Rhizobium Genetics

Abstract

The bacterial family of Rhizobiaceae, consisting of the two genera Rhizobium and Agrobacterium, creates a challenge to geneticists and biochemists be cause of its genetic and regulatory complexity. Both genera live as obligate aerobes saprophytically in soil and both are further characterized by their intimate association and cohabitation with plants. Rhizobia invade by a complex process of recognition and infection the roots of legumes inducing the cortex cells to remeristematisize and to build a new anatomically well-defined structure, the root nodule. The mechanism of this redifferentiation is not understood, but it seems plausible that the microsymbiont interferes directly or indirectly with the plant genome, inducing the growth and differentiation of new plant cells and regulating their structural Organization with the result of nodule formation. This process of genetic regulation of the plant genome by the bacteria continues, the new meristem being located on the tip of the growing nodule. In the inner tissue of the differentiated nodule, the domicile of the bacteria is established and now, by a process which is again not understood, the plant regulates the bacterial genome, turning off the genomic regions responsible for cell propagation and turning on the genes responsible for nitrogen fixation and for delivery of the NH ⁺₄ formed to the plant cytosol. This process is accompanied by shaping the bacterial rods into enlarged branched bacteroids and by the formation of leghemoglobin which is the genetical product of both partners, the plant producing the globin (Sidloi Lumbroso et al., 1978) and the bacteria the heme cofactor (Cutting and Schulman, 1969). The leghemoglobin, being located in the plant cytosol and outside on the membrane surrounding the bacteroids, (Desh et al., 1976) transports oxygen to the ATP generating machinery of the bacteroids necessary for the N² reduction. As a result of the Rhizobium invasion the plant utilizes the bacteria to cover its own nitrogen supply, whereas the differentiated bacteroids finally die: the plant genetically parasitizes the bacteria.