Changes In The Population Of Bacteria Possessing Quorum Sensing Under Rehmannia Glutinosa Monoculture And Their Action Mechanism

Rehmannia glutinosa L.,a perennial medicinal plant,belongs to the family of Scrophulariaceae and has a very high medicinal value.However,consecutive monoculture of this plant results in serious decline in both biomass and quality of underground tuberous roots.Previous studies suggested that the changes in rhizosphere microbial community structure were closely related to the growth and development of plants.Soil is an extremely complex black box,with a complicated interactions between soil microorganisms,such as quorum-sensing(QS)and quorum-quenching(QQ).However,there are few reports about the roles of quorum sensing and quorum quenching in the formation of consecutive monoculture problems of R.glutinosa.Therefore,this study explored the changes in the composition and abundance of quorum-sensing bacteria and quorum-quenching bacteria under consecutive monoculture of R.glutinosa by streak plate method using Chromobacterler violaceum CV026,96-wells liquid culture method,etc.Furthermore,the effects of key allelochemicals in R.glutinosa root exudates on the growth of QS and QQ bacteria were detected to study the relationships between plant and microbes,and to clarify the action mode of quorum sensing and quorum quenching in the formation of consecutive monoculture problems of R.glutinosa.The main conclusions are listed as follows:65 out of 200 strains randomly selected from the newly planted soil of Rehmannia glutinosa were detected as quorum-sensing(QS)bacteria,mainly consisting of Pseudomonas spp.(58.5%).However,34 out of 200 strains randomly selected from the consecutively monocultured soil of R.glutinosa were detected as QS bacteria,mainly consisting of Enterobacter spp.(73.5%).OD value assay showed that phenolic acids mixture showed concentration-dependent inhibitory effects on the growth of beneficial QS bacteria such as Pseudomonas brassicacearum SZ50,et al.While phenolic acids mixture showed concentration-dependent promoting effects on the growth of harmful QS bacteria such as Enterobacter ludwigii BT23,et al.Furthermore,the composition and abundance of quorum-quenching bacteria that could degrade beneficial Pseudomonas brassicacearum AHL signaling molecule were detected by 96-well liquid culture method.The results showed that the population of bacteria quenching P.brassicacearum AHL significantly higher in two-year monoculture soil and replant disease soil than in newly planted soil.Molecular identification found that the quorum-quenching bacteria screened from two-year monoculture soil and replant disease soil mainly consist of Acinetobacter spp.In addition,it was found that phenolic acids mixture identified in the root exudates of R.glutinosa could significantly promote the growth of Acinetobacter sp.,Enterobacter sp.Moreover,these quorum-quenching bacteria showed strong pathogenicity to the tissue culture seedlings of R.glutinosa.In conclusion,consecutive monoculture of R.glutinosa affected the composition and population of quorum-sensing bacteria and quorum-quenching bacteria in rhizosphere.The decrease of beneficial quorum-sensing bacteria(i.e.Pseudomonas brassicacearum)was not only related to the inhibition of phenolic acids in root exudates of R.glutinosa,but also to the increase of specific quorum-quenching bacteria under monoculture.It's speculated that the imbalance between quorum-sensing bacteria and quorum-quenching bacteria led to consecutive monoculture problems of R.glutinosa.This study preliminarily clarify the roles of quorum-sensing and quorum-quenching in the formation of consecutive monoculture problems of R.glutinosa,which provides theoretical basis for exploring the underlying mechanism of replant disease and corresponding control strategy.