The PTS(Ntr)-KdpDE-KdpFABC Pathway Contributes to Low Potassium Stress Adaptation and Competitive Nodulation of Sinorhizobium fredii

The rhizobium-legume symbiosis is essential for sustainable agriculture by reducing nitrogen fertilizer input, but its efficiency varies under fluctuating soil conditions and resources. The nitrogen-related phosphotransferase system (PTS(Ntr)) consisting of PtsP, PtsO, and PtsN is required for optimal nodulation and nitrogen fixation efficiency of the broad-host-range Sinorhizobium fredii CCBAU45436 associated with diverse legumes, though the underlying mechanisms remain elusive. This work characterizes the PtsN-KdpDE-KdpFABC pathway that contributes to low potassium adaptation and competitive nodulation of CCBAU45436. Among three PtsN, PtsN(1) is the major functional homolog. The unphosphorylated PtsN(1) binds the sensory kinase KdpD through a non-canonical interaction with the GAF domain of KdpD, while the region covering HisKA-HATPase domains mediates the interaction of KdpD with the response regulator KdpE. KdpE directly activates the kdpFABC operon encoding the conserved high-affinity potassium uptake system. Disruption of this signaling pathway leads to reduced nodule number, nodule occupancy, and low potassium adaptation ability, but without notable effects on rhizoplane colonization. The induction of key nodulation genes NIN and ENOD40 in host roots during early symbiotic interactions is impaired when inoculating the kdpBC mutant that shows delayed nodulation. The nodulation defect of the kdpBC mutant can be rescued by supplying replete potassium. Potassium is actively consumed by both prokaryotes and eukaryotes, and components of the PTS(Ntr)-KdpDE-KdpFABC pathway are widely conserved in bacteria, highlighting the global importance of this pathway in bacteria-host interactions.