Over several years it has become clear that bacteria are able to communicate with each other to coordinately regulate gene expression according to population density, in a process which has been termed ‘quorum sensing’. Indeed Quorum sensing (QS) is a cell-to-cell signaling mechanism in which bacteria coordinate the expression of certain genes in response to their population density by producing, releasing and detecting diffusible signal molecules. The signaling molecules can be sensed by cells and this allows the whole population to initiate a concerted action once a critical concentration (corresponding to a particular cell density) has been achieved. Gram negative bacteria like Pectobacterium carotovorum produce N-acyl-homoserine lacton (N-AHL) as their signaling molecule. In this bacterium, AHLs regulate virulence gene expression including genes coding for extracellular enzymes, toxins and motility. Interrupting QS system known as quorum quenching (QQ) is a novel strategy to control of plant pathogenic bacteria. QQ is implementable through to ways: 1) plant protection by means of AHL degrading bacteria and 2) heterologous expression of AHL degrading enzymes in bacteria or plant tissues. In this thesis the effect of aiiA gen, isolated from Bacillus sp. A24 and Bacillus sp . DMS133, on pathogenicity of P. carotovorum, the causal agent of soft-rot in potato and other crops, was studied. In biological control test both two transformed bacteria influenced enzymatic activity of EMPCC and reduced its pathogenicity on potato tubers. Heterologous expression of aiiA gen in EMPCC caused degradation of AHLs and severely reduced accumulation of signaling molecules in bacterial population, which prevented pigmentation of Chromobacterium violaceum CV026 bioreporter strain in co-culture test. Morphological tests revealed that the expression of aiiA gen attenuated pathogenicity of EMPCC as this bacteria uses QS system to produce virulence factors. In addition to, the expression level of virulence genes in aiiA transformed bacteria using Real Time PCR technique was evaluated. Real-time PCR results showed high expression of hrpc , hrpn and nip genes in both transformed bacteria against wild type strain. Study of differences in expression of plants cell wall degrading enzymes revealed that there is a little difference between two transformed strains and as morphological tests showed both two genes can severely reduce pathogenicity of P. carotovorum. Key words: Quorum sensing, signal molecules, N-acyl-homoserine lacton, quorum quenching
