The Disease Prevention Potential And Mechanism Of Streptomyces 702 On Rice

Streptomyces 702, a strain owning an independent intellectual property right, was isolated and screened from soil by the Applied Microbiology Laboratory of Jiangxi Agricultural University (JAU). Ag-antibiotic 702, an antifungal component produced by Streptomyces 702, had been proved to be a new structural compound of polyene macrolide. With Rhizoctonia solani as the tested pathogens, this thesis focused on studying the bacteriostatic mechanism as well as the disease prevention and growth promotion mechanism of Ag-antibiotic 702, from different perspectives of cells, subcellular, physiological and biochemical study, and molecular biology. The main results and conclusions are given as follows:Results of physical and chemical properties showed that Ag-antibiotic 702 was the light yellow amorphous powder, and its melting point and Rotation was 170-173.5℃, and [α]D23.7=+4.3425°(c=1.09 in MeOH), respectively. UV spectrum showed that Ag-antibiotic 702 had three peaks at 318 nm,303 nm,290 nm, which was in accordance with the characters of tatar-alkene antibiotic. IR spectrum showed that Ag-antibiotic 702 contained C=O, C=C, C-O, OH and CH2. Results of stability showed that Ag-antibiotic 702 was stable to thermal, but its activity began to decline over 60℃. Its activity was stable when pH was 5 or 6, but unstable at strong acid and alkali conditions.Firstly, the inhibiting effects of Ag-antibiotic 702 on the mycelial growth, sclerotia formation and germination, and spore germination were studied, respectively. Additionally, the effects of Ag-antibiotic 702 on mycelium’s ultramicroscopic morphology and structure were observed by using Electron microscopy. It was found out that Ag-antibiotic 702 had an obvious inhibition action on hypha growth, with mycelium shriveled and shrunken, membrane contour unclear and damaged, organelle seriously damaged and vacuolated. The formation, germination of sclerotia and spore germination were also inhibited apparently by Ag-antibiotic 702.Additionally, compared with the experiment without Ag-antibiotic 702 treatment, when R. solani was dealt with Ag-antibiotic 702, the following results about R.solani were obtained:the electric conductivity was increased by 72.2%; The content of proteins, nucleic acid and Mg2+, K+ leaked from cells were all enhanced; The content of ergosterol was decreased by 92.0%. Ag-antibiotic 702 broke the complete structure of the cell membrane, destroyed the normal physiological function, and inhibited the bacterial respiratory metabolism and energy metabolism. Moreover, Ag-antibiotic 702 could decrease the ability of pathogenic fungi to clear the lipid peroxide and the free radical. With the increase of Ag-antibiotic 702 concentrations, the activities of the carboxy methyl cellulose enzyme (Cx), polygalacturonase (PG) and pectin methyl galacturonase (PMG) of R. solani were all decreased to varying degrees.When processed by Ag-antibiotic 702, the rice leaves exhibited obvious resistance to Rhizoctonia solani. Furthermore, the activities of peroxidase (POD), polyphenol oxidase (PPO), phenylalanine ammonia-lyase (PAL), catalase (CAT) were all increased in the rice leaves. However, the activity changes of SOD had no significant variety. The β-1,3-glucanase activity improved conspicuously and there were multiple protein spots in up-regulation after Ag-antibiotic 702 sprayed on rice leaf.The germination rates, together with the growth of radicle and buds were both improved by treatment with low concentration of Ag-antibiotic 702. Amylase activity and protein content first rise then fall with the increasing of Ag-antibiotic 702 concentration. The root vigor was improved, while the sugar content was reduced. After Streptomyces 702 agent was applied, the soil bacteria and mold counts were reduced, while the number of actinomycetes was increased. It was most noticeable that Streptomyces 702 decreased the amount the nitrosococcus bacteria and nitrifying bacteria, but significantly increased aerobic cellulose decomposing bacteria. Changes in soil microbial physiological groups accelerated the soil nutrient cycling, and enhanced the nutrient uptake of rice, thus contributing to its growth.In summary, based on the inhibiting mechanism of polyene macrolidest antibiotics on pathogenic fungi, the antifungal mechanism of Ag-antibiotic 702 can be preliminarily inferred:After the Ag-antibiotic 702 through the cell wall, the ergosterols, or antibiotics, were formed by mutual combination with ergosterols in the fungal cell membrane, which increased the permeability of the bacterial cell membrane, and resulted in a decreased activity and an insufficient energy supply, thereby inhibiting bacterial respiratory metabolism as well as the energy metabolism. Ultimately there appeared serious injury of pathogenic fungi cells, membrane damage, inhibited growth and development, and serious deformation of mycelium morphology, until death. Furthermore, Ag-antibiotic 702 can induce rice resistance enzyme to promote seed germination. The soil processed with Streptomyces 702 can improve the quality of seedlings, promote the growth of beneficial microorganisms, and inhibit the survival of harmful microbes.