When bacteria invade host cells, they may use the host's own cell machinery to infect and persist. Bacteria also use host molecules as signals. These signal exchanges are critical to pathogenesis and to symbiosis. We study Sinorhizobium meliloti, an alpha- proteobacterium, which establishes a beneficial nitrogen fixing symbiosis with host plants including the model legume Medicago truncatula. Molecular study of this bacteria's invasion into its host has yielded basic information and technical approaches applicable to series mammalian diseases such as Brucella. This biological process is also important because it allows certain plant crops such as soybean and pea to grow without exogenous N-fertilizer: the process is thus critical to human and animal nutrition. The interaction of bacterium and host plant is characterized by species specificity and by the complexity of a developmental process in which (1) the host build a root nodule, which is a multi-tissue organ, and (2) the bacteria invade through layers of nodule and occupy target host cells as a long-term intracellular symbiont. We propose to carry out a comprehensive project aimed to identify genes, regulators and signals involved in intermediate stages of symbiosis (infection, invasion, and release into target cells). A particular advantage of this system is that the bacterium and the host can each be grown independently, and can be used for forward genetic screens. This has yielded a bounty of mutants in both the bacterium and in the host, and our project has the special prospect of using plant developmental mutants as an explicit tool for analyzing the bacteria's mechanisms of infection and differentiation within the host tissues.
We aim to analyze how the bacteria differentiate during several sequential steps of symbiosis. We will build on some unexpected recent results, including (1) the finding that specific RNA polymerase sigma factors are required for successful symbiosis (2) the observation that some previously known regulators have unexpected functions in multiple bacterial behaviors, and (3) the discovery that one of our plant mutants is defective in producing host peptides that are required for expression of bacterial symbiosis genes. This work will yield insights into how bacteria infect host cells.

Public Health Relevance

When bacteria interact with humans or other higher organisms, some cause disease but others establish a symbiosis that benefits the host. Increasingly, evidence is showing that symbioses are crucial for human health. Our work will advance the understanding of symbiosis by studying the interaction of rhizobial bacteria with a legume plant host;this is a sophisticated, practical system for discovering the intricate communication and coordination mechanisms used by bacteria and their hosts.

National Institute of Health (NIH)
National Institute of General Medical Sciences (NIGMS)
Research Project (R01)
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Special Emphasis Panel (ZRG1-GGG-B (02))
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Singh, Shiva P
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Stanford University
Schools of Arts and Sciences
United States
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Lang, Claus; Smith, Lucinda S; Long, Sharon R (2018) Characterization of Novel Plant Symbiosis Mutants Using a New Multiple Gene-Expression Reporter Sinorhizobium meliloti Strain. Front Plant Sci 9:76
Lehman, Alisa P; Long, Sharon R (2018) OxyR-Dependent Transcription Response of Sinorhizobium meliloti to Oxidative Stress. J Bacteriol 200:
Barnett, Melanie J; Long, Sharon R (2018) Novel Genes and Regulators That Influence Production of Cell Surface Exopolysaccharides in Sinorhizobium meliloti. J Bacteriol 200:
Lang, Claus; Barnett, Melanie J; Fisher, Robert F et al. (2018) Most Sinorhizobium meliloti Extracytoplasmic Function Sigma Factors Control Accessory Functions. mSphere 3:
Long, Sharon R (2016) SnapShot: Signaling in Symbiosis. Cell 167:582-582.e1
Ichida, Hiroyuki; Long, Sharon R (2016) LDSS-P: an advanced algorithm to extract functional short motifs associated with coordinated gene expression. Nucleic Acids Res 44:5045-53
Wippel, Kathrin; Long, Sharon R (2016) Contributions of Sinorhizobium meliloti Transcriptional Regulator DksA to Bacterial Growth and Efficient Symbiosis with Medicago sativa. J Bacteriol 198:1374-83
Barnett, Melanie J; Long, Sharon R (2015) The Sinorhizobium meliloti SyrM regulon: effects on global gene expression are mediated by syrA and nodD3. J Bacteriol 197:1792-806
Lang, Claus; Long, Sharon R (2015) Transcriptomic Analysis of Sinorhizobium meliloti and Medicago truncatula Symbiosis Using Nitrogen Fixation-Deficient Nodules. Mol Plant Microbe Interact 28:856-68
Peck, Melicent C; Fisher, Robert F; Bliss, Robert et al. (2013) Isolation and characterization of mutant Sinorhizobium meliloti NodD1 proteins with altered responses to luteolin. J Bacteriol 195:3714-23

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