Archivio Journal of Plant Pathology

Abstract:

The histidine-reversible antibiotic pantocin A is produced by Pantoea vagans strain C9-1 and Pantoea agglomerans strains Eh252, EH318 and P10c and contributes significantly to efficacy of biological control of fire blight of pear and apple flowers caused by Erwinia amylovora. Antibiosis by pantocin A is observed on solidified defined media, such as MGA (MOPS gluconate medium amended with asparagine), but not on the solidified peptide-rich medium lysogeny broth (LB). It was not known if lack of antibiosis on LB was due to lack of pantocin A biosynthesis or to histidine in the medium reversing the effects of pantocin A. We used a green fluorescent protein (GFP) promoter probe in C9-1 to monitor transcription of the pantocin A biosynthesis gene paaA. GFP expression was higher in MGA than in LB. Addition of yeast extract or tryptone, components in LB, to MGA significantly decreased transcription of paaA. The observed lack of inhibition of E. amylovora by C9-1 in LB was positively correlated with paaA transcription. Addition of histidine or arginine, amino acids scarce on pome fruit stigmas, to MOPS gluconate medium (MG) decreased transcription of paaA compared to MG without amino acids. In contrast, asparagine and proline, amino acids that are abundant on pome fruit stigmas supported bacterial growth and transcription of paaA, which is in agreement with findings that pantocin A production is a key component of fire blight control by Pantoea spp. on floral stigmas.