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

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.

Biosynthesis and differential spatial distribution of the 3-deoxyanthocyanidins apigenidin and luteolinidin at the interface of a plant-cyanobacteria symbiosis exposed to cold

Aquatic ferns of the genus Azolla (Azolla) form highly productive symbioses with filamentous cyanobacteria fixing N2 in their leaf cavities, Nostoc azollae. Stressed symbioses characteristically turn red due to 3-deoxyanthocyanidin (DA) accumulation, rare in angiosperms and of unknown function. To understand DA accumulation upon cold acclimation and recovery, we integrated laser-desorption-ionization mass-spectrometry-imaging (LDI-MSI), a new Azolla filiculoides genome-assembly and annotation, and dual RNA-sequencing into phenotypic analyses of the symbioses. Azolla sp. Anzali recovered even when cold-induced DA-accumulation was inhibited by abscisic acid. Cyanobacterial filaments generally disappeared upon cold acclimation and Nostoc azollae transcript profiles were unlike those of resting stages formed in cold-resistant sporocarps, yet filaments re-appeared in leaf cavities of newly formed green fronds upon cold-recovery. The high transcript accumulation upon cold acclimation of AfDFR1 encoding a flavanone 4-reductase active in vitro suggested that the enzyme of the first step in the DA-pathway may regulate accumulation of DAs in different tissues. However, LDI-MSI highlighted the necessity to describe metabolite accumulation beyond class assignments as individual DA and caffeoylquinic acid metabolites accumulated differentially. For example, luteolinidin accumulated in epithelial cells, including those lining the leaf cavity, supporting a role for the former in the symbiotic interaction during cold acclimation.

Species-specific responses of marine bacteria to environmental perturbation

Environmental perturbations shape the structure and function of microbial communities. Oil spills are a major perturbation and resolving spills often requires active measures like dispersant application that can exacerbate the initial disturbance. Species-specific responses of microorganisms to oil and dispersant exposure during such perturbations remain largely unknown. We merged metatranscriptomic libraries with pangenomes to generate Core-Accessory Metatranscriptomes (CA-Metatranscriptomes) for two microbial hydrocarbon degraders that played important roles in the aftermath of the Deepwater Horizon oil spill. The Colwellia CA-Metatranscriptome illustrated pronounced dispersant-driven acceleration of core (~41%) and accessory gene (~59%) transcription, suggesting an opportunistic strategy. Marinobacter responded to oil exposure by expressing mainly accessory genes (~93%), suggesting an effective hydrocarbon-degrading lifestyle. The CA-Metatranscriptome approach offers a robust way to identify the underlying mechanisms of key microbial functions and highlights differences of specialist-vs-opportunistic responses to environmental disturbance.

Phenanthrene stress response and phytoremediation potential of free-floating fern Azolla filiculoides Lam

In this study, the potential of Azolla filiculoides, a freshwater fern species, on phenanthrene phytoremediation and biodegradation was investigated. Furthermore, the effect of phenanthrene on growth performance, photosynthetic activity and biosynthesis, and accumulation of secondary metabolites of A. filiculodes were evaluated. Plants were grown in a nitrogen-free Hoagland and exposed to different phenanthrene concentrations (0, 1, 5, and 10 mg/L). Exposure to 10 mg/L phenanthrene caused a significant reduction (42%) in Azolla filiculoides growth compared to control on day 14. The photosynthetic pigment content of A. filiculoides treated with 1 and 5 mg/L was almost the same as the control, while 10 mg/L phenanthrene was significantly reduced. In comparison to unplanted controls, the biodegradation percentages obtained from the planted growth medium were found to be 88, 69, and 60%, respectively, for the application of 1, 5, and 10 mg/L phenanthrene. Data on plant growth, photosynthetic pigments, secondary metabolite contents, and biodegradation percentages indicated the tolerance level and the effective phytoremediation potential of A. filiculoides for phenanthrene was <10 mg/L. The results indicated that A. filiculoides is highly effective in phytoremediation of low concentrations of phenanthrene pollution in a short time.

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.