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Peronospora salviae‐officinalis, the causal agent of downy mildew on common sage, is an obligate biotrophic pathogen. It grows in the intercellular spaces of the leaf tissue of sage and forms intracellular haustoria to interface with host cells. Although P. salviae‐officinalis was described as a species of its own 10 years ago, the infection process remains obscure. To address this, a histological study of various infection events, from the adhesion of conidia on the leaf surface to de novo sporulation is presented here. As histological studies of oomycetes are challenging due to the lack of chitin in their cell wall, we also present an improved method for staining downy mildews for confocal laser scanning microscopy as well as evaluating the potential of autofluorescence of fixed nonstained samples. For staining, a 1:1 mixture of aniline blue and trypan blue was found most suitable and was used for staining of oomycete and plant structures, allowing discrimination between them as well as the visualization of plant immune responses. The method was also used to examine samples of Peronospora lamii on Lamium purpureum and Peronospora belbahrii on Ocimum basilicum, demonstrating the potential of the presented histological method for studying the infection processes of downy mildews in general.

Glioblastoma multiforme is an aggressive cancer type with poor patient outcomes. Interestingly, we reported previously a novel association between the little studied paucimannosidic N-linked glycoepitope and glioblastoma. Paucimannose has only recently been detected in vertebrates where it exhibits a very restricted tumor-specific expression. Herein, we demonstrate for the first time a very high protein paucimannosylation in human grade IV glioblastoma and U-87MG and U-138MG glioblastoma cells. Furthermore, we revealed the involvement of paucimannosidic epitopes in tumorigenic processes including cell proliferation, migration, invasion and adhesion. Finally, we identified AHNAK which is discussed as a tumor suppressor as the first paucimannose-carrying protein in glioblastoma and show the involvement of AHNAK in the observed paucimannose-dependent effects. This study is the first to provide evidence of a protective role of paucimannosylation in glioblastoma, a relationship that with further in vivo support may have far reaching benefits for patients suffering from this often fatal disease.

Graphical abstract, Highlights • Suggestion to simplify and unify the nomenclature of phage tail-like particles. • Discovery of kosakonicin, a new bacteriocin and tailocin. • Microscopy of kosakonicin from Kosakonia radicincitans DSM 16656. • Discovery of multiple tail fiber genes in the kosakonicin gene cluster. • Discovery of large genetic diversity in the kosakonicin tail fiber locus among ten Kosakonia strains., Type VI secretion systems and tailocins, two bacterial phage tail-like particles, have been reported to foster interbacterial competition. Both nanostructures enable their producer to kill other bacteria competing for the same ecological niche. Previously, type VI secretion systems and particularly R-type tailocins were considered highly specific, attacking a rather small range of competitors. Their specificity is conferred by cell surface receptors of the target bacterium and receptor-binding proteins on tailocin tail fibers and tail fiber-like appendages of T6SS. Since many R-type tailocin gene clusters contain only one tail fiber gene it was appropriate to expect small R-type tailocin target ranges. However, recently up to three tail fiber genes and broader target ranges have been reported for one plant-associated Pseudomonas strain. Here, we show that having three tail fiber genes per R-type tailocin gene cluster is a common feature of several strains of Gram-negative (often plant-associated) bacteria of the genus Kosakonia. Knowledge about the specificity of type VI secretion systems binding to target bacteria is even lower than in R-type tailocins. Although the mode of operation implicated specific binding, it was only published recently that type VI secretion systems develop tail fiber-like appendages. Here again Kosakonia, exhibiting up to three different type VI secretion systems, may provide valuable insights into the antagonistic potential of plant-associated bacteria. Current understanding of the diversity and potential of phage tail-like particles is fragmentary due to various synonyms and misleading terminology. Consistency in technical terms is a precondition for concerted and purposeful research, which precedes a comprehensive understanding of the specific interaction between bacteria producing phage tail-like particles and their targets. This knowledge is fundamental for selecting and applying tailored, and possibly engineered, producer bacteria for antagonizing plant pathogenic microorganisms.

Plant growth-promoting bacteria (PGPB) affect host physiological processes in various ways. This study aims at elucidating the dependence of bacterial-induced growth promotion on the plant genotype and characterizing plant metabolic adaptations to PGPB. Eighteen Arabidopsis thaliana accessions were inoculated with the PGPB strain Kosakonia radicincitans DSM 16656(T). Colonisation pattern was assessed by enhanced green fluorescent protein (eGFP)-tagged K. radicincitans in three A. thaliana accessions differing in their growth response. Metabolic impact of bacterial colonisation was determined for the best responding accession by profiling distinct classes of plant secondary metabolites and root exudates. Inoculation of 18 A. thaliana accessions resulted in a wide range of growth responses, from repression to enhancement. Testing the bacterial colonisation of three accessions did not reveal a differential pattern. Profiling of plant secondary metabolites showed a differential accumulation of glucosinolates, phenylpropanoids and carotenoids in roots. Analysis of root exudates demonstrated that primary and secondary metabolites were predominantly differentially depleted by bacterial inoculation. The plant genotype controls the bacterial growth promoting traits. Levels of lutein and beta-carotene were elevated in inoculated roots. Supplementing a bacterial suspension with beta-carotene increased bacterial growth, while this was not the case when lutein was applied, indicating that beta-carotene could be a positive regulator of plant growth promotion.

Cell-cell fusion in fungi is required for colony formation, nutrient transfer and signal transduction. Disruption of genes required for hyphal fusion in Epichloe festucae, a mutualistic symbiont of Lolium grasses, severely disrupts the host interaction phenotype. They examined whether symB and symC, the E. festucae homologs of Podospora anserina self-signaling genes IDC2 and IDC3, are required for E. festucae hyphal fusion and host symbiosis. Deletion mutants of these genes were defective in hyphal cell fusion, formed intra-hyphal hyphae, and had enhanced conidiation. SymB-GFP and SymC-mRFP1 localize to plasma membrane, septa and points of hyphal cell fusion. Plants infected with ΔsymB and ΔsymC strains were severely stunted. Hyphae of the mutants colonized vascular bundles, were more abundant than wild type in the intercellular spaces and formed intra-hyphal hyphae. Although these phenotypes are identical to those previously observed for cell wall integrity MAP kinase mutants no difference was observed in the basal level of MpkA phosphorylation or its cellular localization in the mutant backgrounds. Both genes contain binding sites for the transcription factor ProA. Collectively these results show that SymB and SymC are key components of a conserved signaling network for E. festucae to maintain a mutualistic symbiotic interaction within L. perenne.

Summary In both Sordaria macrospora and Neurospora crassa, components of the conserved STRIPAK (striatin-interacting phosphatase and kinase) complex regulate cell–cell fusion, hyphal network development and fruiting body formation. Interestingly, a number of Epichloe festucae genes that are required for hyphal cell–cell fusion, such as noxA, noxR, proA, mpkA and mkkA, are also required for the establishment of a mutualistic symbiotic interaction with Lolium perenne. To determine whether MobC, a homologue of the STRIPAK complex component MOB3 in S. macrospora and N. crassa, is required for E. festucae hyphal fusion and symbiosis, a mobC deletion strain was generated. The ΔmobC mutant showed reduced rates of hyphal cell–cell fusion, formed intrahyphal hyphae and exhibited enhanced conidiation. Plants infected with ΔmobC were severely stunted. Hyphae of ΔmobC showed a proliferative pattern of growth within the leaves of Lolium perenne with increased colonization of the intercellular spaces and vascular bundles. Although hyphae were still able to form expressoria, structures allowing the colonization of the leaf surface, the frequency of formation was significantly reduced. Collectively, these results show that the STRIPAK component MobC is required for the establishment of a mutualistic symbiotic association between E. festucae and L. perenne, and plays an accessory role in the regulation of hyphal cell–cell fusion and expressorium development in E. festucae.

Summary Epichloë festucae forms a mutualistic symbiotic association with Lolium perenne. This biotrophic fungus systemically colonizes the intercellular spaces of aerial tissues to form an endophytic hyphal network. E. festucae also grows as an epiphyte, but the mechanism for leaf surface colonization is not known. Here we identify an appressorium‐like structure, which we call an expressorium that allows endophytic hyphae to penetrate the cuticle from the inside of the leaf to establish an epiphytic hyphal net on the surface of the leaf.We used a combination of scanning electron, transmission electron and confocal laser scanning microscopy to characterize this novel fungal structure and determine the composition of the hyphal cell wall using aniline blue and wheat germ agglutinin labelled with Alexafluor‐488.Expressoria differentiate immediately below the cuticle in the leaf blade and leaf sheath intercalary cell division zones where the hyphae grow by tip growth. Differentiation of this structure requires components of both the NoxA and NoxB NADPH oxidase complexes. Major remodelling of the hyphal cell wall occurs following exit from the leaf.These results establish that the symbiotic association of E. festucae with L. perenne involves an interconnected hyphal network of both endophytic and epiphytic hyphae., See also the Commentary on this article by Genre & Lanfranco, 211: 5–7.

Epichloë festucae is an endophyte of the agriculturally important perennial ryegrass. This species systemically colonises the aerial tissues of this host where its growth is tightly regulated thereby maintaining a mutualistic symbiotic interaction. Recent studies have suggested that small secreted proteins, termed effectors, play a vital role in the suppression of host defence responses. To date only a few effectors with important roles in mutualistic interactions have been described. Here we make use of the fully assembled E. festucae genome and EffectorP to generate a suite of 141 effector candidates. These were analysed with respect to their genome location and expression profiles in planta and in several symbiosis-defective mutants. We found an association between effector candidates and a class of transposable elements known as MITEs, but no correlation with other dynamic features of the E. festucae genome, such as transposable element-rich regions. Three effector candidates and a small GPI-anchored protein were chosen for functional analysis based on their high expression in planta compared to in culture and their differential regulation in symbiosis defective E. festucae mutants. All three candidate effector proteins were shown to possess a functional signal peptide and two could be detected in the extracellular medium by western blotting. Localization of the effector candidates in planta suggests that they are not translocated into the plant cell, but rather, are localized in the apoplastic space or are attached to the cell wall. Deletion and overexpression of the effector candidates, as well as the putative GPI-anchored protein, did not affect the plant growth phenotype or restrict growth of E. festucae mutants in planta. These results indicate that these proteins are either not required for the interaction at the observed life stages or that there is redundancy between effectors expressed by E. festucae.

The recent worldwide discovery of plant growth-promoting (PGP) Kosakonia radicincitans in a large variety of crop plants suggests that this species confers significant influence on plants, both in terms of yield increase and product quality improvement. We provide a comparative genome analysis which helps to unravel the genetic basis for K. radicincitans' motility, competitiveness and plant growth-promoting capacities. We discovered that K. radicincitans carries multiple copies of complex gene clusters, among them two flagellar systems and three type VI secretion systems (T6SSs). We speculate that host invasion may be facilitated by different flagella, and bacterial competitor suppression by effector proteins ejected via T6SSs. We found a large plasmid in K. radicincitans DSM 16656T, the species type strain, that confers the potential to exploit plant-derived carbon sources. We propose that multiple copies of complex gene clusters in K. radicincitans are metabolically expensive but provide competitive advantage over other bacterial strains in nutrient-rich environments. The comparison of the DSM 16656T genome to genomes of other genera of enteric plant growth-promoting bacteria (PGPB) exhibits traits unique to DSM 16656T and K. radicincitans, respectively, and traits shared between genera. We used the output of the in silico analysis for predicting the purpose of genomic features unique to K. radicincitans and performed microarray, PhyloChip, and microscopical analyses to gain deeper insight into the interaction of DSM 16656T, plants and associated microbiota. The comparative genome analysis will facilitate the future search for promising candidates of PGPB for sustainable crop production.

Natural hosts for the fungal endophyte Epichloe festucae include Festuca rubra (fine fescue) and Festuca trachyphylla (hard fescue). Some strains also form stable associations with Lolium perenne (perennial ryegrass). L. perenne is a suitable host to study fungal endophyte-grass interactions, such as endophytic fungal growth within the plant and epiphyllous growth on the plant surface. Here we provide a detailed protocol based on work by, for artificial inoculation of E. festucae into L. perenne, and newly developed staining and visualization techniques for observing endophytic and epiphyllous hyphae and the expressorium, an appressorium-like structure used by the fungus to exit the plant. The staining method uses a combination of glucan binding aniline blue diammonium salt (AB) and chitin binding wheat germ agglutinin-conjugated Alexa Fluor®488 -(WGA-AF488). This protocol will be a useful tool to study Epichloe-grass interactions, particularly the comparison of different Epichloe-grass associations, various endophyte-host developmental stages, as well as the analysis of mutant Epichloe strains.