SUMMARY: The expression of R-gene resistance in tissue cultures of Solanum species was investigated. Tissue aggregates (4 mm. diam.) of var. Majestic (rr) and Orion (RI) and other R-gene Solanum lines behaved in a similar way to the intact plants from which they were derived, in response to race 4. Tissue aggregates of var. Orion supported good growth of race I, to which the intact Orion plant is susceptible, but did not support more than rudimentary growth of race 4, to which the intact Orion plant is resistant. Sectioning revealed that race 4 made general growth in Majestic tissues, but was almost totally excluded from Orion tissues. A slide technique showed that Orion tissue suspensions, unlike those of Majestic, inhibited the growth of P. race 4 germ tubes. This led to a demonstration that toxicity to zoospores was developed in Orion but not in Majestic tissue cultures in response to infection with race 4. It was concluded that R-genes may be expressed in tissue culture, that the failure of the Orion tissue to support growth of may be due to expression of the RI gene, and that this may operate through the development of post infectional toxicity.


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