Christina Kennedy
Division of Plant Pathology and Microbiology
Professor

Marley Building, Room 341F
Phone: (520) 621-9835
Email: ckennedy@u.arizona.edu
 

Visit these web-sites to learn more about Dr. Kennedy and her research

 
   
Background and Interests
 
Research:
Biological nitrogen fixation is important in agriculture because it provides a source of fixed nitrogen for growth of plants that is non-polluting and not dependent on fossil fuels for production. The enzyme that converts dinitrogen gas to ammonium, nitrogenase, requires the products of approximately 20 nif genes for its synthesis and activity. The free-living soil bacterium Azotobacter vinelandii is a model organism for the study of molecular mechanisms that regulate the expression of nif genes because it is amenable to genetic manipulation. In addition, a wealth of knowledge is available concerning nif genes and nitrogenase biochemistry and physiology in this organism.
The nif genes are not expressed if sufficient fixed N is available in the environment. In A. vinelandii this regulation involves an activator of nif gene transcription, NifA, and another protein, NifL, that inhibits NifA activity if plentiful fixed N (or oxygen) is available. NifL a sensor for fixed N status and our work has demonstrated the involvement of the glnD and glnK gene products which form a cascade of interactions leading to NifL function. Understanding how fixed N regulates nitrogenase availability is necessary for devising strategies to increase the amount of ammonium sythesized by nitrogen fixing bacteria with the potential to be used in agriculture.

We are excited this year to be involved in a project to sequence the Azotobacter vinelandii genome. The Department of Energy has agreed to include A. vinelandii in its second microbial genome sequencing month in February 2002. There will be several laboratories from around the world involved in the final annotation and mining of this sequence. The importance of A. vinelandii to issues of energy transductions and C metabolism is explained in this link.

Acetobacter diazotrophicus
Acetobacter diazotrophicus, an endophyte of sugarcane: analysis of nif genes and plant growth promotion properties

Acetobacter diazotrophicus, also known as Gluconacetobacter diazotrophicus, is a nitrogen-fixing bacterial species associated with sugarcane, Saccharum officinarum, and has been isolated from plants grown in both American continents, including the Caribbean, and in Australia. The endophytic colonization of sugarcane by A. diazotrophicus represents the first established case in which a monocot plant hosts a diazotrophic bacterial species which in turn supplies nutritionally significant amounts of fixed N for plant growth.

The association between A. diazotrophicus and sugarcane provides a model system for monocot/diazotroph symbiosis that is now well-developed in the PI's laboratory. This system will allow experimentation to answer fundamental questions concerning the colonization process and the factors involved in the endophytic establishment, effective biological nitrogen fixation, and growth stimulation of sugarcane by A. diazotrophicus. The current objectives are 1) to determine the contribution of BNF by A. diazotrophicus colonizing sugarcane by growth and 15N isotope incorporation studies conducted from young to mature plant growth stages; 2) to learn whether production of IAA or other plant growth-promoting factors is also significant in enhancement of sugarcane growth by A. diazotrophicus; 3) to characterize the process and mechanism by which A. diazotrophicus colonizes sugarcane after inoculation of sterile young plants by bacteria tagged with fluorescent markers; and 4) to characterize genes in A. diazotrophicus that are important for its ability to colonize the intercellular spaces and xylem vessels of sugarcane. The results will provide insight into how other grass and cereal plants might benefit from endophytic colonization by nitrogen-fixing bacteria.

 
Selected Publications
 
Kennedy C (2002) Genus Beijerinckia, Genus Derxia, Genus Agromonas in Garrity et al (eds) Bergey's Manual of Systematic Bacteriology (in press)

Kennedy C, Rudnick P (2002) Genus Azomonas in Garrity et al (eds) Bergey's Manual of Systematic Bacteriology (in press)

Kennedy C, Rudnick P, MacDonald M, Melton T (2002) Genus Azotobacter in Garrity et al (eds) Bergey's Manual of Systematic Bacteriology (in press)

Rudnick P, Kunz C, Gunatilaka MK, Hines ER, Kennedy C (2002) The role of GlnK in the NifL-mediated regulation of NifA activity in Azotobacter vinelandii. J Bacteriol (in press)
Colnaghi R, He L, Green A, Yan D, Rudnick P, Larson E, Kennedy C (2001) The lethality of glnD null mutations in Azotobacter vinelandii is suppressible by prevention of glutamine synthetase adenylylation Microbiology 147:1267-1276.

Sevilla M, Gunapala N, Burris RH, Kennedy C (2001) Enhancement of growth and N content in sugarcane plants inoculated with Acetobacter diazotrophicus. Mol Plant Microb Interac 14:358-366.

Rudnick PA, Arcondeguy T, Kennedy CK, Kahn D (2001) glnD and mviN are genes of an essential operon in Sinorhizobium meliloti. J. Bacteriol. 183:2682-2685.

Lee S, Meletzus D, Sevilla M, Kennedy C (2000) Characterization of a major nif/fix gene cluster in Acetobacter diazotrophicus, an endophyte of sugarcane. J.Bacteriol. 182:7088-7091

Sevilla M, Kennedy C (2000) Colonization of rice and other cereals by Acetobacter diazotrophicus, an endophyte of sugarcane In Ladha JK, Reddy PM (eds) The Quest for Nitrogen Fixation in Rice. IRRI press, Los Banos, The Phillipines, 354 p.

Sevilla M, Kennedy C (2000) Genetic analysis of nitrogen fixation and plant-growth stimulating properties of Acetobacter diazotrophicus, an endophyte of sugarcane. In: Triplett, E. W. (ed) Prokaryotic Nitrogen Fixation. Horizon Scientific Press, Wyndham UK, pp 737-760
Teixeira KRS, Morgan TV, Meletzus D, Galler R, Baldani JI, Kennedy C (1999) Identification, sequencing and structural analysis of a nifA-like gene of Acetobacter diazotrophicus. An.Acad.Bras.Cienc. 71:521-530

Hill S, He L, Kennedy C (1999) Physiological characterization of an Azotobacter vinelandii nifU deletion mutant and its spontaneous Nif+ revertants that overproduce cytochrome bd. FEMS Microbiol Letts 175:185-191

Teixeira KRS, Wulling M, Morgan T, Galler R, Zellermann, EM, Baldani JI, Kennedy C, Meletzus D (1999) Molecular analysis of the chromosomal region encoding the nifA and nifB genes of Acetobacter diazotrophicus FEMS Microbiol Letts 176:301-309

Meletzus D, Rudnick P, Doetsch N, Green A, Kennedy C (1998) Characterization of the glnK amtB operon of Azotobacter vinelandii. J Bacteriol 180:3260-3264


 

 
       
 
 


Plant Sciences Home

Department Directory|Seminar Schedule|Course Home Pages
UA Home |UA Admissions|College of Agriculture and Life Sciences|Site maintained by: Mario Marquez

Mailing Address: The University of Arizona
Department of Plant Sciences
Forbes Building
P.O. Box 210036
Tucson, AZ 85721