Abstract\ud Background and aims\ud \ud Endophytic bacteria have been often studied as biological control agents of plant pathogens and many of their secondary metabolites involved in antagonism are Non Ribosomal Peptides (NRPs). In this study, the molecular basis of the biocontrol properties of the endophyte Pseudomonas synxantha, isolated from Actinidia chinensis, against Pseudomonas syringae pv. actinidiae (Psa), the causal agent of bacterial kiwi canker, has been investigated.\ud Methods\ud \ud Antagonism-deficient mutants of P. synxantha strain DLS65 were generated by insertion of minitransposon mTn5-GNm in its genome. Southern blot analysis allowed the selection of single transposon insertion-mutants. Amplification of the transposon flanking regions by means of arbitrary and single primer PCR in selected mutants was perfomed to obtain amplicons for sequencing purposes.\ud Results\ud \ud Sequencing results of the amplicons obtained from three antagonism-deficient mutants led to the localization of the transposonin three genes, which implies their involvement in the antagonism of P. synxantha: an acyl-homoserine lactone acylase gene (pvdQ), a glucose-6-phosphate dehydrogenase gene (zwf) and an mbtH-like gene were identified. It is known from the literature that these three genes are involved directly or indirectly in NRPs synthesis.\ud Conclusions\ud \ud We suggest that a molecule with antibiotic properties, produced by NRP synthetases, contributes to the antagonistic activity of P. synxantha.\ud Keyword

Abstract:BACKGROUND:The bacterial canker, caused by Pseudomonas syringae pv. actinidiae, is the most severe disease of cultivated Actinidia spp. The pathogen is systemic and not easily controlled by agrochemical means. OBJECTIVE:Our aim was to search, select and identify, among kiwifruit bacterial endophytes, possible antagonists able to control Pseudomonas syringae pv. actinidiae. METHODS:Several kiwifruit production areas were inspected, in order to find host plants without any disease symptom inside severely affected orchards. From those plants, endophytes were isolated, selected, tested for their ability to inhibit the growth of the pathogen and identified. RESULTS:A set of 65 different bacterial endophytes was isolated and tested: several of them were able to inhibit the growth of Pseudomonas syringae pv. actinidiae in vitro. None of the antagonists proved to possess either ice nucleation activity or transferable copper resistance. Taxonomically, antagonists belonged to the families of Pseudomonadaceae and Enterobacteriaceae. CONCLUSIONS:Effective bacterial antagonists were found as endophytes in kiwifruit plants and bearing features of safety of use and negligible risk for the crop. Such finding makes it possible to select the most prospective of them, in order to develop efficient biopesticides able control the bacterial canker in commercial orchards