B.S. 1972, University of Missouri-Rolla
Ph.D. 1977, University of California-Berkeley
Postdoctoral Fellow, University of Wisconsin-Madison
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.
The bacterial components that induce the plant roots to develop nodules and how nitrogen fixation occurs in the mature symbiosis are basically understood. We are studying an aspect of symbiosis that is not as well understood -- the process of infection: how the bacteria enter the developing organ through a symbiotic structure known as the
infection thread and how the bacteria are released from the infection thread to multiply such that they fill plant cells in the nodule interior. Our approach has been to isolate bacterial mutants defective in symbiosis. By studying their deficiencies, we have identified bacterial components that are required in the infection process.
One such component is the O-antigen portion of bacterial lipopolysaccharide, an abundant molecule on the bacterial surface, which we have shown to be required throughout infection from the very beginning. It is likely that this molecule serves more than one role in the symbiosis. We are testing specific hypotheses that the O antigen protects against host defenses, acts as a ligand for plant receptors that promote growth of the infection thread, and aids in entering plant cells at the end of infection. Aside from its role in symbiosis, we are studying the biosynthesis of the O-antigen. 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.
Recently, we have become interested in the respiratory chains that the bacteria construct during symbiosis. When the bacteria are fixing nitrogen in the interior of the nodule, they make a chain similar to that of mitochondria, except that the terminal oxidase has very high affinity for oxygen. However, this particular respiratory chain is not required for infection, and we are asking how the respiratory chain differs during infection and whether there are strict requirements for certain respiratory components. In addition, we are investigating how the changes in respiratory chains are regulated.
We are continuing to examine certain other molecules that our genetic analyses have revealed to be important in infection or later stages of the symbiosis. Besides gaining a basic understanding of how symbiosis works, a long-term goal is to determine the minimal characteristics that might be needed on the bacterial side in order to engineer symbiosis with plants other than legumes.
Ojeda, K.J., Simonds, L., and Noel, K.D. 2013. Roles of Predicted Glycosyltransferases in the Biosynthesis of the Rhizobium etli CE3 O Antigen. J. Bacteriol. 195(9): 1949-58.
Ardissone, S., Noel, K.D., Klement, M., Broughton, W.J., and Deakin, W.J. 2011. Synthesis of the flavonoid-induced lipopolysaccharide of Rhizobium Sp. strain NGR234 requires rhamnosyl transferases encoded by genes rgpF and wbgA. Mol Plant Microbe Interact. 24:1513-1521.
Ardissone, S., Kobayashi, H., Kmabara, K., Rummel, C., Noel, K.D., Walker, G., Broughton, W.J., and Deakin, W.J. 2011. Role of BacA in lipopolysaccharide synthesis, peptide transport, and nodulation by Rhizobium Sp. strain NGR234. J Bacteriol. 193:2218-2228.
J.M. Box and K.D. Noel. 2011. Controlling the expression of rhizobial genes during nodule development with elements and an inducer of the lac operon. Mol. Plant-Microbe Interact. 24:478-486.
Ojeda, K., J.M. Box, and K.D. Noel. 2010. Genetic Basis for Rhizobium etli CE3 O-Antigen O-Methyl Moieties that Vary According to Growth Conditions. J. Bacteriol. 192: 679–690.
Noel, K.D. 2009. Rhizobia. In: Encyclopedia of Microbiology, 3rd edition, ed. Schaechter M. Oxford: Elsevier, pp. 261-277
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.
Zac Lunak (Ph.D. student)
Dr. Noel is NOT currently accepting new students into his lab
BIOL 8801 - Prokaryotic Molecular Genetics
BIOL 8953 - Special Topics in Biochemistry and Genetics