What stories can the Frankia genomes start to tell us?

Tracking the trajectory of frankia research through bibliometrics: trends and future directions

Frankia represent a unique group of filamentous, sporangia-forming bacteria, renowned for their exceptional capacity to establish symbiotic partnerships with actinorhizal plants. The objective of this paper is to offer quantitative insights into the current state of frankia research and its future potential. A comprehensive bibliometric analysis covering the years 2000-2022 was conducted using Scopus and SciVal. A steady increase in both annual publication and international collaboration has been observed, particularly since 2013. Research performance metrics for the last 5 years (2018-2022) indicate China and India as leaders with high Field-Weighted Citation Impact scores. This analysis highlighted prominent authors, research groups, and the evolving research landscape, suggesting an increasing focus on molecular and genomic aspects. The genomic era has transformed our understanding of frankia biology, highlighting their significance in diverse ecological and agricultural contexts. This study comprehensively maps the evolving landscape of frankia research, emphasizing key milestones that have catalysed international interest in frankia-actinorhizal research, expanding our perception of frankia's capabilities beyond its traditional symbiotic role. As research in this field progresses, a deeper comprehension of frankia-plant interactions, symbiotic signalling, and the intricacies of metabolic pathways holds the promise of revealing innovative techniques for optimizing nitrogen fixation and broadening the spectrum of host plants.

Genomic Insights of Alnus-Infective Frankia Strains Reveal Unique Genetic Features and New Evidence on Their Host-Restricted Lifestyle

The present study aimed to use comparative genomics to explore the relationships between Frankia and actinorhizal plants using a data set made of 33 Frankia genomes. The determinants of host specificity were first explored for "Alnus-infective strains" (i.e., Frankia strains belonging to Cluster Ia). Several genes were specifically found in these strains, including an agmatine deiminase which could possibly be involved in various functions as access to nitrogen sources, nodule organogenesis or plant defense. Within "Alnus-infective strains", Sp+ Frankia genomes were compared to Sp- genomes in order to elucidate the narrower host specificity of Sp+ strains (i.e., Sp+ strains being capable of in planta sporulation, unlike Sp- strains). A total of 88 protein families were lost in the Sp+ genomes. The lost genes were related to saprophytic life (transcriptional factors, transmembrane and secreted proteins), reinforcing the proposed status of Sp+ as obligatory symbiont. The Sp+ genomes were also characterized by a loss of genetic and functional paralogs, highlighting a reduction in functional redundancy (e.g., hup genes) or a possible loss of function related to a saprophytic lifestyle (e.g., genes involved in gas vesicle formation or recycling of nutrients).

Paradigms of actinorhizal symbiosis under the regime of global climatic changes: New insights and perspectives

Nitrogen occurs as inert and inaccessible dinitrogen gaseous form (N₂ ) in the atmosphere. Biological nitrogen fixation is a chief process that makes this dinitrogen (N₂ ) accessible and bioavailable in the form of ammonium (NH₄ ⁺ ) ions. The key organisms to fix nitrogen are certain prokaryotes, called diazotrophs either in the free-living form or establishing significant mutual relationships with a variety of plants. On such examples is ~95-100 MY old incomparable symbiosis between dicotyledonous trees and a unique actinobacterial diazotroph in diverse ecosystems. In this association, the root of the certain dicotyledonous tree (~25 genera and 225 species) belonging to three different taxonomic orders, Fagales, Cucurbitales, and Rosales (FaCuRo) known as actinorhizal trees can host a diazotroph, Frankia of order Frankiales. Frankia is gram-positive, branched, filamentous, sporulating, and free-living soil actinobacterium. It resides in the specialized, multilobed, and coralloid organs (lateral roots but without caps), the root nodules of actinorhizal tress. This review aims to provide systematic information on the distribution and the phylogenetic diversity of hosts from FaCuRo and their micro-endosymbionts (Frankia spp.), colonization mechanisms, and signaling pathways. We also aim to provide details on developmental and physiological imperatives for gene regulation and functional genomics of symbiosis, phenomenal restoration ecology, influences of contemporary global climatic changes, and anthropogenic impacts on plant-Frankia interactions for the functioning of ecosystems and the biosphere.

Proposal of 'Candidatus Frankia alpina', the uncultured symbiont of Alnus alnobetula and A. incana that forms spore-containing nitrogen-fixing root nodules

The members of the genus Frankia are, with a few exceptions, a group of nitrogen-fixing symbiotic actinobacteria that nodulate mostly woody dicotyledonous plants belonging to three orders, eight families and 23 genera of pioneer dicots. These bacteria have been characterized phylogenetically and grouped into four molecular clusters. One of the clusters, cluster 1 contains strains that induce nodules on Alnus spp. (Betulaceae), Myrica spp., Morella spp. and Comptonia spp. (Myricaceae) that have global distributions. Some of these strains produce not only hyphae and vesicles, as other cluster 1 strains do, but also numerous sporangia in their host symbiotic tissues, hence their phenotype being described as spore-positive (Sp+). While Sp+ strains have resisted repeated attempts at cultivation, their genomes have recently been characterized and found to be different from those of all described species, being markedly smaller than their phylogenetic neighbours. We thus hereby propose to create a 'Candidatus Frankia alpina' species for some strains present in nodules of Alnus alnobetula and A. incana that grow in alpine environments at high altitudes or in subarctic environments at high latitudes.

Plant Growth-Promoting Active Metabolites from Frankia spp. of Actinorhizal Casuarina spp

In agriculture, plant growth enrichment via plant growth stimulating microbes has been recognized as an emergency, it is used as an alternatives to chemical pesticides and growth stimulants. The phytopathogens cause various diseases such as blister bark; stem cankers, and pink and brown rot diseases besides affect the growth frequency of Casuarina spp. toward biotic and abiotic stresses. Bio-control and plant growth-promoting potential of native Frankia isolates from Casuarina spp. in Tamil Nadu, India, was not much explored. Hence, in the present study, we are investigating the plant growth improvement activity and phytopathogen control in Casuarina spp. The Frankia sp. DDNSF-01 and Frankia casuarinae DDNSF-02 were isolated and identified from the root nodules of Casuarina spp. Additionally, it is recognized for plant growth promoter activity and in vitro antimicrobial activity against phytopathogens including Pseudomonas sp. and Colletotrichum sp. The plant growth regulators including IAA, siderophore, ammonia production, and phosphate solubilization were found out. Therefore, the formation of the most significant plant growth-promoting phytohormone IAA was confirmed by UV, FT-IR, TLC, HPLC, HPTLC, and NMR spectrum. Bioactive metabolites including methyl 4-hydroxybenzoate, dodecanoic acid, and some novel flavonoids were identified. Therefore, various growth regulators such as L-aspartic acid, ¹H-indole-3-carboxaldehyde were confirmed by GC-MS spectra. The present findings conclude Frankia spp. as efficient plant growth enhancement mediator and also inhibit the phytopathogens.

Draft genome sequences for three unisolated Alnus-infective Frankia Sp+ strains, AgTrS, AiOr and AvVan, the first sequenced Frankia strains able to sporulate in-planta

Actinobacteria from genus Frankia are able to form symbiotic associations with actinorhizal plants including alders. Among them, Sp+ strains are characterized by their ability to differentiate numerous sporangia inside host plant cells (unlike "Sp-" strains unable of in-planta sporulation). Here, we report the first genome sequences of three unisolated Sp+ strains: AgTrS, AiOr and AvVan obtained from Alnus glutinosa, A. incana and A. alnobetula (previously known as viridis), respectively (with genome completeness estimated at more than 98%). They represent new Frankia species based on Average Nucleotide Identity (ANI) calculations, and the smallest Alnus-infective Frankia genomes so far sequenced (~5 Mbp), with 5,178, 6,192 and 5,751 candidate protein-encoding genes for AgTrS, AiOr and AvVan, respectively.

Analysis and preliminary characterisation of the cytochrome P450 monooxygenases from Frankia sp. EuI1c (Frankia inefficax sp.)

Frankia bacteria are nitrogen fixing species from the Actinobacterium phylum which live on the root nodules of plants. They have been hypothesised to have significant potential for natural product biosynthesis. The cytochrome P450 monooxygenase complement of Frankia sp. EuI1c (Frankia inefficax sp.), which comprises 68 members, was analysed. Several members belonged to previously uncharacterised bacterial P450 families. There was an unusually high number of CYP189 family members (21) suggesting that this family has undergone gene duplication events which are classified as "blooms". The likely electron transfer partners for the P450 enzymes were also identified and analysed. These consisted of predominantly [3Fe-4S] cluster containing ferredoxins (eight), a single [2Fe-2S] ferredoxin and a couple of ferredoxin reductases. Three of these CYP family members were produced and purified, using Escherichia coli as a host, and their substrate range was characterised. CYP1027H1 and CYP150A20 bound a broad range of norisoprenoids and terpenoids. CYP1074A2 was highly specific for certain steroids including testosterone, progesterone, stanolone and 4-androstene-3,17-dione. It is likely that steroids are the physiological substrates of CYP1074A2. These results also give an indication that terpenoids are the likely substrates of CYP1027H1 and CYP150A2. The large number of P450s belonging to distinct families as well as the associated electron transfer partners found in different Frankia strains highlights the importance of this family of enzymes has in the secondary metabolism of these bacteria.

Genomic Insights Into Plant-Growth-Promoting Potentialities of the Genus Frankia

This study was designed to determine the plant growth promoting (PGP) potential of members of the genus Frankia. To this end, the genomes of 21 representative strains were examined for genes associated directly or indirectly with plant growth. All of the Frankia genomes contained genes that encoded for products associated with the biosynthesis of auxins [indole-3-glycerol phosphate synthases, anthranilate phosphoribosyltransferases (trpD), anthranilate synthases, and aminases (trpA and B)], cytokinins (11 well-conserved genes within the predicted biosynthetic gene cluster), siderophores, and nitrogenases (nif operon except for atypical Frankia) as well as genes that modulate the effects of biotic and abiotic environmental stress (e.g., alkyl hydroperoxide reductases, aquaporin Z, heat shock proteins). In contrast, other genes were associated with strains assigned to one or more of four host-specific clusters. The genes encoding for phosphate solubilization (e.g., low-affinity inorganic phosphate transporters) and lytic enzymes (e.g., cellulases) were found in Frankia cluster 1 genomes, while other genes were found only in cluster 3 genomes (e.g., alkaline phosphatases, extracellular endoglucanases, pectate lyases) or cluster 4 and subcluster 1c genomes (e.g., NAD(P) transhydrogenase genes). Genes encoding for chitinases were found only in the genomes of the type strains of Frankia casuarinae, F. inefficax, F. irregularis, and F. saprophytica. In short, these in silico genome analyses provide an insight into the PGP abilities of Frankia strains of known taxonomic provenance. This is the first study designed to establish the underlying genetic basis of cytokinin production in Frankia strains. Also, the discovery of additional genes in the biosynthetic gene cluster involved in cytokinin production opens up the prospect that Frankia may have novel molecular mechanisms for cytokinin biosynthesis.

Transcription factors network in root endosymbiosis establishment and development

Root endosymbioses are mutualistic interactions between plants and the soil microorganisms (Fungus, Frankia or Rhizobium) that lead to the formation of nitrogen-fixing root nodules and/or arbuscular mycorrhiza. These interactions enable many species to survive in different marginal lands to overcome the nitrogen-and/or phosphorus deficient environment and can potentially reduce the chemical fertilizers used in agriculture which gives them an economic, social and environmental importance. The formation and the development of these structures require the mediation of specific gene products among which the transcription factors play a key role. Three of these transcription factors, viz., CYCLOPS, NSP1 and NSP2 are well conserved between actinorhizal, legume, non-legume and mycorrhizal symbioses. They interact with DELLA proteins to induce the expression of NIN in nitrogen fixing symbiosis or RAM1 in mycorrhizal symbiosis. Recently, the small non coding RNA including micro RNAs (miRNAs) have emerged as major regulators of root endosymbioses. Among them, miRNA171 targets NSP2, a TF conserved in actinorhizal, legume, non-legume and mycorrhizal symbioses. This review will also focus on the recent advances carried out on the biological function of others transcription factors during the root pre-infection/pre-contact, infection or colonization. Their role in nodule formation and AM development will also be described.

Taxonomy and systematics of plant probiotic bacteria in the genomic era

Recent decades have predicted significant changes within our concept of plant endophytes, from only a small number specific microorganisms being able to colonize plant tissues, to whole communities that live and interact with their hosts and each other. Many of these microorganisms are responsible for health status of the plant, and have become known in recent years as plant probiotics. Contrary to human probiotics, they belong to many different phyla and have usually had each genus analysed independently, which has resulted in lack of a complete taxonomic analysis as a group. This review scrutinizes the plant probiotic concept, and the taxonomic status of plant probiotic bacteria, based on both traditional and more recent approaches. Phylogenomic studies and genes with implications in plant-beneficial effects are discussed. This report covers some representative probiotic bacteria of the phylum Proteobacteria, Actinobacteria, Firmicutes and Bacteroidetes, but also includes minor representatives and less studied groups within these phyla which have been identified as plant probiotics.

Pb2+ tolerance by Frankia sp. strain EAN1pec involves surface-binding

Several Frankia strains have been shown to be lead-resistant. The mechanism of lead resistance was investigated for Frankia sp. strain EAN1pec. Analysis of the cultures by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDAX) and Fourier transforming infrared spectroscopy (FTIR) demonstrated that Frankia sp. strain EAN1pec undergoes surface modifications and binds high quantities of Pb+2. Both labelled and unlabelled shotgun proteomics approaches were used to determine changes in Frankia sp. strain EAN1pec protein expression in response to lead and zinc. Pb2+ specifically induced changes in exopolysaccharides, the stringent response, and the phosphate (pho) regulon. Two metal transporters (a Cu2+-ATPase and cation diffusion facilitator), as well as several hypothetical transporters, were also upregulated and may be involved in metal export. The exported Pb2+ may be precipitated at the cell surface by an upregulated polyphosphate kinase, undecaprenyl diphosphate synthase and inorganic diphosphatase. A variety of metal chaperones for ensuring correct cofactor placement were also upregulated with both Pb+2 and Zn+2 stress. Thus, this Pb+2 resistance mechanism is similar to other characterized systems. The cumulative interplay of these many mechanisms may explain the extraordinary resilience of Frankia sp. strain EAN1pec to Pb+2. A potential transcription factor (DUF156) binding site was identified in association with several proteins identified as upregulated with heavy metals. This site was also discovered, for the first time, in thousands of other organisms across two kingdoms.

Permanent Draft Genome Sequence of Frankia sp. Strain BR, a Nitrogen-Fixing Actinobacterium Isolated from the Root Nodules of Casuarina equisetifolia

Frankia sp. strain BR is a member of Frankia lineage Ic and is able to reinfect plants of the Casuarinaceae family. Here, we report a 5.2-Mbp draft genome sequence with a G+C content of 70.0% and 4,777 candidate protein-encoding genes.

Permanent Draft Genome Sequence for Frankia sp. Strain EI5c, a Single-Spore Isolate of a Nitrogen-Fixing Actinobacterium, Isolated from the Root Nodules of Elaeagnus angustifolia

Frankia sp. strain EI5c is a member of Frankia lineage III, which is able to reinfect plants of the Eleagnaceae, Rhamnaceae, Myricaceae, and Gymnostoma, as well as the genus Alnus. Here, we report the 6.6-Mbp draft genome sequence of Frankia sp. strain EI5c with a G+C content of 72.14 % and 5,458 candidate protein-encoding genes.

Draft Genome Sequence of Frankia Strain G2, a Nitrogen-Fixing Actinobacterium Isolated from Casuarina equisetifolia and Able To Nodulate Actinorhizal Plants of the Order Rhamnales

Frankia sp. strain G2 was originally isolated from Casuarina equisetifolia and is characterized by its ability to nodulate actinorhizal plants of the Rhamnales order, but not its original host. It represents one of the largest Frankia genomes so far sequenced (9.5 Mbp).

Permanent Draft Genome Sequence of Frankia sp. Strain Allo2, a Salt-Tolerant Nitrogen-Fixing Actinobacterium Isolated from the Root Nodules of Allocasuarina

Frankia sp. strain Allo2 is a member of Frankia lineage Ib, which is able to reinfect plants of the Casuarinaceae family, and exhibits a high level of salt tolerance compared to other isolates. Here, we report the 5.3-Mbp draft genome sequence of Frankia sp. strain Allo2 with a G+C content of 70.0% and 4,224 candidate protein-encoding genes.

Permanent Draft Genome Sequence for Frankia sp. Strain CeD, a Nitrogen-Fixing Actinobacterium Isolated from the Root Nodules of Casuarina equistifolia Grown in Senegal

Frankiastrain CeD is a member ofFrankialineage Ib that is able to reinfect plants of theCasuarinafamilies. Here, we report a 5.0-Mbp draft genome sequence with a G+C content of 70.1% and 3,847 candidate protein-encoding genes.

Permanent Draft Genome Sequences for Two Variants of Frankia sp. Strain CpI1, the First Frankia Strain Isolated from Root Nodules of Comptonia peregrina

Frankia stains CpI1-S and CpI1-P are members of Frankia lineage Ia that are able to reinfect plants of the Betulaceae and Myricaceae families. Here, we report two 7.6-Mbp draft genome sequences with 6,396 and 6,373 candidate protein-coding genes for CpI1-S and CpI1-P, respectively.

Permanent Draft Genome Sequence of Frankia sp. Strain AvcI1, a Nitrogen-Fixing Actinobacterium Isolated from the Root Nodules of Alnus viridis subsp. crispa Grown in Canada

Frankia strain AvcI1, isolated from root nodules of Alnus viridis subsp. crispa, is a member of Frankia lineage Ia, which is able to reinfect plants of the Betulaceae and Myricaceae families. Here, we report a 7.7-Mbp draft genome sequence with a G+C content of 72.41% and 6,470 candidate protein-encoding genes.

Permanent Draft Genome Sequence of Frankia sp. Strain ACN1ag, a Nitrogen-Fixing Actinobacterium Isolated from the Root Nodules of Alnus glutinosa

Frankia strain ACN1^ag is a member of Frankia lineage Ia, which are able to re-infect plants of the Betulaceae and Myricaceae families. Here, we report a 7.5-Mbp draft genome sequence with a G+C content of 72.35% and 5,687 candidate protein-encoding genes.

Draft Genome Sequence of Frankia sp. Strain DC12, an Atypical, Noninfective, Ineffective Isolate from Datisca cannabina

Frankia sp. strain DC12, isolated from root nodules of Datisca cannabina, is a member of the fourth lineage of Frankia, which is unable to reinfect actinorhizal plants. Here, we report its 6.88-Mbp high-quality draft genome sequence, with a G+C content of 71.92% and 5,858 candidate protein-coding genes.

Candidatus Frankia Datiscae Dg1, the Actinobacterial Microsymbiont of Datisca glomerata, Expresses the Canonical nod Genes nodABC in Symbiosis with Its Host Plant

Frankia strains are nitrogen-fixing soil actinobacteria that can form root symbioses with actinorhizal plants. Phylogenetically, symbiotic frankiae can be divided into three clusters, and this division also corresponds to host specificity groups. The strains of cluster II which form symbioses with actinorhizal Rosales and Cucurbitales, thus displaying a broad host range, show suprisingly low genetic diversity and to date can not be cultured. The genome of the first representative of this cluster, Candidatus Frankia datiscae Dg1 (Dg1), a microsymbiont of Datisca glomerata, was recently sequenced. A phylogenetic analysis of 50 different housekeeping genes of Dg1 and three published Frankia genomes showed that cluster II is basal among the symbiotic Frankia clusters. Detailed analysis showed that nodules of D. glomerata, independent of the origin of the inoculum, contain several closely related cluster II Frankia operational taxonomic units. Actinorhizal plants and legumes both belong to the nitrogen-fixing plant clade, and bacterial signaling in both groups involves the common symbiotic pathway also used by arbuscular mycorrhizal fungi. However, so far, no molecules resembling rhizobial Nod factors could be isolated from Frankia cultures. Alone among Frankia genomes available to date, the genome of Dg1 contains the canonical nod genes nodA, nodB and nodC known from rhizobia, and these genes are arranged in two operons which are expressed in D. glomerata nodules. Furthermore, Frankia Dg1 nodC was able to partially complement a Rhizobium leguminosarum A34 nodC::Tn5 mutant. Phylogenetic analysis showed that Dg1 Nod proteins are positioned at the root of both α- and β-rhizobial NodABC proteins. NodA-like acyl transferases were found across the phylum Actinobacteria, but among Proteobacteria only in nodulators. Taken together, our evidence indicates an Actinobacterial origin of rhizobial Nod factors.

Phylogenetic analysis of vitamin B12-related metabolism in Mycobacterium tuberculosis

Comparison of genome sequences from clinical isolates of Mycobacterium tuberculosis with phylogenetically-related pathogens Mycobacterium marinum, Mycobacterium kansasii, and Mycobacterium leprae reveals diversity amongst genes associated with vitamin B12-related metabolism. Diversity is generated by gene deletion events, differential acquisition of genes by horizontal transfer, and single nucleotide polymorphisms (SNPs) with predicted impact on protein function and transcriptional regulation. Differences in the B12 synthesis pathway, methionine biosynthesis, fatty acid catabolism, and DNA repair and replication are consistent with adaptations to different environmental niches and pathogenic lifestyles. While there is no evidence of further gene acquisition during expansion of the M. tuberculosis complex, the emergence of other forms of genetic diversity provides insights into continuing host-pathogen co-evolution and has the potential to identify novel targets for disease intervention.

Molecular responses of Frankia sp. strain QA3 to naphthalene

The Frankia-actinorhizal plant symbiosis plays a significant role in plant colonization in soils contaminated with heavy metals and toxic aromatic hydrocarbons. The molecular response of Frankia upon exposure to soil contaminants is not well understood. To address this issue, we subjected Frankia sp. strain QA3 to naphthalene stress and showed that it could grow on naphthalene as a sole carbon source. Bioinformatic analysis of the Frankia QA3 genome identified a potential operon for aromatic compound degradation as well as several ring-hydroxylating dioxygenases. Under naphthalene stress, the expression of these genes was upregulated. Proteome analysis showed a differential protein profile for cells under naphthalene stress. Several protein spots were analyzed and used to identify proteins involved in stress response, metabolism, and energy production, including a lignostilbene dioxygenase. These results provide a model for understanding the molecular response of Frankia to common soil pollutants, which may be required for survival and proliferation of the bacterium and their hosts in polluted environments.

The ins and outs of metal homeostasis by the root nodule actinobacterium Frankia

BACKGROUND

Frankia are actinobacteria that form a symbiotic nitrogen-fixing association with actinorhizal plants, and play a significant role in actinorhizal plant colonization of metal contaminated areas. Many Frankia strains are known to be resistant to several toxic metals and metalloids including Pb(2+), Al(+3), SeO2, Cu(2+), AsO4, and Zn(2+). With the availability of eight Frankia genome databases, comparative genomics approaches employing phylogeny, amino acid composition analysis, and synteny were used to identify metal homeostasis mechanisms in eight Frankia strains. Characterized genes from the literature and a meta-analysis of 18 heavy metal gene microarray studies were used for comparison.

RESULTS

Unlike most bacteria, Frankia utilize all of the essential trace elements (Ni, Co, Cu, Se, Mo, B, Zn, Fe, and Mn) and have a comparatively high percentage of metalloproteins, particularly in the more metal resistant strains. Cation diffusion facilitators, being one of the few known metal resistance mechanisms found in the Frankia genomes, were strong candidates for general divalent metal resistance in all of the Frankia strains. Gene duplication and amino acid substitutions that enhanced the metal affinity of CopA and CopCD proteins may be responsible for the copper resistance found in some Frankia strains. CopA and a new potential metal transporter, DUF347, may be involved in the particularly high lead tolerance in Frankia. Selenite resistance involved an alternate sulfur importer (CysPUWA) that prevents sulfur starvation, and reductases to produce elemental selenium. The pattern of arsenate, but not arsenite, resistance was achieved by Frankia using the novel arsenite exporter (AqpS) previously identified in the nitrogen-fixing plant symbiont Sinorhizobium meliloti. Based on the presence of multiple tellurite resistance factors, a new metal resistance (tellurite) was identified and confirmed in Frankia.

CONCLUSIONS

Each strain had a unique combination of metal import, binding, modification, and export genes that explain differences in patterns of metal resistance between strains. Frankia has achieved similar levels of metal and metalloid resistance as bacteria from highly metal-contaminated sites. From a bioremediation standpoint, it is important to understand mechanisms that allow the endosymbiont to survive and infect actinorhizal plants in metal contaminated soils.

In-planta sporulation phenotype: a major life history trait to understand the evolution of Alnus-infective Frankia strains

Two major types of Frankia strains are usually recognized, based on the ability to sporulate in-planta: spore-positive (Sp+) and spore-negative (Sp-). We carried out a study of Sp+ and Sp- Frankia strains based on nodules collected on Alnus glutinosa, Alnus incana and Alnus viridis. The nodules were phenotyped using improved histology methods, and endophytic Frankia strain genotype was determined using a multilocus sequence analysis approach. An additional sampling was done to assess the relation between Sp+ phenotype frequency and genetic diversity of Frankia strains at the alder stand scale. Our results revealed that (i) Sp+ and Sp- Alnus-infective Frankia strains are genetically different even when sampled from the same alder stand and the same host-plant species; (ii) there are at least two distinct phylogenetic lineages of Sp+ Frankia that cluster according to the host-plant species and without regard of geographic distance and (iii) genetic diversity of Sp+ strains is very low at the alder stand scale compared with Sp- strains. Difference in evolutionary history and genetic diversity between Sp+ and Sp- Frankia allows us to discuss the possible ecological role of in-planta sporulation.

Actinorhizal root nodule symbioses: what is signalling telling on the origins of nodulation?

Two groups of bacteria are able to induce the formation of nitrogen-fixing nodules: proteobacteria called rhizobia, which associate with Legumes or Parasponia and actinobateria from the genus Frankia which are able to interact with ∼220 species belonging to eight families called actinorhizal plants. Legumes and different lineages of actinorhizal plants differ in bacterial partners, nodule organogenesis and infection patterns and have independent evolutionary origins. However, recent technical achievements are revealing a variety of conserved signalling molecules and gene networks. Actinorhizal interactions display several primitive features and thus provide the ideal opportunity to determine the minimal molecular toolkit needed to build a nodule and to understand the evolution of root nodule symbioses.