| Summary: | Colonization of a biofertilizer <i>Bacillus</i> sp. OYK strain, which was isolated from a soil, was compared with three rhizospheric and endophytic <i>Bacillus</i> sp. strains to evaluate the colonization potential of the <i>Bacillus</i> sp. strains with a different origin. Surface-sterilized seeds of tomato (<i>Solanum lycopersicum</i> L. cv. Chika) were sown in the sterilized vermiculite, and four <i>Bacillus</i> sp. strains were each inoculated onto the seed zone. After cultivation in a phytotron, plant growth parameters and populations of the inoculants in the root, shoot, and rhizosphere were determined. In addition, effects of co-inoculation and time interval inoculation of <i>Bacillus</i> sp. F-33 with the other endophytes were examined. All <i>Bacillus</i> sp. strains promoted plant growth except for <i>Bacillus</i> sp. RF-37, and populations of the rhizospheric and endophytic <i>Bacillus</i> sp. strains were 1.4–2.8 orders higher in the tomato plant than that of <i>Bacillus</i> sp. OYK. The plant growth promotion by <i>Bacillus</i> sp. F-33 was reduced by co-inoculation with the other endophytic strains: <i>Klebsiella</i> sp. Sal 1, <i>Enterobacter</i> sp. Sal 3, and <i>Herbaspirillum</i> sp. Sal 6., though the population of <i>Bacillus</i> sp. F-33 maintained or slightly decreased. When <i>Klebsiella</i> sp. Sal 1 was inoculated after <i>Bacillus</i> sp. F-33, the plant growth-promoting effects by <i>Bacillus</i> sp. F-33 were reduced without a reduction of its population, while when <i>Bacillus</i> sp. F-33 was inoculated after <i>Klebsiella</i> sp. Sal 1, the effects were increased in spite of the reduction of its population. <i>Klebsiella</i> sp. Sal 1 colonized dominantly under both conditions. The higher population of rhizospheric and endophytic <i>Bacillus</i> sp. in the plant suggests the importance of the origin of the strains for their colonization. The plant growth promotion and colonization potentials were independently affected by the co-existing microorganisms.
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