Hetrocystous cyanobacteria living within symbiosomes : genetic and physiological aspects

Sammanfattning: The objective of the thesis was to get a deeper genetic and physiological insight into the two heterocystous cyanobacterial species known today to live within symbiosomes: the marine cyanobacterium Richelia and the terrestrial cyanobacterium Nostoc. Symbiosomes are organelle-like structures formed by algal and plant hosts that accommodate symbiotic microbes inside their cellular compartment. Both cyanobacterial species are contained in photosynthesizing hosts, however while Richelia forms a symbioses with unicellular diatoms (algae), Nostoc is hosted by multi-cellular higher plants, among them species of the angiosperm genus Gunnera. Special emphasis was on the Nostoc-angiosperm symbiosis as it can be reconstituted in vitro. Phylogenetic analysis using the hetR gene showed Richelia and Nostoc to be separate lineages in the cyanobacterial genetic radiation, hence, symbiosome structures in conjunction with heterocystous cyanobacteria have evolved at least twice. In contrast to the Nostoc-angiosperm symbiosis, the Richelia-diatom symbiosis has a distinct species to host specificity and is possibly evolving towards a permanent organelle. Similar to Richelia, Nostoc is known as epiphytic or endophytic on its host, Nostoc is in addition occurring as free-living, it is speculated that the epiphytic character is a key to how cyanobiont symbiosomes may have evolved. Although the genetic cyanobiont specificity is lower and the genetic diversity is higher in the Nostoc-angiosperm symbiosis, the isolation of the infection process expressed cgt gene shows that there are regions in the Nostoc genome that are, although not unique to symbiosis forming strain, regions that are typical for symbiosis forming strains. Hexoses are shown to be carbohydrates that induce cellular responses such as heterocyst differentiation and nitrogen fixation in the symbiotically competent Nostoc PCC 9229 when contained in darkness. The data suggest that a hexose-type compound is likely to be supplied by the angiosperm host to the cyanobiont living in darkness in return for the fixed nitrogen. Other carbohydrates were shown to support cell survival. Two key metabolic genes, coding for the enzymes, sucrose synthase and glutamine synthetase were isolaled from the host: Gunnera, neither gene showed an altered expression pattern by the presence of Nostoc inside the tissue. It is speculated that once the Nostoc symbiosome is established, Nostoc is perceived as a pseudo-plastid by its host and that nitrogen fixation and other cellular processes are intimately controlled by Gunnera. This is possible as Nostoc and plastids are genetically related and cyanobacterial genes and metabolic pathways are present in all plants.