Genetic hydridization of root-nodule bacteria (Rhizobium).

Abstract

Symbiotic nitrogen fixation between legumes and Rhizobium is a complex process involving biochemical, physiological and morphological changes in both partners. In Klebsiella the broad outline of organization and control of genes concerned with nitrogen fixation and assimilation of ammonium has been worked out (Streicher and Valentine, 1973; Brill, 1975). In this case the problems are more amenable to genetical and biochemical analysis since the organism expresses its nitrogen-fixing phenotype in the free-living state. The organization and control of nif genes in Rhizobium are likely to be at least as complex as in Klebsiella, but nitrogen fixation within the nodule is only the culmination of a long chain of events, involving recognition and penetration of root hairs, the induction of infection threads and nodule development, and morphogenetic changes in bacteroid formation. Most, if not all, of these steps are likely to be controlled by genes both in the host and in the bacterium.