Research
Since the start in the spring of 1996, the Baum lab has embarked on three major research avenues to explore the compatible interactions between cyst nematodes (Heterodera spp.) and their host plants. The compatible interaction describes the event in which a nematode does not encounter effective resistance responses from the plant and is able to complete its life cycle. At the heart of such a compatible interaction is the formation and maintenance of a plant-derived feeding site, called a syncytium in the case of the cyst nematodes. Syncytium formation and maintenance are requirements for the successful infection by cyst nematodes.
Unfortunately, researchers have not yet resolved the molecular and physiological events necessary for syncytium formation and maintenance. A working model for these events, however, has been proposed, and is shown at right.
It is very likely that cyst nematode secretions function as molecular signals to initiate the events leading to syncytium formation. Following, it is logical that the host plant needs to perceive and transduce these signals. Finally, the signaling events result in physiological changes that are caused and/or accompanied by host plant gene expression changes. The three research avenues pursued in the Baum lab follow the three events leading to a successful compatible interaction outlined above:
1) Identification and functional characterization of cyst nematode secretory proteins = effectors = parasitism proteins.
2) Identification and functional characterization of host genes that change expression in response to cyst nematode infection.
3) Genetic analyses of the compatible cyst nematode-host interaction by identifying and characterizing plant mutants with altered susceptibilities to cyst nematodes.
Nematode Secretions
This research is conducted in very close collaboration with the research groups of Dr. E. L. Davis, North Carolina State University, Dr. R. S. Hussey, University of Georgia, Dr. M. Goellner Mitchum, University of Missouri, Dr. Xiaohong Wang, Cornell University and Dr. Axel Elling, Washington State University. The research groups involved in this project frequently meet and work as a true team. This collaboration has developed a very successful functional genomics approach to further this research area. In short, the cytoplasm of cyst nematode gland cells, i.e., the cells giving rise to the sought-after secretions, is microaspirated and then used to generate gland-specific cDNA libraries using PCR technologies. These cDNA libraries were mined by a combination of various bioinformatic and molecular technologies including DNA microchip analyses, yeast-selection of secretory proteins, in situ hybridization, etc. As a result, this project has revealed a large panel of new gland-expressed genes, which now are functionally characterized. For example, effector genes (=parasitism genes) are expressed in the host plant to assess phenotypic effects and to test functional hypotheses. Also, yeast-two-hybrid studies are conducted to identify plant proteins interacting with the nematode parasitism proteins. Increasingly, a complex picture of how the nematode manipulates the host plant cell biology is becoming apparent.
Plant Gene Expression Changes
Plants react to cyst nematode infection by a variety of gene expression changes. Besides wound, defense, and other responses, these gene expression changes are involved in syncytium formation and maintenance. We have used Differential Display of mRNA to identify panels of cDNA clones that change mRNA abundance in response to cyst nematode infection. These cDNA clones were derived from the soybean - soybean cyst nematode (Heterodera glycines) and the Arabidopsis thaliana - sugar beet cyst nematode (Heterodera schachtii) pathosystems. We are now further characterizing these cDNAs with regard to their functions during plant development and pathogenesis using a variety of reverse genetics approaches.
Mutant Screening and Analyses
Cyst nematodes rely on plant factors to be successful. A mutant screen is a well-suited approach to identify such plant factors. For this goal we have established a mutant screen procedure utilizing the forward genetics advantages of the model plant Arabidopsis thaliana. We have identified several mutants and we are scrutinizing a variety of established A. thaliana mutants to answer specific questions of cyst nematode - plant interactions.
