Microbes are key determinants of plant population size, density, and diversity in all terrestrial biomes. In species-rich tropical forests, fungi and fungus-like organisms mediate plant population dynamics and community structure. Most studies of plant-fungal dynamics have focused on seedlings and saplings, with little consideration of seeds. Yet, the survival of seeds is a critical first step to the recruitment of individuals to the next generation. This research aims to test the hypothesis that beneficial and antagonistic plant-fungal interactions at the seed stage are important to tropical tree recruitment. The researchers will provide inclusive, cross-disciplinary research training and mentorship from the US and Latin America. The leadership team will ensure that all trainees, students, and postdoctoral associates, will benefit from the rich scientific community of their institutions and the Smithsonian Tropical Research Institute in Panama, including opportunities for professional growth, networking, collaboration, and participation in the Inclusion, Diversity, Equity and Access initiative at the Smithsonian Tropical Research Institute. In addition, the investigators will initiate course-based undergraduate research modules and continue a long-standing undergraduate field course in Panama. Finally, the team will give bilingual presentations for students and the public in regional, national (US, Panama), and international settings, and showcase their research by hosting Seed Ecology IX, a biennial international meeting, in Panama in 2024. Understanding the processes that influence tropical forest biodiversity is important at a global scale. Fungal pathogens are major sources of mortality in the tropics and essential to understanding tropical tree population size, density, reproductive success, and ultimately, species diversity. By combining the study of seed defenses and fungal symbioses, the project will uncover the potentially crucial but hidden drivers of tree recruitment in tropical forests. This research will (1) use two novel approaches (untargeted metabolomics and assays of inducible defenses) to understand how seed chemistry influences susceptibility to fungal infection, pathogenicity, and corresponding fungal responses; (2) move beyond local geographical scales to explore how the physical environment and maternal genotype influence fungal community composition and host susceptibility at a landscape scale, spanning the Panama Canal watershed; (3) combine lab and field approaches to test the hypothesis that priority effects in low diversity microbial systems will strongly influence the outcome of plant-microbial interactions (known as the primary symbiont hypothesis); and (4) support diverse training, outreach, and educational activities in the US and beyond. Collectively these elements will contribute to resolving the intrinsic factors (seed defenses) and extrinsic contingencies (additional microbial partners, different environmental conditions, variable genetic background of plants), that determine the outcome of seed-fungal interactions in the tropics. Thus, this project will elucidate how fungi influence plant recruitment, with relevance to diverse wild- and agro-ecosystems where plants regenerate from seeds. The project includes research training at the undergraduate, graduate student and postdoctoral levels at the University of South Florida, University of Illinois, Utah State University and University of Arizona. 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.