Since 2016, the availability of many Frankia genomes (Tisa et al. 2016) covering the major Frankia clades (Gtari et al. 2013), and the incorporation of comparative genome data, omniLog phenoarrays, morphology, chemotaxonomy and host plant range knowledge have enabled species definitions according to conventional nomenclature. Gtari et al. (2019) summarise current Frankia taxonomy and provide keys features of the 11 species with validly published names that have been proposed within the genus Frankia. In this regard Nouioui et al. (2019) described a new species Frankia torreyi with CpI1T, the first member of the genus Frankia isolated in axenic condition (Callaham et al. 1978), as type strain.

Table 1 International meetings on Frankia and actinorhizal plants
Full size table

Establishing a Frankia actinorhizal symbiosis requires mutual recognition and specific molecular communication between the two partners. While most of symbiotic signaling pathways known for legume and mycorrhizal symbioses have counterparts in the actinorhizal symbiosis (Hocher et al. 2006, 2011; Demina et al. 2013; Griesmann et al. 2018), Frankia nodulating factor is different from known rhizobial and mycorrhizal chitin derivative factors (Cérémonie et al. 1998, 1999; Ghelue et al. 1997; Svistoonoff et al. 2010; Chabaud et al. 2016; Cissoko et al. 2018) and remains uncharacterised due to the continuing inablility to genetically manipulate Frankia. In this regard, Pesce et al. (2019) developed a colony PCR technique that provides a routine procedure for rapidly identifying Frankia strains that may expedite the process of identifying Frankia strains or potential transformants. In addition, Hocher et al. (2019) reported recent technical achievements for several actinorhizal species and major discoveries in Frankia-actinorhizal signaling pathways.

While the overall clustering of the four Frankia lineages remains conserved between each locus analysed with different levels of resolution (Gtari et al. 2013), the precise evolutionary history of the four clusters remains unclear and inconsistent. This topology is has been explored using a variety of approaches. In this context Sarkar et al. (2019) and Sen et al. (2019) used unusual phylogenetic approaches. Sarkar et al. (2019) selected 100 actinobacteria, including Frankia, and built a phylogenetic tree based on the presence and absence of protein domains. The resulting phylogeny clustered the actinobacteria mainly according to their niche rather than their taxonomic classification. Sen et al. (2019) reported a phylogenetic analysis based on putative carbohydrate active enzymes (CAZymes) that are capable of breaking complex polysaccharides into simpler forms. Phylogenetic and evolutionary analyses showed that, in symbiotic Frankia strains, CAZymes are evolving slower than the other potentially highly expressed genes, whereas similar genes from asymbiotic Frankia strains showed little variation in their evolutionary constraints compared to other potentially highly expressed genes.

Isolation of several new strains was a fascinating feature of the meeting. A contributed paper by Gueddou et al. (2019) reports a second Cluster-2 isolate, designated BMG5.30, which is able to reinfect its host plant. Its genome sequence shows that it is more closely related to that Frankia coriariae BMG5.1 than the two other Frankia genomes from Datisca. Based on comparative genomics, Frankia sp. strain BMG5.30 is a new member of the species F. coriariae. Ghodhbane-Gtari et al. (2019) reported the isolation of two non-Frankia actinobacteria from root nodules of Casuarina glauca growing in Tunisia that have been assigned to the genus Nocardia with one strain that is the type strain representing a new species designated Nocardia casuarinae (Ghodhbane-Gtari et al. 2014). The two strains have several important traits that benefit actinorhizal plants. Their ability to produce auxins, cause root hair deformation, and induce nodule-like structures with beneficial growth promoting effects on the development of the C. glauca plants supports this idea. Coinfection studies also showed that BMG51109 caused an earlier onset of nodulation.

A number of contributions dealt with the environmental biology of Frankia. Kucho et al. (2019) studied Frankia communities in disturbed land on Mt. Ontake following the 1984 earthquake that caused a devastating landslide using nifH amplicon sequences. The authors demonstrated that a diverse Frankia community was detected in the most advanced revegetation area, suggesting that revegetation positively affects Frankia diversity. Rehan et al. (2018) reported the mechanism of detoxification and reduction of selenite to elemental red selenium by Frankia inefficax strain EuI1c. The involvement of a periplasmic-binding protein homolog, putative sulfate ABC transporter and extracellular ligand-binding receptor was demonstrated.

Within the current and predicted scenarios of climate changes and human population growth, and their concomitant impact on the availability of arable land, actinorhizal plants should be seen as strong candidates to mitigate some of the negative impacts thanks to their mutualistic symbiosis. Ribeiro-Barros et al. (2019) reviewed the distribution, conservation and uses of actinorhizal species native to or introduced in Africa and their relevance for the developing economies of many African countries, which are highly sensitive to climate and anthropogenic disturbances.

Zhong et al. (2019) reviewed Chinese inoculation expertise aimed at improving Casuarina spp. survival and productivity, mainly in the “Great Green Wall” that represents vast plantations of Casuarina along 3000 km long and 0.5–5 km width on the coast fronting the South China Sea.

Finally we are thankful to all attendees to Hammamet for the friendly atmosphere and productive meeting, to the authors that contributed to this special issue and to the Frankiologists who acted as anonymous reviewers. A special thank is expressed to Editor-in-Chief Professor Iain Sutcliffe and the Editorial office of Antonie van Leeuwenhoek.

On to Japan-2020 for the next Frankia-actinorhizal plants meeting!