BIOLOGICAL CONTROL OF BACTERIAL SOFT ROT

Bacterial soft rots (Erwinia spp.) (BSR) are economically important diseases affecting a wide range of horticultural crops. Erwinia herbicola Eh252, an epiphytic bacterium originally selected for its ability to control fireblight in apple orchards (Vanneste, 1996), when grown on minimal media plates inhibits the growth of those Erwinia species causing BSR [E. carotovora subsp. carotovora (Ecc), E. carotovora subsp. atroseptica and E. chrysanthemi)]. When potato slices were treated with a suspension of Eh252 before inoculation with Ecc, development of BSR was inhibited (Vanneste et al., 1994). To study interaction between Eh252 and the BSR pathogens, we modified the bioassay of Michalik et al. (1992) to measure susceptibility of carrot to BSR. Carrot slices which had been treated with Eh252 prior to inoculation with Ecc had a significantly lower incidence of BSR than those treated with 10 mM MgSO₄. BSR was also inhibited by dipping carrot slices in a suspension of Eh252 before placing them in a compost infected with Ecc (Cornish and Vanneste, 1996). This compost mimics one of the environments under which BSR naturally develops.
Eh252 produces a peptide antibiotic, a major agent involved in the control of fireblight (Vanneste et al., 1992). Transposon-induced mutants of Eh252 which had lost the ability to produce the peptide (Ant^– mutants) were less effective than Eh252 in reducing BSR on potato and carrot slices.
Clones of Escherichia coli carrying gene(s) necessary for antibiotic production were identified in a gene library of Eh252 by their ability to inhibit growth of E. amylovora. Plasmids isolated from these clones of E. coli restored to Ant mutants of Eh252 their antibiotic production and their ability to control BSR on carrot slices, indicating that the antibiotic is a factor in inhibition of BSR. Analysis of nested deletions and transposon-induced mutants revealed that only a ca. 2.2 kb fragment was necessary to complement all Ant^– mutants of Eh252 and to confer antibiotic production to E. coli. Analysis of the sequence of this DNA fragment revealed that a 62 base pair segment shared 57 % homology with the gene mccA isolated from E. coli and the inverted repeat sequences that follow this gene. mccA codes for the peptide structure of microcin C7, a modified heptapeptide which inhibits protein synthesis in the Enterobacteriaceae (Gonzalez-Pastor et al., 1995). Analysis of the predicted amino acid sequence of this 2.2 Kb DNA fragment from Eh252 revealed that a section of 81 amino acids has 51% identity and 67% similarity with MccB from E. coli, which is believed to adenylate the heptapeptide MccA. Together, these results suggest that the inhibitory compound produced by Eh252 is a post-translationally modified peptide, possibly adenylated at the C terminus.

International Postharvest Science Conference Postharvest 96

J.L. Vanneste, J. Yu, D.A. Cornish

https://doi.org/10.17660/ActaHortic.1998.464.109