The quorum sensing is a phenomenon which controls the production of virulence factors in some bacteria. But the correlation between the ice nucleation activities which is one of the most important virulence factor in P.syringae with this system in unknown. In order to investigate that, in spring of 2013, 248 samples (including leaves and flowers) were collected from twigs of stone fruit trees in Khorasan Razavi province. Ice nucleation active bacteria, were identified and among them the P. syringae isolates were distinguished by LOPAT tests. Among virulence factors three of them such as swarming motility, susceptibility to hydrogen peroxide and pyoverdine production were studied. The swarming motility of the isolates with high ice nucleation activity were significantly more than those isolates which indicated low ice nucleation activity. There was no significant different in susceptibility to hydrogen peroxide and pyoverdine production between high and low ice nucleation active isolates. The production of homoserine lactones in six selected P. syringae belonged to the different groups of ice nucleation activity were extracted and detected using Chromobacter violaceum CV026 which was overlaid on Thin-layer chromatography plates. Our results indicated direct correlation between ice nucleation activity and the amount of homoserine lactone production and confirmed that the hypermotile isolates have high ice nucleation activity.

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