Abstract
Food systems face new climatic and socioecological challenges and farmers need a diversity of new plant varieties to respond to these. While plant breeding is important, institutional innovations in seed systems are critical to ensure that new traits and varieties make their way into farmers' fields. This Perspective reviews the state of knowledge on seed system development, outlining insights emerging from the literature that can help navigate the way forward. We synthesize evidence on the contributions and limitations of the different actors, activities, and institutions pertaining to all seed systems smallholder farmers use, formal and informal. To do so, we structure our analysis on three functions-variety development and management, seed production, and seed dissemination-and two contextual factors-seed governance and food system drivers-that can be used to describe any seed system. Our review reveals the strengths and weaknesses of the activities of different actors along the entire chain of functions and demonstrates the multifaceted efforts to strengthen seed systems. We document that a new agenda for seed system development is taking root, based on the view that formal and farmers' seed systems are complementary. Because needs differ from crop to crop, farmer to farmer, and between agroecological and food system contexts, a variety of pathways are needed to ensure farmers' seed security. While the complexity of seed systems eludes a simple roadmap, we conclude by planting a "signpost" with principles to guide efforts to develop resilient and inclusive seed systems.