Roles of flavonoids and the transcriptional regulator Ttsl in the activation of the type III secretion system of Bradyrhizobium elkanii SEMIA587

Bradyrhizobium elkanii SEMIA587 is a symbiotic nitrogen-fixing bacterium of the group commonly called rhizobia, which induce nodule formation in legumes, and is widely used in Brazilian commercial inoculants of soybean. In response to flavonoid compounds released by plant roots, besides Nod factors, other molecular signals are secreted by rhizobia, such as proteins secreted by type III secretion systems (T3SSs). Rhizobial T3SSs are activated by the transcription regulator Ttsl, which binds to sequences present in the promoter regions of T3SS genes via a conserved sequence called the tts box. To study the role of the T3SS of B. elkanii SEMIA587, tts/was mutated. Protein secretion and flavonoid induction analysis, as well as nodulation tests, were performed with the wild-type and mutant strains. The results obtained showed that B. elkanii SEMIA587 secretes at least two proteins (NopA and NopL, known rhizobial T3SS substrates) after genistein induction, whilst supernatants of the tts/mutant did not contain these Nops. Unusually for rhizobia, the promoter region of the B. elkanii SEMIA587 tts/gene contains a tts box, which is responsive to flavonoid induction and to which Ttsl can bind. Nodulation tests performed with three different leguminous plants showed that the B. elkanii SEMIA587 tts/mutant displays host-dependent characteristics; in particular, nodulation of two soybean cultivars, Peking and EMBRAPA 48, was more efficient when Ttsl of B. elkanii was functional.