REGULATION OF DEFENSE SIGNALING IN TOMATO

Michigan State University, East Lansing MI

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Abstract

The long-term goal of this proposed research is to understand the molecular signaling pathways that control the expression of plant defense genes. A useful model system for studying these pathways is the systemic induction of plant anti-herbivore defense genes in response to wounds inflicted by phytophagous insects. In tomato plants, the signal transduction pathway that couples wounding to the systemic activation of target genes is regulated by an 18-amino-acid peptide called systemin. Recent results indicate that systemin exerts its effects on gene activity by up-regulating the synthesis of the fatty acid-derived hormone jasmonic acid (JA), a potent regulator of stress-induced genes in virtually all plant species examined. The focus of this proposal is to exploit genetic strategies to develop this model and to identify genes involved in systemin-mediated signal transduction. The proposed experimental plan relies on our recent success in isolating tomato defenseless (def) mutants that fail to express defense genes in response to wounding and elicitation by systemin. Four specific objectives are proposed. First, two existing mutants (def1 and def2) will be characterized with regard to parameters associated with an active wound response including JA biosynthesis and induction of wound response genes. Second, the chromosomal position of the DEF1 and DEF2 loci will be mapped. Third, at least one DEF gene involved in systemin- mediated signaling will be cloned. And fourth, genetic screens will be performed to identify new classes of wound response mutants including a) mutants in which the wound response is constitutive in the absence of wound stimuli and b) mutants that are insensitive to the signaling action of JA. Identification and characterization of wound response mutants will provide insight into the genetic and biochemical complexity of plant defense signaling pathways. Molecular cloning and subsequent analysis of the genes involved should contribute to an understanding of basic biological processes that operate in both plants and animals, including the biosynthesis and perception of polypeptide signals, the regulation of fatty acid signaling cascades, and cell-cell communication during host defense.