Rhizobium: nitrogen fixing bacteria
Rhizobia are one of several group of bacteria capable of ‘fixing’ nitrogen, i.e. converting dinitrogen gas into ammonia and then into organic molecules such as amino acids. Because of this ability, nitrogen fixing bacteria are significant conduits between an extremely large pool of nitrogen in the atmosphere and living things who otherwise could only obtain nitrogen by recycling it from existing pools of organic nitrogen (e.g. amino acids, ammonia, nitrate and nitrite). Unlike many nitrogen fixing bacteria that can fix nitrogen when ‘free-living‘ (i.e. when not living inside a host plant) Rhizobia can only fix nitrogen when associated with a plant that provides it with carbohydrates. The carbohydrates provide energy for a process that requires substantial inputs of energy (both ATP and the reducing power of NADH). Rhizobia only associate with legumes, members of the pea family. (But not all legumes associate with Rhizobia and some that have nitrogen fixing associates may have bacteria other than Rhizobia).
Taxonomy and Phylogeny
Rhizobia are members of the Domain Bacteria. They are gram negative bacteria that are usually flagellated and motile. The ability to associate with legumes, like the ability to fix nitrogen, is NOT thought to be significant phylogenetically. The ability to fix nitrogen appears to have evolved separately several times (i.e. convergent evolution) as evidenced by its presence in Archaea, Cyanobacteria (see Nostoc) and several other bacterial groups not phylogenetically related. Similarly, the ability to associate with legumes (which might be considered a type of parasitism) is thought to have been transferred horizontally and consequently is not a good indicator of phylogeny (which reflects vertical gene transfer). The Rhizobia group is thus considered to be paraphyletic.
Structure
Rhizobia are rod shaped bacteria, 0.8 um in diameter and 2 um in length, often with flagellae. They assume a different shape when inside their host, being irregularly shaped or often ‘Y’ -shaped. Their presence nearby a root induces a novel structure within root hair cells called an infection thread. Sensing the presence of Rhizobia, root hairs curl and bacteria are lodged in the crook of the curl. At this point the root cell wall is degraded and the bacteria proliferate in a space outside the root hair cell membrane. A tubular ‘infection thread’ is then produced and grows down the outside of the root hair and into the root itself. The thread is bounded cell wall materials and essentially is an elongate invagination of the cell wall, with materials contributed both by the plant and by the bacteria. The infection thread eventually fuses with cell membrane at its base, adjacent to the root cortex. The infection thread then extends to enter (infect) cortical cells, inside of which the bacteria proliferate. As the thread develops the cortical cells de-differentiated and become meristematic, producing the tumor (nodu