Mechanism of resistance of tomato against tobacco mosaic virus using protoplasts

The mechanism of resistance to tobacco mosaic virus (TMV) of two resistant entries, Acc. 549 and TMC 106, was elucidated using protoplasts. These entries exhibited symptomless reaction upon TMV inoculations unlike the susceptible varieties, Marikit and VC II-1, which showed severe mosaic symptoms. More protoplasts were obtained using younger leaves and leaf sections than older and whole leaves. The isolated protoplasts of both susceptible varieties showed positive reaction with very intense yellow color in ELISA test. This implied a high virus concentration on the protoplasts and that the virus may have rapidly multiplied and spread from cell to cell within the leaf tissues. In the resistant entries, protoplasts of Acc. 549 gave slight yellow color reaction in contrast with the protoplast of TMC 106, which yielded a negative colorless reaction in the ELISA tests. In infectivity assay, TMV from susceptible varieties (Marikit and VC II-1) induced higher number of local lesion of Nicotina glutinosa ranging from 7 to 8.5 week post-inoculation and more than 15 lesions after three weeks. The number of lesions could hardly be counted since the lesions coalesced after 3 weeks. In the resistant entries, TMV of Acc. 549 elicited only 2.5 lesions in the first week and 0 lesion in the succeeding 2 to 4 weeks. In the other resistant entry (TMC 106), the virus did not produce any lesion to the local lesion host. Such result suggests that TMV was able to enter Acc. 549 only for a certain period of time (one week) but was not able to multiply or spread from cell to cell within the plant. The virus particles were localized within the small leaf area as evidenced by necrotic local lesion in N. glutinosa. Non-formation of lesions from 2 to 4 wk-old inoculum may suggest that the virus may have died inside the cell. In TMC 106, the virus was not able to enter the plant while in Marikit and VC II-1, TMV was able to invade and to progress in the plant. Results of ELISA test for protoplast and infectivity assay confirmed the presence of TMV susceptible plants and the resistance of both Acc. 549 and TMC 106 to the virus