Frankia inefficax sp. nov., an actinobacterial endophyte inducing ineffective, non nitrogen-fixing, root nodules on its actinorhizal host plants

Effects of pepper-maize intercropping on the physicochemical properties, microbial communities, and metabolites of rhizosphere and bulk soils

BACKGROUND

Intercropping increases land use efficiency and farmland ecological diversity. However, little is understood about whether and how soil biota, metabolites, and nutrients change under interspecific competition among plants. Thus, this study aimed to explore the changes in the physicochemical properties, microbial communities, and metabolites of rhizosphere and bulk soils of pepper monocropping and pepper-maize intercropping systems.

RESULTS

Intercropping significantly increased the contents of available phosphorus (AP) and available potassium (AK), and decreased the pH value, whereas it had little effect on the total nitrogen (TN) and organic matter (OM) in the rhizosphere and bulk soils, compared with those in monocropping pepper. Moreover, the OM content was higher in rhizosphere soil than in bulk soil. The microbial community structures and metabolite profiles also differed between the two systems. The diversity of bacteria and fungi increased in intercropped pepper. The relative abundances of Actinobacteria, Chloroflexi, Cyanobacteria, and Ascomycota were higher while those of Proteobacteria, Planctomycetes, Mucoromycota, and Basidiomycota were significantly lower in the rhizosphere and bulk soils from the intercropping system than in those from the monocropping system. Linear discriminant analysis revealed that the predominant bacteria and fungi in the rhizosphere soil from the intercropping system belonged to the order Sphingomonadales and genera Nitrospira, Phycicoccus and Auricularia, whereas those in the bulk soil from the intercropping system belonged to the phylum Acidobacteria and genera Calocera, Pseudogymnoascus, and Trichosporon. Intercropping promoted the secretion of flavonoids, alkaloids, and nucleotides and their derivatives in the rhizosphere soil and significantly increased the contents of organoheterocyclic compounds in the bulk soil. Furthermore, the AP and AK contents, and pH value had strong positive correlations with bacteria. In addition, co-occurrence network analysis also showed that asebogenin, trachelanthamidine, 5-methyldeoxycytidine, and soil pH were the key factors mediating root-soil-microbe interactions.

CONCLUSION

Intercropping can alter microbial community structures and soil metabolite composition in rhizosphere and bulk soils, enhancing soil nutrient contents, enriching soil beneficial microbes and secondary metabolites (flavonoids and alkaloids) of intercropped pepper, and provided a scientific basis for sustainable development in the pepper-maize intercropping system.

A root nodule microbiome sequencing data set from red alder (Alnus rubra Bong.)

There have been frequent reports of more than one strain of the nitrogen-fixing symbiont, Frankia, in the same root nodule of plants in the genus Alnus, but quantitative assessments of their relative contributions have not been made to date. Neither has the diversity of other microbes, having potential functional roles in symbiosis, been systematically evaluated. Alnus rubra root nodule microbiota were studied using Illumina short read sequencing and kmer-based read classification. Single end 76 bp sequencing was done to a median depth of 96 million reads per sample. Reads were assigned to taxa using KrakenUniq, with taxon abundances being estimated using its companion program Bracken. This was the first high resolution study of Alnus root nodules using next generation sequencing (NGS), quantifying multiple Cluster 1 A Frankia strains in single nodules, and in some cases, a Cluster 4 strain. Root nodules were found to contain diverse bacteria, including several genera containing species known to have growth-promoting effects. Evidence was found for partitioning of some bacterial strains in older versus younger lobes.

Frankia nepalensis sp. nov., a non-infective non-nitrogen-fixing isolate from root nodules of Coriaria nepalensis Wall

Strains CN4T, CN6, CN7 and CNm7 were isolated from root nodules of Coriaria nepalensis from Murree in Pakistan. They do not form root nodules on C. nepalensis nor on Alnus glutinosa although they deformed root hairs of Alnus. The colonies are bright red-pigmented, the strains form hyphae and sporangia but no N2-fixing vesicles and do not fix nitrogen in vitro. The peptidoglycan of strain CN4T contains meso-diaminopimelic acid; whole cell sugars consist of ribose, mannose, glucose, galactose and rhamnose. Diphosphatidylglycerol, phosphatidylglycerol, phosphatidylinositol and two unknown lipids represent the major polar lipids; MK-9(H4) and MK-9(H6) are the predominant menaquinones (>15 %), and iso-C16 : 0 and C17 : 1ω8c are the major fatty acids (>15 %). The results of comparative 16S rRNA gene sequence analyses indicated that strain CN4T is most closely related to Frankia saprophytica CN 3T. An MLSA phylogeny using amino acids sequences of AtpD, DnaA, FtsZ, Pgk and RpoB, assigned the strain to cluster 4 non-nodulating species, close to F. saprophytica CN 3T , Frankia asymbiotica M16386T and Frankia inefficax EuI1cT with 0.04 substitutions per site, while that value was 0.075 with other strains. Digital DNA-DNA hybridization (dDDH) and average nucleotide identity (ANI) values between CN4T and all species of the genus Frankia with validly published names were below the defined threshold for prokaryotic species demarcation, with dDDH and ANI values at or below 27.8 and 83.7 %, respectively. The four strains CN4T, CN6, CN7 and CNm7 had dDDH (98.6-99.6 %) and ANI values that grouped them as representing a single species. CN4T has a 10.76 Mb genome. CN4T was different from its close phylogenetic neighbours with validly published names in being red-pigmented, in having several lantibiotic-coding clusters, a carbon monoxide dehydrogenase cluster and a clustered regularly interspaced short palindromic repeats (CRISPR) cluster. The results of phenotypic, physiological and phylogenomic analyses confirmed the assignment of strain CN4T (=DSM 114740T = LMG 32595T) to a novel species, with CN4T as type strain, for which the name Frankia nepalensis sp. nov. is proposed.

Frankia alni Carbonic Anhydrase Regulates Cytoplasmic pH of Nitrogen-Fixing Vesicles

A phyloprofile of Frankia genomes was carried out to identify those genes present in symbiotic strains of clusters 1, 1c, 2 and 3 and absent in non-infective strains of cluster 4. At a threshold of 50% AA identity, 108 genes were retrieved. Among these were known symbiosis-associated genes such as nif (nitrogenase), and genes which are not know as symbiosis-associated genes such as can (carbonic anhydrase, CAN). The role of CAN, which supplies carbonate ions necessary for carboxylases and acidifies the cytoplasm, was thus analyzed by staining cells with pH-responsive dyes; assaying for CO₂ levels in N-fixing propionate-fed cells (that require a propionate-CoA carboxylase to yield succinate-CoA), fumarate-fed cells and N-replete propionate-fed cells; conducting proteomics on N-fixing fumarate and propionate-fed cells and direct measurement of organic acids in nodules and in roots. The interiors of both in vitro and nodular vesicles were found to be at a lower pH than that of hyphae. CO₂ levels in N₂-fixing propionate-fed cultures were lower than in N-replete ones. Proteomics of propionate-fed cells showed carbamoyl-phosphate synthase (CPS) as the most overabundant enzyme relative to fumarate-fed cells. CPS combines carbonate and ammonium in the first step of the citrulline pathway, something which would help manage acidity and NH₄⁺. Nodules were found to have sizeable amounts of pyruvate and acetate in addition to TCA intermediates. This points to CAN reducing the vesicles' pH to prevent the escape of NH₃ and to control ammonium assimilation by GS and GOGAT, two enzymes that work in different ways in vesicles and hyphae. Genes with related functions (carboxylases, biotin operon and citrulline-aspartate ligase) appear to have undergone decay in non-symbiotic lineages.

Frankia colletiae sp. nov., a nitrogen-fixing actinobacterium isolated from Colletia cruciata

A nitrogen-fixing actinobacterium strain (Cc1.17^T) isolated from a root nodule of Colletia cruciata was subjected to polyphasic taxonomic studies. The strain was characterized by the presence of meso-diaminopimelic acid in its peptidoglycan, galactose, glucose, mannose, rhamnose, ribose and xylose as cell-wall sugars, phosphatidylinositol, diphosphatidylglycerol, glycophospholipids, phosphatidylglycerol, glycophospholipid and uncharacterized lipids as its polar lipids, and C_16 : 0, iso-C_16 : 0, C_{17 : 1} ω9 and C_18 : 1 ω9 as major fatty acids (>10 %). Strain Cc1.17^T showed 16S rRNA gene sequence similarities of 97.4-99.8 % to validly named Frankia species. Phylogenetic trees based on 16S rRNA gene and genome sequences placed strain Cc1.17^T in a new lineage within the genus Frankia. Digital DNA-DNA hybridization and average nucleotide identity values between strain Cc1.17^T and its closest phylogenomic neighbours were well below the thresholds recommended for prokaryotic species delineation. Therefore, strain Cc1.17^T (=DSM 43829^T=CECT 9313^T) merits recognition as the type strain of a new species for which the name Frankia colletiae sp. nov. is proposed.

Novel species of Frankia, Frankia gtarii sp. nov. and Frankia tisai sp. nov., isolated from a root nodule of Alnus glutinosa

The status of four Frankia strains isolated from a root nodule of Alnus glutinosa was established in a polyphasic study. Taxogenomics and phenotypic features show that the isolates belong to the genus Frankia. All four strains form extensively branched substrate mycelia, multilocular sporangia, vesicles, lack aerial hyphae, but contain meso-diaminopimelic acid as the diamino acid of the peptidoglycan, galactose, glucose, mannose, ribose, xylose and traces of rhamnose as cell wall sugars, iso-C_16:0 as the predominant fatty acid, diphosphatidylglycerol, phosphatidylglycerol, phosphatidylinositol as the major polar lipids, have comparable genome sizes to other cluster 1, Alnus-infective strains with structural and accessory genes associated with nitrogen fixation. The genome sizes of the isolates range from 7.0 to 7.7 Mbp and the digital DNA G + C contents from 71.3 to 71.5 %. The four sequenced genomes are rich in biosynthetic gene clusters predicted to express for novel specialized metabolites, notably antibiotics. 16S rRNA gene and whole genome sequence analyses show that the isolates fall into two lineages that are closely related to the type strains of Frankia alni and Frankia torreyi. All of these taxa are separated by combinations of phenotypic properties and by digital DNA:DNA hybridization scores which indicate that they belong to different genomic species. Based on these results, it is proposed that isolates Agncl-4^T and Agncl-10, and Agncl-8^T and Agncl-18, be recognised as Frankia gtarii sp. nov. and Frankia tisai sp. nov. respectively, with isolates Agncl-4^T (=DSM 107976^T = CECT 9711^T) and Agncl-8^T (=DSM 107980^T = CECT 9715^T) as the respective type strains.

Taxogenomic status of phylogenetically distant Frankia clusters warrants their elevation to the rank of genus: A description of Protofrankia gen. nov., Parafrankia gen. nov., and Pseudofrankia gen. nov. as three novel genera within the family Frankiaceae

The genus Frankia is at present the sole genus in the family Frankiaceae and encompasses filamentous, sporangia-forming actinomycetes principally isolated from root nodules of taxonomically disparate dicotyledonous hosts named actinorhizal plants. Multiple independent phylogenetic analyses agree with the division of the genus Frankia into four well-supported clusters. Within these clusters, Frankia strains are well defined based on host infectivity range, mode of infection, morphology, and their behaviour in culture. In this study, phylogenomics, overall genome related indices (OGRI), together with available data sets for phenotypic and host-plant ranges available for the type strains of Frankia species, were considered. The robustness and the deep radiation observed in Frankia at the subgeneric level, fulfilling the primary principle of phylogenetic systematics, were strengthened by establishing genome criteria for new genus demarcation boundaries. Therefore, the taxonomic elevation of the Frankia clusters to the rank of the genus is proposed. The genus Frankia should be revised to encompass cluster 1 species only and three novel genera, Protofrankia gen. nov., Parafrankia gen. nov., and Pseudofrankia gen. nov., are proposed to accommodate clusters 2, 3, and 4 species, respectively. New combinations for validly named species are also provided.

Actinobacteria Community and Their Antibacterial and Cytotoxic Activity on the Weizhou and Xieyang Volcanic Islands in the Beibu Gulf of China

Weizhou Island and Xieyang Island are two large and young volcanic sea islands in the northern part of the South China Sea. In this study, high-throughput sequencing (HTS) of 16S rRNA genes was used to explore the diversity of Actinobacteria in the Weizhou and Xieyang Islands. Moreover, a traditional culture-dependent method was utilized to isolate Actinobacteria, and their antibacterial and cytotoxic activities were detected. The alpha diversity indices (ACE metric) of the overall bacterial communities for the larger island (Weizhou) were higher than those for the smaller island (Xieyang). A beta diversity analysis showed a more dispersive pattern of overall bacterial and actinobacterial communities on a larger island (Weizhou). At the order level, Frankiales, Propionibacteriales, Streptomycetales, Micrococcales, Pseudonocardiales, Micromonosporales, Glycomycetales, Corynebacteriales, and Streptosporangiales were the predominant Actinobacteria. A total of 22.7% of the OTUs shared 88%-95% similarity with some known groups. More interestingly, 15 OTUs formed a distinct and most predominant clade, and shared identities of less than 95% with any known families. This is the first report about this unknown group and their 16S rRNA sequences obtained from volcanic soils. A total of 268 actinobacterial strains were isolated by the culture-dependent method. Among them, 55 Streptomyces species were isolated, representing that 76.6% of the total. S. variabilis and S. flavogriseus were the most abundant. Moreover, some rare Actinobacteria were isolated. These included Micromonospora spp., Nocardia spp., Amycolatopsis spp., Tsukamurella spp., Mycobacterium spp., and Nonomuraea spp. Among them, eight Streptomyces spp. exhibited antibacterial activity against Bacillus cereus. Only three strains inhibited the growth of Escherichia coli. Four strains showed good activity against aquatic pathogenic bacterial strains of Streptococcus iniae. The cytotoxicity assay results showed that 27 strains (10.07%) exhibited cytotoxic activity against HeLa and A549 cell lines. Many actinobacterial strains with cytotoxic activity were identified as rare Actinobacteria, which illustrated that volcanic islands are vast reservoirs for Actinobacteria with promising antibacterial and cytotoxic activity. This study may significantly improve our understanding of actinobacterial communities on volcanic islands. The isolated Actinobacteria showed promising prospects for future use.

Draft genomes of non-nitrogen-fixing Frankia strains

In this study, we describe the genomes of two novel candidate species of non-nitrogen fixing Frankia that were isolated from the root nodules of Coriaria nepalensis and Alnus glutinosa, genospecies CN and Ag, respectively. Comparative genomic analyses revealed that both genospecies lack genes essential for nitrogen-fixation and possess genes involved in the degradation of plant cell walls. Additionally, we found distinct biosynthetic gene clusters in each genospecies. The availability of these genomes will contribute to the study of the taxonomy and evolution of actinorhizal symbioses.

Nutrient availability is a dominant predictor of soil bacterial and fungal community composition after nitrogen addition in subtropical acidic forests

Nutrient addition to forest ecosystems significantly influences belowground microbial diversity, community structure, and ecosystem functioning. Nitrogen (N) addition in forests is common in China, especially in the southeast region. However, the influence of N addition on belowground soil microbial community diversity in subtropical forests remains unclear. In May 2018, we randomly selected 12 experimental plots in a Pinus taiwanensis forest within the Daiyun Mountain Nature Reserve, Fujian Province, China, and subjected them to N addition treatments for one year. We investigated the responses of the soil microbial communities and identified the major elements that influenced microbial community composition in the experimental plots. The present study included three N treatments, i.e., the control (CT), low N addition (LN, 40 kg N ha^-1 yr^-1), and high N addition (HN, 80 kg N ha^-1 yr^-1), and two depths, 0−10 cm (topsoil) and 10−20 cm (subsoil), which were all sampled in the growing season (May) of 2019. Soil microbial diversity and community composition in the topsoil and subsoil were investigated using high-throughput sequencing of bacterial 16S rDNA genes and fungal internal transcribed spacer sequences. According to our results, 1) soil dissolved organic carbon (DOC) significantly decreased after HN addition, and available nitrogen (AN) significantly declined after LN addition, 2) bacterial α-diversity in the subsoil significantly decreased with HN addition, which was affected significantly by the interaction between N addition and soil layer, and 3) soil DOC, rather than pH, was the dominant environmental factor influencing soil bacterial community composition, while AN and MBN were the best predictors of soil fungal community structure dynamics. Moreover, N addition influence both diversity and community composition of soil bacteria more than those of fungi in the subtropical forests. The results of the present study provide further evidence to support shifts in soil microbial community structure in acidic subtropical forests in response to increasing N deposition.

Candidatus Frankia nodulisporulans sp. nov., an Alnus glutinosa-infective Frankia species unable to grow in pure culture and able to sporulate in-planta

We describe a new Frankia species, for three non-isolated strains obtained from Alnus glutinosa in France and Sweden, respectively. These strains can nodulate several Alnus species (A. glutinosa, A. incana, A. alnobetula), they form hyphae, vesicles and sporangia in the root nodule cortex but have resisted all attempts at isolation in pure culture. Their genomes have been sequenced, they are significantly smaller than those of other Alnus-infective species (5Mb instead of 7.5Mb) and are very closely related to one another (ANI of 100%). The name Candidatus Frankia nodulisporulans is proposed. The GenBank/EMBL/DDBJ accession numbers for the 16S rRNA gene and draft genome sequences reported in this study for AgTrS, AgUmASt1 and AgUmASH1 are MT023539/LR778176/LR778180 and NZ_CADCWS000000000.1/CADDZU010000001/CADDZW010000001, respectively.

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.

Frankia soli sp. nov., an actinobacterium isolated from soil beneath Ceanothus jepsonii

Actinobacterial strain Cj^T was directly isolated from soil beneath Ceanothus jepsonii growing in the USA. The strain formed cell structures typical of the genus Frankia including extensive hyphae, vesicles and sporangia, and it effectively nodulated members of the actinorhizal Colletieae, Elaeagnaceae and Myricaceae. The whole-cell hydrolysate of strain Cj^T was rich in meso-diaminopimelic acid and galactose, glucose, mannose, xylose, ribose and a trace of rhamnose. Tbe polar lipid profile contained phosphatidylinositol, phosphatidylglycerol, diphosphatidylglycerol and glycophospholipid. The menaquinone was predominantly MK-9(H₄). The fatty acid profile predominantly consisted of C_17 : 1ω8c, iso-C_16 : 0, C_15:0, C_16 : 0 and C_17 : 0. A multilocus sequence analysis phylogeny based on atp1, ftsZ, dnaK, gyrA and secA gene sequences positioned the strain within Elaeagnaceae- and Colletieae-nodulating species together with Frankia elaeagni DSM 46783^T, Frankia discariae DSM 46785^T and Frankia irregularis DSM 45899^T. Pairwise 16S rRNA gene sequence similarities showed that strain Cj^T was most closely related to F. discariae DSM 46785^T (99.78 %) while their digital DNA-DNA hybridization value was 41.1 %. Based on the overall analyses, strain Cj^T (=DSM 100623^T=CECT 9041^T) warrants classification as the type strain of a novel species, for which the name Frankia soli sp. nov. is proposed.

Frankia-Enriched Metagenomes from the Earliest Diverging Symbiotic Frankia Cluster: They Come in Teams

Frankia strains induce the formation of nitrogen-fixing nodules on roots of actinorhizal plants. Phylogenetically, Frankia strains can be grouped in four clusters. The earliest divergent cluster, cluster-2, has a particularly wide host range. The analysis of cluster-2 strains has been hampered by the fact that with two exceptions, they could never be cultured. In this study, 12 Frankia-enriched metagenomes of Frankia cluster-2 strains or strain assemblages were sequenced based on seven inoculum sources. Sequences obtained via DNA isolated from whole nodules were compared with those of DNA isolated from fractionated preparations enhanced in the Frankia symbiotic structures. The results show that cluster-2 inocula represent groups of strains, and that strains not represented in symbiotic structures, that is, unable to perform symbiotic nitrogen fixation, may still be able to colonize nodules. Transposase gene abundance was compared in the different Frankia-enriched metagenomes with the result that North American strains contain more transposase genes than Eurasian strains. An analysis of the evolution and distribution of the host plants indicated that bursts of transposition may have coincided with niche competition with other cluster-2 Frankia strains. The first genome of an inoculum from the Southern Hemisphere, obtained from nodules of Coriaria papuana in Papua New Guinea, represents a novel species, postulated as Candidatus Frankia meridionalis. All Frankia-enriched metagenomes obtained in this study contained homologs of the canonical nod genes nodABC; the North American genomes also contained the sulfotransferase gene nodH, while the genome from the Southern Hemisphere only contained nodC and a truncated copy of nodB.

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.

An update on the taxonomy of the genus Frankia Brunchorst, 1886, 174AL

Since the recognition of the name Frankia in the Approved Lists of bacterial names (1980), few amendments have been given to the genus description. Successive editions of Bergey's Manual of Systematics of Archaea and Bacteria have broadly conflicting suprageneric treatments of the genus without any advances for subgeneric classification. This review focuses on recent results from taxongenomics and phenoarray approaches to the positioning and the structuring of the genus Frankia. Based on phylogenomic analyses, Frankia should be considered the single member of the family Frankiaceae within the monophyletic order, Frankiales. A polyphasic strategy incorporating genome to genome data and omniLog^® phenoarrays, together with classical approaches, has allowed the designation and an amended description of a type strain of the type species Frankia alni, and the recognition of at least 10 novel species covering symbiotic and non symbiotic taxa within the genus. Genome to phenome data will be shortly incorporated in the scheme for proposing novel species including those recalcitrant to isolation in axenic culture.

Simple colony PCR procedure for the filamentous actinobacteria Frankia

Molecular analysis of the filamentous actinobacteria Frankia is laborious because of the slow growth rate and required biomass needed for these techniques. An efficient and simple colony PCR protocol for Frankia was developed that saved time for analysis of any Frankia strains growing on a plate. Previously, it took 5-6 weeks to get the correct size Frankia colonies on plates and then a minimum of 5 weeks of growth in liquid culture for DNA extraction. With this technique, these colonies could be screened after 5-6 weeks of growth by colony PCR. The procedure used a combination of mechanical and heat treatments and required no added buffers or chemicals. Our results demonstrate rapid and efficient PCR.

Frankia canadensis sp. nov., isolated from root nodules of Alnus incana subspecies rugosa

Strain ARgP5^T, an actinobacterium isolated from a root nodule present on an Alnus incana subspecies rugosa shrub growing in Quebec City, Canada, was the subject of polyphasic taxonomic studies to clarify its status within the genus Frankia. 16S rRNA gene sequence similarities and ANI values between ARgP5^T and type strains of species of the genus Frankiawith validly published names were 98.8 and 82 % or less, respectively. The in silico DNA G+C content was 72.4 mol%. ARgP5^T is characterised by the presence of meso-A2pm, galactose, glucose, mannose, rhamnose (trace), ribose and xylose as whole-organism hydrolysates; MK-9(H8) as predominant menaquinone; diphosphatidylglycerol, phosphatidylinositol and phosphatidylglycerol as polar lipids and iso-C16 : 0 and C17 : 1ω8c as major fatty acids. The proteomic results confirmed the distinct position of ARgP5^T from its closest neighbours in Frankiacluster 1. ARgP5^T was found to be infective on two alder (Alnus glutinosa and Alnusalnobetula subsp. crispa) and on one bayberry (Morella pensylvanica) species and to fix nitrogen in symbiosis and in pure culture. On the basis of phylogenetic (16S rRNA gene sequence), genomic, proteomic and phenotypic results, strain ARgP5^T (=DSM 45898=CECT 9033) is considered to represent a novel species within the genus Frankia for which the name Frankia canadensis sp. nov., is proposed.

Frankia torreyi sp. nov., the first actinobacterium of the genus Frankia Brunchorst 1886, 174AL isolated in axenic culture

Strain CpI1^T was, in 1978, the first isolate of the genus Frankia to be obtained from Comptonia peregrina root nodules. In this study, a polyphasic approach was performed to identify the taxonomic position of strain CpI1^T among the members of the genus Frankia. The strain contains meso-diaminopimelic acid as the diagnostic diamino acid and galactose, glucose, mannose, rhamnose, ribose and xylose as cell wall sugars. The polar lipids were found to consist of phosphatidylinositol, diphosphatidylglycerol, glycophospholipids, phosphatidylglycerol, an aminophospholipid and unidentified phospholipids and lipids. The predominant menaquinone was identified as MK-9 (H₈), while the major fatty acid are iso-C_16:0 and C_17:1ω 8c. The 16S rRNA gene sequence identity varies from 97.4 to 99.6% with the type strains of currently described Frankia species. Phylogenetic analyses based on 16S rRNA gene sequences and multi-locus sequence analysis (MLSA) using atp1, ftsZ, dnaK, gyrA and secA gene sequences showed that strain CpI1^T is closely related to Frankia alni ACN14a^T. The genome size of strain CpI1^T is 7.6 Mb with a digital DNA G+C content of 72.4%. Digital DNA:DNA hybridization (values between strain CpI1^T and its close phylogenetic relative F. alni ACN14a^T was 44.1%, well below the threshold of 70% for distinguishing between bacterial genomic species. Based on the phenotypic, phylogenetic and genomic data, strain CpI1^T (= DSM44263^T = CECT9035^T) warrants classification as the type strain of a novel species, for which the name Frankia torreyi sp. nov. is proposed.

Frankia irregularis sp. nov., an actinobacterium unable to nodulate its original host, Casuarina equisetifolia, but effectively nodulates members of the actinorhizal Rhamnales

A red pigmented actinobacterium designated G2^T, forming extremely branched vegetative hyphae, vesicles and mutilocular sporangia, was isolated from Casuarina equisetifolia nodules. The strain failed to nodulate its original host plant but effectively nodulated members of actinorhizal Rhamnales. The taxonomic position of G2^T was determined using a polyphasic approach. The peptidoglycan of the strain contained meso-diaminopimelic acid as diagnostic diamino acid, galactose, glucose, mannose, rhamnose, ribose and xylose. The polar lipid pattern consisted of phosphatidylinositol (PI), diphosphatidylglycerol (DPG), glycophospholipids (GPL1-2), phosphatidylglycerol (PG), aminophospholipid (APL) and unknown lipids (L). The predominant menaquinones were MK-9 (H4) and MK-9 (H6) while the major fatty acids were iso-C16 : 0, C17 : 1ω8c and C15 : 0. The size of the genome of G2^T was 9.5 Mb and digital DNA G+C content was 70.9 %. The 16S rRNA gene showed 97.4-99.5 % sequence identity with the type strains of species of the genus Frankia. Digital DNA -DNA hybridisation (dDDH) values between G2^T and its nearest phylogenetic neighbours Frankia elaeagniand Frankia discariaewere below the threshold of 70 %. On the basis of these results, strain G2^T (=DSM 45899^T=CECT 9038^T) is proposed to represent the type strain of a novel species Frankia irregularis sp. nov.

Frankia saprophytica sp. nov., an atypical, non-infective (Nod-) and non-nitrogen fixing (Fix-) actinobacterium isolated from Coriaria nepalensis root nodules

Strain CN3^T, a Coriaria nepalensis isolate, appears to form hyphae and sporangia typical of members fo the genus Frankia. However, it failed to form vesicles, to reduce acetylene and to induce nodules on its original host plant. A polyphasic approach was used here to determine the taxonomic status of strain CN3^T. The 16S rRNA gene sequence of strain CN3^T showed the highest sequence identity with Frankia asymbiotica type strain M16386^T (99.4 %). Digital DNA-DNA hybridization between strains CN3^T and M16386^T was 25.7 %, which is clearly below the accepted cut-off point of 70 %. The G+C content of DNA was 71.8 mol%. Whole-cell hydrolysates of strain CN3^T were rich in meso-diaminopimelic acid. Cell-wall sugars were composed of galactose, glucose, mannose, rhamnose and traces of ribose. The polar lipid profile contained phosphatidylinositol, phosphatidylglycerol, diphosphatidylglycerol, phosphoglycolipids, phospholipid, six uncharacterized glycolipids and two uncharacterized lipids. The predominant menaquinone (>25 %) was MK-9(H6). Major fatty acids (>15 %) of strain CN3^T consisted of iso-C16 : 0, C17 : 1ω8c and C15 : 0. Based on 16S rRNA gene phylogeny, genome sequence analysis and phenotypic results, strain CN3^T (=DSM 105290^T=CECT 9314^T) is proposed to represent the type strain of a novel species, Frankia saprophytica sp. nov.

Frankia asymbiotica sp. nov., a non-infective actinobacterium isolated from Morella californica root nodule

The taxonomic status of strain M16386^T, a nitrogen-fixing but non-nodulating isolate from Morella californica, was established on the basis of a polyphasic approach. The strain grows as branched hyphae, with vesicles and non-motile productive multilocular sporangia. It metabolizes short fatty acids, TCA cycle intermediates and carbohydrates as carbon sources, and fixes nitrogen in the absence of combined nitrogen source in the growth media. Chemotaxonomic traits of strain M16386^T are consistent with its affiliation to the genus Frankia. The characteristic diamino acid in the cell wall is meso-diaminopimelic acid. Strain M16386^T contains phosphatidylinositol, phosphatidylglycerol, diphosphatidylglycerol, glycophospholipid and phospholipid as polar lipids; MK-9(H4) and MK-9(H6) as the predominant menaquinones; iso-C16 : 0 and C17 : 1ω8c as major fatty acids; and galactose, glucose, mannose, rhamnose and ribose as whole-cell sugars. Strain M16386^T showed 98.2 % 16S rRNA gene sequence similarity with its closest phylogenetic neighbour, Frankia inefficaxDSM 45817^T. Based on these results, strain M16386^T (=DSM 100626^T=CECT 9040^T) is designated the type strain of a novel species of the genus Frankia,for which the name Frankia asymbiotica sp. nov. is proposed.

Permanent draft genome sequence of Frankia sp. NRRL B-16219 reveals the presence of canonical nod genes, which are highly homologous to those detected in Candidatus Frankia Dg1 genome

Frankia sp. NRRL B-16219 was directly isolated from a soil sample obtained from the rhizosphere of Ceanothus jepsonii growing in the USA. Its host plant range includes members of Elaeagnaceae species. Phylogenetically, strain NRRL B-16219 is closely related to "Frankia discariae" with a 16S rRNA gene similarity of 99.78%. Because of the lack of genetic tools for Frankia, our understanding of the bacterial signals involved during the plant infection process and the development of actinorhizal root nodules is very limited. Since the first three Frankia genomes were sequenced, additional genome sequences covering more diverse strains have helped provide insight into the depth of the pangenome and attempts to identify bacterial signaling molecules like the rhizobial canonical nod genes. The genome sequence of Frankia sp. strain NRRL B-16219 was generated and assembled into 289 contigs containing 8,032,739 bp with 71.7% GC content. Annotation of the genome identified 6211 protein-coding genes, 561 pseudogenes, 1758 hypothetical proteins and 53 RNA genes including 4 rRNA genes. The NRRL B-16219 draft genome contained genes homologous to the rhizobial common nodulation genes clustered in two areas. The first cluster contains nodACIJH genes whereas the second has nodAB and nodH genes in the upstream region. Phylogenetic analysis shows that Frankia nod genes are more deeply rooted than their sister groups from rhizobia. PCR-sequencing suggested the widespread occurrence of highly homologous nodA and nodB genes in microsymbionts of field collected Ceanothus americanus.

Genomic, transcriptomic, and proteomic approaches towards understanding the molecular mechanisms of salt tolerance in Frankia strains isolated from Casuarina trees

Background

Soil salinization is a worldwide problem that is intensifying because of the effects of climate change. An effective method for the reclamation of salt-affected soils involves initiating plant succession using fast growing, nitrogen fixing actinorhizal trees such as the Casuarina. The salt tolerance of Casuarina is enhanced by the nitrogen-fixing symbiosis that they form with the actinobacterium Frankia. Identification and molecular characterization of salt-tolerant Casuarina species and associated Frankia is imperative for the successful utilization of Casuarina trees in saline soil reclamation efforts. In this study, salt-tolerant and salt-sensitive Casuarina associated Frankia strains were identified and comparative genomics, transcriptome profiling, and proteomics were employed to elucidate the molecular mechanisms of salt and osmotic stress tolerance.

Results

Salt-tolerant Frankia strains (CcI6 and Allo2) that could withstand up to 1000 mM NaCl and a salt-sensitive Frankia strain (CcI3) which could withstand only up to 475 mM NaCl were identified. The remaining isolates had intermediate levels of salt tolerance with MIC values ranging from 650 mM to 750 mM. Comparative genomic analysis showed that all of the Frankia isolates from Casuarina belonged to the same species (Frankia casuarinae). Pangenome analysis revealed a high abundance of singletons among all Casuarina isolates. The two salt-tolerant strains contained 153 shared single copy genes (most of which code for hypothetical proteins) that were not found in the salt-sensitive(CcI3) and moderately salt-tolerant (CeD) strains. RNA-seq analysis of one of the two salt-tolerant strains (Frankia sp. strain CcI6) revealed hundreds of genes differentially expressed under salt and/or osmotic stress. Among the 153 genes, 7 and 7 were responsive to salt and osmotic stress, respectively. Proteomic profiling confirmed the transcriptome results and identified 19 and 8 salt and/or osmotic stress-responsive proteins in the salt-tolerant (CcI6) and the salt-sensitive (CcI3) strains, respectively.

Conclusion

Genetic differences between salt-tolerant and salt-sensitive Frankia strains isolated from Casuarina were identified. Transcriptome and proteome profiling of a salt-tolerant strain was used to determine molecular differences correlated with differential salt-tolerance and several candidate genes were identified. Mechanisms involving transcriptional and translational regulation, cell envelop remodeling, and previously uncharacterized proteins appear to be important for salt tolerance. Physiological and mutational analyses will further shed light on the molecular mechanism of salt tolerance in Casuarina associated Frankia isolates.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-4056-0) contains supplementary material, which is available to authorized users.

Permanent Draft Genome Sequence for Frankia sp. Strain Cc1.17, a Nitrogen-Fixing Actinobacterium Isolated from Root Nodules of Colletia cruciata

Frankia sp. strain Cc1.17 is a member of the Frankia lineage 3, the organisms of which are able to reinfect plants of the Eleagnaceae, Rhamnaceae, and Myricaceae families and the genera Gynmnostoma and Alnus. Here, we report the 8.4-Mbp draft genome sequence, with a G+C content of 72.14% and 6,721 candidate protein-coding genes.

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.

Frankia coriariae sp. nov., an infective and effective microsymbiont isolated from Coriaria japonica

The taxonomic description of a nitrogen-fixing actinobacterium, strain BMG5.1T, as a novel species within the genus Frankia was based on a polyphasic approach. The strain was isolated from the root nodules of Coriaria japonica, and it fulfilled Koch's postulates by inducing effective nodules on Coriaria spp. and Datisca spp. Based on phenotypic and chemotaxonomic features, strain BMG5.1T is distinguishable from all other species of the genus Frankia. It is characterized by the presence of phosphatidylinositol, diphosphatidylglycerol, phosphatidylglycerol and glycophospholipids in its polar lipids; galactose, glucose, mannose and a trace of ribose as cellular sugars; meso-diaminopimelic acid as cell-wall peptidoglycan; C18 : 1ω9c as major fatty acid (>30 %); and MK-9(H6) (44.7 %) as predominant isoprenolog (>30 %). The 16S rRNA gene sequence similarities and the digital DNA-DNA hybridization values between the described novel strain and strains of the other species of the genus Frankia correspond to a range of 97-98.4 % and 22.1-24 %, respectively. The DNA G+C content is 70.2  mol%. On the basis of these results, strain BMG5.1T (=CECT 9032T=DSM 100624T) is proposed as the type strain of a novel species of the genus Frankia, named Frankia coriariae sp. nov.

Frankia discariae sp. nov.: an infective and effective microsymbiont isolated from the root nodule of Discaria trinervis

Strain BCU110501^T was the first isolate reported to fulfill Koch's postulates by inducing effective nodules on its host plant of origin Discaria trinervis (Rhalmnaceae). Based on 16S rRNA gene sequence similarities, the strain was found to be most closely related to the type strain of Frankia elaeagni DSM 46783^T (98.6%) followed by F. alni DSM 45986^T (98.2%), F. casuarinae DSM 45818^T (97.8%) and F. inefficacies DSM 45817^T (97.8%). Digital DNA:DNA hybridizations (dDDH) between strain BCU110501^Tand the type strains of other Frankia species were clearly below the cutoff point of 70%. The G+C content of DNA is 72.36%. The cell wall of strain BCU110501^T contained meso-diaminopimelic acid and the cell sugars were galactose, glucose, mannose, xylose and ribose. Polar lipids were phosphatidylinositol (PI), diphosphatidylglycerol (DPG), glycophospholipid (GPL_1-3), phosphatidylglycerol (PG) and an unknown lipid (L). The major fatty acids of strain BCU110501^T consisted of iso-C16:0, C17:1 w8c and C16:0. Major menaquinones were MK9 (H₄), MK9 (H₆) and MK9 (H₂). Based on these analyses, strain BCU110501^T (=DSM 46785^T=CECT 9042^T) should be classified as the type strain of a novel Frankia species, for which the name Frankia discariae sp. nov. is proposed.