Fig 6.

A proposed hypothetical symbiotic role of BjaR₁ in B. diazoefficiens at the free-living and symbiotic state. (A) In the wild-type strain, BjaR₁ positively influences nodD2 expression, and bacterial nitrogen metabolism likely via the NtrBC two-component regulatory system. (B) Inactivation of BjaR₁ notably suppresses these physiological processes. Bacteria may employ this QS regulator to coordinate their metabolic function, enhancing adaptability to the free-living microenvironment. (D) Loss of BjaR₁ function during symbiosis leads to excessive nodulation and nitrogen fixation, triggered by the alleviation of NodD2’s inhibitory influence on isoflavonoid (i.e., genistein)-mediated nodulation gene induction, and the suppression of nitrogen metabolism (or central intermediate metabolism) in bacteroids. However, the symbiotic outcomes in soybeans are compromised, as evidenced by the remarkably reduced plant dry weight. (C) The soybean-mediated suppression of BjaI/BjaR₁ activity in the WT strain is crucial for efficient symbiotic nitrogen fixation, providing tight control over these two fundamental processes. The double arrow symbolizes the extent of BjaR₁’s positive influence on NodD2 and NtrBC activities, both of which are known to potentially regulate genomic modules associated with bacterial nodulation and nitrogen metabolism. The diagram does not illustrate the regulatory positive feedback loop between BjaR₁ and BjaI. G, genistein.