Enhancing Nodulation Competitiveness Of An Inoculum Strain By Coinoculation With A Nonnodulating Antibioticproducing Strain In Fieldgrown Soybean

A.L. Iniguez1, E.A. Robleto2, M.K. Chelius1, E.W. Triplett1

'Dept of Agronomy, University of Wisconsin, Madison, WI, USA

2Tufts University, Boston, MA, USA

Commercially available Bradyrhizobium japonicum inoculants do not prevent nodulation by indigenous strains on field-grown soybeans. Thus, legume productivity is limited to the N2 fixation capability of indigenous strains. In previous work, we successfully employed the use of an antibiotic-producing Rhizobium etli strain to reduce nodulation of Phaseolus vulgaris by indigenous rhizobia in the field (Robleto et al. 1998). The reported antibiotic, Trifolitoxin (TFX) is a small peptide antibiotic that is post-translationally modified. However, TFX does not inhibit bradyrhizobia. Here we summarize our efforts to identify bacterial strains that inhibit most strains of Bradyrhizobium for use in new soybean inoculum systems. Bradyrhizobium sp. IRj2179a was found to have inhibitory properties to a large number of bradyrhizobia. Soybean inoculum strains were tested for resistance or sensitivity to IRj2179a through inhibition assays. Only 3 of the 290 soybean-nodulating strains (USDA strains 4, 54 and 61) in our collection showed zones of inhibition (sensitive to IRj2179a). Although IRj2179a does not nodulate soybean, in this study it was used to co-inoculate soybean seeds with a resistant strain USDA 61 and a sensitive strain USD A 26. This experiment was conducted in the field in West Madison research station of the University of Wisconsin-Madison. The results showed that adding IRj2179a to the USDA 61 inoculum enhanced the competitiveness of USDA 61 versus indigenous bradyrhizobia. In addition, IRj2179a improved the ability of USDA 61 to compete for nodulation versus an inoculum strain, USDA 26, which is sensitive to IRj2179a. This system may be of broad application due to the wide range of bradyrhizobia that are inhibited by IRj2179a. Nevertheless, we need to definitively determine whether antibiotic production is required for the effects on competition observed with IRj2179a. In order to determine the role of the antibiotic, we will need to develop and test antibiotic-minus mutants of IRj2179a. Finally, if the antibiotic is involved in competitiveness, the antibiotic needs to be isolated and characterized, and genes necessary for antibiotic production should be cloned and sequenced.

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