Bacterial wilt-resistant plants were obtained using a tomato tissue culture system. A virulent strain ofPseudomonas solanacearum secreted some toxic substances into the culture medium. Leaf explant-derived callus tissues which were resistant to these toxic substances in the culture filtrate were selectedin vitro and regenerated into plants. These plants expressed bacterial wilt resistance at the early infection stage to suppress or delay the growth of the inoculated bacteria. On the other hand, complete resistance was obtained in self-pollinated progeny of regenerants derived from non-selected callus tissues. These plants showed a high resistance when inoculated with this strain, and were also resistant when planted in a field infested with a different strain of the pathogen.

Tobacco mosaic virus-resistant tobacco was selected in vitro using callus tissues induced from axillary buds of systemically infected tobacco plants. Callus lines in which the virus was continuously multiplying were first isolated and redifferentiated into shoots. By the procedure, non-diseased, healthy shoots were successfully isolated from diseased shoots, which showed typical mosaic symptoms of the virus, and regenerated into intact plants.These regenerated plants showed resistance to virus inoculation, and selfed progeny of virus-resistant regenerants segregated the resistance and susceptibility according to the Mendelian system.

The bacterial wilt resistant line LNSR-7 of tomato was isolated from self-pollinated progenies of leaf-callus derived regenerants by directly inoculating a bacterial wilt pathogen Pseudomonas solanacearum into injured roots of tested plants. The subsequent selfpollinated progenies of the line were examined for their fruit quality and resistance expression under natural cultivation conditions in a pathogen-infested tomato field. During three generations of progenies, the tomato plants showing both the bacterial wilt resistance and the high fruit qualities comparable to the parental cultivar were selected in order to fix commercial characteristics of the line. The stable inheritance of the resistance in the subsequent self-pollinated progenies was further examined by directly inoculating the pathogen into the roots of test plants. Inoculated plants were planted in soil heavily infested with the pathogen to ensure exposure to the pathogen. Under these artificial inoculation conditions, the selected line was shown to be highly resistant to the disease. The resistance mechanism in the line was analyzed by examining multiplication and translocation of the pathogen in planta. The precise monitoring of infection behavior of the pathogen was successfully achieved using the genetically marked P. solanacearum. Consequently the present line LNSR-7 strictly limited secondary multiplication and translocation of the pathogen and suppressed the wilt induction by the pathogen.

In somaclonal tissues obtained from systemically TMV‐infected tobacco plants, a relation between changes of TMV amounts and the callus growth was examined. The culture medium was suitable for maintaining a constant concentration of TMV as well as active callus growth. By using the shake‐culture method, somaclonal tissues were separated into two classes on the basis of callus sizes. In large callus tissues, TMV amounts were constant during subculturing but the tissues did not either grow or release the newly divided cells after the last subculture. On the other hand, smaller callus tissues grew markedly and the TMV amounts were conspicuously lowered. After shake‐subculture of smaller tissues, they were successfully regenerated to plantlets. None of the plantlets expressed any mosaic symptoms, while plantlets from the original somaclones showed severe mosaic symptoms of TMV in leaflets. Thus, the present report describes the successful production of virus‐free plantlets from infected somaclonal callus cultures.

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