Faba bean root exudates alter pea root colonization by the oomycete Aphanomyces euteiches at early stages of infection.

Aphanomyces euteiches is an oomycete pathogen that causes the pea root rot. We investigated the potential role of early belowground defense in pea (susceptible plant) and faba bean (tolerant plant) at three days after inoculation. Pea and faba bean were inoculated with A. euteiches zoospores. Root colonization was examined. Root exudates from pea and faba bean were harvested and their impact on A. euteiches development were assessed by using in vitro assays. A. euteiches root colonization and the influence of the oomycete inoculation on specialized metabolites patterns and arabinogalactan protein (AGP) concentration of root exudates were also determined. In faba bean root, A. euteiches colonization was very low as compared with that of pea. Whereas infected pea root exudates have a positive chemotaxis index (CI) on zoospores, faba bean exudate CI was negative suggesting a repellent effect. While furanoacetylenic compounds were only detected in faba bean exudates, AGP concentration was specifically increased in pea.This work showed that early in the course of infection, host susceptibility to A. euteiches is involved via a plant-species specific root exudation opening new perspectives in pea root rot disease management.

Identification of Tomato Ve1 Homologous Proteins in Flax and Assessment for Race-Specific Resistance in Two Fiber FlaxCultivars against Verticillium dahliae Race 1.

In the last decade, the soil borne fungal pathogen Verticillium dahliae has had an increasingly strong effect on fiber flax (Linum usitatissimum L.), thus causing important yield losses in Normandy, France. Race-specific resistance against V. dahliae race 1 is determined by tomato Ve1, a leucine-rich repeat (LRR) receptor-like protein (RLP). Furthermore, homologous proteins have been found in various plant families. Herein, four homologs of tomato Ve1 were identified in the flax proteome database. The selected proteins were named LuVe11, LuVe12, LuVe13 and LuVe14 and were compared to other Ve1. Sequence alignments and phylogenic analysis were conducted and detected a high similarity in the content of amino acids and that of the Verticillium spp. race 1 resistance protein cluster. Annotations on the primary structure of these homologs reveal several features of tomato Ve1, including numerous copies of a 28 amino acids consensus motif [XXIXNLXXLXXLXLSXNXLSGXIP] in the LRR domain. An in vivo assay was performed using V. dahliae race 1 on susceptible and tolerant fiber flax cultivars. Despite the presence of homologous genes and the stronger expression of LuVe11 compared to controls, both cultivars exhibited symptoms and the pathogen was observed within the stem. Amino acid substitutions within the segments of the LRR domain could likely affect the ligand binding and thus the race-specific resistance. The results of this study indicate that complex approaches including pathogenicity tests, microscopic observations and gene expression should be implemented for assessing race-specific resistance mediated by Ve1 within the large collection of flax genotypes.

Effect of a Bacillus subtilis strain on flax protection against Fusarium oxysporum and its impact on the root and stem cell walls.

In Normandy, flax is a plant of important economic interest because of its fibres. Fusarium oxysporum, a telluric fungus, is responsible for the major losses in crop yield and fibre quality. Several methods are currently used to limit the use of phytochemicals on crops. One of them is the use of plant growth promoting rhizobacteria (PGPR) occurring naturally in the rhizosphere. PGPR are known to act as local antagonists to soil-borne pathogens and to enhance plant resistance by eliciting the induced systemic resistance (ISR). In this study, we first investigated the cell wall modifications occurring in roots and stems after inoculation with the fungus in two flax varieties. First, we showed that both varieties displayed different cell wall organization and that rapid modifications occurred in roots and stems after inoculation. Then, we demonstrated the efficiency of a Bacillus subtilis strain to limit Fusarium wilt on both varieties with a better efficiency for one of them. Finally, thermo-gravimetry was used to highlight that B. subtilis induced modifications of the stem properties, supporting a reinforcement of the cell walls. Our findings suggest that the efficiency and the mode of action of the PGPR B. subtilis is likely to be flax variety dependent.

A Flavor Lactone Mimicking AHL Quorum-Sensing Signals Exploits the Broad Affinity of the QsdR Regulator to Stimulate Transcription of the Rhodococcal qsd Operon Involved in Quorum-Quenching and Biocontrol Activities.

In many Gram-negative bacteria, virulence, and social behavior are controlled by quorum-sensing (QS) systems based on the synthesis and perception of N-acyl homoserine lactones (AHLs). Quorum-quenching (QQ) is currently used to disrupt bacterial communication, as a biocontrol strategy for plant crop protection. In this context, the Gram-positive bacterium Rhodococcus erythropolis uses a catabolic pathway to control the virulence of soft-rot pathogens by degrading their AHL signals. This QS signal degradation pathway requires the expression of the qsd operon, encoding the key enzyme QsdA, an intracellular lactonase that can hydrolyze a wide range of substrates. QsdR, a TetR-like family regulator, represses the expression of the qsd operon. During AHL degradation, this repression is released by the binding of the γ-butyrolactone ring of the pathogen signaling molecules to QsdR. We show here that a lactone designed to mimic quorum signals, γ-caprolactone, can act as an effector ligand of QsdR, triggering the synthesis of qsd operon-encoded enzymes. Interaction between γ-caprolactone and QsdR was demonstrated indirectly, by quantitative RT-PCR, molecular docking and transcriptional fusion approaches, and directly, in an electrophoretic mobility shift assay. This broad-affinity regulatory system demonstrates that preventive or curative quenching therapies could be triggered artificially and/or managed in a sustainable way by the addition of γ-caprolactone, a compound better known as cheap food additive. The biostimulation of QQ activity could therefore be used to counteract the lack of consistency observed in some large-scale biocontrol assays.

Verticillium Wilt on Fiber Flax: Symptoms and Pathogen Development In Planta.

Fiber flax (Linum usitatissimum L.), an important crop in Normandy (France), is increasingly affected by Verticillium wilt caused by the soilborne fungus Verticillium dahliae. This disease leads to nonnegligible yield losses and depreciated fibers that are consequently difficult to upgrade. Verticillium wilt is a major threat to a broad range of agriculture. In this study, susceptible fiber flax cultivar Adélie was infected by VdLu01 (isolated from fiber flax, this study) or green fluorescent protein-tagged VdLs17 (transformed and provided by the department of Plant Pathology, University of California, Davis). Between 3 and 4 weeks postinoculation, wilting symptoms on leaves were first observed, with acropetal growth during the following weeks. Pathogen development was tracked by confocal laser-scanning microscopy during the asymptomatic and symptomatic stages. First, conidia germination led to the development of hyphae on root epidermis; more particularly, on the zone of cell differentiation and around emerging lateral roots, while the zone of cell division and the root tip were free of the pathogen. At 3 days postinoculation, the zone of cell differentiation and lateral roots were embedded into a fungal mass. Swelling structures such as appressoria were observed at 1 week postinoculation. At 2 weeks postinoculation and onward, the pathogen had colonized xylem vessels in roots, followed by the stem and, finally, leaves during the symptomatic stage. Additionally, observations of infected plants after retting in the field revealed microsclerotia embedded inside the bast fiber bundle, thus potentially contributing to weakening of fiber. All of these results provide a global account of V. dahliae development when infecting fiber flax.

Oxidoreductases provide a more generic response to metallic stressors (Cu and Cd) than hydrolases in soil fungi: new ecotoxicological insights.

The present study investigates the effect of metals on the secretion of enzymes from 12 fungal strains maintained in liquid cultures. Hydrolases (acid phosphatase, ß-glucosidase, ß-galactosidase, and N-acetyl-ß-glucosaminidase) and ligninolytic oxidoreductases (laccase, Mn, and lignin peroxidases) activities, as well as biomass production, were measured in culture fluids from fungi exposed to Cu or Cd. Our results showed that all fungi secreted most of the selected hydrolases and that about 50% of them produced a partial oxidative system in the absence of metals. Then, exposure of fungi to metals led to the decrease in biomass production. At the enzymatic level, Cu and Cd modified the secretion profiles of soil fungi. The response of hydrolases to metals was contrasted and complex and depended on metal, enzyme, and fungal strain considered. By contrast, the metals always stimulated the activity of ligninolytic oxidoreductases in fungal strains. In some of them, oxidoreductases were specifically produced following metal exposure. Fungal oxidoreductases provide a more generic response than hydrolases, constituting thus a physiological basis for their use as biomarkers of metal exposure in soils.

A Sensitive Assay for Rapid Detection and Quantification of Aphanomyces euteiches in Soil.

Aphanomyces euteiches is a widespread oomycete pathogen causing root rot in a wide range of leguminous crops. Losses can reach up to 100% for pea culture and there is currently no registered pesticide for its control. Crop management remains the most efficient tool to control root rot, and avoidance of infested soil seems to be the optimal solution. A test was developed to identify fields suitable for pea crops, consisting of the determination of the inoculum potential of soil using baiting plants. A new rapid, specific, and sensitive molecular method is described allowing the quantification of less than 10 oospores per gram of soil. This challenge is achieved by a real-time polymerase chain reaction procedure targeting internal transcribed spacer 1 from the ribosomal DNA operons. A preliminary study based on typical soils from northwestern France demonstrated that the A. euteiches oospore density in soil is related to the inoculum potential. Furthermore, this method has proved sensitive enough to accurately study the influence of biotic factors that may govern the actual emergence of root rot.

Effect of arabinogalactan proteins from the root caps of pea and Brassica napus on Aphanomyces euteiches zoospore chemotaxis and germination.

Root tips of many plant species release a number of border, or border-like, cells that are thought to play a major role in the protection of root meristem. However, little is currently known on the structure and function of the cell wall components of such root cells. Here, we investigate the sugar composition of the cell wall of the root cap in two species: pea (Pisum sativum), which makes border cells, and Brassica napus, which makes border-like cells. We find that the cell walls are highly enriched in arabinose and galactose, two major residues of arabinogalactan proteins. We confirm the presence of arabinogalactan protein epitopes on root cap cell walls using immunofluorescence microscopy. We then focused on these proteoglycans by analyzing their carbohydrate moieties, linkages, and electrophoretic characteristics. The data reveal (1) significant structural differences between B. napus and pea root cap arabinogalactan proteins and (2) a cross-link between these proteoglycans and pectic polysaccharides. Finally, we assessed the impact of root cap arabinogalactan proteins on the behavior of zoospores of Aphanomyces euteiches, an oomycetous pathogen of pea roots. We find that although the arabinogalactan proteins of both species induce encystment and prevent germination, the effects of both species are similar. However, the arabinogalactan protein fraction from pea attracts zoospores far more effectively than that from B. napus. This suggests that root arabinogalactan proteins are involved in the control of early infection of roots and highlights a novel role for these proteoglycans in root-microbe interactions.

Association between border cell responses and localized root infection by pathogenic Aphanomyces euteiches.

BACKGROUND AND AIMS: The oomycete Aphanomyces euteiches causes up to 80 % crop loss in pea (Pisum sativum). Aphanomyces euteiches invades the root system leading to a complete arrest of root growth and ultimately to plant death. To date, disease control measures are limited to crop rotation and no resistant pea lines are available. The present study aims to get a deeper understanding of the early oomycete-plant interaction at the tissue and cellular levels. METHODS: Here, the process of root infection by A. euteiches on pea is investigated using flow cytometry and microscopic techniques. Dynamic changes in secondary metabolism are analysed with high-performance liquid chromatography with diode-array detection. KEY RESULTS: Root infection is initiated in the elongation zone but not in the root cap and border cells. Border-cell production is significantly enhanced in response to root inoculation with changes in their size and morphology. The stimulatory effect of A. euteiches on border-cell production is dependent on the number of oospores inoculated. Interestingly, border cells respond to pathogen challenge by increasing the synthesis of the phytoalexin pisatin. CONCLUSIONS: Distinctive responses to A. euteiches inoculation occur at the root tissue level. The findings suggest that root border cells in pea are involved in local defence of the root tip against A. euteiches. Root border cells constitute a convenient quantitative model to measure the molecular and cellular basis of plant-microbe interactions.

Secretion profiles of fungi as potential tools for metal ecotoxicity assessment: a study of enzymatic system in Trametes versicolor.

The relationship between the expression of extracellular enzymatic system and a metal stress is scarce in fungi, hence limiting the possible use of secretion profiles as tools for metal ecotoxicity assessment. In the present study, we investigated the effect of Zn, Cu, Pb and Cd, tested alone or in equimolar cocktail, on the secretion profiles at enzymatic and protein levels in Trametesversicolor. For that purpose, extracellular hydrolases (acid phosphatase, ß-glucosidase, ß-galactosidase and N-acetyl-ß-glucosaminidase) and ligninolytic oxidases (laccase, Mn-peroxidase) were monitored in liquid cultures. Fungal secretome was analyzed by electrophoresis and laccase secretion was characterized by western-blot and mass spectrometry analyses. Our results showed that all hydrolase activities were inhibited by the metals tested alone or in cocktail, whereas oxidase activities were specifically stimulated by Cu, Cd and metal cocktail. At protein level, metal exposure modified the electrophoretic profiles of fungal secretome and affected the diversity of secreted proteins. Two laccase isoenzymes, LacA and LacB, identified by mass spectrometry were differentially glycosylated according to the metal exposure. The amount of secreted LacA and LacB was strongly correlated with the stimulation of laccase activity by Cu, Cd and metal cocktail. These modifications of extracellular enzymatic system suggest that fungal oxidases could be used as biomarkers of metal exposure.

Insights into the development of fungal biomarkers for metal ecotoxicity assessment: case of Trametes versicolor exposed to copper.

The relationship between the physiological state of fungi and the response of their functional system to metals is not known, limiting the use of fungal enzymes as tools for assessing metal ecotoxicity in terrestrial ecosystems. The present study attempts to establish how the development phases modulate the secretion of enzymes in the filamentous fungus Trametes versicolor after exposure to Cu. For that purpose, extracellular hydrolases (acid and alkaline phosphatases, aryl-sulfatase, beta-glucosidase, beta-galactosidase, and N-acetyl-beta-glucosaminidase) and oxidoreductases (laccase, manganese and lignin peroxidases) were monitored in liquid cultures for 2 weeks. Copper was added during either the growth or the stationary phases at 20 or 200 ppm. Results of the present study showed that Cu at the highest concentration modifies the secretion of enzymes, regardless of the development phase to which the fungus was exposed. However, the sensitivity of enzyme responses to Cu depended on the phase development and the type of secreted enzyme. In a general way, the production of hydrolases was decreased by Cu, whereas that of oxidoreductases was highly increased. Furthermore, lignin peroxidase was not detected in control cultures and was specifically produced in the presence of Cu. In conclusion, fungal oxidoreductases may be enzymatic biomarkers of copper exposure for ecotoxicity assessment.

Development of a molecular method to detect and quantify Aphanomyces euteiches in soil.

A real-time PCR assay using 136F/211R primers and 161T TaqMan probe for the detection and quantification of Aphanomyces euteiches in soil is presented. The specificity of primers was tested on 105 different A. euteiches isolates, mainly from France. A calibration curve was established with a plasmid pHS1 resulting from the target region cloned into the pCR4 Topo vector (Invitrogen). The target copy number was evaluated and was constant whatever the isolate. A DNA-based method was able to discriminate between different artificial infestation levels in soil with small SDs thus validating the relevance of the extraction and amplification method in soil samples. Furthermore, a good correlation was observed between inoculum quantity in soil estimated by qPCR and root rot severity in plant evaluated by bioassays. These steps are essential when considering the feasibility of using a DNA-based method as a fast and accurate way to evaluate inoculum quantity in soil.

Alteration of raw-milk cheese by Pseudomonas spp.: monitoring the sources of contamination using fluorescence spectroscopy and metabolic profiling.

Thirty Pseudomonas spp. strains isolated from milk, water, cheese centre and cheese surface in two traditional workshops manufacturing raw milk St. Nectaire cheese were characterised by fluorescence spectroscopy and Biolog metabolic profiling. Factorial discriminant analysis (FDA) of the two data sets revealed clear linkages between groups of isolates. In the first workshop, milk could be incriminated as the sole source of cheese contamination. In the second one, milk and cheese centre isolates were found similar but surface cheese contaminants could be linked to a secondary contamination originating from water. Thus, it is possible to characterise, differentiate and trace Pseudomonas spp. strains using the fluorescence and metabolic profiling techniques. In addition, the two data sets were found highly correlated by canonical correlation analysis (CCA). Fluorescence spectroscopy however showed several advantages because of its low cost and processing speed.