Tn-seq profiling reveals that NodS of the beta-rhizobium Paraburkholderia phymatum is detrimental for nodulating soybean

The beta-rhizobial strain Paraburkholderia phymatum STM815 ^T 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 world’s 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 .