title: A Chemotaxis Receptor Modulates Nodulation during the Azorhizobium caulinodans-Sesbania rostrata Symbiosis.
abstract: Azorhizobium caulinodans ORS571 is a free-living nitrogen-fixing bacterium which can induce nitrogen-fixing nodules both on the root and the stem of its legume host Sesbania rostrata This bacterium, which is an obligate aerobe that moves by means of a polar flagellum, possesses a single chemotaxis signal transduction pathway. The objective of this work was to examine the role that chemotaxis and aerotaxis play in the lifestyle of the bacterium in free-living and symbiotic conditions. In bacterial chemotaxis, chemoreceptors sense environmental changes and transmit this information to the chemotactic machinery to guide motile bacteria to preferred niches. Here, we characterized a chemoreceptor of A. caulinodans containing an N-terminal PAS domain, named IcpB. IcpB is a soluble heme-binding protein that localized at the cell poles. An icpB mutant strain was impaired in sensing oxygen gradients and in chemotaxis response to organic acids. Compared to the wild-type strain, the icpB mutant strain was also affected in the production of extracellular polysaccharides and impaired in flocculation. When inoculated alone, the icpB mutant induced nodules on S. rostrata, but the nodules formed were smaller and had reduced N2-fixing activity. The icpB mutant failed to nodulate its host when inoculated competitively with the wild-type strain. Together, the results identify chemotaxis and sensing of oxygen by IcpB as key regulators of the A. caulinodans-S. rostrata symbiosis.
  
  
  
  Bacterial chemotaxis has been implicated in the establishment of various plant-microbe associations, including that of rhizobial symbionts with their legume host. The exact signal(s) detected by the motile bacteria that guide them to their plant hosts remain poorly characterized. Azorhizobium caulinodans ORS571 is a diazotroph that is a motile and chemotactic rhizobial symbiont of Sesbania rostrata, where it forms nitrogen-fixing nodules on both the roots and the stems of the legume host. We identify here a chemotaxis receptor sensing oxygen in A. caulinodans that is critical for nodulation and nitrogen fixation on the stems and roots of S. rostrata These results identify oxygen sensing and chemotaxis as key regulators of the A. caulinodans-S. rostrata symbiosis.
authors.affiliation: Key Laboratory of Coastal Biology and Bioresource Utilization, Yantai Institute of Coastal Zone Research, Chinese Academy of Sciences, Yantai, China School of Resource and Environment, University of Chinese Academy of Sciences, Beijing, China.
authors.forename: Nan
authors.initials: N
authors.lastname: Jiang
authors.affiliation: Key Laboratory of Coastal Biology and Bioresource Utilization, Yantai Institute of Coastal Zone Research, Chinese Academy of Sciences, Yantai, China.
authors.forename: Wei
authors.initials: W
authors.lastname: Liu
authors.affiliation: Key Laboratory of Coastal Biology and Bioresource Utilization, Yantai Institute of Coastal Zone Research, Chinese Academy of Sciences, Yantai, China.
authors.forename: Yan
authors.initials: Y
authors.lastname: Li
authors.affiliation: Key Laboratory of Coastal Biology and Bioresource Utilization, Yantai Institute of Coastal Zone Research, Chinese Academy of Sciences, Yantai, China School of Resource and Environment, University of Chinese Academy of Sciences, Beijing, China.
authors.forename: Hailong
authors.initials: H
authors.lastname: Wu
authors.affiliation: School of Mechatronics Engineering, Beijing Institute of Technology, Beijing, China.
authors.forename: Zhenhai
authors.initials: Z
authors.lastname: Zhang
authors.affiliation: Biochemistry, Cellular and Molecular Biology Department, University of Tennessee, Knoxville, Tennessee, USA.
authors.forename: Gladys
authors.identifier: 0000-0002-9238-4640
authors.initials: G
authors.lastname: Alexandre
authors.affiliation: Institut Pasteur, Paris, France.
authors.forename: Claudine
authors.initials: C
authors.lastname: Elmerich
authors.affiliation: Key Laboratory of Coastal Biology and Bioresource Utilization, Yantai Institute of Coastal Zone Research, Chinese Academy of Sciences, Yantai, China zhxie@yic.ac.cn.
authors.forename: Zhihong
authors.initials: Z
authors.lastname: Xie
mesh_terms: D020599:Azorhizobium caulinodans / Q000502:physiology*; D002264:Carboxylic Acids / Q000378:metabolism; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism; D002633:Chemotaxis; D055786:Gene Knockout Techniques; D000081062:Heme-Binding Proteins; D006420:Hemeproteins / Q000235:genetics / Q000378:metabolism; D008124:Locomotion; D010100:Oxygen / Q000378:metabolism; D055170:Plant Root Nodulation; D011956:Receptors, Cell Surface / Q000235:genetics / Q000378:metabolism*; D049931:Sesbania / Q000382:microbiology* / Q000502:physiology*; D013559:Symbiosis
chemical_list: D002264:Carboxylic Acids; D002352:Carrier Proteins; D000081062:Heme-Binding Proteins; D006420:Hemeproteins; D011956:Receptors, Cell Surface; D010100:Oxygen