Key Stage5 –
Fixation with intimacy
Rhizobia and legumes: fixation with intimacy
Excerpt from an article in Microbiology Today3995 – 99 (May 2012)
J. Allan Downie & Philip P. Poole
Nitrogen is an essential building block of all life but eukaryotes have never evolved a biochemical pathway to fix atmospheric nitrogen. However, some plants have developed intimate symbiotic relationships that enable direct transfer of fixed nitrogen from bacteria.
Prokaryotes are the only life forms that have the nitrogenase enzyme that reduces N2. Ammonia produced by nitrogenase is assimilated by the enzyme glutamine synthetase, which forms glutamine from glutamate and ammonia. The ‘fixed’ nitrogen in the glutamine is then used in the biosynthesis of the other nitrogen-containing building blocks of life.
Ultimately, eukaryotes depend on bacterially fixed nitrogen for growth, and it is surprising that no eukaryotes have evolved the ability to fix nitrogen. However, some plants have evolved symbiotic associations with bacteria, which supply ammonia to the plants in exchange for a supply of carbon and other nutrients. These symbiotic associations can be so intimate that the bacteria within the plants can be considered to be equivalent to organelle-like structures in which N2 is reduced. This is analogous to the other prokaryote-derived organelles, mitochondria and chloroplasts, which deal with oxygen and carbon dioxide. However, the nitrogen-fixing bacteria are not inherited and so must re-infect the roots many times.
Rhizobia–legume symbiosis
The best understood nitrogen-fixing symbiosis is that which occurs between rhizobia and legumes. The legumes develop root outgrowths called nodules, often several hundred on a root system (and in some legumes nodules also form on the stems). Each nodule contains many nitrogen-fixing bacteria (108–1010), all of which are usually clonal, derived from a single infecting bacterium.
The benefits of growing legumes have long been recognized; for example, the Roman, Theophrastus (370–285 BC) wrote ‘Beans are not a burdensome crop to the ground: they even seem to manure it’. In the last 20 years or so, research in rhizobial and legume molecular genetics, biochemistry and cell biology has given significant insights into the signalling that occurs between the symbiotic partners to allow this symbiosis to be established.
Unlike pathogenic infections of plants and animals, rhizobial infection of legumes is actively promoted by the plants, most of which produce specialized tunnel-like infection structures called ‘infection threads’ in response to signals from the bacteria. The bacteria grow at the tips of the infection threads, which grow down through root cells, eventually depositing the bacteria into plant cells in the growing nodule meristem. In the mature nodule a plant membrane surrounds the bacteria. The plant provides the bacteria with dicarboxylic acids as a carbon and energy source that they use to reduce N2 to NH3, which they export to the plant for assimilation into amino acids.