Developmental Genetics of the Rhizobium/Legume Symbiosis

Abstract

INTRODUCTION
Rhizobium is the genus of gram-negative bacteria that establishes nitrogen-fixing symbioses with plants in the family Leguminoseae. An important feature of the symbiosis is host-range specificity; each Rhizobium species nodulates a specific and limited “cross-inoculation” group of legumes. Nitrogen fixation occurs in nodules, which are specialized organs that usually form on the roots. Inside the nodules, the Rhizobium are contained within plant cells and differentiate into “bacteroids,” which reduce dinitrogen (N₂) to ammonia. Both bacterial and plant mutants that block nodulation at different morphological stages have been isolated indicating that nodule formation is a multistep process requiring genetic input from both the legume host and the bacterial endosymbiont (for recent reviews, see Vincent 1980; Verma and Long 1983).

Because bacteroids are intracellular, they can be considered to be highly specialized nitrogen-fixing organelles. One interesting feature of bacteroid physiology is that the ammonium produced by N₂ reduction cannot be assimilated by the bacteroids (Brown and Dilworth 1975; Robertson and Farnden 1980) and is exported to the plant cytosol (Bergersen 1965; Bergersen and Turner 1967). Except for one species (Dreyfus et al. 1983), free-living Rhizobium, which derepress nitrogen fixation (nif) genes, are also incapable of assimilating the ammonium that they produce by N₂ reduction (Kurz and LeRue 1975; McComb et al. 1975; Pagan et al. 1975; O’Gara and Shanmugam 1976; Bergersen and Turner 1978). A major unanswered question about bacteroid formation is whether it is directed by a temporally ordered developmental program such as occurs during the process of spore formation...