Microbial Signaling Influences Soil Nitrogen Mineralization and Plant Nitrogen AvailabilityEPA Grant Number: U916157
Title: Microbial Signaling Influences Soil Nitrogen Mineralization and Plant Nitrogen Availability
Investigators: DeAngelis, Kristen M.
Institution: University of California - Berkeley
EPA Project Officer: Zambrana, Jose
Project Period: January 1, 2003 through January 1, 2006
Project Amount: $98,687
RFA: STAR Graduate Fellowships (2003) Recipients Lists
Research Category: Fellowship - Microbiology , Academic Fellowships , Biology/Life Sciences
The objectives of this research project are to: (1) understand the magnitude and importance of quorum sensing (QS) to plant health and nutrient cycling in the rhizosphere soil; and (2) demonstrate how relevant and important QS is to rhizosphere nitrogen (N) mineralization. N is generally most limiting to plant growth in terrestrial ecosystems; enters soil mainly as chitin, proteins, peptidoglycans, and DNA; and up to one-half of total soil N can exist in these forms. Because plants generally cannot access organic N, they rely on microbial decomposition to release it as inorganic, or mineralized, N. In Gram-negative bacteria, the first step towards N mineralization is the secretion of digestive extracellular enzymes, which is under the control of QS. Gram-negative QS signaling via secreted acylated homoserine lactones is prominent in the rhizosphere. Although there is some evidence of plant interference in prokaryotic signaling, the mechanisms and significance of interference are not well understood. Preliminary data indicate that although there are higher N mineralization and bacterial numbers in the rhizosphere, plants can compete effectively for inorganic nitrogen from bacteria.
I propose three testable hypotheses to determine the extent of the influence of QS on rhizosphere N mineralization and plant N availability: (1) QS has an effect on N mineralization in soil; (2) interspecific QS signaling in the rhizosphere affects N mineralization; and (3) QS-induced N mineralization is a major source of plant N. I recreated a natural grassland system with Avena barbata (wild oat) seedlings and its natural soil from Hopland, California. My methods will include nitrate and ammonium green fluorescent protein, ice biological sensors, and acyl-homoserine lactone sensors of varying sensitivities and specificities; allowing me to survey QS in the living rhizosphere and to link QS activity to mineralized N. I will assay soil enzyme activities in the rhizosphere and conduct genetic community analysis using terminal restriction fragment length polymorphism in response to added signal. The fate of 15N-labeled macromolecular organic compounds will be determined in pot experiments when QS is artificially induced or suppressed. I also will test for changes in rhizosphere microbial-community behavior and plant growth in signal-producing plants compared to wildtype plants. These studies should elucidate the mechanisms of plant N accumulation and the controls of microbial N mineralization in the rhizosphere. This research is particularly pertinent because QS disruption is a proposed method of biological control of plant bacterial infection; many genes associated with pathogenesis are induced in a quorum. In addition, QS may affect N mineralization and increase crop dependence on fertilizer N, augmenting nitrogen nonpoint pollution of surface waters.