The Expression of R-gene Resistance to Phytophthora infestans in Tissue Cultures of Solanum tuberosum

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Abstract
The expression of R-gene resistance in tissue cultures of Solanum species was investigated. Tissue aggregates (4 mm. diam. of Solanum tuberosum cv. ''Majestic'' (rr) and ''Orion'' (R I) and other R-gene Solanum lines behaved in a similar way to the intact plants from which they were derived, in response to Phytophthora infestans race 4. Tissue aggregates of S. tuberosum cv. ''Orion'' supported good growth of P. infestans 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 P. infestans 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. infestans race 4 germ tubes. This led to a demonstration that toxicity to P. infestans zoospores was developed in Orion but not in Majestic tissue cultures in response to infection with P. infestans race 4. R-genes may be expressed in tissue culture. The failure of the ''Orion'' tissue to support growth of P. infestans may be due to expression of the R I gene, and this may operate through the development of post infectional toxicity.