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Biological Mechanism For The Effects Of Soil Bacterial Community And Root Exudates On Bacterial Wilt Of Tomato

Posted on:2018-04-19Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y A GuFull Text:PDF
GTID:1360330575977153Subject:Plant Nutrition
Abstract/Summary:PDF Full Text Request
Soil-borne pathogen,which is responsible for important losses in crop production,can have severe impacts on plant health.Although extensively studied,pathogenic interactions represent only a fraction of the overall plant-microbe interactions.Natural plants interact closely with a diverse community of soil microbes that form a shield against pathogens.Before causing disease,pathogens must break this line of defense.On the other hand,plant roots secret a high diversity of low-molecular weight compounds(i.e.,root exudates)into the rhizosphere.Plant-derived root exudates modulate plant-microbe interactions and play an important role in modulating the composition of soil bacterial community.In this study,we used tomato bacterial wilt as disease model to reveal the effects of bacterial wilt of tomato on soil bacterial community composition,the effects of soil bacterial community composition on bacterial wilt of tomato,and the role of root exudates in interactions between plant health and soil bacterial community.These studies may help us to decipher the characteristics of healthy soil microbiome and to explore the method to modulate soil microbiome for plant health.Our main results are listed as followed:1.We conducted a in situ field experiment and collected rhizospheric soil by nondestructive sampling method and our results suggested that initial bacterial community composition was the most strongly factor determining health condition instead of total bacterial abundance,pathogen abundance and soil physicochemical properties in initial soil.Significant different soil microbiome composition but decay of similarity associated with healthy and diseased plants was observed during plant growth.By contrast,healthy plants were mainly attributed to the enrichment of subgroup of Proteobacteria and Bacteroidetes in initial soil.However,most of the OTUs which were associated with healthy plants rapidly disappeared during further microbiome development.Additionally,Bacillus spp.and Pseudomonas spp.isolated from plants that would later remain healthy were in average more antagonistic to the pathogen.2.By conducting a greenhouse experiment with homogenized soil and collecting rhizospheric soil by nondestructive sampling method,we profiled the succession of rhizosphere microbiome associated with healthy and diseased plants.Based on wilt symptom and pathogen density in stem crown,plants were separated into healthy,diseased and latently infected plants.R.solanacearum populations were similar among rhizosphere of the three health statuses in initial soil and they were elevated both in healthy and infected(diseased and latently infected)tomato plant rhizosphere across time.However,significant variation in R.solanacearum population was observed among the three health statuses at the end of the greenhouse experiment.Clear clustering of soil bacterial community composition and soil physical-chemical properties by different tomato plant developmental stages was revealed and significant relationship existed between rhizosphere microbiome and soil physical-chemical data.Clear differences of pathogen infection were observed at two reproductive stages and the impact of pathogen infection on rhizosphere microbiome is independent of the change of soil physical-chemical properties.Plant health condition and the interaction between health condition and plant developmental stages had significant effects on the relative abundance of 7 bacterial phyla including Chloroflexi,Gemmatimonadetes,Betaproteobacteria,Gammaproteobacteria etc.Comparative analyses indicated that the composition of Gemmatimonadetes,Chlamydiae,and Firmicutes showed the rapidest responses to pathogen invasion.3.By collecting tomato plant secretions in the absence and presence of R.solanacearum and adding them to an unplanted soil,we demonstrated shifts in root exudate profile and an elevated secretion of caffeic acid triggered by R.solanacearum invasion.The shifts in root exudate profile further affected the composition of soil bacterial community and increased caffeic acid directly inhibited the growth of R.solanacearum QL-Rs1115.4.Structural equation modeling revealed that the ratio of amino acids and organic acids in root exudates indirectly affected the incidence of bacterial wilt through changing the soil bacterial community composition,while the ratio of amino acids showed a direct and positive correlation with the incidence of tomato bacterial wilt.On the other hand,substrate richness in root exudates indirectly altered the incidence of bacterial wilt through changing the soil bacterial community composition,while substrate richness had no direct effects on the incidence of bacterial wilt.In addition,the ratio of amino acids in root exudates showed a significant positive correlation with soil total bacterial abundance and pathogen abundance,and the ratio of amino acids showed a significant positive correlation with soil total bacterial abundance.5.We showed that biochar can significantly decrease bacterial wilt disease incidence by attracting pathogen both directly and indirectly via adsorption of root exudates that exert strong chemotactic signal towards the pathogen.Furthermore,biochar suppressed the swarming motility of the pathogen,which likely directly decreased pathogen virulence and potentially prevented the pathogen from escaping the biochar pores.Lastly,the adsorption of tomato root exudates could indirectly reduce pathogen invasion by intensifying resource competition with other bacteria in more natural settings.In conclusion,we show that initial soil bacterial community can affect plant health and,on the contrary,bacterial wilt of tomato can change the rhizosphere bacterial community.Root exudates had great influence on soil bacterial community composition and the incidence of bacterial wilt.We could decrease disease incidence through root exudates addition or adsorption.
Keywords/Search Tags:Bacterial wilt of tomato, Biochar, High-throughput sequencing, Rhizosphere, Root exudates, Bacterial community composition
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