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 remained facultative, 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 Geosiphon and the angiosperm Gunner a. Close cell–cell contact and the development of special structures ensure efficient nutrient exchange between the partners. The mobile nutrients are normal products of the donor cells, although their production is increased in symbiosis. Establishment of cyanobacterial–plant symbioses differs from chloroplast evolution. In these symbioses, the cyanobiont undergoes structural–functional changes suited to its role 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 during 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 cyanobacterial–plant symbioses. Finally we outline areas that require special attention for future research. Not only will these provide information of academic interest but they will also help to improve the use of Azolla as green manure, to enable us to establish artificial N2-fixing associations with cereals such as rice, and to allow the manipulation of free-living cyanobacteria for photobiological ammonia or hydrogen production or for use as biofertilizers.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1046/j.1469-8137.2000.00720.x

Tansley Review No. 116: Cyanobacterium-plant symbioses.

TheAzolla-Anabaena symbiosis is outstanding due to its high productivity combined with its ability to fix nitrogen at high rates. Because of this, in recent decades, countless studies have been conducted on this association, but with insufficient synthesis and coordination. This paper, therefore, attempts to review and synthesize past and recent findings concerning the biology and utilization ofAzolla in hopes that this will facilitate increased future collaborative research on this “green gold mine.” It reviews the taxonomy, distribution, morphology, physiology, and reproduction ofAzolla as well as new developments in its manifold uses.

https://www.jstor.org/stable/pdfplus/4354285.pdf

Azolla: A Review of Its Biology and Utilization

The phytogeographic distribution of Cainozoic ferns is reported based upon a critical re-appraisal of the macrofossil and mesofossil record also taking account of evidence from a few highly diagnostic spores. Well-documented circum-Arctic Cainozoic floras show ferns (Woodwardia, Onoclea, Osmunda, Coniopteris and to a lesser extentAzolla) distributed around the pole to very high paleolatitudes. Some ferns are shared between the mid-paleolatitudes of North America and Europe as would be predicted from the distributions of other biota. Evidence for the composition of Cainozoic fern floras is minimal in some regions (e.g., Antarctica, Central and South America, Africa, India, South East Asia), so the absence of fern fossils from these areas has no biogeographical significance. Matoniaceae were abundant in the preceding Mesozoic. However, the absence of Cainozoic macrofossils, and the fact that no CainozoicMatonisporites spores areMatonia-like, indicates that Matoniaceae had attained their modern relict distribution by, or very early in, the Cainozoic. The important Mesozoic families Marattiaceae and Dipteridaceae are also not represented by Cainozoic macrofossils. They probably also showed Cainozoic restriction but spores are not sufficiently diagnostic to enable testing of this hypothesis. Other ferns, which were also important in the Mesozoic (e.g., Dicksoniaceae, Gleicheniaceae), have patchy, equivocal, or inadequately published Cainozoic records. The dispersed spore record may provide an opportunity to track Cainozoic Gleicheniaceae but this approach is not without problems. Most well-represented Cainozoic fern families, genera and subgenera show widespread Cainozoic ranges, typically with considerable range extensions over their living relatives, both onto other continents and north and south to higher paleolatitudes. These include Schizaeaceae (Lygodium, Anemia, and the extinctRuffordia), Osmundaceae (Osmunda), Pteridaceae (Acrostichum), Thelypteridaceae (Cyclosorus), Lophosoriaceae (Lophosoria), Cyatheaceae (theCnemidaria/Cyathea decurrens clade) and the heterosporous water fernAzolla (Azollaceae). A few well-represented ferns show Cainozoic distributions similar to those of the present day (e.g.,Salvinia [Salviniaceae] andCeratopteris [Pteridaceae] (the latter by the Neogene and based only on spores]) but even these had slightly broader ranges in the Cainozoic. Some Cainozoic ferns have apparently local distributions, e.g.,Blechnum dentatum (Blechnaceae) in Europe; and others are so far represented at only one or few sites, e.g.,Dennstaedtiopsis (Dennstaedtiaceae),Botrychium (Ophioglossales),Grammitis (Grammitidaceae), andMakotopteris andRumohra (Dryopteridaceae). Cainozoic fossils assigned toDryopteris (and some other dryopteroids) require revision along with those of Thelypteridaceae, the latter having high potential to provide useful paleobiogeographic evidence, at least of theCyclosorus group. Cainozoic records of Hymenophyllaceae and Polypodiaceae are here considered unconfirmed.

Cainozoic ferns and their distribution

Enormous quantities of the free-floating freshwater fern Azolla grew and reproduced in situ in the Arctic Ocean during the middle Eocene, as was demonstrated by microscopic analysis of microlaminated sediments recovered from the Lomonosov Ridge during Integrated Ocean Drilling Program (IODP) Expedition 302. The timing of the Azolla phase (~48.5 Ma) coincides with the earliest signs of onset of the transition from a greenhouse towards the modern icehouse Earth. The sustained growth of Azolla , currently ranking among the fastest growing plants on Earth, in a major anoxic oceanic basin may have contributed to decreasing atmospheric p CO 2 levels via burial of Azolla -derived organic matter. The consequences of these enormous Azolla blooms for regional and global nutrient and carbon cycles are still largely unknown. Cultivation experiments have been set up to investigate the influence of elevated p CO 2 on Azolla growth, showing a marked increase in Azolla productivity under elevated (760 and 1910 ppm) p CO 2 conditions. The combined results of organic carbon, sulphur, nitrogen content and 15 N and 13 C measurements of sediments from the Azolla interval illustrate the potential contribution of nitrogen fixation in a euxinic stratified Eocene Arctic. Flux calculations were used to quantitatively reconstruct the potential storage of carbon (0.9‐3.5 10 18 gC) in the Arctic during the Azolla interval. It is estimated that storing 0.9 10 18 to 3.5 10 18 g carbon would result in a 55 to 470 ppm drawdown of p CO 2 under Eocene

http://bujakresearch.com/wp-content/uploads/2014/05/Speelman-et-al.-2009.pdf

The Eocene Arctic Azolla bloom: environmental conditions, productivity and carbon drawdown.

Recent phylogenetic studies have provided compelling evidence that confirms the once disputed hypothesis of monophyly for heterosporous leptosporangiate ferns (Marsileaceae and Salviniaceae). Hypotheses for relationships among the three genera of Marsileaceae (Marsilea, Regnellidium, and Pilularia), however, have continued to be in conflict. The phylogeny of Marsileaceae is investigated here using information from morphology and rbcL sequence data. In addition, relationships among all heterosporous ferns, including the whole‐plant fossil Hydropteris pinnata are reconsidered. Data sets of 71 morphological and 1239 rbcL characters for 23 leptosporangiate ferns, including eight heterosporous ingroup taxa and 15 homosporous outgroup taxa, were subjected to maximum parsimony analysis. Morphological analyses were carried out both with and without the fossil Hydropteris, and it was excluded from all analyses with rbcL data. An annotated list of the 71 morphological characters is provided in the appendix. For com...

https://www.jstor.org/stable/10.1086/314177

Phylogeny of Marsileaceous Ferns and Relationships of the Fossil Hydropteris pinnata Reconsidered.

The systematics of the aquatic fern genusAzollaLam. is revised at the supraspecific level. Published morphological data are reviewed, and cytological and cladistic analyses of the genus conducted, enabling a discussion of the putative evolutionary trends in the genus, and a revision of the current taxonomy. A phylogenetically more acceptable subgeneric and sectional classification is proposed, viz. subg.Azolla sect.Azolla (comprisingA. filiculoidesLam.,A. rubraR. Br.,A caroliniana auct. nonWilld.,A. microphylla auct. nonKaulf. andA. mexicanaPresl), subg.Azolla sect.Rhizosperma (Mey.)Mett. (comprisingA. pinnataR. Br.), and subg.Tetrasporocarpia subg. nov. (comprisingA. niloticaDecne. exMett.).

Keith Fowler

Papers

The supraspecific taxonomy and evolution of the fern genus Azolla (Azollaceae)

A morphological taxonomic revision of Azolla Lam. section Rhizosperma (Mey.) Mett. (Azollaceae)