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David Demezas
Oklahoma State University
Visiting Assistant Professor, Dept. of Microbiology and Molecular Genetics
Address: 307 Life Sciences East, OSU, Stillwater, OK 74078
Phone: 405-744-6411 Fax: 405-744-6790
Email: david.demezas@okstate.edu
URL: http://microbiology.okstate.edu/faculty/demed
Ph.D., Microbiology, 1986, Oregon State University  
Postdoctoral: Molecular Microbial Ecology, 1986-1989, Division of Plant Industry, CSIRO, Australia
Molecular Microbial Ecology, 1989-1992, Center for Microbial Ecology, Michigan State University
Research Emphasis: Plant-Microbe Interactions, Molecular Microbial Ecology
Related Activities: Teaching Biology and Microbiology
 
Research Description:

Detailed studies of microbial communities associated with the rhizosphere/rhizoplane of plants are important because of the impacts that bacteria may have on plant development and vigor. For example some species of bacteria produce phytohormones that enhance root elongation, while other species produce toxins, such as HCN, which retard root development. Improving our understanding of the diversity and dynamics of these microbial communities during the life cycle of a plant will allow us to develop strategies to better manage them to promote plant development and growth.

Microbial populations, like communities, are diverse; there is considerable intraspecific variation. I have studied the genetic diversity of two specific populations of plant associated bacteria, namely Rhizobium and Pseudomonas fluorescens. These two species of bacteria have very different life histories. Rhizobium is a specialist, it makes its living by forming nodules on specific legumes. In contrast, Pseudomonas fluorescens is a generalist, it must compete with numerous other soil-borne bacteria in the rhizosphere for its living. I am interested in the phenotypic and genotypic diversity of these species of bacteria. How is it maintained, is genetic exchange and recombination common among soil bacteria? What is the role of phenotypic and genotypic diversity in making these species successful rhizosphere inhabitants? And what is the role of the environment in maintaining phenotypic and genotypic diversity?

Another aspect of microbial ecology that has received very little attention in the past is the ecology of plasmids harbored by soil bacteria We know very little about most plasmids; the notable exceptions are the symbiotic plasmid of Rhizobium spp. and the Ti plasmid of Agrobacterium. I am interested in three aspects of plasmid ecology. Firstly, what is the genetic diversity within a plasmid family? Secondly, what role do plasmids play in the evolution of new genotypes of bacteria? And lastly, do plasmids play a role in saprophytic or rhizosphere competence of soil bacteria.
 
Recent Publications:

Bidlack JE, Rao SC, Demezas DH (2001) Nodulation, nitrogenase activity, and dry weight of chickpea and pigeon pea cultivars using different Bradyhizobium strains. J Plant Nutr 24:549-560.

Zhao Y, Damicone JP, Demezas DH, Rangaswamy V, Bender CL (2000) Bacterial leaf spot on leafy crucifers in Oklahoma caused by Pseudomonas syringae pv. maculicola. Plant Dis 84:1015-1020.

Zhao Y, Damicone JP, Demezas DH, Bender CL (2000) Bacterial leaf spot diseases of leafy crucifers in Oklahoma caused by pathovars of Xanthomonas campestris. Plant Dis 84:1008-1014.

Rao SC, Bidlack JE, Demezas DH (1997) Chickpea and pigeonpea in wheat-legume cropping systems of the Southern Great Plains, USA. International Chickpea and Pigeonpea Newsletter 3:25-27.

Demezas DH, Bell J (1996) Evaluation of low molecular weight RNA profiles and ribotyping to differentiate some Bacillus species. System Appl Microbiol 18: 582-589.
 
Selected Additional Publications:

Demezas DH (1997) Fingerprinting bacterial genomes using restriction fragment length polymorphisms. In Bacterial Genomes: physical structure and analysis (de Bruijn FJ, Lupski JR, Weinstock G, eds) Chapman & Hall, New York.

Demezas DH, Reardon TB, Gibson AH, Watson JM (1995) Diversity and genetic relationships among a natural population of Rhizobium leguminosarum bv. trifolii isolated from Trifolium subterraneum L.. Mol Ecol 4: 209-220.

Sundin GW, Demezas DH, Bender CL (1994) The evolution of copper and streptomycin resistance in Pseudomonas syringae. Appl Environ Microbiol 60:4421-4431.

Demezas DH, Reardon TB, Watson JM, Gibson AH (1991) Genetic relationships among Rhizobium strains revealed by enzyme electrophoretic and restriction fragment length polymorphism analyses. Appl Environ Microbiol 57:3489-3495.

Gibson AH, Demezas DH, Gault RR, Bhuvaneswari TV, Brockwell J (1990). Genetic stability in rhizobia in the field. Plant Soil 129:37-44.