Is there foul play in the leaf pocket? The metagenome of floating fern Azolla reveals endophytes that do not fix N2 but may denitrify

Unravelling the Activity and Presence of N2O Reducers on Urban Greening Tree Leaves

Nitrous oxide (N2O) is a potent greenhouse gas and can be biotically emitted from soils, water, and the less recognised plant leaves. Leaves can produce N2O and may host N2O-reducing microbes that use it as a respiratory substrate, potentially mitigating climate warming. This study examines the ecophysiology of N2O reducers in the plant phyllosphere. Anoxic microcosm experiments, quantification of N2O reduction kinetics, and analysis of nosZ gene governing N2O reduction were conducted to assess the activity and presence of N2O reducers in leaf epiphytes from various canopy positions of Photinia fraseri, an urban greenery plant. Results revealed canopy position-dependent N2O reduction activity in the leaf microbiota. We identified previously unrecognised atypical Clade II nosZ gene in the phyllosphere microbiome, with its absolute abundance positively correlated with N2O reduction activity, highlighting its significance in this process. Sequencing of bacterial and archaeal 16S rRNA genes revealed keystone taxa as primary drivers of N2O reduction activity. These findings underscore the functional potential for N2O reduction and the presence of the Clade II nosZ group within epiphytic microbes. This work provides insights into the ecophysiology of epiphytic N2O reducers and underpins the development of leaf-based microbial solutions for N2O mitigation under future warming.

The nitrogen-fixing fern Azolla has a complex microbiome characterized by varying degrees of cophylogenetic signal

PREMISE

Azolla is a genus of floating ferns that has closely evolved with a vertically transmitted obligate cyanobacterium endosymbiont-Anabaena azollae-that fixes nitrogen. There are also other lesser-known Azolla symbionts whose role and mode of transmission are unknown.

METHODS

We sequenced 112 Azolla specimens collected across the state of California and characterized their metagenomes to identify the common bacterial endosymbionts and assess their patterns of interaction.

RESULTS

Four genera were found across all samples, establishing that multiple Azolla endosymbionts were consistently present. We found varying degrees of cophylogenetic signal across these taxa as well as varying degrees of isolation by distance and of pseudogenation, which demonstrates that multiple processes underlie how this endosymbiotic community is constituted. We also characterized the entire Azolla leaf pocket microbiome.

CONCLUSIONS

These results show that the Azolla symbiotic community is complex and features members at potentially different stages of symbiosis evolution, further supporting the utility of the Azolla microcosm as a system for studying the evolution of symbioses.

Non-pathogenic microbiome associated to aquatic plants and anthropogenic impacts on this interaction

The microbiota associated with aquatic plants plays a crucial role in promoting plant growth and development. The structure of the plant microbiome is shaped by intricate interactions among hosts, microbes, and environmental factors. Consequently, anthropogenic pressures that disrupt these interactions can indirectly impact the ecosystem services provided by aquatic plants, such as CO2 fixation, provision of food resources, shelter to animals, nutrient cycling, and water purification. Presently, studies on plant-microbiota interactions primarily focus on terrestrial hosts and overlook aquatic environments with their unique microbiomes. Therefore, there is a pressing need for a comprehensive understanding of plant microbiomes in aquatic ecosystems. This review delves into the overall composition of the microbiota associated with aquatic plant, with a particular emphasis on bacterial communities, which have been more extensively studied. Subsequently, the functions provided by the microbiota to their aquatic plants hosts are explored, including the acquisition and mobilization of nutrients, production of auxin and related compounds, enhancement of photosynthesis, and protection against biotic and abiotic stresses. Additionally, the influence of anthropogenic stressors, such as climate change and aquatic contamination, on the interaction between microbiota and aquatic plants is discussed. Finally, knowledge gaps are highlighted and future directions in this field are suggested.

The crane fly glycosylated triketide δ-lactone cornicinine elicits akinete differentiation of the cyanobiont in aquatic Azolla fern symbioses

The restriction of plant-symbiont dinitrogen fixation by an insect semiochemical had not been previously described. Here we report on a glycosylated triketide δ-lactone from Nephrotoma cornicina crane flies, cornicinine, that causes chlorosis in the floating-fern symbioses from the genus Azolla. Only the glycosylated trans-A form of chemically synthesized cornicinine was active: 500 nM cornicinine in the growth medium turned all cyanobacterial filaments from Nostoc azollae inside the host leaf-cavities into akinetes typically secreting CTB-bacteriocins. Cornicinine further inhibited akinete germination in Azolla sporelings, precluding re-establishment of the symbiosis during sexual reproduction. It did not impact development of the plant Arabidopsis thaliana or several free-living cyanobacteria from the genera Anabaena or Nostoc but affected the fern host without cyanobiont. Fern-host mRNA sequencing from isolated leaf cavities confirmed high NH4-assimilation and proanthocyanidin biosynthesis in this trichome-rich tissue. After cornicinine treatment, it revealed activation of Cullin-RING ubiquitin-ligase-pathways, known to mediate metabolite signaling and plant elicitation consistent with the chlorosis phenotype, and increased JA-oxidase, sulfate transport and exosome formation. The work begins to uncover molecular mechanisms of cyanobiont differentiation in a seed-free plant symbiosis important for wetland ecology or circular crop-production today, that once caused massive CO2 draw-down during the Eocene geological past.

Genome Engineering by RNA-Guided Transposition for Anabaena sp. PCC 7120

In genome engineering, the integration of incoming DNA has been dependent on enzymes produced by dividing cells, which has been a bottleneck toward increasing DNA insertion frequencies and accuracy. Recently, RNA-guided transposition with CRISPR-associated transposase (CAST) was reported as highly effective and specific in Escherichia coli. Here, we developed Golden Gate vectors to test CAST in filamentous cyanobacteria and to show that it is effective in Anabaena sp. strain PCC 7120. The comparatively large plasmids containing CAST and the engineered transposon were successfully transferred into Anabaena via conjugation using either suicide or replicative plasmids. Single guide (sg) RNA encoding the leading but not the reverse complement strand of the target were effective with the protospacer-associated motif (PAM) sequence included in the sgRNA. In four out of six cases analyzed over two distinct target loci, the insertion site was exactly 63 bases after the PAM. CAST on a replicating plasmid was toxic, which could be used to cure the plasmid. In all six cases analyzed, only the transposon cargo defined by the sequence ranging from left and right elements was inserted at the target loci; therefore, RNA-guided transposition resulted from cut and paste. No endogenous transposons were remobilized by exposure to CAST enzymes. This work is foundational for genome editing by RNA-guided transposition in filamentous cyanobacteria, whether in culture or in complex communities.

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.

Comparative analysis of the endophytic bacteria inhabiting the phyllosphere of aquatic fern Azolla species by high-throughput sequencing

BACKGROUND

Azolla is a small floating fern living in symbiosis with nitrogen-fixing cyanobacteria and provides a variety of important ecosystem benefits. Previous studies have presented that Azolla harbors diverse bacteria that may play a key role in host fitness and productivity. However, the characteristics of endophytic bacteria inhabiting the phyllosphere of different species of Azolla have not yet been fully understood.

RESULTS

In this study, the 16S ribosomal DNA (rDNA) V5-V7 region of bacteria was determined by Illumina high-throughput sequencing platform to study the diversity and richness of endophytic bacterial communities in the phyllosphere of five Azolla species collected from different countries. A total of 1150 operational taxonomic units (OTUs) were detected for the endophytic bacteria community. According to the α diversity indices, the diversity of bacteria was ordered as Azolla imbricata > A. pinnata > A. filiculoides > A. mexicana > A. caroliniana. The PCoA results displayed that the bacterial communities of A. mexicana and A. caroliniana shared the highest similarity, followed by the similarity between A. pinnata and A. imbricata, and they were significantly distinct from the community of A. filiculoides. The dominant bacteria of Azolla mainly belonged to the phylum of Proteobacteria, followed by Actinobacteria, Chlorobillobacteria, and Firmicutes. In detail, the relative abundance of Proteobacteria in A. imbricata was 52.23%, whereas it was more than 80.00% in the other four species of Azolla. Notably, Herbaspirillum (45.91%, 44.08%) and Methylophilus (29.97%, 37.96%) were the main genera inhabiting A. mexicana and A. caroliniana respectively. Ferrovibrio (18.54%) and Rhizobium (16.68%) were the dominant genera inhabiting A. filiculoides. The group of unidentified genera (41.63%, 44.92%) consisted most of the bacteria in A. imbricata and A. pinnata respectively. Further analysis suggested that the significant different bacteria identified in LDA Effect Size analysis existed Azolla species-specific patterns.

CONCLUSIONS

In summary, all results suggested that the diversity and composition of the endophytic bacterial communities were different in Azolla species.

Evolutionary genomic insights into cyanobacterial symbioses in plants

Photosynthesis, the ability to fix atmospheric carbon dioxide, was acquired by eukaryotes through symbiosis: the plastids of plants and algae resulted from a cyanobacterial symbiosis that commenced more than 1.5 billion years ago and has chartered a unique evolutionary path. This resulted in the evolutionary origin of plants and algae. Some extant land plants have recruited additional biochemical aid from symbiotic cyanobacteria; these plants associate with filamentous cyanobacteria that fix atmospheric nitrogen. Examples of such interactions can be found in select species from across all major lineages of land plants. The recent rise in genomic and transcriptomic data has provided new insights into the molecular foundation of these interactions. Furthermore, the hornwort Anthoceros has emerged as a model system for the molecular biology of cyanobacteria-plant interactions. Here, we review these developments driven by high-throughput data and pinpoint their power to yield general patterns across these diverse symbioses.

Bacterial Microbiome in the Phyllo-Endosphere of Highly Specialized Rock Spleenwort

Bacteria communities associated with plants have been given increasing consideration because they are arguably beneficial to their host plants. To understand the ecological and evolutionary impact of these mutualistic associations, it is important to explore the vast unknown territory of bacterial genomic diversity and their functional contributions associated with the major branches of the tree-of-life. Arguably, this aim can be achieved by profiling bacterial communities by applying high throughput sequencing approaches, besides establishing model plant organisms to test key predictions. This study utilized the Illumina Miseq reads of bacterial 16S rRNA sequences to determine the bacterial diversity associated with the endosphere of the leaves of the highly specialized rock spleenwort Asplenium delavayi (Aspleniaceae). By documenting the bacterial communities associated with ferns collected in natural occurrence and cultivation, this study discovered the most species-rich bacterial communities associated with terrestrial ferns reported until now. Despite the substantial variations of species diversity and composition among accessions, a set of 28 bacterial OTUs was found to be shared among all accessions. Functional analyses recovered evidence to support the predictions that changes in bacterial community compositions correspond to functional differentiation. Given the ease of cultivating this species, Asplenium delavayi is introduced here as a model organism to explore the ecological and evolutionary benefits created by mutualistic associations between bacteria and ferns.

Exposure to different light intensities affects emission of volatiles and accumulations of both pigments and phenolics in Azolla filiculoides

Many agronomic trials demonstrated the nitrogen-fixing ability of the ferns Azolla spp. and its obligate cyanobiont Trichormus azollae. In this study, we have screened the emission of volatile organic compounds (VOCs) and analyzed pigments (chlorophylls, carotenoids) as well as phenolic compounds in Azolla filiculoides-T. azollae symbionts exposed to different light intensities. Our results revealed VOC emission mainly comprising isoprene and methanol (~82% and ~13% of the overall blend, respectively). In particular, by dissecting VOC emission from A. filiculoides and T. azollae, we found that the cyanobacterium does not emit isoprene, whereas it relevantly contributes to the methanol flux. Enhanced isoprene emission capacity (15.95 ± 2.95 nmol m^-2  s^-1 ), along with increased content of both phenolic compounds and carotenoids, was measured in A. filiculoides grown for long-term under high (700 μmol m^-2  s^-1 ) rather than medium (400 μmol m^-2  s^-1 ) and low (100 μmol m^-2  s^-1 ) light intensity. Moreover, light-responses of chlorophyll fluorescence demonstrated that A. filiculoides was able to acclimate to high growth light. However, exposure of A. filiculoides from low (100 μmol m^-2  s^-1 ) to very high light (1000 μmol m^-2  s^-1 ) did not affect, in the short term, photosynthesis, but slightly decreased isoprene emission and leaf pigment content whereas, at the same time, dramatically raised the accumulation of phenolic compounds (i.e. deoxyanthocyanidins and phlobaphenes). Our results highlight a coordinated photoprotection mechanism consisting of isoprene emission and phenolic compounds accumulation employed by A. filiculoides to cope with increasing light intensities.

The diversity and community structure of symbiotic cyanobacteria in hornworts inferred from long-read amplicon sequencing

PREMISE

Nitrogen-fixing endosymbioses with cyanobacteria have evolved independently in five very different plant lineages. Expanding knowledge of these symbioses promises to improve the understanding of symbiosis evolution and broaden the toolkit for agricultural engineering to reduce artificial fertilizer use. Here we focused on hornworts, a bryophyte lineage in which all members host cyanobacteria, and investigated factors shaping the diversity of their cyanobiont communities.

METHODS

We sampled hornworts and adjacent soils in upstate New York throughout the hornwort growing season. We included all three sympatric hornwort species in the area, allowing us to directly compare partner selectivity. To profile cyanobacteria communities, we established a metabarcoding protocol targeting rbcL-X with PacBio long reads.

RESULTS

The hornwort cyanobionts detected were phylogenetically diverse, including clades that do not contain other known plant symbionts. We found significant overlap between hornwort cyanobionts and soil cyanobacteria, a pattern not previously reported in other plant-cyanobacteria symbioses. Cyanobiont communities differed between host plants only centimeters apart, but we did not detect an effect of sampling time or host species on the cyanobacterial community structure.

CONCLUSIONS

This study expands the phylogenetic diversity of known symbiotic cyanobacteria. Our analyses suggest that hornwort cyanobionts have a tight connection to the soil background, and we found no evidence that time within growing season, host species, or distance at the scale of meters strongly govern cyanobacteria community assembly. This study provides a critical foundation for further study of the ecology, evolution, and interaction dynamics of plant-cyanobacteria symbiosis.

Comparative genomic insights into culturable symbiotic cyanobacteria from the water fern Azolla

Species of the floating, freshwater fern Azolla form a well-characterized symbiotic association with the non-culturable cyanobacterium Nostoc azollae, which fixes nitrogen for the plant. However, several cyanobacterial strains have over the years been isolated and cultured from Azolla from all over the world. The genomes of 10 of these strains were sequenced and compared with each other, with other symbiotic cyanobacterial strains, and with similar strains that were not isolated from a symbiotic association. The 10 strains fell into three distinct groups: six strains were nearly identical to the non-symbiotic strain, Nostoc (Anabaena) variabilis ATCC 29413; three were similar to the symbiotic strain, Nostoc punctiforme, and one, Nostoc sp. 2RC, was most similar to non-symbiotic strains of Nostoc linckia. However, Nostoc sp. 2RC was unusual because it has three sets of nitrogenase genes; it has complete gene clusters for two distinct Mo-nitrogenases and an alternative V-nitrogenase. Genes for Mo-nitrogenase, sugar transport, chemotaxis and pili characterized all the symbiotic strains. Several of the strains infected the liverwort Blasia, including N. variabilis ATCC 29413, which did not originate from Azolla but rather from a sewage pond. However, only Nostoc sp. 2RC, which produced highly motile hormogonia, was capable of high-frequency infection of Blasia. Thus, some of these strains, which grow readily in the laboratory, may be useful in establishing novel symbiotic associations with other plants.

Azolla ferns testify: seed plants and ferns share a common ancestor for leucoanthocyanidin reductase enzymes

Questions about in vivo substrates for proanthocyanidin (PA) biosynthesis and condensation have not been resolved and wide gaps in the understanding of transport and biogenesis in 'tannosomes' persist. Here we examined the evolution of PA biosynthesis in ferns not previously reported, asking what PAs are synthesised and how. Chemical and gene-expression analyses were combined to characterise PA biosynthesis, leveraging genome annotation from the floating fern Azolla filiculoides. In vitro assay and phylogenomics of PIP-dehydrogenases served to infer the evolution of leucoanthocyanidin reductase (LAR). Sporophyte-synthesised (epi)catechin polymers, averaging only seven subunits, accumulated to 5.3% in A. filiculoides, and 8% in A. pinnata biomass dry weight. Consistently, a LAR active in vitro was highly expressed in A. filiculoides. LAR, and paralogous fern WLAR-enzymes with differing substrate binding sites, represent an evolutionary innovation of the common ancestor of fern and seed plants. The specific ecological niche of Azolla ferns, a floating plant-microbe mat massively fixing CO₂ and N₂ , shaped their metabolism in which PA biosynthesis predominates and employs novel fern LAR enzymes. Characterisation of in vivo substrates of these LAR, will help to shed light on the recently assigned and surprising dual catalysis of LAR from seed plants.

Far-Red Light-Induced Azolla filiculoides Symbiosis Sexual Reproduction: Responsive Transcripts of Symbiont Nostoc azollae Encode Transporters Whilst Those of the Fern Relate to the Angiosperm Floral Transition

Water ferns of the genus Azolla and the filamentous cyanobacteria Nostoc azollae constitute a model symbiosis that enabled the colonization of the water surface with traits highly desirable for the development of more sustainable crops: their floating mats capture CO₂ and fix N₂ at high rates using light energy. Their mode of sexual reproduction is heterosporous. The regulation of the transition from the vegetative phase to the spore forming phase in ferns is largely unknown, yet a prerequisite for Azolla domestication, and of particular interest as ferns represent the sister lineage of seed plants. Sporocarps induced with far red light could be crossed so as to verify species attribution of strains from the Netherlands but not of the strain from the Anzali lagoon in Iran; the latter strain was assigned to a novel species cluster from South America. Red-dominated light suppresses the formation of dissemination stages in both gametophyte- and sporophyte-dominated lineages of plants, the response likely is a convergent ecological strategy to open fields. FR-responsive transcripts included those from MIKC^C homologues of CMADS1 and miR319-controlled GAMYB transcription factors in the fern, transporters in N. azollae, and ycf2 in chloroplasts. Loci of conserved microRNA (miRNA) in the fern lineage included miR172, yet FR only induced miR529 and miR535, and reduced miR319 and miR159. Phylogenomic analyses of MIKC^C TFs suggested that the control of flowering and flower organ specification may have originated from the diploid to haploid phase transition in the homosporous common ancestor of ferns and seed plants.

Azolla filiculoides L. as a source of metal-tolerant microorganisms

The metal hyperaccumulator Azolla filiculoides is accompanied by a microbiome potentially supporting plant during exposition to heavy metals. We hypothesized that the microbiome exposition to selected heavy metals will reveal metal tolerant strains. We used Next Generation Sequencing technique to identify possible metal tolerant strains isolated from the metal-treated plant (Pb, Cd, Cr(VI), Ni, Au, Ag). The main dominants were Cyanobacteria and Proteobacteria constituting together more than 97% of all reads. Metal treatment led to changes in the composition of the microbiome and showed significantly higher richness in the Pb-, Cd- and Cr-treated plant in comparison with other (95–105 versus 36–44). In these treatments the share of subdominant Actinobacteria (0.4–0.8%), Firmicutes (0.5–0.9%) and Bacteroidetes (0.2–0.9%) were higher than in non-treated plant (respectively: 0.02, 0.2 and 0.001%) and Ni-, Au- and Ag-treatments (respectively: <0.4%, <0.2% and up to 0.2%). The exception was Au-treatment displaying the abundance 1.86% of Bacteroidetes. In addition, possible metal tolerant genera, namely: Acinetobacter, Asticcacaulis, Anabaena, Bacillus, Brevundimonas, Burkholderia, Dyella, Methyloversatilis, Rhizobium and Staphylococcus, which form the core microbiome, were recognized by combining their abundance in all samples with literature data. Additionally, the presence of known metal tolerant genera was confirmed: Mucilaginibacter, Pseudomonas, Mycobacterium, Corynebacterium, Stenotrophomonas, Clostridium, Micrococcus, Achromobacter, Geobacter, Flavobacterium, Arthrobacter and Delftia. We have evidenced that A. filiculoides possess a microbiome whose representatives belong to metal-resistant species which makes the fern the source of biotechnologically useful microorganisms for remediation processes.

The Study on the Cultivable Microbiome of the Aquatic Fern Azolla Filiculoides L. as New Source of Beneficial Microorganisms

The aim of the study was to determine the still not completely described microbiome associated with the aquatic fern Azolla filiculoides. During the experiment, 58 microbial isolates (43 epiphytes and 15 endophytes) with different morphologies were obtained. We successfully identified 85% of microorganisms and assigned them to 9 bacterial genera: Achromobacter, Bacillus, Microbacterium, Delftia, Agrobacterium, and Alcaligenes (epiphytes) as well as Bacillus, Staphylococcus, Micrococcus, and Acinetobacter (endophytes). We also studied an A. filiculoides cyanobiont originally classified as Anabaena azollae; however, the analysis of its morphological traits suggests that this should be renamed as Trichormus azollae. Finally, the potential of the representatives of the identified microbial genera to synthesize plant growth-promoting substances such as indole-3-acetic acid (IAA), cellulase and protease enzymes, siderophores and phosphorus (P) and their potential of utilization thereof were checked. Delftia sp. AzoEpi7 was the only one from all the identified genera exhibiting the ability to synthesize all the studied growth promoters; thus, it was recommended as the most beneficial bacteria in the studied microbiome. The other three potentially advantageous isolates (Micrococcus sp. AzoEndo14, Agrobacterium sp. AzoEpi25 and Bacillus sp. AzoEndo3) displayed 5 parameters: IAA (excluding Bacillus sp. AzoEndo3), cellulase, protease, siderophores (excluding Micrococcus sp. AzoEndo14), as well as mineralization and solubilization of P (excluding Agrobacterium sp. AzoEpi25).

Cyanobiont diversity in six Azolla spp. and relation to Azolla-nutrient profiling

Illumina-Miseq^®-based cyanobiont diversity and biomass were analyzed in six Azolla spp. Results revealed that 93-98% of total operational taxonomic units (OTUs) belong to Nostacaceae followed by Cylindrospermopsis with about 1-6% OTUs. The taxonomy of Azolla-cyanobiont is a long-term debate within the scientific community. Morphological and biochemical-based reports indicated the presence of Anabaena, Nostoc and/or Trichormus azollae as abundant Azolla-cyanobionts, however, molecular data did not support the abundance of Anabaena and/or Nostoc. To understand furthermore, the cyanobiont diversity in six species of Azolla (A. microphylla, A. mexicana, A. filiculoides, A. caroliniana, A. pinnata and A. rubra) was analyzed based on 16S rRNA Illumina-MiSeq sequencing. Additionally, biomass and nutrient profiling of Azolla spp. were analyzed and correlated with cyanobiont diversity. Illumina-MiSeq data revealed that 99.6-99.9% of total operational taxonomic units (OTUs) belonged to Nostocophycideae (class), Nostocales (order) and Nostacaceae (family). At genus level, the unassigned affiliation (93.4-97.9%) under Nostacaceae family was abundant followed by Cylindrospermopsis OTUs (1.1-6.0%). Interestingly, A. pinnata harboured maximum Cylindrospermopsis OTUs and also recorded higher biomass (40.67 g m^-2 day^-1), whereas crude protein (25.9%) and antioxidants (76.9%) were recorded to be higher in A. microphylla. Biplot analysis revealed that A. pinnata and its cyanobiont abundance were positively correlated with neutral and acid detergent fibers. Overall, the present findings deepened the understanding about cyanobiont in Azolla and its relations with Azolla nutrient profiling.

Fern genomes elucidate land plant evolution and cyanobacterial symbioses

Ferns are the closest sister group to all seed plants, yet little is known about their genomes other than that they are generally colossal. Here, we report on the genomes of Azolla filiculoides and Salvinia cucullata (Salviniales) and present evidence for episodic whole-genome duplication in ferns-one at the base of 'core leptosporangiates' and one specific to Azolla. One fern-specific gene that we identified, recently shown to confer high insect resistance, seems to have been derived from bacteria through horizontal gene transfer. Azolla coexists in a unique symbiosis with N2-fixing cyanobacteria, and we demonstrate a clear pattern of cospeciation between the two partners. Furthermore, the Azolla genome lacks genes that are common to arbuscular mycorrhizal and root nodule symbioses, and we identify several putative transporter genes specific to Azolla-cyanobacterial symbiosis. These genomic resources will help in exploring the biotechnological potential of Azolla and address fundamental questions in the evolution of plant life.