K. DALE NOEL
Professor

WLS 409
(414) 288-1475
email: dale.noel@marquette.edu

Molecular Biology of Root Nodule Bacteria

Bacteria known as rhizobia induce leguminous plants to form organs called root nodules in which these bacteria carry out nitrogen fixation.  Aside from benefiting both partners, this symbiosis is of tremendous importance in agriculture and global ecology.  Like fixation of carbon dioxide by primary producers such as green plants, nitrogen fixation is essential to sustaining life at populations above a small fraction of what the earth currently supports.  Globally in the symbiosis with legumes, rhizobia are estimated to carry out well over half of the total biological nitrogen fixation on land.

How nitrogen fixation occurs in the mature symbiosis is basically understood.  In addition, over the past three decades much has been learned about the early events of  the development of the nodule as an organ.  Each partner produces specific classes of chemicals that the other partner senses and to which it responds.  A key example is Nod factor, a glycolipid from the bacteria, which triggers cell division that generates the plant cells that compose the nodule.  Not as well understood is the molecular biology of infection, the process by which the plant allows the bacteria to enter the developing organ through a fascinating symbiotic structure, the infection thread.  Nor is it known how the infection process is coordinated with later events in development to bring about the mature, functioning root nodule.  These issues relating to the infection process are ones with which research in my laboratory has been concerned.  

We study Rhizobium etli, whose host in symbiosis is the common bean, Phaseolus vulgaris.  Like many other investigators in this field, our studies begin with the isolation of bacterial mutants.  We are interested in ones that are defective in infection and later steps in nodule development.  By identifying the bacterial molecules affected by the mutations, we deduce that these molecules are important in the infection process.   Over the years this approach has led us to show that symbiosis is particularly dependent on purine biosynthesis or derivatives of a particular purine intermediate, a cytochrome bc1 respiratory complex that is not required outside the nodule, and other bacterial components affected by mutants not yet studied.

In recent years, most of our studies have dealt with mutants affected in the major bacterial lipopolysaccharide, a molecule that is abundantly distributed on the bacterial surface. The O-antigen portion of this molecule is essential for infection.  It is likely that this molecule serves more than one role in the symbiosis.  With approaches that combine biochemistry and genetics, we are testing specific hypotheses regarding whether the O antigen protects against host defenses, acts as a ligand for plant receptors that activate pathways needed for growth of the infection thread, provides essential contact with plant membrane during endocytosis at the end of infection, and/or provides necessary surface features during active nitrogen fixation.

Aside from its role in symbiosis, we are studying the biosynthesis of this O-antigen.  Our genetic analysis of the O-antigen led eventually to the sequence of the 30,000-nucleotide cluster of genes required for O-antigen synthesis.  In addition, we have mutated almost every gene in this cluster.  Our present goals are to match the genes with their biosynthetic functions and to study poorly understood types of polysaccharide modification, such as the several O-methylations of the O-antigen sugars.  Almost all O antigens are synthesized by one or the other of two alternative pathways, and this one apparently is synthesized by the lesser known of these pathways.  With the genetic and biochemical tools we have developed for studying it, this O antigen is a good model to use in elucidating this lesser known pathway.

Another major emphasis in the lab is to understand how the infection process is coordinated with development of the nodule. Using bacterial mutants whose defect in infection could be suppressed chemically, we have shown that a certain limited extent of bacterial infection is required so that subsequent development on the plant side gives true nodule anatomy. We want to know how the bacteria produce this developmental effect and whether specific molecules from the bacteria are required at this point in development. Partly to address this question, we are developing a technique for "turning on" or "turning off" any bacterial gene at any point in nodule development. With this technique we will be able to ask whether a bacterial function is required throughout development or only at one period in nodule development.

Selected References:

D’Haeze, W., C. Leoff, G. Freshour, K.D. Noel, and R. W. Carlson.  2007. Rhizobium etli CE3 bacteroid lipopolysaccharides are structurally similar but not identical to those produced by cultured CE3 bacteria.  J. Biol. Chem. 282:17101–17113.

Noel, K.D., J.M. Box, and V. J. Bonne.  2004.  2-O-Methylation of fucosyl residues of a rhizobial lipopolysaccharide is increased in response to host exudate and is eliminated in a symbiotically defective mutant. App. Env. Microbiol. 70:1537-1544.

Jahn, O.J., G. Davila, D. Romero, and K.D. Noel. 2003.  BacS: an abundant bacteroid protein in Rhizobium etli whose expression ex planta requires nifA. Mol. Plant-Microbe Interact. 16:65-73.

Duelli, D.M., Tobin, A., Box, J.M., Kumar Kolli, V.S., Carlson, R.W., and K.D. Noel.  2001.  Genetic locus required for antigenic maturation of Rhizobium etli CE3 lipopolysaccharide. J. Bacteriol. 183:6054-6064.

Noel, K.D. and D.M. Duelli.  2000.  Rhizobium lipopolysaccharide and its role in symbiosis. In: Prokaryotic Nitrogen Fixation: A Model System for Analysis of a Biological Process (E.W. Triplett, ed.). Horizon Scientific Press, Wymondham, U.K. pp. 415-431.

Current Graduate Students

PhD

• Kristylea Ojeda

Graduate Courses

• Biology 232 - Bacterial Physiology
• Biology 323 - Special Topics in Biochemistry and Genetics