Mechanism Of Rhizosphere Colonization And Induction Of Systemic Resistance By Bacillus Amyloliquefaciens

Overapplication of chemical feritlizer caused soil degradation in China.Plant growth-promoting rhizobacteria(PGPR)could play important role in healty and yield of crops in agricultural production.Application of PGPR would contribute to the reduction of usage of chemical fertilizer and pesticides.It is valuable to study the behavior of PGPRs in rhizosphere.However,the beneficial effect of PGPR in agricultural application is not that well as in lab.It is generally believed that the efficiency of the beneficial effect of PGPRs is highly restricted with its lack colonization on root surface.Root colonization is divided into two processes,chemotaxis toward the root and the subsequent biofilm formation on the root surface.In this study,the communication between the PGPR and plant that affect chemotaxis,biofilm formation and induced systemic resistance are studied.The results for the thesis are summarized:1.In comparison with other root-secreted compounds,D-galactose in the root exudates of cucumber was found to be the strongest chemoattractant for Bacillus amyloliquefaciens SQR9.Exogenous D-galactose also enhanced root colonization by SQR9.In addition,the secretion of D-galactose by cucumber roots could be induced by the inoculation of SQR9,indicating that D-galactose may be an important signal in the mutual interaction.Further results from the knockout of methyl-accepting chemotaxis proteins(MCPs)demonstrated that McpA was the only MCP responsible for sensing D-galactose.Interestingly,D-galactose significantly enhanced the biofilm formation of SQR9 in an McpA-dependent manner.The MCP-dependent induction of biofilm formation by an external signal is novel in the crosstalk between the signal transduction of chemotaxis and biofilm formation.This study advances the knowledge of the interaction between plants and bacteria and identifies the novel downstream signal transduction of MCPs.2.The signal molecules in root exudates sensed by the plant growth-promoting rhizobacteria(PGPR)is the key point to regulate the colonization of PGPRs.Phosphorylated SpoOA is an important global transcriptional regulator that controls the colonization and sporulation in Bacillus.In this study,the KinD,the original phosphate donor of SpoOA,was investigated for its ligands in root exudates of cucumber by both non-targeted ligand fishing method and the targeted SPR detection method.We spermine and guanosine were ligands of KinD with KD of 213μM and 51μM,respectively,which was supported by the SPR experiment Besides,calcium L-threonate,N-acetyl-L-aspartic acid,sodium decanoic acid and parabanic acid could bind KinD weakly.After then,the 3-dimensional binding models have been constructed to confirm the result and for deeply understanding the interactions between root secreted signal and KinD.We showed that spermine and guanosine bind to different site of KinD.We also identified that neither malate nor pyruvate as ligands of KinD,indicating the different binding pref’erence between B.amyloliquefacie.ns and B.subtilis3.Nine mutants of the plant growth-promoting rhizobacterium Bacillus amyloliquefaciens SQR9 deficient in producing the extracellular compounds were tested for the induction of systemic resistance against Pseudomonas syringae pv.tomato DC3000 and Botrytis cinerea and the transcription of the salicylic acid,jasmonic acid,and ethylene signaling pathways in Arabidopsis.Deficiency in producing any of these compounds in SQR9 significantly weakened the induced plant resistance against these phytopathogens.These SQR9-produced elicitors induced different plant defense genes.Overall,these elicitors of SQR9 could act synergistically to induce plant systemic resistance against different phytopathogens through different signaling pathway genes,and the bacterial antibiotics are major contributors to the induction.