| Bacteria can communicate by producing and responding to small diffusible molecules that act as signals. As this phenomenon occurs in a cell-density-dependent manner, it has been termed quorum sensing (QS). Many Gram-positive and Gram-negative bacterias utilize quorum sensing systems to control gene expression and activate behaviours that are beneficial under the particular condition encountered. The main problem in applying biocontrol bacteria is that the disease suppression and growth promotion afforded by the bacteria are frequently not consistent in the field. Root colonization by biocontrol bacteria has been considered as a prerequisite for a successful biological control, and the variable root colonization by a biocontrol agent was considered as the main reason for its inconsistent performance. To date, the preponderance of studies on biocontrol bacteria have been conducted with Gram-negative bacteria, mostly on Pseudomonas spp. Bacillus strains are widely used as biocontrol agents. However, the relationship between quorum sensing and colonization in biocontrol Bacillus strains has not been well studied.B. cereus 905 isolated from wheat rhizosphere exhibited significant plant growth promoting effects. One commercial product developed from 905 has been applied to wheat fields after being registered as biopesticide. Although the results from previous study show that 905 is an efficient colonizer of wheat roots, our knowledge about the mechanisms of its plant growth enhancing activity is still very limited. In this work, we report the colonization of 905 could be significantly affected by quorum sensing related gene disruption. which will enhance analysis of mechanisms of biocontrol and be critical for the development of a genetically engineered biocontrol agent. The main contents and results of this dissertation are as followed.In this study we detected the role of PlcR-PapR and LuxS/AI-2 QS systems in B. cereus 905. By use of the knock out system constructed, the mutant strains, which lacking the QS genes, were created. Analysis of these mutant strains and its complementary strains revealed differences in biological characteristics. The PlcR-PapR system positively regulated swarming motility, root colonization, biosurfactant production and negatively regulated submerged biofilm formation. But the LuxS/AI-2 system did not seem to have any affect. The two systems act independently of each other. These results suggest that the two QS systems in B. cereus 905 may be very different from other B. cereus isolated from other environment. And then RNA-seq transcriptomic analysis was used to investigate the cellular response to PlcR-PapR system in B. cereus 905.The genome of B. cereus 905 was sequenced and then analyzed using softwares. The Next-Generation Sequencing(NGS) based RNA-seq reflected the transcript levels of genes were significantly changed (p≤0.05). The GO cluster analysis revealed that the differentially expressed genes were related with many metabolic processes. A considerable number of genes involved in motility, biofilm formation, chemotaxis, bacteriocin, polyketide, protease, superoxide dismutase, cellulase, and chitinase, whose expression were up- or down-regulated. Previous study showed that the ability of 905 to colonize wheat roots depends on various factors including motility and biofilm formation. The colonization mechanisms regulated by the PlcR-PapR system were suggested. These results enriched the knowledge of the PlcR-PapR QS system and raised new insight for comprehensive exploration of the relationship between QS system and colonization. |
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