ISHS Acta Horticulturae 273: V International Workshop on Fireblight
ISOLATION OF AVIRULENT MUTANTS OF ERWINIA AMYLOVORA CFBP1430 |
| Authors: | M.-A. Barny, J. Laurent, J.-P. Paulin |
| Abstract:
The Erwinia amylovora strain CFBP1430 is a natural isolate showing a very stable virulence level on different host species of fire blight. For this reason this strain was used to isolate avirulent mutants after transposon mutagenesis. Stable insertions were obtained by using MuPR13, a phage Mu derivative which confers resistance to chloramphenicol and is completely deleted for transposition functions: 6000 independent Cm* clones were recovered. The analysis of auxotrophs (frequency and distribution) showed that the insertions of MuPR13 were roughly random in E. amylovora. More than 4000 Cm  clones were screened to detect those which were no longer able to produce exudate on apple calli (Cal- clones). Among the Cal- mutants, 46 presented a unique insertion of MuPR13 and were further studied on apple and pear seedlings.
Depending on their behaviour on both types of seedlings the Cal- mutants can be divided into three groups: 5 Cal- mutants are as virulent as the wild type strain, 17 showed a reduction of virulence and 24 were totally avirulent.
In addition we have used another pathogenicity test on axenicly grown unrooted pear microcuttings.
When Cal- mutants were tested on this material the two first groups showed the same behaviour, probably because axenic pear microcuttings are more susceptible to fire blight, and the third group remained unchanged proving the validity of this test to study avirulent mutants.
Finally, avirulent mutants have been- tested for hypersensitive response on tobacco plants: 8 of them were still able to produce HR and 16 were not.
These mutants are now studied at the molecular level.
Most of them appeared to be clustered in the same genomic region.
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