Biocontrol Activities And Mechanisms Of Myxococcus Xanthus Against Tomato Bacterial Wilt

Myxobacteria are microbial predators and widely distributed in soil.They are capable of preying on a variety of microorganisms,including bacteria,yeast,fungi,and microalgae,through characteristic “wolf-pack” collective behavior.Therefore,myxobacteria are of great significance in the study of microbial ecology.In tomato agricultural production,bacterial wilt is a common devastating soil-borne bacterial disease,causing huge losses to tomato agricultural production.The pathogen of tomato bacterial wilt was known as Ralstonia solanacearum(RS),which is a gram-negative bacterium.At present,administration of chemical agents is the dominating approach to control tomato bacterial wilt.However,long-term use of chemical agents will cause damage to natural ecosystems,and some of the damages are difficult to repair.In this context,the biological methods to control tomato bacterial wilt are of important practical significance.Previously,we used RS as the prey to induce and isolate myxobacteria from soil samples,and recruited 50 myxobacteria strains,of which strain Myxococcus xanthus GDMCC 801043 effectively prey on RS on solid medium plates.In this study,the biocontrol effect of M.xanthus GDMCC 801043 against tomato bacterial wilt were studied by pot experiments;the inhibitory effect of secondary metabolites and the lytic effect of extracellular enzymes produced by M.xanthus GDMCC 801043 on RS were also investigated.The main results are as follows:(1)According to the pot experiment,M.xanthus GDMCC 801043 significantly reduced the disease incidence and disease index of tomato bacterial wilt,and the biological control efficiency reached 81.9%.(2)The secondary metabolites of M.xanthus GDMCC 801043 were extracted from the supernatant of fermentation and also the cells with ethyl acetate.Experimental results showed that secondary metabolites are ineffective on inhibiting R.solanacearum RSH.Therefore,secondary metabolites of M.xanthus GDMCC 801043 were considered not involved in the biocontrol of tomato bacterial wilt.(3)In the plate experiment,M.xanthus GDMCC 801043 was found to be able to hunt R.solanacearum RSH-gm efficiently.It was observed by scanning electron microscope that the cells of RSH-gm that were predated by M.xanthus GDMCC 801043 broked into incomplete fragments,and found that M.xanthus GDMCC 801043 would secrete a large amount of extracellular filamentous substances in the process of predation of RSH-gm cells,which was speculated to be exopolysaccharides or proteins.(4)The protein components of M.xanthus GDMCC 801043 fermentation supernatant were extracted using the method of ammonium sulfate precipitation.After redissolution,dialysis and ultrafiltration,crude enzyme solution was obtained.The integrity of R.solanacearum RSH-gm cells treated with crude enzyme solution was found to be damaged with contents outflowed,as observed by transmission electron microscopy.The substrate spectrum analysis of the crude enzyme solution revealed that enzymes showed activities of hydrolyzing β-1,4、β-1,6-glycoside hydrolase and lipase.Therefore,extracellular hydrolase secreted by M.xanthus GDMCC 801043 were deduced to play an important role in biocontrol of tomato bacterial wilt.In summary,M.xanthus GDMCC 801043 showed good biocontrol potential against tomato bacterial wilt,and significantly reduced the incidence and disease index of tomato bacterial wilt.Myxobacteria can lyse RS cells using the secreted extracellular hydrolase.This study revealed the good application potential of myxobacteria in the control of plant diseases,and provided a desirable strain for developing the biocides.Furthermore,this study preliminarily clarified the biocontrol mechanism of myxobacteria against tomato bacterial wilt,what provided a reference for further studying the molecular mechanism.