23 - The Plasmids of Rhizobium and Symbiotic Nitrogen Fixation

Abstract

This chapter discusses the plasmids of Rhizobium and symbiotic nitrogen fixation. The interaction between members of the genus Rhizobium and leguminous plants is specific. Legumes belonging to a given cross-inoculation group can be nodulated effectively only by certain rhizobia. A high degree of specificity is displayed to the formation of root hair infection threads, and plant proteins, called lectins, play a major role in initial Rhizobium–legume interface with their carbohydrate-binding specificity. A comprehensive analysis of symbiotic properties of rhizobia should help improve the agronomic input of atmospheric nitrogen in soils. This includes the specific host-recognition system, the ability to induce host cell multiplication, and to fix dinitrogen, and the production of bacteriocins that probably play a role in the regulation of the dynamics of Rhizobium populations in the soil microniches. The use of improved biological resources represents an actual alternative to escape the high-energy demanding synthesis of nitrogen fertilizers. Moreover, the nitrogen fixed in planta is completely available as nutrient, in contrast to the combined nitrogen that is applied as fertilizer.