University Distinguished Prof,
MSU Foundation Professor

Department of Biochemistry & Molecular Biology
howeg@msu.edu

Research: Molecular and Biochemical Basis of Plant-Insect Interactions

Research in our laboratory is aimed at understanding how plants respond to insect herbivory and other forms of wound stress. We use both tomato (Solanum lycopersicum) and Arabidopsis (Arabidopsis thaliana) as experimental model systems for three related areas of investigation: (1) we are elucidating the mechanism of synthesis and action of the plant hormone jasmonate; (2) we are studying how jasmonate-regulated defensive compounds thwart insect attack; and (3) we are studying the development and metabolic function of glandular trichomes in tomato. These projects provide training in several areas of modern plant biology, including: analysis of protein-protein and receptor-hormone interactions; transcriptional networks; plant development; genetics of plant-insect interactions; protein biochemistry/proteomics; metabolism and metabolomics; and crop improvement for insect resistance.

Molecular Mechanism of Jasmonate Signaling

Herbivorous insects use diverse feeding strategies to obtain nutrients from their host plants. Rather than acting as passive victims in these interactions, plants cope with herbivory through the production of myriad specialized metabolites and proteins that exert toxic or anti-feedant affects on herbivores, or volatile substances that act indirectly by attracting predators of the herbivore. This highly dynamic form of immunity is initiated by the recognition of insect oral secretions and signals from injured plant cells. The plant hormone jasmonate (JA) plays a conserved and central role in this process by regulating genome-wide changes in gene expression.

A long-term objective of our research is to elucidate the molecular mechanism by which JA controls gene expression. A combination of genetic, cell biological, molecular, and biochemical analyses indicates that the core signal transduction chain linking JA synthesis to hormone-induced changes in gene expression consists of four components: a bioactive JA signal, the SCF-type E3 ubiquitin ligase SCFCOI1, JAsmonate ZIM-domain (JAZ) repressor proteins that are targeted by SCFCOI1 for degradation via the ubiquitin/26S proteasome pathway, and transcription factors (TFs) that promote the expression of JA-responsive genes (Fig. 1). Recent studies from our lab indicate that the F-box protein COI1 is a critical component of the JA receptor, and that jasmonoyl-isoleucine (JA-Ile), an amino acid-conjugated form of JA, is a natural ligand for this receptor system. A major unanswered question we seek to address is how the specificity of receptor-ligand and JAZ-TF interactions regulates the diversity of JA-mediated responses.

Plant Anti-Insect Proteins

A central question in plant-insect interaction research concerns the identity of JA-regulated compounds that thwart the ability of herbivores to colonize, consume, or reproduce on plants. Although plant secondary metabolites have traditionally been viewed as the major determinants of host plant utilization by insects, proteins are also an important component of the plant’s defensive response. Wound-inducible proteinase inhibitors that impair digestive proteases in the insect gut provide one of the best examples of a defensive protein whose synthesis is tightly regulated by the JA pathway.

We are using proteomic analysis of insect gut content and feces (frass) to identify the plant’s defensive protein arsenal. This novel approach is based on the premise that defensive proteins are relatively resistant to gut proteases and, as a consequence, are highly enriched during passage of the food bolus through the insect (Fig. 2). Application of this procedure to tomato-reared M. sexta larvae led to the identification of JA-regulated isoforms of arginase and threonine deaminase, which degrade the essential amino acids arginine and threonine, respectively, in the caterpillar gut. We hypothesize that arginase and threonine deaminase are components of a multitiered defensive system that functions to deplete the availability of essential amino acids in the insect gut. With funding from the USDA, we are using this proteomics platform to systemically identify anti-insect proteins from a broad range of crop plants.  Results obtained from this research are expected to provide new tools to improve pest tolerance in crop plants.

Glandular Trichomes in the Solanum

Glandular trichomes (GTs) populate the aerial surfaces of approximately 30% of all vascular plant species. These uni- and multi-cellular appendages play a critical role in plant protection against insects, and various abiotic stress conditions as well. A remarkable feature of GTs is their capacity to synthesize, store, and secrete large amounts of secondary metabolites. Because they are not essential for plant viability, GTs provide a unique opportunity to study complex and specialized metabolic pathways that operate within the confines of a simple and highly accessible developmental structure. Many GT-borne compounds have significant commercial value as pharmaceuticals, fragrances, food additives, and natural pesticides. For this reason, the prospect of exploiting GTs as “chemical factories” to produce high-value plant products has recently captured the attention of plant biochemists and biotechnologists alike.

Tomato and related species in the Solanum produce a variety of GT types on the surface of leaves, stems, and reproductive structures (Figure 3). The occurrence of multiple types of GTs within a single species provides a unique opportunity to understand the regulation of development of each class of trichome, and to identify the major biosynthetic pathways operating in each type. We are involved in a collaborative NSF-funded Plant Genome Project (http://www.trichome.msu.edu/) to study the morphogenesis, metabolic pathways, and function of GTs in cultivated tomato and its closely related wild species. The long-term goal of this collaborative project is to lay a foundation for a complete understanding of the network of genes and proteins involved in the development and metabolic function of GTs in Solanum spp. Work in our lab is currently focused on the characterization of several tomato mutants that are defective in GT development and metabolism. This line of investigation builds on our previous research showing that the JA/COI1 signaling pathway in tomato is required for the normal development and metabolic function of GTs.