Nitrogen-fixing microorganisms have been found in roots or other organs of many species of plants with which they establish symbiotic associations. In some cases, the diazo-trophic partner is a cyanobacterium, such as Anabaena in coralloid roots of Cycads or Nostoc in stems of Gunnera. Particularly interesting is the symbiosis between the cyanobacterium Anabaena azollae and the leaves of the aquatic fern Azolla. This symbiosis is permanent and hereditary and is the only known mutualistic symbiosis between a pteridophyte and a diazotrophic prokaryote. The Anabaena-Azolla association has received great attention for its potentiality as a biofertilizer to substitute chemical nitrogen compounds. In rice fields, for instance, it can fix over 1 kgNha- d~ , providing sufficient nitrogen to allow sustainable rice cultivation. Other symbioses having cyanobacteria as the N2-fixing phycobiont are those of lichens.
However, the symbiotic associations of current ecological importance for wide diffusion and the large nitrogen supply to the ecosystems are those between N2-fixing bacteria and roots of higher plants and, in particular, the rhizobia-legume and Frankia-dicotyledon symbioses. Another association of particular interest is that established by endophytic diazotrophic bacteria with cereals.
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