Roots Of Cycads Research Articles

Phylometagenomics of cycad coralloid roots reveals shared symbiotic signals.

Cycads are known to host symbiotic cyanobacteria, including Nostocales species, as well as other sympatric bacterial taxa within their specialized coralloid roots. Yet, it is unknown if these bacteria share a phylogenetic origin and/or common genomic functions that allow them to engage in facultative symbiosis with cycad roots. To address this, we obtained metagenomic sequences from 39 coralloid roots sampled from diverse cycad species and origins in Australia and Mexico. Culture-independent shotgun metagenomic sequencing was used to validate sub-community co-cultures as an efficient approach for functional and taxonomic analysis. Our metanalysis shows a host-independent microbiome core consisting of seven bacterial orders with high species diversity within the identified taxa. Moreover, we recovered 43 cyanobacterial metagenome-assembled genomes, and in addition to Nostoc spp., symbiotic cyanobacteria of the genus Aulosira were identified for the first time. Using this robust dataset, we used phylometagenomic analysis to reveal three monophyletic cyanobiont clades, two host-generalist and one cycad-specific that includes Aulosira spp. Although the symbiotic clades have independently arisen, they are enriched in certain functional genes, such as those related to secondary metabolism. Furthermore, the taxonomic composition of associated sympatric bacterial taxa remained constant. Our research quadruples the number of cycad cyanobiont genomes and provides a robust framework to decipher cyanobacterial symbioses, with the potential of improving our understanding of symbiotic communities. This study lays a solid foundation to harness cyanobionts for agriculture and bioprospection, and assist in conservation of critically endangered cycads.

Root System Radius of Three Cycas Species

Knowledge about the position of tree roots is needed to understand tree biology and inform management decisions. Cycad species produce pachycaul stems and are not represented in the published studies on the relationships between aboveground tree traits and root system spread. We measured the radius of root egress from 6-year-old Cycas edentata de Laub., Cycas micronesica K.D. Hill, and Cycas nitida K.D. Hill & A. Lindstrom plants in an ex situ cycad germplasm collection into an 8-m-wide buffer zone in which no trees were growing. We then determined the relationships of root system radius to aboveground plant traits. The root system radius extended 5.2 to 7.5 m from the base of the stems, and was 3.5 times the plant height, 32 to 36 times the stem diameter, and 3.6 to 4.2 times the radius of the canopy dripline. Values for root radius in relation to plant height and canopy radius exceeded published data for leptocaul tree species, but values for stem diameter were less than these data. These findings on the location of cycad roots provide adaptive management cycad knowledge to inform horticulture decisions such as avoidance of roots during soil disturbance and placement of fertilizers and systemic pesticides.

Diversity, Phylogeny and Antagonistic Activity of Fungal Endophytes Associated with Endemic Species of Cycas (Cycadales) in China.

The culture-based approach was used to characterize the fungal endophytes associated with the coralloid roots of the endemic Cycas debaoensis and Cycas fairylakea from various population sites in China. We aim to determine if the assemblages of fungal endophytes inside these endemic plant hosts are distinct and could be explored for bioprospecting. The isolation method yielded a total of 284 culturable fungal strains. Identification based on the analysis of the internal transcribed spacer (ITS) rDNA showed that they belonged to two phyla, five classes, eight orders and 22 families. At least 33 known genera and 62 different species were confirmed based on >97% ITS sequence similarity. The most frequent and observed core taxa in the two host species regardless of their population origin were Talaromyces, Penicillium, Fusarium, Pochonia and Gliocladiopsis. Seventy percent was a rare component of the fungal communities with only one or two recorded isolates. Contrary to common notions, diversity and fungal richness were significantly higher in C. debaoensis and C. fairylakea collected from a botanical garden, while the lowest was observed in C. debaoensis from a natural habitat; this provides evidence that garden management, and to a minor extent, ex-situ conservation practice, could influence fungal endophyte communities. We further selected nineteen fungal isolates and screened for their antagonistic activities via a co-cultivation approach against the phytopathogens, Diaporthe sp. and Colletotrichum sp. Among these, five isolates with high ITS similarity matches with Hypoxylon vinosupulvinatum (GD019, 99.61%), Penicillium sp. (BD022, 100%), Penicillifer diparietisporus (GD008, 99.46%), Clonostachys rogersoniana (BF024, 99.46%) and C. rosea (BF011, 99.1%), which showed exceptional antagonistic activities against the phytopathogenic fungi with a significant inhibition rate of 70–80%. Taken together, our data presented the first and most comprehensive molecular work on culturable fungal endophytes associated with the coralloid roots of cycads. Our study also demonstrated that about 5% of fungal endophytes were not detected by the high-throughput sequencing approach, implying the equal importance of a culture-dependent approach to study fungal communities of cycads. We further highlighted the potential role of endemic and rare plants to discover and isolate unique plant-associated fungal taxa with excellent biocontrol properties.

Full-Length 16S rRNA and ITS Gene Sequencing Revealed Rich Microbial Flora in Roots of Cycas spp. in China.

Cycads have developed a complex root system categorized either as normal or coralloid roots. Past literatures revealed that a great diversity of key microbes is associated with these roots. This recent study aims to comprehensively determine the diversity and community structure of bacteria and fungi associated with the roots of two Cycas spp. endemic to China, Cycas debaoensis Zhong & Chen and Cycas fairylakea D.Y. Wang using high-throughput amplicon sequencing of the full-length 16S rRNA (V1-V9 hypervariable) and short fragment ITS region. The total DNA from 12 root samples were extracted, amplified, sequenced, and analyzed. Resulting sequences were clustered into 61 bacteria and 2128 fungal OTUs. Analysis of community structure revealed that the coralloid roots were dominated mostly by the nitrogen-fixer Nostocaceae but also contain other non-diazotrophic bacteria. The sequencing of entire 16S rRNA gene identified four different strains of cyanobacteria under the heterocystous genera Nostoc and Desmonostoc. Meanwhile, the top bacterial families in normal roots were Xanthobacteraceae, Burkholderiaceae, and Bacillaceae. Moreover, a diverse fungal community was also found in the roots of cycads and the predominating families were Ophiocordycipitaceae, Nectriaceae, Bionectriaceae, and Trichocomaceae. Our results demonstrated that bacterial diversity in normal roots of C. fairylakea is higher in richness and abundance than C. debaoensis. On the other hand, a slight difference, albeit insignificant, was noted for the diversity of fungi among root types and host species as the number of shared taxa is relatively high (67%). Our results suggested that diverse microbes are present in roots of cycads which potentially interact together to support cycads survival. Our study provided additional knowledge on the microbial diversity and composition in cycads and thus expanding our current knowledge on cycad-microbe association. Our study also considered the possible impact of ex situ conservation on cyanobiont community of cycads.

Highly diverse endophytes in roots of Cycas bifida (Cycadaceae), an ancient but endangered gymnosperm.

As an ancient seed plant, cycads are one of the few gymnosperms that develop a root symbiosis with cyanobacteria, which has allowed cycads to cope with harsh geologic and climatic conditions during the evolutionary process. However, the endophytic microbes in cycad roots remain poorly identified. In this study, using next-generation sequencing techniques, we investigated the microbial diversity and composition of both the coralloid and regular roots of Cycas bifida (Dyer) K.D. Hill. Highly diverse endophytic communities were observed in both the coralloid and regular roots. Of the associated bacteria, the top five families were the Nostocaceae, Sinobacteraceae, Bradyrhizobiaceae, Bacillaceae, and Hyphomicrobiaceae. The Nectriaceae, Trichocomaceae, and Incertae sedis were the predominant fungal families in all root samples. A significant difference in the endophytic bacterial community was detected between coralloid roots and regular roots, but no difference was observed between the fungal communities in the two root types. Cyanobacteria were more dominant in coralloid roots than in regular roots. The divergence of cycad root structures and the modified physiological processes may have contributed to the abundance of cyanobionts in coralloid roots. Consequently, the colonization of cyanobacteria inhibits the assemblage of other endophytes. Our results contribute to an understanding of the species diversity and composition of the cycad-endophyte microbiome and provide an abbreviated list of potential ecological roles of the core microbes present.

Rethinking cycad metabolite research.

Cycads are among the most ancient of extant Spermatophytes, and are known for their numerous pharmacologically active compounds. One compound in particular, β-methylamino-L-alanine (BMAA), has been implicated as the cause of amyotrophic lateral sclerosis/Parkinson dementia complex (ALS/PDC) on Guam. Previous studies allege that BMAA is produced exclusively by cyanobacteria, and is transferred to cycads through the symbiotic relationship between these cyanobacteria and the roots of cycads. We recently published data showing that Cycas micronesica seedlings grown without endophytic cyanobacteria do in fact increase in BMAA, invalidating the foundation of the BMAA hypothesis. We use this example to suggest that the frenzy centered on BMAA and other single putative toxins has hindered progress. The long list of cycad-specific compounds may have important roles in signaling or communication, but these possibilities have been neglected during decades of attempts to force single metabolites into a supposed anti-herbivory function. We propose that an unbiased, comprehensive approach may be a more appropriate means of proceeding with cycad biochemistry research.

Root contraction in Cycas and Zamia (Cycadales) determined by gelatinous fibers.

• Reaction wood (RW) in seed plants can induce late and usually secondary changes in organ orientation. Conifers produce compression wood (CW), generated by compression tracheids, which generate a push force. Angiosperms produce tension wood (TW), generated by tension wood fibers (TWF) often described as "gelatinous fibers," which exert a pull force. Usually RW is produced eccentrically, but it can occur concentrically, as in aerial roots of Ficus. However, gymnosperms can produce gelatinous fibers (tension fibers, TF), as in cortical and secondary phloem tissues (Gnetum). TFs are therefore limited neither to wood, xylem, nor angiosperms. Here we demonstrate that TFs in secondary phloem are involved in contraction of roots of cycads and compare them with TFs of Ficus.• We sectioned root material of cycads at various stages of seedling development using simple staining and histochemical procedures to follow the course of secondary phloem development. Aerial roots of Ficus were compared with the cycad root material.• Tension fibers (gelatinous fibers) occur extensively and continuously in the secondary phloem in roots that undergo contraction. Older tissues, but notably the xylem, become distorted by contraction. TFs in cycads correspond in cell wall features to TFs that occur in Ficus, but do not occur in secondary xylem. The individual fibers visibly contract.• Tissue contraction in Cycas and Zamia corresponds to that found in angiosperms and Gnetum and further broadens the scope of the activity of tension tissues. This finding possibly indicates that gelatinous fibers originated at a very early period of seed plant evolution.

Rethinking cycad metabolite research

Cycads are among the most ancient of extant Spermatophytes, and are known for their numerous pharmacologically active compounds. One compound in particular, β-methylamino-L-alanine (BMAA), has been implicated as the cause of amyotrophic lateral sclerosis/parkinson dementia complex (ALS/PDC) on Guam. Previous studies allege that BMAA is produced exclusively by cyanobacteria, and is transferred to cycads through the symbiotic relationship between these cyanobacteria and the roots of cycads. We recently published data showing that Cycas micronesica seedlings grown without endophytic cyanobacteria do in fact increase in BMAA, invalidating the foundation of the BMAA hypothesis. We use this example to suggest that the frenzy centered on BMAA and other single putative toxins has hindered progress. The long list of cycad-specific compounds may have important roles in signaling or communication, but these possibilities have been neglected during decades of attempts to force single metabolites into a supposed anti-herbivory function. We propose that an unbiased, comprehensive approach may be a more appropriate means of proceeding with cycad biochemistry research.

First record of two cyanoprokaryotes, Oscillatoria (Oscillatoriales) and Nostoc (Nostocales), endophytic within the angiosperm Alternanthera sessilis (Amaranthaceae) from India

Reports of endophytic or endosymbiotic cyanoprokaryotes in higher plants are not uncommon, but only a few genera are known to occur in such conditions. Of them Nostoc and Anabaena are most common. They are known in the roots of cycads and in the stem nodules of an angiosperm Gunnera. Calothrix and Aphanocapsa are the only other genera of cyanoprokaryotes, which have been reported to grow endosymbiotically in the roots of Encephalartos hildebrandtii and Cycas beddomei, respectively. There are, until now, no reports about an association of non-heterocystous members of cyanoprokaryotes and other plants. While working on the biodiversity of cyanoprokaryotes of Burdwan district of West Bengal an Oscillatoria species (O. obscura Bruhl et Biswas) and a Nostoc [N. punctiforme (KutZ.) hariot ex Born. et flah.] species were recovered in endophytic conditions occurring within the cortical cells of the angiosperm Alternanthera sessilis r. Br. (Family Amaranthaceae). It is interesting to note that this is also the first report of the occurrence of Nostoc sp. in an angiosperm other than Gunnera.

A novel cyanobacterial toxin (BMAA) with potential neurodegenerative effects

The non-protein amino acid beta-N-methyl-amino-L-alanine (BMAA) is a neurotoxin that was recently found to be produced by most cyanobacteria. The neurotoxin was discovered in 1967 in the seeds of the cycad Cycas micronesica, but this BMAA may originate from the symbiotic cyanobacterium Nostoc, which inhabits the roots of cycads. BMAA is thought to be the cause of the deadly neurodegenerative disease amyotrophic lateral sclerosis/parkinsonism dementia complex (ALS/PDC), common among the Chamorro people of Guam. It was demonstrated that the Chamorros, in all probability, have been exposed to high levels of BMAA through dietary consumption of flying foxes which fed mainly on cycads seeds. BMAA production may be a common conserved evolutionary feature among cyanobacteria and due to their wide global distribution, the toxin may be a common concern and potentially involved in provoking degenerative diseases worldwide. BMAA may likewise be bioaccumulated in other cyanobacterial based food webs within ecosystems outside Guam, and it is proposed that such webs may exist in the Baltic Sea, with its massive occurrence of cyanobacteria (blooms).

Production of the Neurotoxin BMAA by a Marine Cyanobacterium

Diverse species of cyanobacteria have recently been discovered to produce the neurotoxic non-protein amino acid beta-methylamino-L-alanine (BMAA). In Guam, BMAA has been studied as a possible environmental toxin in the diets of indigenous Chamorro people known to have high levels of Amyotrophic Lateral Sclerosis/ Parkinsonism Dementia Complex (ALS/PDC). BMAA has been found to accumulate in brain tissues of patients with progressive neurodegenerative illness in North America. In Guam, BMAA was found to be produced by endosymbiotic cyanobacteria of the genus Nostoc which live in specialized cycad roots. We here report detection of BMAA in laboratory cultures of a free-living marine species of Nostoc. We successfully detected BMAA in this marine species of Nostoc with five different methods: HPLC-FD, UPLC-UV, Amino Acid Analyzer, LC/MS, and Triple Quadrupole LC/MS/MS. This consensus of five different analytical methods unequivocally demonstrates the presence of BMAA in this marine cyanobacterium. Since protein-associated BMAA can accumulate in increasing levels within food chains, it is possible that biomagnification of BMAA could occur in marine ecosystems similar to the biomagnification of BMAA in terrestrial ecosystems. Production of BMAA by marine cyanobacteria may represent another route of human exposure to BMAA. Since BMAA at low concentrations causes the death of motor neurons, low levels of BMAA exposure may trigger motor neuron disease in genetically vulnerable individuals.

Sequence based data supports a single Nostoc strain in individual coralloid roots of cycads

The genetic diversity of cyanobacteria associated with cycads was examined using the tRNA Leu (UAA) intron as a genetic marker. Coralloid roots of both natural populations of the cycad Macrozamia riedlei (Fischer ex Gaudichaud-Beaupré) C.A. Gardner growing in Perth, Australia and cycads growing in greenhouses, also in Perth, were used and their respective cyanobionts analyzed. Several Nostoc strains were found to be involved in this symbiosis, both in natural populations and greenhouse-originated cycads. However, only one strain was present in individual coralloid roots and in individual plants, even when analyzing different coralloid roots from the same plant. Moreover, when examining plants growing close to each other (female plants and their respective offspring) the same cyanobacterium was consistently present in the different coralloid roots. Whether this reflects a selective mechanisms or merely the availability of Nostoc strains remains to be ascertained. The high cyanobacterial diversity in coralloid roots of cycads revealed by PCR fingerprinting is, therefore, contested. In this study, the potential problems of using different methods (e.g., PCR fingerprinting) to study the genetic diversity of symbiotic cyanobacteria, is also addressed.

Sequence based data supports single Nostoc strain in individual coralloid roots of cycads

The genetic diversity of cyanobacteria associated with cycads was examined using the tRNA Leu (UAA) intron as a genetic marker. Coralloid roots of both natural populations of the cycad Macrozamia riedlei (Fischer ex Gaudichaud-Beaupré) C.A. Gardner growing in Perth, Australia and cycads growing in greenhouses, also in Perth, were used and their respective cyanobionts analyzed. Several Nostoc strains were found to be involved in this symbiosis, both in natural populations and greenhouse-originated cycads. However, only one strain was present in individual coralloid roots and in individual plants, even when analyzing different coralloid roots from the same plant. Moreover, when examining plants growing close to each other (female plants and their respective offspring) the same cyanobacterium was consistently present in the different coralloid roots. Whether this reflects a selective mechanisms or merely the availability of Nostoc strains remains to be ascertained. The high cyanobacterial diversity in coralloid roots of cycads revealed by PCR fingerprinting is, therefore, contested. In this study, the potential problems of using different methods (e.g., PCR fingerprinting) to study the genetic diversity of symbiotic cyanobacteria, is also addressed.

Mycorrhizae Are Present in Cycad Roots

We describe the occurrence of arbuscular mycorrhizae in the roots ofZamia pumila andDioon edule. Seedlings were grown on native, unsterilized soil taken from local pinelands of south Florida, whereZ. pumila occurs naturally. Arbuscules, hyphae, hyphal coils, and vesicles occur in the parenchyma cells of the root cortex, especially the half of the cortex next to the stele. Hyphae of the arbuscular mycorrhizal fungi (AMF) occur mainly in longitudinal intercellular spaces and conform to theAcorus type. The finest, ultimate roots have AMF, but these roots are extremely brittle, detach with the slightest disturbance, and are usually lost when plants are uprooted from the ground. No AMF were found in the cortex of coralloid roots. Open image in new window Vovides (1991) previously reported that AMF occur onDioon edule and Ceratozamia mexicana, and we reconfirm this inD. edule. In this species, AMF appear to be mostly associated with the outer and to a lesser extent the inner cortex. However, roots of a potted plant of C.hildae growing in native soil lacked AMF. When grown on low phosphorus soils, legumes are known to require AMF in order for theirRhizobium nodules to fix nitrogen. Without AMF, the legumes are deficient in phosphorus, which inhibits nodule production and nitrogen fixation. It is probable that cycads, with their nitrogen-fixing coralloid roots containingNostoc, may also require AMF for successful nitrogen fixation when phosphorus is limiting.

89 Morphological and molecular characterisation of four nostoc species isolated from desert soils

Four unusual strains of Nostoc species from desert soils of the western United States were chosen for morphological and molecular characterization. The morphological characteristics of the desert Nostoc were compared with the species of Nostoc described in the literature. Although our strains were similar to previously described species such as N. punctiforme, N. paludosum, N. commune, N. microscopicum and N. muscorum, they did not fit any of these taxa well when all characters (including ecological preference) were used. The 16S rRNA gene sequences of the strains CM1‐VF10, CM1‐VF14, CNP‐AK1 and JT2‐VF2 were aligned with published Nostoc sequences from GenBank and other desert Nostoc sequenced at John Carroll University. Pseudanabaena, Cylindrospermum sp. and Anabaena spiroides from GenBank were used as outgroups. In addition, the ITS regions of all four desert taxa, a typical Nostoc commune from European temperate soils, Nostoc punctiforme PCC73102 from cycad roots were compared by examining secondary structure. The combined analysis provided compelling evidence that CM1VF10, CM1VF14, and JT2VF2 were species new to science. While CNP‐AK1 (the phycobiont of the desert soil lichen, Collema tenax) had some significant discontinuities from its closest relative, N. punctiforme, we consider the evidence currently insufficient to separate these two strains. The purported phycobiont of this lichen is N. commune, and we have demonstrated that this is incorrect. One of the desert strains (JT2‐VF2) likely represents a new genus. Morphological autapomorphies, distinctive SSU rRNA gene sequence, and differences in secondary structure of the 16S‐23S ITS provide congruent evidence for this conclusion.

The Structure of Cyanobacterial Communities Formed during the Degradation of Apogeotropic Roots of Cycads

The Structure of Cyanobacterial Communities Formed during the Degradation of Apogeotropic Roots of Cycads

Microbial Complexes Occurring on the Apogeotropic Roots and in the Rhizosphere of Cycad Plants

The microbial complexes of soil, the rhizosphere, and the rhizoplane of the apogeotropic (coralloid) roots of cycad plants were comparatively studied. The aseptically prepared homogenates of the surface-sterilized coralloid roots did not contain bacterial microsymbiont, indicating that in the root tissues the symbiosis is a two-component one (plant–cyanobacteria). At the same time, associated bacteria belonging to different taxonomic groups were detected in increasing amounts in the cycad rhizoplane, rhizosphere, and the surrounding soil. The bacterial communities found in the cycad rhizoplane and the surrounding soil were dominated by bacteria from the genus Bacillus. The saprotrophic bacteria and fungi colonizing the cycad rhizosphere and rhizoplane were dominated by microorganisms capable of degrading the plant cell walls. The local degradation of the cell wall was actually observed on the micrographs of the thin sections of cycad roots in the form of channels through which symbiotic cyanobacterial filaments can penetrate into the cortical parenchyma.

High cyanobacterial diversity in coralloid roots of cycads revealed by PCR fingerprinting

The genetic diversity of symbiotic cyanobacteria in coralloid roots of cycads was examined using PCR fingerprinting with primers derived from repetitive sequences. The highest genetic resolution was achieved using the primer corresponding to the short tandemly repeated repetitive sequences. The cyanobacteria were collected from the coralloid roots of a large number of indigenous cycad plants and used directly in the PCR. It could be demonstrated that numerous cyanobacteria were present in a single coralloid root. Even within a single cluster in the coralloid root, diversity could be observed between the apical, middle and basal regions.

High cyanobacterial diversity in coralloid roots of cycads revealed by PCR fingerprinting

The genetic diversity of symbiotic cyanobacteria in coralloid roots of cycads was examined using PCR fingerprinting with primers derived from repetitive sequences. The highest genetic resolution was achieved using the primer corresponding to the short tandemly repeated repetitive sequences. The cyanobacteria were collected from the coralloid roots of a large number of indigenous cycad plants and used directly in the PCR. It could be demonstrated that numerous cyanobacteria were present in a single coralloid root. Even within a single cluster in the coralloid root, diversity could be observed between the apical, middle and basal regions.

Diversity and Specificity in Cyanobacterial Symbioses

DIVERSITY AND SPECIFICITY IN CYANOBACTERIAL SYMBIOSES U. Rasmussen and C. Johansson U.Rasmussen (corresponding author; e-mailullarasmussen@ botan su se) and C Johansson, Department ofBotany,Stockholm University, 106 91 Stockholm, Sweden INTRODUCTION Cyanobacteria constitute one of the largest sub groups of Gram-negative photosynthetic prokary otes and are of great evolutionary antiquity, dating back to the Precambrian (2.8-3.5 billion years ago). They are found worldwide in highly diverse ecosystems, from aquatic (limnic and marine) habi tats to terrestrial systems and from polar to tropical regions of the globe. In terrestrial systems, nitro gen-fixing Nostoc isby far themost common genus and includes cyanobacteria capable of forming symbioses with a broad range of plants and other organisms. Hosts include fungi (Geosiphon species and lichenised fungi), bryophytes (liverworts and hornworts), aquatic ferns (genus Azolla), gym nosperms (cycads) and angiosperms (genus Gunnera) (Bergman et al. 1996). CELL DIFFERENTIATION IN NOSTOC The key to the success of the genus Nostoc, whether in the free-living form or in symbiosis, is most likely its extreme structural and functional plasticity. Influenced by environmental changes, Nostoc undergoes differentiation into several differ ent cell and filament types (P1. I).When grown under nitrogen-limited conditions or in symbiosis, vegetative cells differentiate to form specialised nitrogen-fixing heterocysts at a frequency of ap proximately 5% in the free-living form and up to 80% in symbiosis with hosts such as Gunnera spe cies. In addition, the entire vegetative filament can differentiate into amotile stage, the hormogonium, which serves as a means of dispersal under unfa vourable conditions and constitutes the 'infective unit' during establishment of the symbiosis. Finally, akinetes are differentiated under adverse conditions and function as spores. Akinete differentiation is an unusual stage in symbiotic associations and is known to play a role only in Azolla, in which akinetes function as cyanobacterial inocula for the next sporophyte generation. In addition, the cyanobacteria must change their metabolism from that of a free-living, phototrophic organism to that of a symbiotic, heterotrophic organism in order to be symbiotically competent. The mechanisms in volved in this shift are largely unknown. SPECIFICITY The specificity of the cyanobacterium-host inter action has been investigated by isolating and grow ing the two symbiotic partners separately and then reconstituting the symbiotic association under lab oratory conditions. Nostoc strains isolated from a cycad, the bryophyte Anthoceros and a lichen all infect Gunnera. Similarly, strains isolated from cycads, Gunnera and a lichen infect Anthoceros species. Hence, Nostoc in general exhibits a very low host specificity, with no correlation to the taxonomic status of the host. Furthermore, the host has a low specificity for its cyanobiont. Even certain strains of Calothrix and Chlorogloeopsis can infect both the homwort Phaeoceros and the liverwort Blasia (West and Adams 1997). Moreover, the cyanobiont shows great adap tive flexibility because the same strain has the capacity to infect different host organs. The cyanobacteria are found extracellularly among the fungal hyphae in lichens, in the cavities of bryophyte thalli and Azolla leaves and in the spe cialised roots of cycads, whereas they are found intracellularly in the fungus Geosiphon and in the stem glands of Gunnera. Taken together, these facts indicate that the host is the (more) selective part ner. Cyanobacteria are unique among symbiotic bacteria in their ability to infect a broad range of hosts and organs. GENETIC DIVERSITY The extreme morphological flexibility of Nostoc makes identification and taxonomy based onmorph ology problematic and unreliable. In the recent literature, many symbiotic isolates (from Gunnera, cycads, bryophytes and the fungus Geosiphon) have been referred to as N. punctiforme, but a pheno type-based characterisation has proved inadequate. No genetic analysis has yet been used to identify the taxonomic status of different symbiotic isolates. At present, the taxonomic interrelatedness of the symbiotic Nostoc strains is largely unknown and does not extend below the genus level. More precise genetic information must therefore be ob tained to establish a more reliable classification system. In recent years, molecular methods have contrbuted to amore detailed classification of the genus Nostoc (Table 1). BIOLOGY AND ENVIRONMENT PPOCEENDINGS OF THE ROYAL IRISHACADEMY, VOL 102B, No 1, 53-56 (2002) 0) ROYAL IRISHACAsEMY 53 BIOLOGY AND ENVIRONMENT P1...