TISSUE CULTURE IN THE PRODUCTION OF NOVEL DISEASE-RESISTANT CROP PLANTS

Multiplication of tobacco mosaic virus in tobacco callus tissues and in vitro selection for viral disease resistance

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.

Selection of bacterial wilt-resistant tomato through tissue culture

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.

TOWARDS THE SELECTION IN VITRO FOR RESISTANCE TO ALTERNARIA BRASSICICOLA (SCHW.) WILTS., IN BRASSICA NAPUS SSP. OLEIFERA (METZG.) SINSK., WINTER OILSEED RAPE

Culture filtrates of Alternaria brassicicola (Schw.) Wilts, were found to be toxic to secondary embryoids of Brassica napus ssp. oleifera (Metzg.) Sinsk. The culture filtrates were partially purified and added to tissue culture media to produce a selection medium. Different secondary embryoid lines of B. napus were found to differ in their sensitivity to the selection medium, and it was possible to select lines which showed increased resistance to the selection medium. Some plants that were regenerated from secondary embryoids were found to be more resistant to the pathogen than were seed-grown plants of the same cultivar, although there was no correlation between sensitivity to the selection medium and susceptibility to the pathogen.

Regeneration of peach plants from callus derived from immature embryos

Peach plants were repeatedly regenerated from immature embryos but not from callus derived from mature embryos. A white, nodular, highly regenerative callus was obtained when friable, primary callus from immature embryos was transferred from medium containing 4.5 μM 2,4-dichlorophenoxyacetic acid and 0.44 μM benzyladenine (BA) to media containing 0.27 μM α-naphthaleneacetic acid (NAA) and 2.2 μM BA. This callus retained its morphogenetic potential for a minimum of three subcultures. Green nodular callus, that lacked regenerative capacity, was produced from primary callus derived from mature embryos. Maximum regeneration of shoots occurred when highly regenerative callus was transferred to a medium in which the NAA concentration was reduced five times and the BA concentration was increased two times. Regenerated shoots were rooted in the dark on a medium containing 28.5 μM indoleacetic acid. Cytogenetic analysis of regenerated plants indicated that all plants were diploid, 2n = 2x = 16. Phenotypic evaluation of regenerated plants, grown under field conditions, is now in progress.

Resistant tobacco plants from protoplast-derived calluses selected for their resistance to Pseudomonas and Alternaria toxins

Protoplast-derived calluses of tobacco (Nicotiana tabacum cv. 'Samsun') were selected for their resistance to toxins from Pseudomonas syringae pv. tabaci, which causes wildfire disease, and from Alternaria alternata pathotype tobacco, which causes brown spot. A number of plants were regenerated from each of the toxin-selected protoplast-derived calluses. A large percentage of the plants obtained from the second selection cycle calluses were resistant to infection by these pathogens. Resistance to wildfire disease, however, seems to be unrelated to resistance to brown spot disease. Variations in the morphological characteristics of the regenerated plants were found. Results of an assay of the R1 generation indicate that the resistance shown by R0 plants against both disease is heritable.

Resistance responses to Phoma lingam of plants regenerated from selected cell and embryogenic cultures of haploid Brassica napus

Resistant plants and plants with reduced susceptibility against the pathogen Phoma lingam could be regenerated from selected callus and embryogenic cultures of haploid rape (Brassica napus) previously treated with mutagens. In the two in vitro selection systems used - absence of fungus growth on the cultures after incubation with parasite spores and resistance to the toxic filtrate - the resistance to the toxin was effective. In addition, some regenerants with increased tolerance were obtained from unselected cultures. Resistance tests on regenerated plants were carried out by inoculation of whole plants in the greenhouse, reproducing as much as possible the infection mechanisms which take place under natural conditions. Preliminary results on resistance of the progeny of single susceptible and tolerant regenerants seem to indicate that the acquired resistances are of a genetic nature.

Somaclonal variation - a novel source of variability from cell cultures for plant improvement

It is concluded from a review of the literature that plant cell culture itself generates genetic variability (somaclonal variation). Extensive examples are discussed of such variation in culture subclones and in regenerated plants (somaclones). A number of possible mechanisms for the origin of this phenomenon are considered. It is argued that this variation already is proving to be of significance for plant improvement. In particular the phenomenon may be employed to enhance the exchange required in sexual hybrids for the introgression of desirable alien genes into a crop species. It may also be used to generate variants of a commercial cultivar in high frequency without hybridizing to other genotypes.

Reversion of Texas male-sterile cytoplasm maize in culture to give fertile, T-toxin resistant plants

Plants carrying Texas male-sterile (Tms) cytoplasm are normally sensitive to Drechslera maydis T-toxin. Tissue cultures were initiated from immature embryos of maize carrying Tms-cytoplasm, and plants were regenerated after selection for resistance to T-toxin. Fertile, T-toxin resistant plants were obtained from the unselected control cultures as well as from the selected material. In addition, one regenerant from an unselected culture was fertile and T-toxin sensitive. The progeny of the regenerants showed the phenotype of the female parent with respect to pollen-fertility, and T-toxin resistance. The data are consistent with the heritable changes observed being the result of the expression of an altered mitochondrial genome.