Update on Nutrient Sharing between Symbionts Nutrient Sharing between Symbionts

In this review, we consider the exchange of nutrients between the host plant and the bacterial microsymbiont in nitrogen-fixing legume root nodules. During nodule formation, the host tissues and the bacterial microsymbiont develop in response to each other to form a specialized tissue that maintains an environment where nitrogen fixation can occur (Brewin, 2004; Mergaert et al., 2006; Prell and Poole, 2006). This complex development will not be considered here but, at the end of the process, specialized, nitrogen-fixing forms of the bacteria, known as bacteroids, reside in the plant cytosol, enclosed within plant-derived membranes. These organelle-like structures are known as symbiosomes; the plant-derived membrane that surrounds the bacteroid is the symbiosome (or peribacteroid) membrane and the space between the two is the symbiosome (or peribacteroid) space. An infected plant cell may be packed with thousands of symbiosomes. The exchange of nutrients that is fundamental to N2 fixation therefore involves metabolism in the plant to provide carbon and nitrogen compounds to the bacteroids and to assimilate the metabolites that bacteroids release. Nutrients transferred between the symbionts must traverse both the symbiosome and the bacteroid membranes. It is clear that there is more than one pattern whereby successful nutrient exchange can take place: There are two basic types of legume nodules, determinate and indeterminate, and there are fundamental differences between the two in how they develop and in their carbon and nitrogen metabolism. This review focuses on the metabolism of carbon and nitrogen compounds in the symbionts and on the exchange of nutrients across the bacteroid and symbiosome membranes. Particular attention is paid to the movement of primary carbon and nitrogen sources and how they are utilized by both bacteroids and the plant.