Molecular mechanisms that boost systemic immunity in plants

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Plants are sessile organisms that lack circulating immune cells. To fight bacterial infections, plants use small, mobile metabolites that travel throughout their vascular system to turn on defenses at sites of attack and throughout the plant body. By activating defenses in uninfected tissues, plants exist in a heightened immune state that limits new pathogen infections. This immune response is known as defense priming or systemic acquired resistance. Recent research identified an amino acid derivative called N-hydroxy-pipecolic acid (NHP) that is the bioactive metabolite required to turn on defense priming. There is sparse information for how NHP affects plant defense and development. Notably, watering or injecting plants with NHP is sufficient to induce defense priming and protect against bacterial and fungal infections. These findings suggest that altering NHP levels in plants or providing NHP to plants to enhance their defense responses may be effective strategies to enhance disease resistance in plants used in agriculture and horticulture. This project will study important aspects of NHP biology. Key questions include: (1) How long does the NHP defense signal last? (2) How is the defense signal turned off? (3) How does NHP signaling impact normal growth and development? (5) Do pathogens manipulate NHP biology to turn off this plant defense system? These questions will be answered by using chemical biology and functional genomic approaches with tomato, an important crop plant. The long-term goal is to investigate possible chemical applications and/or engineering efforts to enhance defense priming in plants to improve plant health. This study will provide intensive research training at the graduate, undergraduate and high school level with special consideration of women, underrepresented minorities, and those from under-resourced backgrounds. The research and outreach activities will also provide hands-on teaching and mentorship training for Stanford graduate and postgraduate students. Systemic acquired resistance (SAR) is a global plant immune response induced at the site of pathogen infection that triggers long-lasting and broad-spectrum disease resistance throughout the plant. A single small metabolite, N-hydroxy-pipecolic acid (NHP), is necessary and sufficient for initiation of this heightened immune state, even in the absence of an initial infection. Interestingly, NHP and its derivatives appear to be mobile metabolites, illuminating the chemical nature of the signals that are required to initiate and amplify defense responses over long distances within the plant body. Moreover, overexpression of the NHP biosynthetic pathway in local tissues alone can protect distal tissues from pathogen infection. These data thus highlight the intriguing possibility for translating a chemical or metabolic engineering approach to prime and/or enhance disease resistance under pathogen pressure. Currently, there is sparse information available regarding the regulation of NHP biosynthesis, dynamics of NHP signaling, and universal impact of this defense priming mechanism on plant growth and health. The goal of this research is to elucidate the temporal dynamics of NHP chemical defense and its effectiveness in protecting important crop plants under native and engineered conditions. This study will test the hypothesis that the regulation of NHP biosynthesis and the duration of the NHP bioactive signal can be titrated to increase disease resistance without compromising plant fitness. The model vegetable tomato, Solanum lycopersicum, will be used to elucidate fundamental aspects of NHP. This research will provide insight to the dynamics of NHP signaling and growth-defense associated with altered NHP production.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.