Directed Succession Mechanism Of Rhizosphere Quorum Sensing Bacteria Under Pseudostellaria Heterophylla Consecutive Monoculture And Its Relationships With Replant Disease

Pseudostellaria heterophylla is a perennial herbaceous plant in the Caryophyllaceae family.Its tuberous roots are widely used in traditional Chinese medicine with high medicinal value and great market demand.However,it suffers from serious consecutive monoculture problems in the agricultural production.As the second genome of plants,rhizosphere microbiome is closely related to plant growth and health.Therefore,this study explored the relationships between P.heterophylla rhizosphere specific microorganisms and the formation of consecutive monoculture problems from the perspective of quorum sensing(QS)and quorum quenching(QQ).The results were shown as follows:Firstly,the changes in composition and abundance of rhizosphere quorum sensing bacteria were analyzed under P.heterophylla consecutive monoculture.The results showed that with the increasing years of monoculture,the number of QS bacteria in rhizosphere soil increased.Among them,potential pathogens such as genus Enterobacter showed an upward trend while the potential beneficial QS bacteria such as genera Bacillus and Burkholderia showed a downward trend under consecutive monoculture,which was consistent with the quantitative PCR results.Furthermore,it was found that QS bacteria including Enterobacter cancerogenus,Enterobacter ludwigii,Enterobacter cloacae and Enterobacter hormaechei,predominant in the rhizosphere of the consecutively-monocultured and diseased P.heterophylla,showed strong pathogenicity to the tissue cultured seedlings of P.heterophylla.In contrast,QS bacteria including Burkholderia cepacia,Bacillus aryabhattai,Bacillus megaterium and Bacillus thuringiensis,predominant in the rhizosphere of newly-plant P.heterophylla,have obvious antagonistic effect on the several fungal pathogens of P.heterophylla(such as Fusarium oxysporum or Sclerotium delphinii).Secondly,the effects of phenolic acids,the key allelopathic compounds in P.heterophylla root exudates,on the chemotaxis and growth of specific QS bacteria and their synthesis in quorum sensing signal molecules(Acyl-homoserine lactones,AHL)were detected.The results showed that the above-mentioned 8 main QS beneficial bacteria and pathogenic bacteria all showed significant chemotaxis to phenolic acids mixture.However,phenolic acids mixture had significant inhibitory effect on the growth of QS beneficial bacteria including B.cepacia,B.megaterium,B.thuringiensis and B.aryabhattai.Moreover,phenolic acids mixture had significant inhibitory effect on the AHL synthesis of B.cepacia and B.aryabhattai.By contrast,phenolic acids mixture significantly promoted the growth and AHL synthesis of QS pathogens including E.hormaechei,E.cloacae,E.ludwigii and E.cancerogenus.Finally,we analyzed the changes in the composition and abundance of QQ bacteria with the ability to degrade the AHL from a probiotic strain B.cepacian(N78)in the rhizosphere soil of P.heterophylla with different monoculture years.The results showed that the number of QQ bacteria increased with the increasing years of monoculture.Among them,the number of QQ bacteria belonging to genus Enterobacter increased in rhizosphere under consecutive monoculture,while the opposite trend was observed for the QQ bacteria belonging to the genera Bacillus and Burkholderia.By contrast,the number of QQ bacteria with the ability to degrade the AHL of a pathogen E.hormaechei(I-128)decreased with the increasing years of monoculture,mainly due to the decrease of the QQ bacteria belonging to the genera Bacillus and Burkholderia.Furthermore,the AHLs were analyzed by TLC and the results showed that the AHLs secreted by B.cepacia(N78)and E.hormaechei(I-128)were different.Meanwhile,to provide microbial resources for control QS pathogens in future,biocontrol effect evaluation of part of QQ bacteria was performed and the results showed that QQ bacteria including B.cepacia,B.aryabhattai,B.megaterium,B.thuringiensis could effectively control the pathogenicity of pathogenic QS bacterium such as E.hormaechei on P.heterophylla seedlings.In conclusion,consecutive monoculture of P.heterophylla resulted in the imbalance of rhizosphere quorum sensing bacterial community,leading to relatively fewer beneficial bacteria(i.e.Bacillus and Burkholderia)and relatively more pathogenic bacteria(i.e.Enterobacter).These shifts was not only due to the inhibitory or promoting effects of root-secreted phenolic acids on beneficial or pathogenic microorganisms,but also due to the changes in the quorum quenching bacterial community under consecutive monoculture.This study uncovered the relationships between the changes in quorum sensing bacteria/quorum quenching bacteria and the formation of consecutive monoculture problems of P.heterophylla,which provided theoretical basis and technical support for the subsequent construction of new strategies for solving consecutive monoculture problems of P.heterophylla.