Nodulation in Legumes and Actinorhizal Plants

Abstract

All nitrogen in living organisms is ultimately derived from atmospheric dinitrogen which gets incorporated into organic compounds by biological or chemical nitrogen fixation. Since biospheric nitrogen is subjected to a rapid turnover by denitrification, maintenance of the biosphere has to be achieved by nitrogen fixation. Biological nitrogen fixation is an energy-consuming process performed by the enzyme nitrogenase which is irreversibly denatured by oxygen. Nitrogenase is formed only by prokaryotes who in some cases fix nitrogen in symbiosis with higher plants. In these symbioses, bacteria are hosted inside plant cells in special organs, the so-called root nodules. The product of nitrogen fixation, ammonium, is exported to the plant, while the plant in turn is providing its symbiont with energy sources. Of the symbiotic nitrogen fixers, two distinct phylogenetic groups are (Azo-, Brady-)Rhizobium and Frankia who fix atmospheric nitrogen in association with higher plants, leading to the Rhizobium-legame symbioses and Frankia-actinorhizal symbioses (Young, 1992). Rhizobium enters symbioses only with leguminous plants (with the exception of Parasponia; Trinick, 1979), while Frankia is able to nodulate a taxonomically diverse group of plants which recently have been found to be closely related amongst each other and with legumes (Soltis et al., 1995). These plants are collectively referred to as actinorhizal plants.KeywordsNitrogen FixationRoot HairInfection ThreadLegume NoduleActinorhizal PlantThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.