Selection for Bacterial Blight Phytotoxin Resistance in Wheat Tissue Culture

Bacterial blight (caused by Pseudomonas syringae pv. syringae) is a foliar disease that occasionally reduces wheat (Triticum aestivum L.) yields in Minnesota and other northern states. Many strains of P. syringae pv. syringae produce a low molecular weight protein, syringomycin (SR), which has nonspecific toxic activity. This study was conducted to determine whether SR was toxic to wheat tissue cultures and whether selection in tissue culture could be a feasible approach for obtaining germplasm with increased bacterial blight resistance. Tissue cultures were established from immature embryos of ‘Angus’, a cuitivar susceptible to bacterial blight. Syringomycin was extracted according to established procedures, and conditions were determined to inhibit tissue culture growth. A bacterial mutant (no. 9-25) incapable of producing SR was isolated from a producing strain (B359) via acridine orange mntagenesis. Polyacrylamide gel electrophoresis of partially purified extracts from B359 SR+, no. 9-25 SR-, and uninoculated control media indicated a single bioactive band inhibitory to Geotrichum candidum in only the B359 SR+ extract. Partially purified extracts from B359 SR+, but not no. 9- 25 SR- or control media inhibited the initiation and growth of tissue cultures from immature Angus embryos. The B359 SR+ extract concentrations that caused about 50% reductions in growth of initiating cultures were then used in attempts to select for resistance in established cultures. Cultures responded to SR selection by an increased tendency to form roots and shoots, but no stable resistant cultures were obtained during the period that regenerable cultures could be maintained. A total of 238 plants (R0) were regenerated from control and selected cultures during five subcultures. Eightyeight progeny plants (R1) from selected cultures were included in preliminary evaluation to compare their bacterial blight reaction to that of three noncultured reference genotypes. A suspension of P. syringae pv. syringae was introduced by pressure into leaf intercellular spaces and the increase in bacterial number was monitored in the leaf tissue. Phenotypic ratings of R1 plants were more variable than those of the three reference genotypes. Most RI plants were as susceptible as Angus or L-l, but five R1 plants had ratings within the range of the resistant cultivar, ‘Len’. Although increased variability is not unexpected in progeny of plants regenerated from tissue culture, tests of advanced generations will be required to establish whether R1 phenotypic differences are heritable.