Study On The Functional Role Of Quorum Sensing Of Bradyrhizobium Diaoefficiens In Symbiosis

Quorum sensing is the exchange of information between bacteria,is the population-denisty-dependent regulation of gene expression in bacteria.It affects the expression of genes through autoinducers and regulates the physiological and biochemical characteristics and biological behavior of microbial populations.The quorun-sensing system is widely found in the bacterial community,including rhizobia with a close symbiotic relationship with various legumes.Bradyrhizobium diaoefficiens,a symbiotic rhizobial strain of soybean,exists as a dominant bacterium in the soil of the major soybean growing areas in China.The study of the quorum-sensing effect system in this bacteria will deepen the understanding of Soybean-B.diaoefficiens,a symbiotic nitrogen fixation system which is extremely important for the global nitrogen ecological cycle and agricultural production.It can provide theoretical basis for the production and application of inoculation of rhizobia preparation.In this dissertation,B.diaoefficiens USDA110,a soybean rhizobia-type bacterium,was taken as the reseach object.Taking the genes blr1062 and blr1063 that involved in the quorum sensing system as the cut-in point,the detail aspects were studied of the system are as follows:1.The construction of gene mutant is the most direct means of studying gene function.In this study,single-gene deletion mutants of the genes blr1062 and blr1063 and the two-gene deletion mutants of the two genes were constructed.Compared with the wild-type strain the mutants were repectively deleted 364,342 and 1003 bases.2.The analysis of physiological and biochemical showed that compared with the wild strain,the growth ability of the three mutants became weaker;in the morphology of the colonies,the mutants of the gene blr1062 and the gene blr1063 were as smooth as the wild strain,while the mutant of the double genes was slightly rougher;although the ability of biofilm-forming of the three mutant strains were significantly lower than of the wild one,the ability of movement of the three mutants strains showed greatly increased;in the experiment of bacteriostatic piece,the tolerance to Cefotaxime sodium of the mutant of the gene blr1063 become lower than the wild and the other two mutants,however,tetracycline and Genistein have no bacteriostasis to the wild and three mutants,and the bacteriostasis of SDS showed no difference between them.3.Analysis the expression of nodulation genes and the experiment of symbiotic inoculation showed that compared with the wild type,the expression of the nodY gene which an important for the synthesis of nodulation factor was significantly enhanced in the three mutant strains.The quorum sensing system may participate in the signaling regulatory pathways of the expression of nodulation genes.However,the inoculation test showed that the mutants in this quorum sensing system did not affect the ability of nodulation of rhizobia significantly.