Genetically marked Rhizobium identifiable as inoculum strain in nodules of soybean plants grown in fields populated with Rhizobium japonicum

Abstract

The fate of an inoculum strain of R. japonicum was studied using a genetically marked strain I-110 subline carrying resistance markers for azide, rifampin and streptomycin (I-110 ARS). At the time of planting into a field populated with R. japonicum, seeds of soybean cultivars Kent and Peking were inoculated with varying cell densities of strain I-110 ARS. At various times during the growing season, surface-sterilized root nodules were examined for the presence of the inoculum strain by plating onto selective media. The recovery of the inoculum strain was unambiguous, varying, in the case of ''Kent'', from about 5% with plants (sampled at 51 days) that were inoculated with 3 .times. 108 cells/cm of row to about 20% with plants (sampled at 90 days) inoculated with 3 .times. 109 cells/cm. The symbiotically incompatible interaction of ''Peking'' and strain 110 in Rhizobium-populated field soil was confirmed by the finding that, at 60 days after planting, only 1 nodule in 360 sampled contained strain I-110 ARS. The use of genetically marked Rhizobium bacteria provided for precise identification of the inoculum strain in nodules of field-grown soybeans.