HOW MANY RHIZOBIUM GENES, IN ADDITION TO NOD, NIF FIX, AND EXO, ARE NEEDED FOR NODULE DEVELOPMENT AND FUNCTION/

The establishment of a nodule during the Rhizobium-legume symbiosis re quires the induction of new developmental programs within each partner . During the initial interaction between Rhizobium and its host legume , operons of bacterial genes are induced to perceive plant-secreted si gnals and, in turn, to synthesize the bacterium's own secreted nodulat ion signals. However, a number of rhizobial genes which are required f or free-living conditions as well as for legume infection have been id entified. Mutations in these genes affect the establishment and/or eff ectiveness of the nitrogen-fixing symbiosis. As the rhizobia encounter different zones of the developing nodule, some genes play very specif ic roles in bacteroid function, whereas others appear to be part of re gulons involved in general cell maintenance. Ln this review, we catalo g these genes and describe their involvement, either direct or indirec t, in the symbiosis.