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Dr. Leland S. Pierson III
Professor & Department Head
120A LF Peterson
979-845-8288
plpm-head@ag.tamu.edu
Education
Ph.D. Microbiology, Washington State University
B.A. Microbiology, University of California, Davis
Research Emphasis:
1. Secondary metabolite production in beneficial bacteria.
A. Molecular regulation of phenazine (PZ) production. My laboratory studies the regulation and function of PZs in the root-associated bacterium Pseudomonas chlororaphis strain 30-84 and the opportunistic pathogen P. aeruginosa. PZs are nitrogen containing heterocyclic compounds originally classified in soil borne bacteria as antibiotics due to the fact that they are required for persistence of the bacterium in the rhizosphere and can inhibit the growth of pathogenic fungi. We showed that PZ production is regulated by a complex sensory network that includes quorum sensing, positive and negative two component regulation, and post-transcriptional regulation. Recent work has implicated additional regulatory systems in the fine-tuning of PZ production.
B. Multiples roles of secondary metabolites. The complex regulatory networks that control PZ production are not consistent with a role solely as antibiotics. We hypothesized that this complex regulation reflects the complex roles that PZs play in the lifestyle of the producing bacterium. We are focused on identifying and understanding additional roles of PZs for strain 30-84 in microbial physiology, microbe-microbe and plant-microbe interactions. Recently, we demonstrated that PZs are essential for biofilm development by strain 30-84 and that mutants unable to produce PZs failed to establish biofilms. We also showed that altering the ratio of the endogenous PZs produced by strain 30-84 has profound effects on cell adhesion, biofilm architecture, and bacterial release. We are studying the roles of phenazine structural derivatives on cell adhesion, biofilm architecture, root colonization and plant nutrient uptake. This work will include microarray analyses of the roles of PZs as signals controlling the expression of additional genes involved in cell adhesion and biofilm formation. The upcoming availability of the genomic sequence of P. chlororaphis strain 30-84 will greatly assist us in our studies.
2. Signaling among microbial populations in vitro and in situ.
Phenazine (PZ) production is regulated via quorum sensing which is dependent on the accumulation of diffusible microbial pheromones. Mutants defective in the production of these signals were rescued for PZ production in vitro and in situ by signals produced by another subpopulation of the wheat rhizosphere community. Additionally, we identified a second subpopulation from the wheat rhizosphere that inhibited PZ production via the production of non-AHL signals that interfered with normal quorum sensing activation of the PZ biosynthetic operon. Thus, PZ production is influenced directly by other members of the rhizosphere community. We are studying some of the negative signals produced by select rhizosphere strains both at the genetic level and are examining their effects on PZ expression by strain 30-84 on plant roots.
3. Microbial communities: involvement in speleothem formation.
My laboratory is part of a joint Microbial Observatories project to analyze microbial communities and their contributions to speleothem formation in Kartchner Caverns, an oligotrophic carbonaceous cave in Arizona. We are characterizing both bacterial and fungal communities via Differential Gradient Gel Electrophoresis analysis and analyzing intra- versus inter-speleothem population variations. We are a metagenomics approach to enable us to link metabolic capabilities and redox mechanisms to mineralization patterns. Our long term goals are to determine community stability, the influence of perturbations such as tourism on cave microbial communities, and the identification of novel redox systems in speleothem-associated bacteria.
Taught previously:
PLP428R/528R. Microbial Genetics (3 units). Comprehensive course covering all aspects of prokaryotic genetics, including DNA structure and function, RNA functions, transcription, translation, phage and transposon biology, gene regulation, including post-transcriptional regulation, genome sequence analysis, cloning, PCR primer design.
PLP428L/528L. Microbial Genetics Laboratory (2 units). Laboratory course incorporating actual research projects. Includes genomic and plasmid isolation, transposon mutagenesis, restriction digestion, cloning, and DNA sequence analysis.
Recent Publications
Maddula VSK, Pierson EA, Pierson, L.S. III. 2008. Altering the ratio of phenazines in Pseudomonas chlororaphis (aureofaciens) strain 30-84: effects on biofilm formation and pathogen inhibition. J. Bacteriol. 190:2759-66.
Pierson, L.S. III. 2008. Microbial Communication: Bacteria/Bacteria and Bacteria/Host. In: Environmental Microbiology, 2nd Ed. Maier, RM, Pepper, IL, and Gerba, CP, eds. Academic Press, San Diego.
Pierson, L.S. III, and EA Pierson. 2007. Roles of Diffusible Signals in Communication among Plant-Associated Bacteria. Phytopathology 97:227-232.
Maddula VSK, Zhang Z, Pierson EA, and Pierson, L.S. III. 2006. Quorum Sensing and Phenazines are Required for Biofilm Formation by Pseudomonas aureofaciens 30-84. Microbial Ecol. 52:289-301.
Paulsen, IT, Press, C., Ravel, J, Kobayashi, DY, Myers, GSA, Mavrodi, DV, DeBoy, RT, Seshadri, R, Ren, Q, Madupu, R, Dodson, RJ, Durkin, AS, Brinkac, LM, Daugherty, SC, Sullivan, SA, Rosovitz, MJ, Gwinn, ML, Zhou, L, Nelson, WC, Weidman, J, Watkins, K, Tran, K, Khouri, H, Pierson, EA, Pierson, L.S. III, Thomashow, LS, and Loper, JE. 2005. Complete genome sequence of the plant commensal Pseudomonas fluorescens Pf-5: insights into the biological control of plant disease. Nature Biotechnology 23:873-878.
Morello, JE, Pierson, EA, and Pierson, L.S. III. 2004. Negative cross-communication among wheat rhizosphere bacteria: Effect on antibiotic production by the biological control bacterium Pseudomonas aureofaciens 30-84. Appl. Environ. Microbiol. 70:3103-3109.
