title: Bacterial quorum sensing and nitrogen cycling in rhizosphere soil.
abstract: Plant photosynthate fuels carbon-limited microbial growth and activity, resulting in increased rhizosphere nitrogen (N) mineralization. Most soil organic nitrogen is macromolecular (chitin, protein, nucleotides); enzymatic depolymerization is likely rate limiting for plant nitrogen accumulation. Analyzing Avena (wild oat) planted in microcosms containing sieved field soil, we observed increased rhizosphere chitinase and protease-specific activities, bacterial cell densities, and dissolved organic nitrogen (DON) compared with bulk soil. Low-molecular-weight (MW) DON (<3000 Da) was undetectable in bulk soil but comprised 15% of rhizosphere DON. Extracellular enzyme production in many bacteria requires quorum sensing (QS), cell-density-dependent group behavior. Because proteobacteria are considered major rhizosphere colonizers, we assayed the proteobacterial QS signals N-acyl-homoserine lactones (AHLs), which were significantly increased in the rhizosphere. To investigate the linkage between soil signaling and nitrogen cycling, we characterized 533 bacterial isolates from Avena rhizosphere: 24% had chitinase or protease activity and AHL production; disruption of QS in seven of eight isolates disrupted enzyme activity. Many Alphaproteobacteria were newly found with QS-controlled extracellular enzyme activity. Enhanced specific activities of nitrogen-cycling enzymes accompanied by bacterial density-dependent behaviors in rhizosphere soil gives rise to the hypothesis that QS could be a control point in the complex process of rhizosphere nitrogen mineralization.
authors: Department of Plant and Microbial Biology, University of California, Berkeley, CA, USA.
authors: Kristen M
authors: KM
authors: DeAngelis
authors: Steven E
authors: SE
authors: Lindow
authors: Mary K
authors: MK
authors: Firestone
mesh_terms: D054742:Acyl-Butyrolactones / Q000378:metabolism; D020561:Alphaproteobacteria / Q000145:classification / Q000235:genetics / Q000302:isolation & purification; D018554:Avena / Q000254:growth & development / Q000382:microbiology*; D001419:Bacteria / Q000145:classification / Q000201:enzymology* / Q000235:genetics / Q000302:isolation & purification; D002688:Chitinases / Q000235:genetics / Q000378:metabolism; D015964:Gene Expression Regulation, Bacterial; D009584:Nitrogen / Q000378:metabolism*; D010447:Peptide Hydrolases / Q000235:genetics / Q000378:metabolism; D018517:Plant Roots / Q000382:microbiology*; D053038:Quorum Sensing; D012987:Soil / Q000032:analysis; D012988:Soil Microbiology
chemical_list: D054742:Acyl-Butyrolactones; D012987:Soil; D002688:Chitinases; D010447:Peptide Hydrolases; D009584:Nitrogen