Genetic and Physiologic Characterization of aBradyrhizobium japonicumMutant Defective in Early Bacteroid Development

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Abstract
We report the identification and cloning of a new symbiotically essential gene from the soybean root nodule bacterium, Bradyhizobium japonicum. The main phenotypic characteristics of mutants with insertions in that gene (e.g., strain 3160) are as follows: 1) mutant 3160 forms numerous tiny, bacteroid-free pseudonodules on soybean and three other legume hosts; 2) the absence of endosymbiotic bacteria is strictly correlated with a selective decrease in the expression of the soybean early nodulin gene ENOD55, whereas ENOD2 and ENOD13 are expressed normally; 3) mutant 3160 does not succeed in bacteroid propagation even in the presence of other B. japonicum strains as coinoculants, whereas it is able to restore the full nodulation capability of coinoculant nodA and nodC mutants; 4) strain 3160 has a leaky folate auxotrophy when grown on minimal medium with nitrate as the sole nitrogen source. The affected gene appears to play an important role in a very early stage of bacteroid and root nodule development.

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