First report in Spain of a variant of Tomato mosaic virus (ToMV) overcoming the Tm-22 resistance gene in tomato (Lycopersicon esculentum)

A naturally-infected tomato plant showing stunting and necrosis in the apical leaves was tested for viruses by enzyme-linked immunosorbent assay (ELISA) using polyclonal antisera (Loewe). The sample tested positive for Tobacco mosaic virus (TMV) and Tomato mosaic virus (ToMV) and negative for Arabis mosaic virus, Cucumber mosaic virus, Pepino mosaic virus, Potato virus X, Potato virus Y, Tomato spotted wilt virus and Tomato bushy stunt virus. Leaf extracts were not infective by mechanical inoculation to Cucumis sativus, and induced necrotic local lesions on Datura stramonium, Petunia hybrida, Chenopodium quinoa, Nicotiana glutinosa, N. tabacum cv. Xanthi, and peppers (Capsicum annuum) cv. `Pekin´ (Fito SA) and `Oman´ (Seminis) which carry the L4 gene for resistance to Pepper mild mottle virus and TMV. Systemic infections produced chlorotic mosaic in Solanum nigrum, tomatoes cv. `Marmande´ and `Roma´ and a non-resistant pepper hybrid C0103-4 (Fito SA), necrotic mosaic and stunting in tomatoes cv. `Bodar´ and `Lisboa´ (Seminis), ref. 537 (Fito SA) carrying the Tm-2² gene, and leaf and stem necrosis in peppers cv. `Delfos´ (Fito SA), `Lido´ and `Atlantic´ (Seminis) carrying the L1 gene for TMV resistance.

A product of the expected size (419 nt) was amplified from nucleic acid extracts obtained from infected tomato cv. `Marmande´ by reverse transcription-polymerase chain reaction (RT-PCR) using the primers 1F (5'-TGCAGCAAAGGTAATAGTAG-3' position 1841-1860) and 1R (5'-CCGAACTTTTGACGAGTTTC-3' position 2280-2289), based on ToMV sequence AJ 417701 (NCBI). Sequence analysis using BLAST (NCBI) showed 99.63% homology with ToMV strain L11A, (AB083196), which is able to overcome the resistance conferred by the tomato Tm22 gene (Yamamoto et al., 2002). No amplicons were produced using TMV-specific primers designed from TMV sequence AJ 011933 (NCBI), suggesting the reaction to the TMV antiserum was due to a cross-reaction.

Important losses in pepper crops caused by ToMV have been reported in Spain (Alonso et al., 1989); however, this is the first report from Spain of a ToMV variant that overcomes the Tm2² resistance gene in tomato.