Nodulation Gene Regulation inBradyrhizobium japonicum: a Unique Integration of Global Regulatory Circuits

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Abstract
Intercellular communication via diffusible chemical signals is well described for bacteria and functions to modulate a num- ber of cellular processes. The perception and interpretation of these signals enables bacteria to sense their environment, lead- ing to the coordinate expression of genes. The result of this communication is the proper and appropriate response of the bacterial community to its surroundings (74). Intercellular re- sponses resulting from such signaling include the control of competence, sporulation, and virulence factor production (4, 29, 76). In some cases, the signaling processes not only occur between microbial populations but also occur between bacteria and members of higher eukaryotes. The symbiotic association between rhizobia and legumes is an example of such an inter- action (68). Bacteria of the genera Rhizobium, Mesorhizobium, Azorhizo- bium, Sinorhizobium, and Bradyrhizobium have the ability to infect the roots of leguminous plants, causing the formation of a new organ (i.e., nodule) and establishing a nitrogen-fixing symbiosis. Infection of the plant requires the products of the bacterial nodulation genes, which encode for the production of a lipochitin nodulation signal. This Nod signal, upon recogni- tion by the plant, induces de novo organogenesis leading to nodule formation. The bacteria invade the root through root hairs and penetrate inside a plant-produced infection thread. Upon endocytosis into an infected cortical cell, the bacteria are enclosed in a membrane-bound symbiosome where they dif- ferentiate into bacteroids that are capable of fixing N2 into a form that the plant can utilize. In return, the bacteroids are supplied with an environment rich in carbon as an energy source. The nature of the rhizobial life cycle presents a number of survival challenges. The bacteria must exist as a saprophyte in the soil, competing against other bacteria for sustenance. When the rhizobium encounters the rhizosphere, this presents another environment, richer in nutrients but with its own unique environmental challenges. In the presence of a com- patible host root, the individual rhizobial cell must compete with members of its own species to gain successful entry into the host root to initiate the symbiosis. Its environment then changes again so that it must adapt to the rigors of the in planta and finally intracellular lifestyle. Due to these successive changes, study of rhizobial ecology has continued to be a major area of research interest. This work has practical implications, since a commercial industry exists to provide rhizobial inocu- lants for agricultural production. For example, the fact that commercial inoculants generally compete poorly against indig- enous rhizobia for nodulation is considered a major limitation to improving the agricultural use of biological nitrogen fixa- tion. In this review, we focus on the symbiotic association of B. japonicum with its legume hosts (primarily soybean), focusing on both signaling and the integration of regulatory processes involved in this association. Specifically, we highlight the reg- ulation of the bacterial nodulation genes, the surprising com- plexity of this regulation, and the integration of this system with the overall physiology and ecology of the organism.

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