| Beneficial becteria, colonized in the rhizosphere of plant or near the roots, can promote the growth of plants, facilitate the absorption and utilization of mineral nutrients, and inhibite the growth and reproduction of pathogen. Strains were isolated from rhizosphere soil, root surface and the interior of root by dilution plate method, and their phosphate solubilization, potassium solubilization and nitrogen-fixing activities were detected. Meanwhile, the capacities of producing indole acetic acid, ACC deaminase and siderophore were assayed. Furthermore, the strains inhibiting the growth of pathogenic fungi were screened by flat confrontation method. Consequently, the inhibition rates and properties of the strains were determinated. Moreover, phylogenetic trees were built based on16S rDNA sequence of the strains to show the genetic diversity of strains producing concentreation of siderophore and rhizospere. Ultimatly, the effects of growth promoting and diseases resistence were measured by plate experiments and pot culture experiments135strains were isolated, among which103strains could grow on phosphate-dissolving medium,106strains could grow on medium with non-solving potassium, and106strains could grow on nitrogen-free medium. At the same time,95strains could grow on all of the three media. Apart from that, IAA production of16strains were above20mg-L-1, and41strains showed high levels of siderophore production up to5+level, accounting for31%of all the strains.Out of135strains,20strains could inhibit the growth of Fusarium oxysporum f. sp. niveum, with14strains having significant inhibiting action against three physiological races (race No.0, No.1and No.2) of the pathogens. The inhibition rates of the strains YGRE3, YGRE24and YGRH7were above25%, having the strongest activities among all strains. Moreover, only the cell-free culture liquid of strain YGRE24showed the inhibiting activity on Fusarium oxysporum f. sp. niveum and the inbibition rate to race No.0, race No.1and race No.2were28.37%,22.86%and17.43%respectively. Mycelial growth of the pathogen was restrained by strain YGRE24. Furthermore, cellulase and protease activities were found in most of16strains with inhibition effects on the pathogen, while β-glucanase activity was just detected only in strain YGRH7. Conversely, chintinase activity was not found in thethese strains. However yellow-green pigments were producted in some strains cultivated on chitin medium.Most strains were identified according to morphology, biophysiological, biochemical characteristics and16S rDNA sequence. Phylogenetic analysis of strains producing concentration of siderophore showed that19strains are Pseudomonas spp., and8strains are Bacillus spp. In addition, other strains were assigned to different genera including Rhizobium radiobacte, Arthrobacter sp., Kocuria sp.. The genuses of Pseudomonas and Bacillus were the main group having high production of siderophore and being antagonistic to pathogenes.According to the plate experiments, the growth of stem and root of watermelon were boosted by growth-promoting bacteria in a certain degree. The pot experiments showed that the effect of disease resistence was obvious in these bacteria strains. Being inoculated with the pathogen, the biomass of the dry and fresh weight of shoot declined manifestly by28.6%,26.3%respectively, and the the dry and fresh weight of root fall by33.5%and27.0%respectively. Neverthless, the decrease of the biomass, e.g., fresh shoot weight, dry shoot weight, fresh root weirht and dry root weight, caused by Fusarium oxysporum f.sp.niveum race No.2, were dramatically eased by46.56%,36.36%,52.65and37.72%, respectively, after inoculated with the disease-resisitent strains FRBN3,. |
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