Enhanced Suppression Of Soil-borne Bacterial Wilt By Applying Prebiotics With Beneficial Bacteria

Soil-borne diseases are characterized by high variety,high infectivity and difficulty in terms of control,which seriously affects crop yields and restricts the sustainable development of agriculture.The use of beneficial bacteria to control soil-borne diseases has become a hot research topic because it is environmental-friendly.However,effects of beneficial bacteria on suppressing soil-borne disease in the field is often unstable,mainly due to the lack of appropriate resources in the rhizosphere for the growth of beneficial bacteria.The beneficial bacteria cannot compete with indigenous microorganisms in the rhizosphere,and thus cannot effectively colonize in the rhizosphere.As a result,its biocontrol efficiency cannot be fully performed.Studies showed that some resources or organic carriers could promote the growth of beneficial bacteria,but these resources are not selectively targeted at beneficial bacteria,and may also promote the growth of soil-borne pathogens.Therefore,providing beneficial bacteria with specific resources which are suitable for their growth can enhance their biocontrol efficiency in the rhizosphere.Some studies on animal intestines demonstrated that the addition of prebiotics can specifically promote the growth and colonization of beneficial bacteria in the intestine,thereby maintaining the health of host.Functions of Rhizosphere and animal intestinal are similar.Some researchers suggested that probiotics can also be applied in the rhizosphere to promote biocontrol efficiency of beneficial bacteria.However,whether this kind of substances can be found and how they enhance the biocontrol capabilities of beneficial bacteria are still unknown.Therefore,in order to screen prebiotics to different beneficial bacteria and examine their ability of enhancing soil-borne diseases suppression,we used three beneficial bacteria:Ralstonia pickettii QL-A6,Pseudomonas fluorescens CHAO and Lysinibacillus sphaericus HR92,and took Ralstonia solanacearum QL-Rs1115 as model pathogen.We screened prebiotics out from 95 kinds of resources and investigated their effects on pathogen suppression and growth of microbes.Effects of prebiotics on antagonistic abilities of beneficial bacteria,production of siderophores and auxin and antagonism and expression of pathogenic genes of pathogen were also measured,including production of siderophores and auxin,antagonism and expression of pathogenic genes of pathogen.Finally,greenhouse experiment was carried out to investigate effects of prebiotics on suppression of bacterial wilt together with beneficial bacteria.The main findings were as follows:1.In vitro experiment,prebiotics were screened based on following three standards.First,resource could improve R.solanacearum inhibition of beneficial bacteria when co-cultured.Second,substances could significantly promote growth of beneficial bacteria.Third,resource had no significant promotive effect on R.solanacearum.Results showed that cycloserine,gallic acid,inulin,etc.fit previous standards.2.We further investigated how probiotics enhanced biocontrol efficiency of beneficial bacteria by measuring several biocontrol traits.By investigating growth rate,production of siderophores and auxin,antagonism,and expression of pathogenic genes after adding prebiotics,we found that cycloserine enhanced growth rate of beneficial bacteria QL-A6,production of iron carrier and auxin,and inhibit the expression of pathogenic genes of R.solanacearum.Gallic acid enhanced the iron carrier and auxin production of beneficial bacteria CHA0,and inhibited the expression of part of the pathogenic genes of R.solanacearum.Inulin can enhanceed the growth rate of beneficial bacteria HR92,antagonism,siderophore and auxin,and inhibited the expression of pathogenic genes fliC、egl、epsB of R.solanacearum.Finally,three prebiotics were selected,which were cycloserine to QL-A6,gallic acid to CHAO and inulin to HR92.3.In the greenhouse experiment results show that when prebiotics and beneficial bacteria are applied to tomato rhizosphere by root irrigation,we found that cycloserine,gallic acid and inulin significantly promoted biocontrol efficiency of beneficial bacteria on bacterial wilt.Meanwhile,gallic acid improved shoot biomass of tomato,while cycloserine and inulin had no effect on it.In addition,results showed that addition of prebiotics altered structure of bacterial community in the rhizosphere.Specifically,cycloserine significantly reduced relative abundance of Proteobacteria.the addition of gallic acid significantly reduced relative abundance of Gemmatimonadetes.Whereas,inulin significantly decreased relative abundance of Patescibacteria,thereby improving overall disease resistance of tomato rhizosphere.In summary,prebiotics of beneficial bacteria QL-A6,CHA0 and HR92 are cycloserine,gallic acid and inulin,respectively.These three probiotics enhanced pathogen suppression of beneficial bacteria,promoted the growth of beneficial bacteria,and affected structure of rhizosphere microbial community,thereby reducing the occurrence of tomato bacterial wilt.