RESEARCH INTEREST

Plant-Nematode Interactions


RESEARCH

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 (Heterodera glycines) and its host plant, soybean. Sedentary endoparasitic nematodes, such as the soybean cyst nematode (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 nearly 1 billion in crop losses annually. After penetrating and migrating through soybean root tissue, SCN induces dramatic modifications of selected root cells to form elaborate feeding cells (syncytia) within plant roots. Growth and development of the nematode is completely dependent on the formation of the syncytium. We are investigating compatible and incompatible soybean-SCN interactions using functional genomic and proteomic approaches. The aim of my research is to understand the molecular basis of plant 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 during the infection process that ultimately gives rise to specialized feeding cells (syncytia) within host plant roots. The underlying molecular mechanisms controlling these processes are largely unknown. We are using laser capture microdissection (LCM) to specifically isolate the contents of nematode-induced feeding cells in roots for cDNA library construction, microarray, and proteomic studies. In soybean resistant to SCN, feeding cell formation is compromised and nematode development is impeded. Gene profiling of different stages of syncytium development will enable us to discover new genes and genetic pathways modulated by nematode invasion for syncytium induction, development and maintenance and to identify new genes involved in SCN resistance. An extensive molecular characterization of gene expression in developing syncytia in resistant and susceptible host plants remains to be completed, and this approach may prove successful to identify additional host targets for engineered resistance.

Identification and Functional Analysis of Nematode Esophageal Gland Secretions
With regard to the nematode, we are focusing on the identification and functional analysis of SCN parasitism genes as part of a Molecular Nematology collaboration with the labs of Dr. Eric Davis (NCSU), Dr. Dick Hussey (UGA), and Dr. Thomas Baum (ISU). Our group is interested in the underlying mechanisms of cyst nematode parasitism, particularly how the cyst nematode modifies plant cells during the formation of a complex feeding site (syncytium) within the host root which is required for nematode 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 nematode esophageal gland stylet secretions are involved in initiating the interaction and modifying plant cells for parasitism. Notable progress is being made to determine the identity and precise nature of the molecules involved in establishing the parasitic interaction. Nematode esophageal gland cell-specific cDNA libraries have been constructed from microaspirated gland cell mRNA using PCR-based approaches and subjected to extensive EST sequence analysis. My laboratory is using molecular genetic approaches to conduct functional analyses of parasitism gene products to determine their role in plant parasitism. One approach we have taken is to identify interactions between nematode secreted proteins and plant proteins, including potential interactions with recently cloned soybean resistance genes that confer resistance to SCN. The goal is to identify the upstream and downstream components of the resistance gene signaling pathways during the SCN-soybean interaction using proteomic and reverse genetic approaches. In addition, we are examining the molecular diversity of SCN to identify differences in the molecular structure of parasitism gene products among H. glycines genotypes that correlate with virulence on resistant soybean.

The Role of Phytohormones in Plant-Nematode Interactions
Phytohormones have been known for decades to modulate plant development; however, the molecular mechanisms involved are only beginning to be discovered. Although not well understood, several lines of evidence suggest considerable interplay and crosstalk among various phytohormones for the modulation of plant growth. 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 (NFS) formation. Several hormone-responsive plant gene promoters have been shown to be upregulated in NFS. It is not entirely clear whether the nematode produces plant hormones for secretion into plant cells, or modulates the level of host phytohormone levels by redirecting normal plant biosynthetic and signaling pathways. We are using the model plant, Arabidopsis thaliana, as a parallel system to dissect the complex plant-nematode interaction. We are interested in elucidating how the nematode alters the complex plant hormone biosynthetic and signaling networks for the development of nematode feeding sites in plant roots. This involves studying the expression and function of genes encoding biosynthetic and catabolic enzymes, hormone signaling pathway components, and response genes in both model plants and soybean.



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