Molecular Signals and Receptors: Communication Between Nitrogen-Fixing Bacteria and Their Plant Hosts

Our understanding of the extent of communication taking place between the plant and its underground microbiome (rhizosphere microbes) as well as with other soil organisms has grown exponentially in the last decade. Much of this information has been obtained from studies of nitrogen-fixing organisms, particularly members of the family Rhizobiaceae (Alphaproteobacteria) that establish nodules on legume roots in which atmospheric nitrogen is converted to plant-utilizable forms. Signals exchanged among organisms in the rhizosphere via quorum sensing (QS) and the responses to these signals have been identified, but it is unclear how they influence the downstream stages of nodulation and nitrogen fixation. An exchange of signal molecules ensures that a high level of specificity takes place to optimize the nitrogen- fixing interaction between host legume and symbiont. Chitin-related molecules appear to be the microbial currency for communication between the symbiotic partners in both mutualistic and pathogenic interactions. Exceptions to the paradigms based on the legume-Rhizobium interaction, including the discovery of Betaproteobacteria (now called beta-rhizobia) that nodulate and fix nitrogen with legumes and the lack of nodulation (nod) genes in certain alpha-rhizobia, particularly those that nodulate Aeschynomene and Arachis, bring into question the universality of some of the previous models. Moreover, new frontiers have opened that examine the coordination of information exchange that is needed for the induction and maintenance of nitrogen fixation and for bacteroid differentiation. Nevertheless, nitrogen-fixing organisms are just one small part of a highly interactive rhizosphere community. The challenge of the next decade will be to understand in greater depth the community dynamics that occur in soil, one of our planet’s most precious yet limited resources, in the hopes of maintaining the key signal webs that are critical not only for the promotion of agriculture but also for the preservation of the environment overall.