Abstract
Bacteria of the family Rhizobiaceae and compatible legumes establish symbiosis to exchanges nitrogen and carbon in root nodules where rhizobial cells differentiate to bacteroids in host cytoplasm. Although the interaction is mutualistic under nitrogen limited conditions, it resembles a pathogenic interaction at some stages including invasion of rhizobial cells to the host cytoplasm. It is now known that at least some of host legumes commence defense reactions. These reactions might exist not only to exclude incompatible and parasitic rhizobial cells but also to regulate the excess nodule formation by compatible rhizobial cells. To evade such host reactions, rhizobial cells seem to utilize a number of tools to actively modulate host functions as well as to passively protect themselves. Our molecular genetic investigation on Mesorhizobium loti, the microsymbiont of a model legume Lotus japonicus, indicated difference in the extent of host aggression between Lotus-M. loti system and Medicago-Sinorhizobium meliloti system.
