Meet the Faculty

Melissa Goellner Mitchum

Assistant Professor
Division of Plant Sciences

Phone: 573-882-6152
Fax: 573-884-9676
E-mail: GoellnerM@missouri.edu
Address: 371H Life Sciences Center
Web site: Mitchum Lab

Education
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

Description
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. In addition to soybean, the Arabidopsis-beet cyst nematode pathosystem is used as a model system to dissect mechanisms of pathogenesis and feeding cell formation. Sedentary endoparasitic nematodes, such as SCN, are the most economically important group of plant-parasitic nematodes. SCN is consistently the most damaging pest of soybean grown in Missouri and throughout the U.S., causing nearly one 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. The Mitchum lab is studying the signal exchange that occurs between the nematode and its host for the formation of feeding cells. The aim of this research is to advance understanding of the molecular basis of pathogenicity and host resistance to cyst nematodes with the long-term goal of developing improved disease resistance strategies.

Host Plant Responses during Compatible and Incompatible Plant-Nematode Interactions

Nematodes induce multifaceted changes in plant cellular metabolism and gene expression that ultimately give rise to specialized feeding cells, called syncytia, within host plant roots. Proper formation of feeding cells is essential for the growth and development of the nematode to reproductive maturity. However, little is known about the molecular mechanisms that guide the development and determine the properties of a syncytium. Determining the expression profiles of syncytially expressed genes promises to significantly advance current understanding of feeding cell development. Moreover, determining how a nematode selects and modifies root cells to undergo pronounced changes in development to form a syncytium will contribute to knowledge of basic plant cell biology.

The difficulty in studying plant gene expression changes within syncytia in the past has stemmed from the fact that these cyst nematode-induced feeding cells make up only a minute fraction of the root cell population. Thus, high-throughput functional genomic studies using microarrays have been limited by their inability to obtain syncytial cell contents apart from surrounding cell types for analysis. Recent technologies such as laser capture microdissection (LCM) now allow for the isolation and cloning of mRNA populations from individual target cells within complex tissues leading to the identification of plant genes involved in cell-specific biological processes. LCM, therefore, is superbly suited to studies of syncytium induction, formation and function.

The lab recently coupled LCM technology with the latest soybean microarray platform available, representing 35,611 soybean genes, to provide a comprehensive gene expression profile of developing cyst nematode feeding cells in a compatible plant-nematode interaction. As a consequence, the lab's approach has provided the most comprehensive profile currently attainable and identified metabolic and regulatory pathways that may play intrinsic roles in syncytium induction, formation and function. The researchers are currently characterizing the function of genes up- and down-regulated in syncytium formation. In addition to understanding the basic biology of the system, they are using this information to identify nematode-responsive plant promoters and host targets of potential use in engineered resistance.

Little is known regarding the molecular mechanisms of soybean resistance to soybean cyst nematode. In soybean resistant to SCN, feeding cell formation is compromised and nematode development is impeded. Using LCM and microarrays, The researchers have directly compared gene expression profiles in developing syncytia of resistant and susceptible soybean differing at major loci controlling SCN resistance to identify components of the soybean resistance response to SCN. At present, they are characterizing the function of these genes to determine their role in resistance. In addition, the lab is collaborating with other groups to confirm the identity and function of candidate SCN resistance genes. Ultimately, they would like to identify both upstream and downstream components of the resistance gene signaling pathways during the SCN-soybean interaction using functional genomic and reverse genetic approaches and apply this knowledge to improve soybean resistance to SCN.

Identification and Functional Analysis of Nematode Parasitism Genes

With regard to the nematode, the mitchum lab has been focusing on the identification and functional analysis of nematode genes encoding esophageal gland secretions (i.e. nematode parasitism genes) as part of a Molecular Nematology collaboration with the labs of Eric Davis (NCSU), Dick Hussey (UGA), Thomas Baum (ISU), and Xiaohong Wang (Cornell). We are interested in elucidating the underlying mechanisms of cyst nematode parasitism, in particular how cyst nematodes utilize esophageal gland secretions to modify plant cells during the formation of a complex feeding site (syncytium) within the host root, which is required for their growth and development.

It is unclear how feeding sites are induced by the nematode and the nature and origin of the stimulus required to elicit the formation of feeding sites has not been identified. However, evidence suggests that esophageal gland stylet secretions encoded by nematode parasitism genes include key molecules involved in initiating the host-nematode interaction and modifying plant cells for parasitism. Notable progress has been made to determine the identity and nature of the molecules involved in establishing the parasitic interaction. Previously, nematode esophageal gland cell-specific cDNA libraries were constructed from microaspirated gland cell mRNA using PCR-based approaches and subjected to extensive EST sequence analysis. Mitchum's lab is using molecular genetic approaches to conduct functional analyses of a subset of these parasitism gene products to determine their role in plant parasitism. They are using novel approaches including RNA interference, ectopic expression in plants, and protein-protein interaction studies. Of significant interest is a class of Heterodera genes encoding secreted CLAVATA3/ESR-like (CLE) peptides that the researchers have shown to be functionally similar to plants CLE signaling peptides in overexpression and complementation studies (Wang et al., 2005). These data suggest that nematode CLE peptides may play an important role in parasitism by functioning as secreted ligand mimics of natural plant signal peptides. The lab is currently conducting detailed functional studies to assess the role of ligand mimicry in plant parasitism.

In addition, little is known regarding the mechanism of nematode virulence (i.e. the ability of a nematode to grow on roots of resistant cultivars). Interestingly, the Mitchum lab has identified differences in the molecular structure of parasitism gene products among H. glycines genotypes that correlate with virulence on resistant soybean. We are examining potential roles 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; however, the molecular mechanisms involved in modulation are only beginning to be discovered. Although not well understood, several lines of evidence suggest considerable interplay and crosstalk among various phytohormones is required to regulate plant growth and development. Morphological and biochemical evidence have shown that local phytohormone levels and hormone response pathways are altered in nematode-infected roots and may play a significant role in nematode feeding site formation. Phytohormone imbalances induced by nematodes likely result in altered expression of cell wall-modifying proteins and enzymes that play a central role in the controlled cell wall architectural modifications observed during feeding cell development (Goellner et al., 2001). Mitchum's lab has shown that a tobacco endo-ß-1,4-glucanase gene promoter (NtCel7) is upregulated within both cyst and root-knot nematode feeding sites during the early stages of their formation in heterologous plants (tomato, soybean and Arabidopsis), and it is responsive to auxin. Such promoters have potential use in driving the expression of “anti-nematode” effectors to engineer plant resistance.

Several other hormone-responsive plant gene promoters have been shown to be upregulated in NFS and both auxin and ethylene-insensitive mutants are less susceptible to cyst nematodes due to impairments in feeding cell development. It is not entirely clear whether the nematode produces plant hormones for secretion into plant cells, or whether it modulates host phytohormone levels by affecting transport or redirecting normal plant biosynthetic and signaling pathways. To answer the question of how the nematode alters the complex plant hormone biosynthetic and signaling networks for the development of nematode feeding sites in plant roots, the lab is studying the expression and function of genes encoding biosynthetic and catabolic enzymes, hormone signaling pathway components, and response genes in both model plants (e.g. Arabidopsis) and soybean.