Molecular signals and receptors: Controlling rhizosphere interactions between plants and other organisms
Rhizosphere interactions are affected by many different regulatory signals. As yet, however, only a few have been identified. Signals, by definition, contain information, react with a receptor, and elicit a response. Signals may thus represent the highest level of evolved response in rhizosphere communities and, in that sense, occupy a supreme control point. At the same time, some signals may function as modulators of downstream responses, rather than on/off switches. To assess these possibilities, several interactions between plants and soil organisms are described, starting with the molecular interactions between leguminous plants and symbiotic bacteria of the family Rhizobiaceae, one of the best-characterized plant-microbe associations in the rhizosphere. We then examine other interactions between plants and soil organisms for overlap and/or connections with the rhizosphere signals utilized in the legume-Rhizobium symbiosis. Whether information currently available reflects the interaction of the organisms in nature or only in the laboratory has not always been determined. Thus, the key ecological issue of how important some of the signals are under field conditions remains to be addressed. Molecular tools now available make this task less daunting than in the past, and thus a new age of experimental field ecology may soon burst forth in rhizosphere studies. By identifying the signals, receptors, and the critical control points, we can better understand the organismal dynamics in this key belowground ecosystem.