Sinorhizobium fredii HH103 flgJ is a flagellar gene induced by genistein in a NodD1- and TtsI- dependent manner

Sinorhizobium fredii HH103 is a broad host-range rhizobial strain able to induce the formation of nitrogen-fixing nodules in dozens of legumes, including soybean. S. fredii HH103 exhibits genistein-induced surface motility. The aim of this work has been to determine whether the flgJ gene, which is inducible by genistein and codes for a flagellar protein, is involved in this motility and is relevant for symbiosis with soybean.

We have generated two independent mutants in the flgJ gene of HH103 and analysed their phenotypes in motility, exopolysaccharide production, biofilm formation, soybean root colonization, symbiosis with soybean, and secretion of effector proteins. We have also further studied the regulation of the expression of flgJ.

We show that the expression of flgJ is driven by a tts box previously not detected, which accounts for its induction by flavonoids and the NodD1 and TtsI transcriptional activators. Inactivation of flgJ led to severe impairments in bacterial motility (swimming and genistein-induced surface motility) as well as to a significant reduction in symbiotic performance with soybean when bacteria are not directly inoculated onto the seedling roots. However, the absence of a functional FlgJ protein did not affect the bacterial ability to colonize soybean roots.

The flgJ gene of S. fredii HH103 connects the nod regulon with the genistein-induced surface motility exhibited by this rhizobial strain.