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Wilfrid Laurier University Faculty of Science
September 20, 2014
 
 
Canadian Excellence

Research



My research centers on the fundamental question of how plant roots sense their environment. I am focussing on two structures, nodules and mycorrhizae, that result from symbiotic interactions occurring between a plant and a soil bacterium or a soil fungus, respectively, and on the mechanism(s) used by the plant to regulate entry to these beneficial microsymbionts. Nodules or mycorrhizae develop and function only if colonization by the appropriate microorganisms is successful. These structures are the result of complex, multi-step processes which are tightly regulated by the plant. The epidermis, the first root layer to be in contact with the microbes, is thought to be very important in sensing the proper symbionts and in rejecting unmatched ones. However, it is not the only check-points for successful colonization. For example, in nodulation, the pericycle, the outermost layer of the vascular tissue, also senses the chemical signals sent by the microsymbiont. The fact that both symbionts enter ultimately into the host cell makes it possible for the plant to tightly control the establishment and development of the symbioses. This control is likely to involve hormones.

I am especially interested in studying the roles played by plant hormones in the establishment and development of these two beneficial interactions. In nodulation, it has been proposed that auxin and cytokinin play an important role in the establishment of the nodule primordium; they are thought to act together in determining the site of nodule initiation. They have also been shown to have an effect on early nodulin gene expression. Ethylene, another plant hormone, has an inhibitory role in nodulation. It blocks most of the rhizobial infection threads in the epidermis and decreases the number of cortical cell divisions.

In my lab, we are now working on two pea mutants.
R50 (sym16) is a pleiotropic mutant which nodulates poorly and has pale green leaves. It also has a partial etiolation phenotype; furthermore, R50 accumulates cytokinins. We propose that the high levels of cytokinins in the mature shoots have an inhibitory effect on the development of nodule primordia. In R50, periclinal divisions do not follow anticlinal divisions, resulting in the formation of flat primordia.
E107 (brz) is a pleiotropic mutant characterized by a decreased ability to form nodules and mycorrhizae. In this mutant, the development of the fungal invasion was found to be similar to that of the rhizobial infection. Both microsymbionts are blocked either at the epidermis or at the interface between the epidermis and the outermost cortical cell layer. Furthermore, both infections are under the control of the shoot. Our results support the hypothesis that mycorrhiza formation and nodulation are regulated by the same processes.

Supported externally by NSERC (Natural Sciences and Engineering Research Council of Canada) Discovery grants and internally by WLU.