Omics of the early molecular dialogue between Frankia alni and Alnus glutinosa and the cellulase synton

Identification and evolution of nsLTPs in the root nodule nitrogen fixation clade and molecular response of Frankia to AgLTP24

Non-specific lipid transfer proteins (nsLTPs) are antimicrobial peptides, involved in several plant biological processes including root nodule nitrogen fixation (RNF). Nodulating plants belonging to the RNF clade establish symbiosis with the nitrogen-fixing bacteria rhizobia (legumes symbiosis model) and Frankia (actinorhizal symbiosis model) leading to root nodule formation. nsLTPs are involved in processes active in early step of symbiosis and functional nodule in both models. In legumes, nsLTPs have been shown to regulate symbiont entry, promote root cortex infection, membrane biosynthesis, and improve symbiosis efficiency. More recently, a nsLTP, AgLTP24 has been described in the context of actinorhizal symbiosis between Alnus glutinosa and Frankia alni ACN14a. AgLTP24 is secreted at an early step of symbiosis on the deformed root hairs and targets the symbiont in the nitrogen-fixing vesicles in functional nodules. nsLTPs are involved in RNF, but their functions and evolutionary history are still largely unknown. Numerous putative nsLTPs were found up-regulated in functional nodules compared to non-infected roots in different lineages within the RNF clade. Here, results highlight that nodulating plants that are co-evolving with their nitrogen-fixing symbionts appear to have independently specialized nsLTPs for this interaction, suggesting a possible convergence of function, which opens perspectives to investigate nsLTPs functions in RNF.

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.

NAD+ Synthetase is Required for Free-living and Symbiotic Nitrogen Fixation in the Actinobacterium Frankia casuarinae

Frankia spp. are multicellular actinobacteria that fix atmospheric dinitrogen (N₂) not only in the free-living state, but also in root-nodule symbioses with more than 200 plant species, called actinorhizal plants. To identify novel Frankia genes involved in N₂ fixation, we previously isolated mutants of Frankia casuarinae that cannot fix N₂. One of these genes, mutant N3H4, did not induce nodulation when inoculated into the host plant Casuarina glauca. Cell lineages that regained the ability to fix N₂ as free-living cells were isolated from the mutant cell population. These restored strains also regained the ability to stimulate nodulation. A comparative ana-lysis of the genomes of mutant N3H4 and restored strains revealed that the mutant carried a mutation (Thr584Ile) in the glutamine-dependent NAD⁺ synthetase gene (Francci3_3146), while restored strains carried an additional suppressor mutation (Asp478Asn) in the same gene. Under nitrogen-depleted conditions, the concentration of NAD(H) was markedly lower in the mutant strain than in the wild type, whereas it was higher in restored strains. These results indicate that glutamine-dependent NAD⁺ synthetase plays critical roles in both free-living and symbiotic N₂ fixation in Frankia.

Draft Genomes of Symbiotic Frankia Strains AgB32 and AgKG'84/4 from Root Nodules of Alnus Glutinosa growing under Contrasted Environmental Conditions

The genomes of two nitrogen-fixing Frankia strains, AgB32 and AgKG'84/4, were isolated from spore-containing (spore+) and spore-free (spore-) root nodules of Alnus glutinosa, but they did not sporulate upon reinfection. The two strains are described as representatives of two novel candidate species. Phylogenomic and ANI analyses indicate that each strain represents a novel species within cluster 1, with genome sizes of 6.3 and 6.7 Mb smaller than or similar to those of other cultivated Alnus-infective cluster 1 strains. Genes essential for nitrogen-fixation, clusters of orthologous genes, secondary metabolite clusters and transcriptional regulators analyzed by comparative genomic analyses were typical of those from Alnus-infective cluster 1 cultivated strains in both genomes. Compared to other cultivated Alnus-infective strains with large genomes, those of AgB32 and AgKG'84/4 had lost 380 or 409 genes, among which one hup cluster, one shc gene and the gvp cluster, which indicates genome erosion is taking place in these two strains.

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.

The Proteogenome of Symbiotic Frankia alni in Alnus glutinosa Nodules

Omics are the most promising approaches to investigate microbes for which no genetic tools exist such as the nitrogen-fixing symbiotic Frankia. A proteogenomic analysis of symbiotic Frankia alni was done by comparing those proteins more and less abundant in Alnus glutinosa nodules relative to N₂-fixing pure cultures with propionate as the carbon source. There were 250 proteins that were significantly overabundant in nodules at a fold change (FC) ≥ 2 threshold, and 1429 with the same characteristics in in vitro nitrogen-fixing pure culture. Nitrogenase, SuF (Fe–Su biogenesis) and hopanoid lipids synthesis determinants were the most overabundant proteins in symbiosis. Nitrogenase was found to constitute 3% of all Frankia proteins in nodules. Sod (superoxide dismutase) was overabundant, indicating a continued oxidative stress, while Kats (catalase) were not. Several transporters were overabundant including one for dicarboxylates and one for branched amino acids. The present results confirm the centrality of nitrogenase in the actinorhizal symbiosis.

Effect of actinorhizal root exudates on the proteomes of Frankia soli NRRL B-16219, a strain colonizing the root tissues of its actinorhizal host via intercellular pathway

Frankia and actinorhizal plants exchange signals in the rhizosphere leading to specific mutual recognition of partners and nitrogen-fixing nodule organogenesis. Frankia soli strain NRRL B-16219, from the Elaeagnus specificity group, colonizes the root tissues of its actinorhizal host through direct intercellular penetration of root epidermis cells and cortex. Here, we studied the early proteogenomic response of strain NRRL B-16219 to treatment with root exudates from compatible Elaeagnus angustifolia, and incompatible Ceanothus thyrsiflorus and Coriaria myrtifolia, host plants grown in nitrogen depleted hydroponic medium. Next-generation proteomics was used to identify the main Frankia proteins differentially expressed in response to the root exudates. No products of the nod genes present in B-16219 were detected. Proteins specifically upregulated in presence of E. angustifolia root exudates include those connected to nitrogen fixation and assimilation (glutamate synthetase, hydrogenase and squalene synthesis), respiration (oxidative phosphorylation and citric acid cycle pathways), oxidative stress (catalase, superoxide dismutase, and peroxidase), proteolysis (proteasome, protease, and peptidase) and plant cell wall degrading proteins involved in the depolymerization of celluloses (endoglucanase, glycosyltransferase, beta-mannanases, glycoside hydrolase and glycosyl hydrolase). Proteomic data obtained in this study will help link signaling molecules/factors to their biosynthetic pathways once those factors have been fully characterized.

Advanced Proteomics as a Powerful Tool for Studying Toxins of Human Bacterial Pathogens

Exotoxins contribute to the infectious processes of many bacterial pathogens, mainly by causing host tissue damages. The production of exotoxins varies according to the bacterial species. Recent advances in proteomics revealed that pathogenic bacteria are capable of simultaneously producing more than a dozen exotoxins. Interestingly, these toxins may be subject to post-transcriptional modifications in response to environmental conditions. In this review, we give an outline of different bacterial exotoxins and their mechanism of action. We also report how proteomics contributed to immense progress in the study of toxinogenic potential of pathogenic bacteria over the last two decades.

Comparative genomics and proteogenomics highlight key molecular players involved in Frankia sporulation

Sporulation is a microbial adaptive strategy to resist inhospitable conditions for vegetative growth and to disperse to colonise more favourable environments. This microbial trait is widespread in Actinobacteria. Among them, Frankia strains are able to differentiate sporangia in pure culture, while others can sporulate even when in symbiosis with sporulation occurring within host cells. The molecular determinants controlling Frankia sporulation have not been yet described. In order to highlight, for the first time, the molecular players potentially involved in Frankia sporulation, we conducted (i) a comparison of protein contents between Frankia spores and hyphae and (ii) a comparative genomic analysis of Frankia proteomes with sporulating and non-sporulating Actinobacteria. Among the main results, glycogen-metabolism related proteins, as well as oxidative stress response and protease-like proteins were overdetected in hyphae, recalling lytic processes that allow Streptomyces cells to erect sporogenic hyphae. Several genes encoding transcriptional regulators, including GntR-like, appeared up-regulated in spores, as well as tyrosinase, suggesting their potential role in mature spore metabolism. Finally, our results highlighted new proteins potentially involved in Frankia sporulation, including a pyrophosphate-energized proton pump and YaaT, described as involved in the phosphorelay allowing sporulation in Bacillus subtilis, leading us to discuss the role of a phosphorelay in Frankia sporulation.