for 24 hours before inoculation. This procedure has been used as a means of increasing the infectivity rate of TSWV. Plants were scored visually for TSWV symptoms up to 4 weeks after inoculation. Plants without obvious symptoms were tested using dot-ELISA to confirm latent infection of TSWV.
The results are presented in Table 1. L. peruvianum was the specie that exhibited higher levels of resistance to all four TSWV isolates. Six lines of L. peruvianum were not infected by at least two isolates treated. A very low percentage of plants developed few nerrotic spots on the upper leaves. Identical response was obtained when these lines were inoculated by grafting (data now shown).
L. pimpinellifolium lines showed low percentages of infection to some isolates but not to others. The susceptible reaction was characterized by mosaic with few necrotic spots on the leaves. L. hirsutum and L. chinense lines also presented relatively low infection rates to some isolates. The symptoms in L. hirsutum lines were mainly characterized by a strong necrotic reaction to TSWV. The others Lycopersicon spp. lines tested in this work were highly susceptible to all TSWV isolates.
Based on our results, the most promising sources of resistance genes are found in L. peruvianum. Additional studies involving a greater and more representative number of TSWV isolates will be necessary to confirm if the resistance presented by some L. peruvianum lines (e.g. LA 372) is isolate-specific. This work is now underway in our research center.
Table 1. Reaction of sixteen wild tomato (Lycopersicon spp.) accessions to mechanical inoculation with four tomato spotted wilt virus (TSWV) isolates.
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Species Line TSWV Isolates (% of Infected Plants)
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DF-T GO-P SC-P SP-T L.
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peruvianum CNPH 201 0.0 NT 0.0 8.3
CNPH 374 0.0 NT 0.0 8.3
LA 111 16.6 16.6 0.0 0.0
LA 372 0.0 0.0 0.0 0.0
LA 385 0.0 33.3 0.0 8.3
LA 1113-1 8.3 0.0 0.0 0.0
L. hirsutum CNPH 417 28.6 28.6 50.0 66.6
WYR 7924 16.6 50.0 33.3 16.6
L. pimpinellifolium CNPH 415 20.0 33.3 75.0 71.4
WYR 3951 33.3 83.3 66.6 83.3
LA 1584 50.0 40.0 50.0 66.6
L. chilense LA 1967 16.6 33.3 8.3 37.5
L. cheesmanii LA 1036 50.0 66.6 100.0 100.0
L. paraviflorum LA 1716 88.8 10.0 100.0 100.0
L. esculentum
var. cerasiflorme LA 1226 75.0 66.6 NT 100.0
LA 1425 100.0 66.6 83.3 100.0
L esculentum
(susceptible control) Toyofuku F1 91.6 100.0 83.3 91.6
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NT = not tested
de Avila, A.C. Diversity of the Tospoviruses. PhD Dissertation. Wageningen Agricultural University, Wageningen, The Netherlands.