Microbial community ecology and bacterial quorum sensing as control points in rhizosphere nitrogen cycling

Group behavior is a common strategy found throughout nature, and many bacterial species are known to regulate population-level behaviors through extracellular signaling in a mechanism known as quorum sensing (QS). In vitro many bacterial species regulate extracellular enzyme production such as chitinase and protease using quorum sensing. Microbial enzyme activities that depolymerize macromolecular organic N are likely the rate-limiting steps in N mineralization in the rhizosphere. Despite the abundance of N in the soil, temperate terrestrial plants are generally N-limited; up to half of total soil N exists as plant-inaccessible macromolecular organic N, mainly as chitin, proteins, peptidoglycans and nucleic acids. Rhizosphere processes fuel heterotrophic cycling of organic N in soil and we know that rates of gross N-mineralization are substantially increased in rhizosphere soil. While we know that rhizosphere bacteria play a major role in N mineralization, the mechanisms through which these bacteria accelerate the conversion of soil organic N to plant-available mineral N are not understood. Microcosms planted with Avena fatua (wild oat) were employed to examine N cycling, microbial community dynamics and QS in rhizosphere root zones (root tip, root hairs and mature root) compared to bulk soil. This research explores microbial community ecology and bacterial QS as control points of N mineralization and plant-available N in the rhizosphere.; Rhizosphere soil was characterized by increased cell density that caused increased N-acyl homoserine lactone (AHL) abundance and QS-controlled behaviors. The biosensor Agrobacterium tumefaciens pAHL-Ice was constructed to respond to a wide range of AHL signals, and detected increased AHL in the rhizosphere compared to bulk soil. Chitinase and protease activity were also increased on a per-cell basis in rhizosphere soil; this increased enzyme activity accompanying increased cell density suggests a QS-mediated effect. Increased dissolved organic nitrogen (DON) and low-molecular weight DON (which was undetectable in the bulk soil) were consistent with the measured increases in specific activities of chitinase and protease. Since the rates of N-mineralization found in rhizosphere soil are predicated on chitinase and protease activity as well as DON pools, it is likely the N-mineralization is also affected by cell-density dependent QS control. In an examination of rhizosphere isolates, 23% of exoenzyme producing isolates activated the AHL biosensor, and seven proteobacterial rhizosphere isolates were found to have exoenzymes under control of QS. These results also support QS control of exoenzymes depolymerizing organic N in the rhizosphere. Additions of two exogenous AHL compounds to soil however either significantly depressed or did not effect rates of chitinase activity and gross N mineralization, confirming (1) that the effect of AHL compounds are highly species- and concentration-dependent and (2) that some of these compounds actively modulate chitinase and N-mineralization in soil. These experiments strongly support the hypothesis that N cycling in the rhizosphere is mediated by density-dependent, QS-mediated microbial activity.; Changes in a subset of 7% of the microbial community accompanied increases in cell density and activity in the rhizosphere, as determined by the 16S rDNA community profile analyses TRFLP and high density phylochip. The results from both methods rejected the null hypothesis that the whole community responded equally to the root moving through the soil. The phylochip data suggest that most taxa doubled once in relative abundance in rhizosphere compared to bulk soil, but a subset doubled as many as ten times. Increased activity of the Proteobacteria was evident by their strong representation in the dynamic subset, and their functional importance in rhizosphere was shown by increased AHL abundance in the rhizosphere. The Actinobacteria and Bacteroidetes are also likely...