Mikrobiol. Z. 2018; 80(6):54-65. Ukrainian.
Survival of Bradyrhizobium japonicum Strains in Soil at their Introduction into Soybean Agrocenosis
Institute of Agricultural Microbiology and Agro-Industrial Manufacture, NAAS of Ukraine
97 Shevchenko Str., Chernihіv, 14027, Ukraine
Objective. The objective of our work was to assess the ability of Bradyrhizobium japonicum strains with slow and intensive growth rate to survive in the soil and maintain their symbiotic activity for a long time. Methods. The survival of inoculum strains in the soil has been studied in the field experiments with soybean (Glycine max (L.) Merr.). The cowpea (Vigna unguiculata (L.) Walp.), mung bean (Vigna radiata (L.) Wilczek) and adzuki beans (Vigna radiata (L.) Wilczek) were used as trap plants for soybean rhizobia. The presence of studied strains in the nodules was determined in reaction of agglutination with specific antisera. Morphological and cultural properties of isolated rhizobia were studied by generally accepted methods. Sequencing of the 16S - 23S rDNA intergenic spacer of rhizobia was performed on ABI 3130 Genetic Analyzer. Results. The B. japonicum strains with different growth rates were introduced into soybean agrocenosis in the first year of experiment. When soybean was grown in monoculture, gradual replacement of slow-growing rhizobia (B. japonicum 46, B. japonicum М8 and B. japonicum 634b) from nodule populations by strain with intensive growth rate B. japonicum KB11 was observed. At the 4th year of the studies, these strains were absent in the soybean nodule, however they remained in the soil as saprophytic microorganisms. Their presence was detected using trap plants, which had symbiotic relationships with the studied microorganisms. The proportion of strains with slow growth rate in the nodules of different cowpea species ranged from 2.1 to 58.3 %. The presence of slow-growing strains in nodule populations of cowpea, mung bean and adzuki bean is proved by microbiological, serological (belonging to serogroups 46, M8, 634b) and molecular genetic methods. The rhizobia from mung bean nodules showed 99.4 - 99.8 % identity to B. japonicum strains (from GenBank) based on nucleotide sequences of ITS region. The highest level of homology (99.9 - 100.0 %) was noted with B. japonicum KC23 strain, which is a typical representative of the serogroup M8. Conclusion. The strains of B. japonicum with slow and intensive growth rates introduced in soybean agrocenosis persist in the soil for a long time. They occupy their ecological niche in the local rhizobial population, however they have different ex planta and in planta strategies. Within five years, the ratio of studied strains in the nodule populations has been significantly changed: from the presence of all strains in the nodules to complete dominance of B. japonicum KB11 strain with intensive growth rates and the transition of slow-growing rhizobia to saprophytic existence.
Keywords: Bradyrhizobium japonicum, isolates, trap-host, 16S-23S rDNA, soybean, cowpea, mung bean, adzuki bean.
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