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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.

Figure 1. JA regulates numerous physiological processes in response to environmental and developmental cues. FACs, fatty acid-amino acid conjugates; GLVs, green leafy volatiles. Figure modified from Howe and Jander (2008) Annu Rev Plant Biol 59: 41-66.

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.

Selected Publications

Katsir L, Chung HS, Koo AJK, Howe GA (2008) Jasmonate signaling: a conserved mechanism of hormone sensing. Curr Opin Plant Biol (In press)

Katsir L, Schilmiller AL, Staswick PE, He SY, Howe GA (2008). COI1 is a critical component of a receptor for jasmonate and the bacterial virulence factor coronatine. Proc Natl Acad Sci USA 105: 7100-7105 Abstract

Howe GA, Jander G (2008) Plant immunity to insect herbivores. Annu Rev Plant Biol 59: 41-66 Abstract

Browse J, Howe GA (2008) Update on jasmonate signaling: New weapons and a rapid response against insect attack. Plant Physiol. 146: 832-383 Full Text

Chung HS, Koo AJK, Gao X, Jayanty S, Thines B, Jones AD, Howe GA (2008) Regulation and function of Arabidopsis JASMONATE-ZIM domain genes in response to wounding and herbivory. Plant Physiol. 146: 952-964 Abstract

Jander G, Howe GA (2008) Plant interactions with arthropod herbivores: State of the field. Plant Physiol. 146: 801-803 Full Text

Chen H, Gonzales-Vigil E, Wilkerson CG, Howe GA (2007) Stability of plant defense proteins in the gut of insect herbivores. Plant Physiol 143: 1954-1967 Abstract

Kandoth PK, Ranf S, Pancholi SS, Jayanty S, Walla MD, Miller W, Howe GA, Lincoln DE, Stratmann JW (2007) Tomato MAPKs LeMPK1, LeMPK2, and LeMPK3 function in the systemin-mediated defense response against herbivorous insects. Proc Natl Acad Sci U S A. 104: 12205-12210 Abstract

Schilmiller AL, Koo AJ, Howe GA (2007) Functional diversification of acyl-coenzyme A oxidases in jasmonic acid biosynthesis and action. Plant Physiol 143: 812-824. Epub 2006 Dec 15. Abstract 

Thines B, Katsir L, Melotto M, Niu Y, Mandaokar A, Liu G, Nomura K, He SY, Howe GA, Browse J (2007) JAZ repressor proteins are targets of the SCF CO11 complex during jasmonate signalling. Nature [Jul 18 online] Publication

Chen H, Jones AD, Howe GA (2006) Constitutive activation of the jasmonate signaling pathway enhances the production of secondary metabolites in tomato. FEBS Lett 580: 2540-2546

Canoles MA, Beaudry RM, Li CY, Howe G (2006) Deficiency of linolenic acid in fad7 mutant tomato changes the volatile profile and sensory perception of disrupted leaf and fruit tissue. J Am Soc Hort Sci 131: 284-289

Koo AJ, Chung HS, Kobayashi Y, Howe GA (2006) Identification of a peroxisomal acyl-activating enzyme involved in the biosynthesis of jasmonic acid in Arabidopsis. J Biol Chem 281: 33511-33520. Epub 2006 Sep 8. Abstract

Powers RA, Rife CL, Schilmiller AL, Howe GA, Garavito RM (2006) Structure determination and analysis of acyl-CoA oxidase (ACX1) from tomato. Acta Cryst D62: 683-686

Chen H, Wilkerson CG, Kuchar JA, Phinney BS, Howe GA (2005) Jasmonate-inducible plant enzymes degrade essential amino acids in the herbivore midgut. Proc Natl Acad Sci USA 102:19237-19242 Abstract

Li C, Schilmiller AL, Liu G, Lee GI, Jayanty S, Sageman C, Vrebalov J, Giovannoni JJ, Yagi K, Kobayashi Y, Howe GA (2005) Role of b-oxidation in jasmonate biosynthesis and systemic wound signaling in tomato. Plant Cell 17: 971-986 Abstract

Schilmiller AL, Howe GA (2005) Systemic signaling in the wound response. Curr Opin Plant Biol 8: 369-377 Abstract

Li L, Zhao Y, McCaig BC, Wingerd BA, Wang J, Whalon ME, Pichersky E, Howe GA (2004) The tomato homolog of COI1 is required for maternal control of seed maturation, jasmonate-signaled defense responses, and glandular trichome development. Plant Cell 16: 126-143 Full Text  [Supplemental Data]

Li  L, Li C, Lee GI, Howe GA (2002) Distinct roles for jasmonic acid synthesis and action in the systemic wound response of tomato. Proc Natl Acad Sci USA 99: 6416-6421 Abstract 

Lee GI, Howe GA (2003) The tomato mutant spr1 is defective in systemin perception and the production of a systemic wound signal for defense gene expression. Plant J 33: 567-576 Abstract

Li C, Liu G, Xu C, Lee GI, Bauer P, Ling HQ, Ganal MW, Howe GA (2003) The tomato Suppressor of Prosystemin-mediated Responses2 (Spr2) gene encodes a fatty acid desaturase required for the biosynthesis of jasmonic acid and the production of a systemic wound signal for defense gene expression. Plant Cell 15: 1646-1661 Abstract

Zhao Y, Thilmony R, Bender C, Schaller A, He SY, Howe GA (2003) Virulence systems of Pseudomonas syringae pv. tomato promote bacterial speck disease in tomato by targeting the jasmonate signaling pathway. Plant J 36: 485-499 Supplemental Data

Li C, Williams MM, Loh Y-T, Lee GI, Howe GA (2002) Resistance of cultivated tomato to cell content-feeding herbivores is regulated by the octadecanoid-signaling pathway. Plant Physiol 130: 494-503 Abstract

Howe GA, Li L, Lee GI, Li C, Shaffer D (2002) Genetic dissection of induced resistance in tomato. In A Schmitt, B Mauch-Mani, eds, Induced Resistance in Plants Against Insects and Diseases. Proceedings of the meeting at Wageningen (The Netherlands), 26-28 April 2002. IOBC/WPRS Bulletin 25(6): 47-52

Howe GA, Schilmiller AL (2002) Oxylipin metabolism in response to stress. Curr Opin Plant Biol  5: 230-236 Abstract

Li L, Li C, Lee GI, Howe GA (2002) Distinct roles for jasmonate synthesis and action in the systemic wound response of tomato. Proc Natl Acad Sci USA 99: 6416-6421 Abstract

Strassner J, Schaller F, Frick UB, Howe GA, Weiler EW, Amrhein N, Macheroux P, Schaller A (2002) Characterization and cDNA-microarray expression analysis of 12-oxophytodienoate reductases reveals differential roles for octadecanoid biosynthesis in the local versus the systemic wound response. Plant J 32: 585-601 Abstract

Itoh A, Schilmiller AL, McCaig BC, Howe GA (2002) Identification of a jasmonate-regulated allene oxide synthase that metabolizes 9-hydroperoxides of linoleic and linolenic acids. J Biol Chem 277: 46051-46058Howe G, Lightner J, Browse J, Ryan C (1996) An octadecanoid pathway mutant (JL5) of tomato is compromised in signaling for defense against insect attack. Plant Cell 8: 2067-2077 Abstract

Froehlich JE, Itoh A, Howe GA (2001) Tomato allene oxide synthase and fatty acid hydroperoxide lyase, two cytochrome P450s involved in oxylipin metabolism, are targeted to different membranes of chloroplast envelope. Plant Physiol 125: 306-317 Abstract

Itoh A, Howe GA (2001) Molecular cloning of a divinyl ether synthase: Identification as a CYP74 cytochrome P-450. J Biol Chem 276: 3620-3627 Abstract

Li L, Howe GA (2001) Alternative splicing of prosystemin pre-mRNA produces two isoforms that are active as signals in the wound response pathway. Plant Mol Biol 46: 409-419 Abstract

Howe GA (2001) Cyclopentenone signals for plant defense: Remodeling the jasmonic acid response. Proc Natl Acad Sci USA 98: 12317-12319 Paper

Li L, Li C, Howe GA (2001) Genetic analysis of wound signaling in tomato: Evidence for a dual role of jasmonic acid in defense and female fertility. Plant Physiol 127: 1414-1417 Paper

Howe GA, Lee GI, Itoh A, Li L, DeRocher A (2000) Cytochrome P450-dependent metabolism of oxylipins in tomato: Cloning and expression of allene oxide synthase and fatty acid hydroperoxide lyase. Plant Physiol 123: 711-724 Abstract

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