AUTOINDUCER IN ERWINIA AMYLOVORA

The fire blight pathogen Erwinia amylovora, produces two types of exopoly saccharides (EPS), named amylovoran and levan. Levan is polymerized from the fructose residues of sucrose by the secreted enzyme levansucrase and has an intermediate effect on virulence. Expression of levansucrase is regulated by several rls-genes and the global regulator H-NS. The acidic EPS amylovoran consists mainly of galactose residues and forms a capsule around the cells to protect them against recognition by plant defense mechanisms. Changes in EPS production may interfere with formation of disease symptoms. The 12 genes of the ams-region responsible for amylovoran synthesis are regulated by rcsA, rcsB and rcsC. In our current work, we investigate the influence of autoinducers (AI) mediated quorum sensing on EPS production. While measuring no notable acyl-homoserine-lactone (AI-1) production, we could demonstrate synthesis of AI-2 by different E. amylovora fruit tree and raspberry strains as well as related species such as Erwinia pyrifoliae, Erwinia billingiae, and Erwinia tasmaniensis. The key enzyme for AI-2 synthesis, LuxS, is highly conserved within these species. To investigate the influence of AI-2 on virulence, luxS-mutants of E. amylovora strains were constructed, which were shown to be AI-2 deficient in a luminescence bioassay with Vibrio harveyi strain MM32. Subsequently, their EPS-production was analyzed. While expression of levansucrase barely changed, production of amylovoran in E. amylovora luxS mutants was strongly delayed as compared to the wild-type. While virulence of the luxS mutant was not impaired in planta, we could identify a plant derived AI-2 like signal that might complement for the luxS mutation. The relevance of AI-2 production for colonization of host plants by E. amylovora is discussed.