| Root exudates play important roles in the communication between plants and their environments. A great deal of documents have elucidated that the content of root exudates had been changed and thus would directly affect on the rhizosphere microbial community when plants grow in varied season or habitat. However, it is not clear how rhizosphere microbial community is affected by plants root exudates and how the effects are fed back to the plants. It has been demonstrated that 3-acetonyl-3-hydroxyoxindole water dispersible granules (ASO) is a chemical inducer of plant systemic acquired resistance (SAR) which can enhance tobacco resistance against tobacco mosaic virus (TMV) and the fungal pathogen Erysiphe cichoracearum (powdery mildew). Here we investigated how the root exudates changed and its influences on the composition and function of the rhizosphere bacteric community after tobacco K326 was sprayed by ASO. In addition, we performed culture-dependent approaches to identify that plant enabled specific rhizosphere bacteria to thrive via root exudates. The major results of this thesis are as follows:1. The composition of volatile organic compounds (VOCs) and organic acids of root exudates was analysed by GC-MS and ion chromatography after tobacco K326 was induced by ASO. The influence of ASO application on the content of VOCs and organic acids was insignificant, but the effects on composition of VOCs were remarkable. For example, content of the antibiotic 2-methyl-butanoic acid and 3-methyl-butanoic acid as well the nicotinyl insecticide myosmine and anataline varied a great. And content the signal molecules acetovanillone, acetosyringone and isatin also dramatically increaded. These results indicated that ASO induced systemic acquired resistance (SAR) triggers a kind of widely protective response of the plants, incuding root exudates which can direct impact on the composition of rhizosphere microorganisms.2. The rhizosphere bacterial community developing in the rhizosphere of K326 was analyzed by 16S rDNA amplicon sequencing after ASO application. Both the growth of tobacco K326 and ASO decreased rhizosphere bacterial diversity. The growth stages had only an impact on bacterial community, while ASO affected on bacterial species composition and function. The function of the dominant bacterial population in the control group was associated with the degradation of complex organic compounds and materials cycle. While in the ASO treated group, the functions of the dominant bacterial population changed into secreting antibiotics and inducing plant systematic resistance. That is to say the dominant bacterial population in the ASO treated tobacco K326 has been going on a succession from materials cycle community to a defence community which guaranteed their functions to keep in step with the plant survival stratage. The results suggested that rhizosphere bacterial community confronted the challenges of environmental changes with plants together as a whole.3. A powdery mildew (E. cichoracearum) resistance bacterial strain was isolated from tobacco cultivated soil with selection medium which containing the root exudates of tobacco K326 induced by ASO. Based on its morphology, physical, biochemical, cellular chemistry and molecular charcters, the bactraial strain was a novel species, and at last to be identified and nomenclatured as Lysobacter erysipheiresistens RS-LYSO-3. RS-LYSO-3 showed an obvious effect against powdery mildew in tobacco and an obvious chemotaxis for ASO treated tobacco root exudates. These results suggested that plant may recruite rhizosphere microorganism to constructure defense system together as a symbiont for going through their environment via transformation root exudates. |
