Summary 449 I. INTRODUCTION 450 II. THE PARTNERS 451 1. Cyanobionts and their role 451 2. Hosts and their role 453 3. Location of cyanobionts in their hosts 455 III. INITIATION AND DEVELOPMENT OF SYMBIOSES 458 1. Initiation of symbioses 458 2. Geosiphon pyriforme 458 3. Cyanolichens 459 4. Liverworts and hornworts 460 5. Azolla 460 6. Cycads 461 7. Gunnera 461 IV. THE SYMBIOSES 462 1. Geographical distribution and ecological significance 462 2. Benefits to the partners 462 (a) Benefits to the cyanobionts 462 (b) Benefits to the hosts 463 3. Duration and stability 463 4. Mode of transmission and perpetuation 463 5. Recognition between the partners 464 6. Specificity and diversity 464 7. Symbiosis‐related genes 465 8. Modifications of the cyanobiont 466 (a) Growth and morphology 466 (b) Photosynthesis and carbon metabolism 467 (c) Glutamine synthetase 467 (d) Heterocysts 469 (e) N2fixation 470 9. Nutrient exchange 471 (a) Carbon 471 (b) Nitrogen 472 V. EVOLUTIONARY ASPECTS 472 VI. ARTIFICIAL SYMBIOSES 474 VII. FUTURE OUTLOOK AND PERSPECTIVES 475 1. Cryptic symbioses 476 2. Developmental profile of symbiotic tissues 476 3. Sensing and signalling 476 4. Genetic aspects 476 5. Physiological and biochemical aspects of nutrient exchange 477 6. Microaerobiosis 477 7. Potential applications 477 Acknowledgements 477 References 477 Cyanobacteria are an ancient, morphologically diverse group of prokaryotes with an oxygenic photosynthesis. Many cyanobacteria also possess the ability to fix N2. Although well suited to an independent existence in nature, some cyanobacteria occur in symbiosis with a wide range of hosts (protists, animals and plants). Among plants, such symbioses have independently evolved in phylogenetically diverse genera belonging to the algae, fungi, bryophytes, pteridophytes, gymnosperms and angiosperms. These are N2‐fixing symbioses involving heterocystous cyanobacteria, particularly Nostoc, as cyanobionts (cyanobacterial partners). A given host species associates with only a particular cyanobiont genus but such specificity does not extend to the strain level. The cyanobiont is located under a microaerobic environment in a variety of host organs and tissues (bladder, thalli and cephalodia in fungi; cavities in gametophytes of hornworts and liverworts or fronds of the Azolla sporophyte; coralloid roots in cycads; stem glands in Gunnera). Except for fungi, the hosts form these structures ahead of the cyanobiont infection. The symbiosis lasts for one generation except in Azolla and diatoms, in which it is perpetuated from generation to generation. Within each generation, multiple fresh infections occur as new symbiotic tissues and organs develop. The symbioses are stable over a wide range of environmental conditions, and sensing–signalling between partners ensures their synchronized growth and development. The cyanobiont population is kept constant in relation to the host biomass through controlled initiation and infection, nutrient supply and cell division. In most cases, the partners have remaine...

Localization of nitrogenase, glutamine synthetase (GS), phycoerythrin (PE) and ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) was studied with immunocytochemical techniques in the cyanobiont and the free-living cultured isolate Nostoc 7801 of Anthoceros punctatus. In both cases nitrogenase was located in heterocysts only and was uniformly distributed within the cell. GS was located both in heterocysts and vegetative cells, with a uniform cellular distribution in each cell type. Whereas heterocysts of Nostoc 7801 had about twofold higher label than vegetative cells, labelling in heterocysts and vegetative cells of the cyanobiont was similar. While the GS content of the vegetative cells of the cyanobiont and Nostoc 7801 was comparable, the apparent GS content of the cyanobiont heterocysts was 60% less than that in Nostoc 7801 heterocysts. PE and RuBisCO were located in vegetative cells only. PE was located on thylakoid membranes and RuBisCO in the cytoplasm and carboxysomes. In each case the pattern of labelling in the cyanobiont and Nostoc 7801 was similar.

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SUMMARYThe energy requirements of akinete germination in A. doliolum were met initially from aerobic oxidation of endogenously stored carbon reserve. 'Phe germination of non-photosynthetic akinetes commenced in light with new protein synthesis followed by tbe simultaneous development of oxygenic photosynthesis, nitrate reductase activity, glutamine synthetase activity and aspartate dehydrogenase (AsDH) activity by 24 h and heterocyst formation and nitrogenase activity by 60 h. Glutamate-oxaloacetate transaminase (GOT) and glutamate-pyruvate transaminase (GP'P) activities were present in mature akinetes and only GPT activity increased during akinete germination. The simultaneous appearance during the course of germination of oxygenic photosynthesis, nitrate reductase activity and glutamine synthetase activity much before that of N^ fixation, implies that oxygenic photosynthesis is more closely associated developmentally with carbon dioxide and nitrate assimilation than witb N2 assimilation. The activities of the transaminases (GOT and GPT) during the initial stages of germination suggest a significant role for these enzymes in amino-acid metabolism associated with germination. The appearance of nitrate reductase activity under N.j-fixing conditions, in the absence of nitrate, suggests that the nitrate assimilating enzyme is not nitrate inducible.

1991. Inorganic nitrogen regulation of glutamate uptake in the cyanobacterium Nosioc muscorum. -Physiol. Piant. 82: 257-260.In Nosioc muscorum (Anabaena ATCC 27893) glutamate was not metabolised as a fixed nitrogen source, rather it functioned as an inhibitor of growth. The latter effect was nitrogen source specific and occurred in Ni-fixing cultures but not in cultures assimilating nitrate or ammonium. NOj-grown cultures lacked heterocysts and tiitrogenase activity and showed a nearly 50% reduction in glutamate uptake rates, as well as in the final extent of glutamate taken up, compared to N,-fixing or nitrogen-limited control cultures. NH4-grown cultures showed a similar response, except that the reduction in glutamate uptake rates and the final extent of glutamate taken up was over 80%. The present results suggest a relation between nitrate/ammonium nitrogen-dependent inhibition of glutamate uptake, probably via repression of the glutamate transport system, and glutamate toxicity.