B.S., Biology, University of Puget Sound, Washington
M.S., Plant Pathology, University of Nebraska, Lincoln
Ph.D., Plant Pathology, North Carolina State University, Raleigh
The major focus of research in the Mitchum Lab is the molecular basis of plant-nematode interactions with an emphasis on the interaction between the soybean cyst nematode (SCN; Heterodera glycines) and its host plant, soybean. Sedentary endoparasitic nematodes, such as SCN, are the most economically important group of plant-parasitic nematodes. SCN is consistently the most damaging pest of soybeans grown in Missouri and throughout the US, causing more than 1 billion dollars in crop losses annually.
After penetrating and migrating through soybean root tissue, SCN induces dramatic modifications of selected cells near the vasculature of the root to form an elaborate feeding cell (called a syncytium). Growth and development of the nematode is completely dependent on the formation of the syncytium from which it derives nutrients to complete its life cycle. We are studying the signal exchange that occurs between the nematode and its host for the formation of feeding cells. In addition to soybean, the Arabidopsis-beet cyst nematode pathosystem is used as a model system to dissect the mechanisms of pathogenesis and feeding cell formation. The aim of this research is to advance our understanding of the molecular basis of pathogenicity and host resistance to cyst nematodes with the long term goal of developing improved disease resistance strategies.
Plant Responses During Compatible and Incompatible Plant-Nematode Interactions
Nematodes induce multifaceted changes in plant cellular metabolism and gene expression during the infection process that ultimately gives rise to specialized feeding cells (syncytia) within host plant roots. The underlying molecular mechanisms controlling these processes remain largely unknown. We have used laser capture microdissection (LCM) to specifically isolate mRNA of nematode-induced feeding cells over a time-course of their development in soybean roots infected with SCN and coupled this with microarray analysis to develop the most comprehensive profile of syncytia-expressed genes to date. We are currently characterizing the function of genes up and down regulated in developing syncytia to assess for direct roles in syncytium induction, development and maintenance. This approach may also prove successful in identifying host targets for engineered resistance.
Little is known regarding the molecular mechanisms of soybean resistance to SCN. In resistant soybean, feeding cells degenerate and nematode development is impeded. We have been collaborating with the lab of Dr. Khalid Meksem (SIU) to confirm the identity and function of resistance genes underlying major SCN resistance QTL. We recently determined the soybean Rhg4 gene encodes a serine hydroxymethyltransferase, which points to a new mechanism of plant resistance to pathogens. We are currently using functional genomic, genetic, and biochemical approaches to elucidate the mechanism of resistance. We have also coupled LCM with microarray analysis to directly compare gene expression profiles in developing syncytia of resistant and susceptible soybean to identify components of the soybean resistance response to SCN. At present, we are characterizing the function of these genes to determine their role in resistance.
Identification and Functional Analysis of Nematode Effector Proteins
With regard to the nematode, we have been focusing on the identification and functional analysis of nematode parasitism genes expressed in the esophageal gland cells that code for stylet-secreted effector proteins, as part of a Molecular Nematology collaboration with the labs of Drs. Eric Davis (NCSU), Dick Hussey (UGA), Thomas Baum (ISU), and Xiaohong Wang (Cornell). Our group is interested in elucidating the underlying mechanisms of cyst nematode parasitism, in particular how cyst nematodes utilize effector proteins to modify plant cells during the formation of a complex feeding site (syncytium) within the host root.
Stylet-secreted effectors are key molecules involved in initiating the interaction and modifying plant cells for parasitism. Notable progress has been made to determine the identity of stylet-secreted effectors involved in establishing the parasitic interaction. Previously, nematode esophageal gland cell-specific cDNA libraries were constructed from microaspirated gland cell mRNA, sequenced, and subjected to bioinformatics analysis. We are conducting functional analyses of several effector proteins to determine their role in plant parasitism. Approaches include RNA interference, ectopic expression in plants, and protein-protein interaction studies. Of particular focus is a class of parasitism genes encoding secreted CLAVATA3/ESR-like (CLE) peptides. These are the first CLE genes identified outside the plant kingdom. In plants, CLE peptides serve as ligands for receptors to mediate plant signaling regulating the balance between stem cell proliferation and differentiation. We are currently conducting detailed functional studies to assess the role of peptide hormone mimicry in nematode parasitism. We are also investigating differences in effector proteins among SCN populations differing in virulence on resistant soybean to elucidate a potential role for these proteins in eliciting, suppressing or evading host plant resistance mechanisms.
The Role of Phytohormones in Plant-Nematode Interactions
Phytohormones have been known for decades to modulate plant development and we continue to make progress in our understanding of the molecular mechanisms controlling these processes. Considerable interplay among various phytohormones for the modulation of plant growth has been identified. Several lines of evidence have shown that local phytohormone levels and hormone response pathways are altered in nematode-infected roots and play a significant role in nematode feeding cell formation. Phytohormone imbalances induced by nematodes likely result in altered expression of cell wall modifying enzymes with a central role in the controlled cell wall architechural modifications observed during feeding cell development, and may play a variety of other roles. Hormone-responsive plant gene promoters have been shown to be upregulated in feeding cells and both auxin and ethylene-insensitive mutants are less susceptible to cyst nematodes due to impairments in feeding cell development. It remains to be determined whether the nematode secretes phytohormone mimics into plant cells, and/or modulates the level of host phytohormone levels by affecting transport or redirecting normal plant biosynthetic and signaling pathways. We are using the model plant, Arabidopsis thaliana, to dissect the role of phytohormones during plant-nematode interactions. We recently identified a nematode stylet-secreted effector that interacts with the auxin influx transporter LAX3, suggesting that nematodes use their effectors to directly regulate aspects of phytohormone transport to form feeding cells. Several ongoing projects are directed at determining how nematodes alter phytohormone biosynthetic and signaling networks for the development of nematode feeding cells in plant roots.