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Motility and chemotaxis are crucial processes for soil bacteria and plant–microbe interactions. This applies to the symbiotic bacterium Rhizobium leguminosarum, where motility is driven by flagella rotation controlled by two chemotaxis systems, Che1 and Che2. The Che1 cluster is particularly important in free‐living motility prior to the establishment of the symbiosis, with a che1 mutant delayed in nodulation and reduced in nodulation competitiveness. The Che2 system alters bacteroid development and nodule maturation. In this work, we also identified 27 putative chemoreceptors encoded in the R. leguminosarum bv. viciae 3841 genome and characterized its motility in different growth conditions. We describe a metabolism‐based taxis system in rhizobia that acts at high concentrations of dicarboxylates to halt motility independent of chemotaxis. Finally, we show how PTSNtr influences cell motility, with PTSNtr mutants exhibiting reduced swimming in different media. Motility is restored by the active forms of the PTSNtr output regulatory proteins, unphosphorylated ManX and phosphorylated PtsN. Overall, this work shows how rhizobia typify soil bacteria by having a high number of chemoreceptors and highlights the importance of the motility and chemotaxis mechanisms in a free‐living cell in the rhizosphere, and at different stages of the symbiosis. [ABSTRACT FROM AUTHOR]

Engineering signalling between plants and microbes could be exploited to establish host‐specificity between plant‐growth‐promoting bacteria and target crops in the environment. We previously engineered rhizopine‐signalling circuitry facilitating exclusive signalling between rhizopine‐producing (RhiP) plants and model bacterial strains. Here, we conduct an in‐depth analysis of rhizopine‐inducible expression in bacteria. We characterize two rhizopine‐inducible promoters and explore the bacterial host‐range of rhizopine biosensor plasmids. By tuning the expression of rhizopine uptake genes, we also construct a new biosensor plasmid pSIR05 that has minimal impact on host cell growth in vitro and exhibits markedly improved stability of expression in situ on RhiP barley roots compared to the previously described biosensor plasmid pSIR02. We demonstrate that a sub‐population of Azorhizobium caulinodans cells carrying pSIR05 can sense rhizopine and activate gene expression when colonizing RhiP barley roots. However, these bacteria were mildly defective for colonization of RhiP barley roots compared to the wild‐type parent strain. This work provides advancement towards establishing more robust plant‐dependent control of bacterial gene expression and highlights the key challenges remaining to achieve this goal. [ABSTRACT FROM AUTHOR]

Rhizobium leguminosarum has two high-affinity Mn2+ transport systems encoded by sitABCD and mntH. In symbiosis, sitABCD and mntH were expressed throughout nodules and also strongly induced in Mn2+-limited cultures of free-living cells. Growth of a sitA mntH double mutant was severely reduced under Mn2+ limitation and sitA and mntH single mutants were more sensitive to oxidative stress. The double sitA mntH mutant of R. leguminosarum was unable to fix nitrogen (Fix-) with legumes belonging to the galegoid clade ( Pisum sativum, Vicia faba and Vicia hirsuta). The presence of infection thread-like structures and sparsely-packed plant cells in nodules suggest that bacteroid development was blocked, either at a late stage of infection thread progression or during bacteroid-release. In contrast, a double sitA mntH mutant was Fix+ on common bean ( Phaseoli vulgaris), a member of the phaseoloid clade of legumes, indicating a host-specific symbiotic requirement for Mn2+ transport. [ABSTRACT FROM AUTHOR]

Motility and chemotaxis are crucial processes for soil bacteria and plant-microbe interactions. This applies to the symbiotic bacterium Rhizobium leguminosarum, where motility is driven by flagella rotation controlled by two chemotaxis systems, Che1 and Che2. The Che1 cluster is particularly important in free-living motility prior to the establishment of the symbiosis, with a che1 mutant delayed in nodulation and reduced in nodulation competitiveness. The Che2 system alters bacteroid development and nodule maturation. In this work, we also identified 27 putative chemoreceptors encoded in the R. leguminosarum bv. viciae 3841 genome and characterized its motility in different growth conditions. We describe a metabolism-based taxis system in rhizobia that acts at high concentrations of dicarboxylates to halt motility independent of chemotaxis. Finally, we show how PTS Ntr influences cell motility, with PTS Ntr mutants exhibiting reduced swimming in different media. Motility is restored by the active forms of the PTS Ntr output regulatory proteins, unphosphorylated ManX and phosphorylated PtsN. Overall, this work shows how rhizobia typify soil bacteria by having a high number of chemoreceptors and highlights the importance of the motility and chemotaxis mechanisms in a free-living cell in the rhizosphere, and at different stages of the symbiosis., (© 2024 The Authors. Environmental Microbiology published by Applied Microbiology International and John Wiley & Sons Ltd.)

Rhizobium leguminosarum has two high-affinity Mn 2+ transport systems encoded by sitABCD and mntH. In symbiosis, sitABCD and mntH were expressed throughout nodules and also strongly induced in Mn 2+ -limited cultures of free-living cells. Growth of a sitA mntH double mutant was severely reduced under Mn 2+ limitation and sitA and mntH single mutants were more sensitive to oxidative stress. The double sitA mntH mutant of R. leguminosarum was unable to fix nitrogen (Fix - ) with legumes belonging to the galegoid clade (Pisum sativum, Vicia faba and Vicia hirsuta). The presence of infection thread-like structures and sparsely-packed plant cells in nodules suggest that bacteroid development was blocked, either at a late stage of infection thread progression or during bacteroid-release. In contrast, a double sitA mntH mutant was Fix + on common bean (Phaseoli vulgaris), a member of the phaseoloid clade of legumes, indicating a host-specific symbiotic requirement for Mn 2+ transport., (© 2017 The Authors. Environmental Microbiology published by Society for Applied Microbiology and John Wiley & Sons Ltd.)