The beta-rhizobial strain Paraburkholderia phymatum STM815T is noteworthy for its wide host range in nodulating legumes, primarily mimosoids (over 50 different species) but also some papilionoids. It cannot, however, nodulate soybean (Glycine max [L.] Merr.), one of the most important crops. Here, we constructed a highly saturated genome-wide transposon library of a P. phymatum strain and employed a transposon sequencing (Tn-seq) approach to investigate the underlying genetic mechanisms of symbiotic incompatibility between P. phymatum and soybean. Soybean seedlings inoculated with the P. phymatum Tn-seq library displayed nodules on the roots that were mainly occupied by different mutants in a gene, nodS, coding for a methyltransferase involved in the biosynthesis of nodulation factors. The construction of a nodS deletion strain and a complemented mutant confirmed that nodS is responsible for the nodulation-incompatibility of P. phymatum with soybean. Moreover, infection tests with different host plants revealed that NodS is necessary for optimal nodulation of common bean (Phaseolus vulgaris), but it is not required for nodulation of its natural host Mimosa pudica. In conclusion, our results suggest that NodS is involved in determining nodulation specificity of P. phymatum.
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