Plants constantly interact with environmental microbes, which result in diverse outcomes. Some microbes act as beneficial symbionts, while others can be harmful pathogens. Interactions between plants and individual microbes have been extensively characterized, however the dynamics between different microbes colonizing the same host remain largely to be explored. Here we develop a tripartite system, where plant-pathogenic root-knot nematodes (RKN, Meloidogyneincognita) and rhizobia (Mesorhizobiumloti) co-infect the same host (Lotusjaponicus), to model inter-specific interactions. Both RKN and rhizobia are known to infect roots and form specialized organs known as galls and nodules, respectively. However, RKNs are seen as agricultural pests while rhizobia are known to be beneficial for crops through nitrogen fixation. Interestingly, plants inoculated with RKN show substantial rhizobial nodulation suppression. This process appeared to occur early, as root-hair curling and infection thread formation were also blocked by RKN infection. Interestingly, this RKN-induced nodulation suppression effect appears to be systemic. Through split-root assays, local RKN infections causes reduction in nodulation even in roots physically separated from RKN. These lines of evidence suggest RKN infection may elicit the production of a mobile signaling molecule that suppresses rhizobia nodulation systematically. Currently further analyses are being performed to identify this mobile signal.
