ISHS Acta Horticulturae 338: VI International Workshop on Fireblight
AVIRULENT TNPHOA MUTANTS OF ERWINIA AMYLOVORA |
| Authors: | R. Cooper, D. Dymock, J. Milner, I. Roberts |
| Abstract:
TnphoA mutagenesis of Erwinia amylovora was performed by the method of Coleman et al. (1991). Pho+ insertions were detected and PhoA expression assayed in response to induction by chlorogenic acid and pear extract; the phenolic was previously identified as a host inducer of some component of virulence in E. amylovora by Cooper et al. (1990). Using this approach a PhoA+ insertion mutant was identified the PhoA expression of which, could be induced by both chlorogenic acid and pear extract. In addition, this mutant displayed reduced virulence to apple seedlings. Several other avirulent capsular mutants were obtained, one of which displayed constitutive PhoA activity and showed reduced killing of suspension-cultured apple cells. None of the insertions mapped to the dsp region identified by other workers and the mutants were HR+. Mutants defective in peptidoglycan biosynthesis were also identified using this procedure. These mutants were altered in cell shape, being cocci as opposed to rods. Complementation and Southern blot analysis using the cloned pbpA/rodA operon from E.coli revealed these mutants had TnphoA insertions in the Erwinia amylovora equivalent of the pbpA/rodA operon. These 'round' mutants were also avirulent in the apple seedling assay but regained virulence when complemented with the whole of the E.coli pbpA/rodA operon. Killing of suspension-cultured apple cells equivalent to that by the wild type was reduced with the 'round' mutants. The possible role of cell surface components and cell size and shape in the disease process will be considered. | |
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