Research on determinants of plant assemblage structure and diversity has traditionally focused on aboveground factors such as seed dispersal, light competition, and interactions with herbivores. While these factors are clearly important, a growing number of studies indicate that interactions with belowground microorganisms also have a large effect on plant assemblages. Our lab studies microorganisms that provide plants with different resources in exchange for carbon derived from photosynthesis. Although these symbioses are very common, their ecological roles have only recently begun to be more fully understood. This is largely because breakthroughs in molecular biology have made it possible to ask more specific questions about these symbionts and their role in plant assemblage dynamics. We use molecular tools, in combination with field and laboratory experiments, to address a broad range of research questions.