Cyanobacterium-plant symbioses

Cyanobacteria are an ancient, morphologically diverse group of prokaryotes with an oxygenic photosynthesis. Many cyanobacteria also possess the abilit y to fix N-2. Although well suited to an independent existence in nature, s ome cyanobacteria occur in symbiosis with a wide range of hosts (protists, animals and plants). Among plants, such symbioses have independently evolve d in phylogenetically diverse genera belonging to the algae, fungi, bryophy tes, pteridophytes, gymnosperms and angiosperms. These are N-2-fixing symbi oses involving heterocystous cyanobacteria, particularly Nostoc, as cyanobi onts (cyanobacterial partners). A given host species associates with only a particular cyanobiont genus but such specificity does not extend to the st rain level. The cyanobiont is located under a microaerobic environment in a variety of host organs and tissues (bladder, thalli and cephalodia in fung i; cavities in gametophytes of hornworts and liverworts or fronds of the Az olla sporophyte; coralloid roots in cycads; stem glands in Gunnera). Except for fungi, the hosts form these structures ahead of the cyanobiont infecti on. The symbiosis lasts for one generation except in;Azolla and diatoms, in which it is perpetuated from generation to generation. Within each generat ion, multiple fresh infections occur as new symbiotic tissues and organs de velop. The symbioses are stable over a wide range of environmental conditio ns, and sensing-signalling between partners ensures their synchronized grow th and development. The cyanobiont population is kept constant in relation to the host biomass through controlled initiation and infection, nutrient s upply and cell division. In most cases, the partners hale remained facultat ive, with the cyanobiont residing extracellularly in the host. However, in the water-fern Azolla and the freshwater diatom Rhopalodia the association is obligate. The cyanobionts occur intracellularly in diatoms, the fungus G eosiphon and the angiosperm Gunnera. Close cell-cell contact and the develo pment of special structures ensure efficient nutrient exchange between the partners. The mobile nutrients are normal products of the donor cells, alth ough their production is increased in symbiosis. Establishment of cyanobact erial-plant symbioses differs from chloroplast evolution. In these symbiose s, the cyanobiont undergoes structural-functional changes suited to its rol e as provider of fixed N rather than fixed C, and the level of intimacy is far less than that of an organelle. This review provides an updated account of cyanobacterial-plant symbioses, particularly concerning developments du ring the past 10 yr. Various aspects of these symbioses such as initiation and development, symbiont diversity, recognition and signalling, structural -functional modifications, integration, and nutrient exchange are reviewed and discussed, as are evolutionary aspects and the potential uses of cyanob acterial-plant symbioses. Finally we outline areas that require special att ention for future research. Not only will these provide information of acad emic interest but they will also help to improve the use of Azolla as green manure, to enable us to establish artificial N-2-fixing associations with cereals such as rice, and to allow the manipulation of free-living cyanobac teria for photobiological ammonia or hydrogen production or for use as biof ertilizers.