Insect pests cause huge losses to farmers in the U.S. and worldwide. Developing improved plants with increased resistance to herbivorous insects is important for increasing yields while reducing production costs and the use of environmentally harmful insecticides. A key for understanding plant defense responses to insects are the underlying signaling pathways that result in upregulation of defense genes. In tomato, potato and other solanaceous plants, the response to chewing insects is mediated by a hormone-like short signaling peptide, systemin. Systemin is essential for successful defense against insects, and its signaling pathway is well defined. A major advance was the recent identification of the systemin receptor SR160, a LRR-receptor kinase. Surprisingly, it was shown that SR160 is identical to tBRI1, the tomato brassinosteroid (BR) receptor. BRs play an important role in plant development. Thus, tomato provides a unique opportunity to study how interactions of the receptor SR160/tBRI1 with systemin and BRs result in either stress or developmental responses. Such dual ligand receptors had not been identified in plants before and may be an entirely new way of signaling.
MAPKs are central for the relay of stress signals to defense genes. They are part of the MAPK cascade, which consists of three functionally linked kinases. The MAPK cascade is often activated in a receptor-dependent manner, but the components that link receptors and MAPK cascades are not known. The focus of the proposed research is to identify the components and mechanisms that connect the receptor SR160/tBRI1 to the systemin-responsive MAPKs. As a first step, systemin-responsive MAPK kinases (MAPKKs) will be identified by employing the yeast two-hybrid system. MAPKKs function directly upstream of MAPKs. Using mutant plants and a tomato protoplast transient transformation system, it will be tested in vivo if these MAPKKs activate MAPKs in response to systemin, and if this requires SR160/tBRI1. It will also be tested, if expression of constitutively active mutants of these MAPKKs results in upregulation of defense genes and increased resistance against herbivorous insects in transgenic plants. Manipulation of MAPK cascades has been realized to be a very promising tool to generate stress resistant crop plants. It is hypothesized that signal transduction for systemin and BRs diverges downstream of the receptor SR160/tBRI1, and that BRs do not activate MAPKs. If BRs do activate MAPKs, these MAPKs will be further characterized. Elucidating the unique mode of signaling via a dual ligand receptor will lead to new insights in signal perception and transduction that is likely to be more common than previously assumed. Taken together, this project will increase the understanding of how signaling mechanisms regulate plant stress responses.
The project will involve a postdoctoral research associate, a graduate student and undergraduate students who will be trained in state-of-the-art methods in plant molecular biology. Access to hands-on research experience for undergraduate students is crucial for the development of their careers. Research related to this project will be incorporated in an upper level class for undergraduate and graduate students, taught by the PI. Underrepresented groups will be actively recruited to participate in the project.
