Symbiotic Interactions of Frankia and Actinorhizal Plants

Actinorhizal plants, a diverse assemblage of dicotyledonous species, engage in a mutualistic partnership with filamentous actinobacteria of the genus Frankia. In this association, Frankia infects plant roots and induces the formation of specialised nodules that house nitrogen‐fixing vesicles. Within these compartments, atmospheric dinitrogen is converted into bioavailable ammonium, enhancing host growth in nutrient‐poor environments. The interaction is governed by a complex molecular dialogue: Frankia expresses nodulation genes analogous to those of rhizobia, producing signalling molecules that trigger root hair deformation and nodule organogenesis via the common symbiotic pathway. Host plants supply photosynthate and maintain the micro‐oxic conditions essential for nitrogenase activity. Recent advances reveal that each Frankia–plant pair exhibits unique physiological and genomic adaptations, including stress‐responsive pathways that confer tolerance to salinity, drought and heavy metals. Cultivation breakthroughs for previously unculturable clusters have facilitated high‐resolution comparative genomics, uncovering gene sets linked to host specificity, vesicle differentiation and metal homeostasis. Globally, this symbiosis underpins ecosystem succession, land reclamation and sustainable forestry, highlighting its environmental and agricultural significance.

Research from Nature Portfolio

Efforts to culture recalcitrant Frankia strains have yielded a slow‐growing isolate of cluster-2 microsymbionts by optimising alkaline growth media. Physiological assays on nodular tissues of Coriaria myrtifolia revealed an obligate requirement for elevated pH, leading to the recovery of a viable strain that produces nitrogen‐fixing vesicles in vitro and fulfils Koch’s postulates on host plants. Genome sequencing of this isolate confirmed its close relationship to uncultured cluster-2 lineages, with gene gains and losses illuminating the genetic basis of auxotrophy and symbiotic competence. This work has opened avenues for systematic studies of early‐diverging Frankia and their global distribution in actinorhizal nodules.

Symbiotic Interactions of Frankia and Actinorhizal Plants publication trend

The graph below shows the total number of articles in symbiotic interactions of frankia and actinorhizal plants across all publications each year (not limited to Nature Index journals).

Technical terms

Actinorhizal plants: Dicotyledonous species capable of forming symbioses with Frankia bacteria.

Frankia: Filamentous, nitrogen‐fixing actinobacteria that induce root nodule formation on actinorhizal hosts.

Root nodule: Plant organ housing intracellular Frankia vesicles for nitrogen fixation.

Nitrogen fixation: Microbial conversion of atmospheric N₂ into bioavailable ammonia.

Vesicle: Membrane‐bound structure within Frankia hyphae where nitrogenase operates under micro‐oxic conditions.

Carbonic anhydrase: Enzyme that catalyses CO₂–bicarbonate interconversion, crucial for pH regulation in nitrogen‐fixing cells.

References

1. Cultivating the uncultured: growing the recalcitrant cluster-2 Frankia strains. Scientific Reports (2015).
2. Frankia alni Carbonic Anhydrase Regulates Cytoplasmic pH of Nitrogen-Fixing Vesicles. International Journal of Molecular Sciences (2023).
3. Genomic Insights Into Plant-Growth-Promoting Potentialities of the Genus Frankia. Frontiers in Microbiology (2019).
4. Frankia-Enriched Metagenomes from the Earliest Diverging Symbiotic Frankia Cluster: They Come in Teams. Genome Biology and Evolution (2019).
5. Use of Frankia and Actinorhizal Plants for Degraded Lands Reclamation. BioMed Research International (2013).