Research On Targeted Prevention And Control Of Banana Wilt By Rhizospheric Chemotactic Bacteri

Banana Fusarium wilt disease is a destructive soil-borne disease.In recent years,a lot of progress has been made in the screening and control of banana wilt using antagonistic biocontrol bacteria.However,the control effects of biocontrol agents for traditional cultivars such as Brail,Williams,etc.are not ideal,probably because the applied antagonist is randomly distributed in the soil,and it is difficult to form a high abundance of antagonist colonies around the root system due to the lack of targeting root colonization.Plants can recruit rhizosphere microorganisms using their root exudates,thereby shaping the rhizosphere microbiome to resist pathogen infection.Therefore,it is of great significance to use root exudates as targeted signaling molecules to cooperate with antagonistic bacteria to prevent and control such root soil-borne diseases.In this study,chemotaxis was used to screen chemotactic strains and targeted signaling molecules,and the synergistic effect of chemotactic strains and targeted signaling molecules on the prevention and control of banana wilt was studied.The main results were as follows:1.A total of 256 plant rhizosphere soil samples were collected from 17 locations across the country,from which 115 bacterial strains were isolated with plate antagonism to strain B2 of Fusarium oxysporum f.sp.cubense race4(FOC4).11 strains with chemotaxis to banana root exudates were screened from the above 115 antagonistic strains,among which the best performing LG14-3 strain showed significant chemotaxis to banana root exudates,glycine and citric acid with the relative chemotaxis index(RCI)of 3.62,7.26 and 8.37,respectively.Because RCI > 2,Brazilian banana root exudates,glycine and citric acid were identified as targeted signaling molecules of strain LG14-3.2.Based on morphological,physiological and biochemical characteristics,and phylogenetic analyses,strain LG14-3 was identified as Bacillus velezensis.The plate antagonism assay showed that Bacillus velezensis LG14-3 had broad-spectrum resistance to seven strains of pathogens,including FOC4 and the bacterial angular spot pathogen of Mango(Xanthomnas citri pv.mangiferaeindicae,XCM),with 78% inhibition against FOC4.Further studies on the growth-promoting potential of this strain showed that strain LG14-3is capable of producing amylase,cellulase,protease,indoleacetic acid(IAA)and iron carriers,and has the ability to fix nitrogen.3.Further studies on the interaction mechanism between chemotactic strains and targeted signaling molecules.The results showed that: the root exudate significantly improved the swarming motility ability of strain LG14-3;glycine(0.25 m M)and citric acid(0.25 m M)enhanced the biofilm formation of LG14-3 strain;Bacillus LG14-3 had colonization advantage and citric acid enhanced the colonization ability of Bacillus velezensis LG14-3 in the banana rhizosphere.In the colonization test,chemotactic strain LG14-3 colonized banana roots significantly more than with other biocontrol bacteria,and the number of chemotactic strains colonized in the citric acid-treated banana roots increased by 353.67% compared to CK(only chemotactic strain treatment),significantly higher than CK(P<0.05),and the root vigor of banana treated with citric acid increased by 34.11%compared to CK,significantly higher than CK(P <0.05).4.A pot biocontrol experiment was conducted with Brazilian plantain as the experimental material to study the prevention and control effect of chemotactic bacteria and the synergistic effect of chemotactic bacteria and targeted signaling molecules(glycine and citric acid)on banana wilt.The results showed that the targeting signal molecule citric acid was able to improve the control of banana wilt by the chemotactic strain.In the biocontrol experiment,the citric acid treatment had the best control effect in the pots,with 75.56%,and the chemotactic strain treatment had a significant growth-promoting effect on banana plants.The soil bacterial community structure of each treatment group was analyzed using 16 S r RNA amplicon sequencing,and the results showed that the chemotactic strain could increase the rhizosphere bacterial species richness;the targeting signal molecules citric acid and glycine could increase the number of rhizosphere Bacillus.