Can oak powdery mildew severity be explained by indirect effects of climate on the composition of the Erysiphe pathogenic complex? B Marçais, D Piou, D Dezette, ML Desprez-Loustau Phytopathology

Abstract

Coinfection by several pathogens is increasingly recognized as an important feature in the epidemiology and evolution of plant fungal pathogens. Oak mildew is induced by 2 closely related Erysiphe invasive species E. alphitoides and E. quercicola, which differ in their mode of overwintering. We investigated how climate influences the co-occurrence of the 2 species in oak young stands and whether this is important for the disease epidemiology We studied the frequency of flag-shoots (i.e. shoots developing from infected buds, usually associated with E. quercicola) in 95 oak regenerations over a 6 year period.. Additionally, in 2012 and 2013, the oak mildew severity and the 2 Erysiphe species relative frequencies were determined both in spring and in autumn in 51 regenerations and 43 one-year-old plantations of oaks. Both the frequency of flag-shoots and the proportion of Erysiphe lesions with E. quercicola presence were related to climate. We showed that survival of E. quercicola was improved after mild winters, with increase of both the flag-shoot frequency and the proportion of Erysiphe lesions with E. quercicola presence in spring. However, disease severity was not related to any complementarity effect between the two Erysiphe species causing oak powdery mildew. By contrast, increased E. alphitoides prevalence in spring was associated with higher oak mildew severity in autumn. Our results point out the critical role of between season transmission and primary inoculum to explain disease dynamics which could be significant in a climate warming context.

Photoreceptors in the dark: a functional white collar-like complex and other putative light-sensing components encoded by the genome of the subterranean fungus Tuber melanosporumR Gerace, B Montanini, M Proietto, E Levati, C De Luca, A Brenna, … Fungal Biology

Abstract

Light is perceived and transduced by fungi, where it modulates processes as diverse as growth and morphogenesis, sexual development and secondary metabolism. A special case in point is that of fungi with a subterranean, light-shielded habitat such as Tuber spp. Using as reference the genome sequence of the black truffle Tuber melanosporum, we employed sequence prediction tools and expression data to gain insight on the photoreceptor systems expressed by this hypogeous ectomycorryzal fungus. These include a chromophore-less opsin preferentially expressed in fruiting bodies, a putative red light-sensing phytochrome that is not expressed at detectable levels in any of the examined developmental conditions, and a nearly canonical two-component (WC-1/WC-2) photoreceptor system similar to the Neurospora white collar complex. The latter is expressed at relatively high levels throughout the different developmental stages of T. melanosporum, except for sexual stage fruiting bodies where it is down-regulated. Various evidences, including the growth arrest phenotype elicited by blue light and the ability of a chimeric Tuber/Neurospora WC-1 protein to complement a N. crassa wc-1^koblind strain suggest that the Tuber white collar complex is likely functional and capable of responding to blue-light. The other putative photoreceptor components we have identified in the T. melanosporum genome, especially the chromophore-less opsin and the likely non-functional phytochrome, may represent the signatures of adaptation to a hypogeous (light-shielded) lifestyle.

Regulation of Differentiation of Nitrogen-Fixing Bacteria by Microsymbiont Targeting of Plant Thioredoxin s1 CW Ribeiro, F Baldacci-Cresp, O Pierre, M Larousse, S Benyamina, … Current Biology

Mineral type and tree species determine the functional and taxonomic structure of forest soil bacterial communities Y Colin, O Nicolitch, MP Turpault, S Uroz Applied and Environmental Microbiology, AEM. 02684-16

ABSTRACT

Although minerals represent important soil constituents, their impact on the diversity and structure of soil microbial communities remains poorly documented. In this study, pure mineral particles with varying chemistries (i.e., obsidian, apatite and calcite) were considered. Each mineral type was conditioned in mesh bags and incubated in soil below different tree stands (beech, coppice with standards and Corsican pine) for 2.5 years to determine the relative impact of mineralogy and mineral weatherability on the taxonomic and functional diversity of mineral-associated bacterial communities. After this incubation period, the minerals and the surrounding bulk soil were collected to determine mass loss and to perform soil analyses, enzymatic assays, and cultivation-dependent and -independent analyses. Notably, our 16S rRNA gene pyrosequencing analyses revealed that, after the 2.5-year incubation period, the mineral-associated bacterial communities strongly differed from those of the surrounding bulk soil for all tree stands considered. When focusing only on minerals, our analyses showed that the bacterial communities associated with calcite, the less recalcitrant mineral type, significantly differed from those that colonized obsidian and apatite minerals. The cultivation-dependent analysis revealed significantly higher abundances of effective mineral weathering bacteria on the most recalcitrant minerals (i.e., apatite and obsidian). Together, our data showed an enrichment of Betaproteobacteria and effective mineral weathering bacteria related to the Burkholderia and Collimonas genera on the minerals, suggesting a key role for these taxa in mineral weathering and nutrient cycling in nutrient-poor forest ecosystems.

IMPORTANCE Forests are usually developed on nutrient-poor and rocky soils, while nutrient-rich soils have been dedicated to agriculture. In this context, nutrient recycling and nutrient access are key processes in such environments. Deciphering how soil mineralogy influences the diversity, structure and function of soil bacterial communities in relation to the soil conditions is crucial to better understanding the relative role of the soil bacterial communities in nutrient cycling and plant nutrition in nutrient-poor environments. The present study determined in detail the diversity and structure of bacterial communities associated with different mineral types incubated for 2.5 years in the soil under different tree species using cultivation-dependent and -independent analyses. Our data showed an enrichment of specific bacterial taxa on the minerals, specifically on the most weathered minerals, suggesting that they play key roles in mineral weathering and nutrient cycling in nutrient-poor forest ecosystems.

The escalatory Red Queen: Population extinction and replacement following arms‐race dynamics in poplar rust A Persoons, KJ Hayden, B Fabre, P Frey, S De Mita, A Tellier, F Halkett Molecular Ecology

Abstract

Host-parasite systems provide convincing examples of Red Queen co-evolutionary dynamics. Yet, a key process underscored in Van Valen’s theory – that arms-race dynamics can result in extinction – has never been documented. One reason for this may be that most sampling designs lack the breadth needed to illuminate the rapid pace of adaptation by pathogen populations. In this study we used a 25-years temporal sampling to decipher the demographic history of a plant pathogen: the poplar rust fungus, Melampsora larici-populina. A major adaptive event occurred in 1994 with the breakdown of R7 resistance carried by several poplar cultivars widely planted in Western Europe since 1982. The corresponding virulence rapidly spread in M. larici-populina populations, and nearly reached fixation in northern France, even on susceptible hosts. Using both temporal records of virulence profiles and temporal population genetic data, our analyses revealed that (i) R7 resistance breakdown resulted in the emergence of a unique and homogeneous genetic group, the so-called cultivated population, which predominated in northern France for about 20 years, (ii) selection for Vir7 individuals brought with it multiple other virulence types via hitchhiking, resulting in an overall increase in the population-wide number of virulence types and (iii) – above all – the emergence of the cultivated population superseded the initial population which predominated at the same place before R7 resistance breakdown. Our temporal analysis illustrates how antagonistic co-evolution can lead to population extinction and replacement, hence providing direct evidence for the escalation process which is at the core of Red Queen dynamics.

Ecologically Different Fungi Affect Arabidopsis Development: Contribution of Soluble and Volatile Compounds S Casarrubia, S Sapienza, H Fritz, S Daghino, M Rosenkranz, … PLOS ONE 11 (12), e0168236

Abstract

Plant growth and development can be influenced by mutualistic and non-mutualistic microorganisms. We investigated the ability of the ericoid endomycorrhizal fungus Oidiodendron maius to influence growth and development of the non-host plant Arabidopsis thaliana. Different experimental setups (non-compartmented and compartmented co-culture plates) were used to investigate the influence of both soluble and volatile fungal molecules on the plant phenotype. O. maius promoted growth of A. thaliana in all experimental setups. In addition, a peculiar clumped root phenotype, characterized by shortening of the primary root and by an increase of lateral root length and number, was observed in A. thaliana only in the non-compartmented plates, suggesting that soluble diffusible molecules are responsible for this root morphology. Fungal auxin does not seem to be involved in plant growth promotion and in the clumped root phenotype because co-cultivation with O. maius did not change auxin accumulation in plant tissues, as assessed in plants carrying the DR5::GUS reporter construct. In addition, no correlation between the amount of fungal auxin produced and the plant root phenotype was observed in an O. maius mutant unable to induce the clumped root phenotype in A. thaliana. Addition of active charcoal, a VOC absorbant, in the compartmented plates did not modify plant growth promotion, suggesting that VOCs are not involved in this phenomenon. The low VOCs emission measured for O. maius further corroborated this hypothesis. By contrast, the addition of CO2 traps in the compartmented plates drastically reduced plant growth, suggesting involvement of fungal CO2 in plant growth promotion. Other mycorrhizal fungi, as well as a saprotrophic and a pathogenic fungus, were also tested with the same experimental setups. In the non-compartmented plates, most fungi promoted A. thaliana growth and some could induce the clumped root phenotype. In the compartmented plate experiments, a general induction of plant growth was observed for most other fungi, especially those producing higher biomass, further strengthening the role of a nonspecific mechanism, such as CO2 emission.

Screening for N-AHSL-Based-Signaling Interfering Enzymes. S Uroz, PM Oger Metagenomics: Methods and Protocols, 271-286

Abstract

Quorum sensing (QS)-based signaling is a widespread pathway used by bacteria for the regulation of functions involved in their relation to the environment or their host. QS relies upon the production, accumulation and perception of small diffusable molecules by the bacterial population, hence linking high gene expression with high cell population densities. Among the different QS signal molecules, an important class of signal molecules is the N-acyl homoserine lactone (N-AHSL). In pathogens such as Erwinia or Pseudomonas, N-AHSL based QS is crucial to overcome the host defenses and ensure a successful infection. Interfering with QS-regulation allows the algae Delisea pulcra to avoid surface colonization by bacteria. Thus, interfering the QS-regulation of pathogenic bacteria is a promising antibiotic-free antibacterial therapeutic strategy. To date, two N-AHSL lactonases and one amidohydrolase families of N-ASHL degradation enzymes have been characterized and have proven to be efficient in vitro to control N-AHSL-based QS-regulated functions in pathogens. In this chapter, we provide methods to screen individual clones or bacterial strains as well as pool of clones for genomic and metagenomic libraries, that can be used to identify strains or clones carrying N-ASHL degradation enzymes.