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Projet MICADO : http://www.agence-nationale-recherche.fr/projet-anr/?solr=run&tx_lwmsuivibilan_pi2%5BCODE%5D=ANR-11-JSV7-0003; International audience; Chemosynthetic mats involved in cycling sulfur compounds are often found in hydrothermal vents, cold seeps and whale falls. However, there are only few records of wood fall mats, even though the presence of hydrogen sulfide at the wood surface should create a perfect niche for sulfide-oxidizing bacteria. Here we report the growth of microbial mats on wood incubated under conditions that simulate the Mediterranean deep-sea temperature and darkness. We used amplicon and metagenomic sequencing combined with fluorescence in situ hybridization to test whether a microbial succession occurs during mat formation and whether the wood fall mats present chemosynthetic features. We show that the wood surface was first colonized by sulfide-oxidizing bacteria belonging to the Arcobacter genus after only 30 days of immersion. Subsequently, the number of sulfate reducers increased and the dominant Arcobacter phylotype changed. The ecological succession was reflected by a change in the metabolic potential of the community from chemolithoheterotrophs to potential chemolithoautotrophs. Our work provides clear evidence for the chemosynthetic nature of wood fall ecosystems and demonstrates the utility to develop experimental incubation in the laboratory to study deep-sea chemosynthetic mats.

Ectomycorrhizal fungal community is largely integrated into the overall diversity and ecosystem processes. This study aims to determine the below-ground structure of ectomycorrhizal fungi community associated with Uapaca bojeri and Sarcolaena oblongifolia in highland sclerophyllous forest of Madagascar and to assess the impact of shared ectomycorrhizal symbiont on tree seedling recruitment and plant succession facilitation. Ectomycorrhizal fungi were identified based on morphotyping and RFLP analysis of rDNA internal transcribed spacer (ITS)-large subunit followed by sequence analysis of each RFLP pattern. Impacts of shared ectomycorrhizal symbionts on seedling development of U. bojeri were investigated in soil microcosm collected at different distances from naturally established of both host plant species. The ECM fungal communities associated with both plants showed high similarity and were dominated by the genus Russula. Expressed as RFLP types, 21 taxa of ECM fungi were collected from root tips. More than half of these ECM taxa were shared between the two plant species. The better development of U. bojeri seedling was observed in soil samples naturally influenced by S. oblongifolia and located between 5 and 10 m from established U. bojeri where seedlings were colonized particularly by shared ectomycorrhizal symbionts. This study demonstrates that natural establishment of secondary colonizing hosts may contribute to tree seedling recruitment and development by providing compatible ectomycorrhizal symbionts.

Adansonia madagascariensis fruits were found in May 2011 on the beach of Anjiabe in the north of Madagascar. Marks found on the fruits and the absence of this species on this coast indicated that the fruits had been in the sea for a long time. The viability of seeds contained in these fruits was assessed and compared to the viability of seeds collected from under trees. The results show that the time spent in the sea did not affect their germination potential and that germination is possible, and even improves, in tidal zones. These initial results confirm the hypothesis of marine hydrochory for this species. They show that baobab fruits can remain in the sea without affecting the viability of seeds to colonize new areas. This could explain the frequent occurrence of this species along the coast of Madagascar and, more rarely, on the Mayotte coast.

Pterocarpus officinalis Jacq. is a legume tree native to the Caribbean islands and South America growing as a dominant species in swamp forests. To analyze (i) the genetic diversity and (ii) the symbiotic properties of its associated nitrogen-fixing soil bacteria, root nodules were collected from P. officinalis distributed in 16 forest sites of the Caribbean islands and French Guiana. The sequencing of the 16S-23S ribosomal RNA intergenic spacer region (ITS) showed that all bacteria belonged to the Bradyrhizobium genus. Bacteria isolated from insular zones showed very close sequence homologies with Bradyrhizobium genospecies V belonging to the Bradyrhizobium japonicum super-clade. By contrast, bacteria isolated from continental region displayed a larger genetic diversity and belonged to B. elkanii super-clade. Two strains from Puerto Rico and one from French Guiana were not related to any known sequence and could be defined as a new genospecies. Inoculation experiments did not show any host specificity of the Bradyrhizobium strains tested in terms of infectivity. However, homologous Bradyrhizobium sp. strain-P. officinalis provenance associations were more efficient in terms of nodule production, N acquisition, and growth than heterologous ones. The dominant status of P. officinalis in the islands may explain the lower bacterial diversity compared to that found in the continent where P. officinalis is associated with other leguminous tree species. The specificity in efficiency found between Bradyrhizobium strains and host tree provenances could be due to a coevolution process between both partners and needs to be taken in consideration in the framework of rehabilitation plantation programs.

International audience; Azospirillum is a plant growth-promoting rhizobacterium (PGPR) able to enhance growth and yield of cereals such as rice, maize and wheat. The growth-promoting ability of some Azospirillum strains appears to be highly specific to certain plant species and cultivars. In order to ascertain the specificity of the associative symbiosis between rice and Azospirillum, the physiological response of two rice cultivars, Nipponbare and Cigalon, inoculated with two rice-associated Azospirillum was analyzed at two levels: plant growth response and plant secondary metabolic response. Each strain of Azospirillum (Azospirillum lipoferum 4B isolated from Cigalon and Azospirillum sp. B510 isolated from Nipponbare) preferentially increased growth of the cultivar from which it was isolated. This specific effect is not related to a defect in colonization of host cultivar as each strain colonizes effectively both rice cultivars, either at the rhizoplane (for 4B and B510) and inside the roots (for B510). The metabolic profiling approach showed that, in response to PGPR inoculation, profiles of rice secondary metabolites were modified, with phenolic compounds such as flavonoids and hydroxycinnamic derivatives being the main metabolites affected. Moreover, plant metabolic changes differed according to Azospirillum strain \texttimes cultivar combinations; indeed, 4B induced major secondary metabolic profile modifications only on Cigalon roots, while B510, probably due to its endophytic feature, induced metabolic variations on shoots and roots of both cultivars, triggering a systemic response. Plant secondary metabolite profiling thereby evidences the specific interaction between an Azospirillum strain and its original host cultivar.

Leaf scald, a sugarcane disease caused by Xanthomonas albilineans, has been largely studied since its discovery in 1911. Numerous data were obtained and reported, from isolation, transmission and diagnosis of the pathogen to the complete sequence of its genome. Although X. albilineans was thought to be exclusively transmitted by infected cuttings and contaminated cutting implements for more than 80 years, the existence of an epiphytic life of this pathogen was reported in the late 1990s. We highlight herein the role of the epiphytic populationsof X. albilineans in plant and field contamination and the impact of environmental conditions, especially rainfall, on these populations of the pathogen. Data obtained from experimental research in Guadeloupe showed the capacity of X. albilineans, when rainfall is sufficient, to spread by aerial means within the sugarcane canopy in a field or between fields. This new epidemiological trait helps X. albilineans in contaminating rapidly healthy sugarcane plants and, in certain geographical locations, it can be much more important than disease spread by cutting implements. These new epidemiological data of sugarcane leaf scald lead us to propose a revisited infection cycle for X. albilineans.

The genome of Xanthomonas albilineans, the causal agent of sugarcane leaf scald, contains a cluster of genes called rpf for regulation of pathogenicity factors. This cluster of genes encodes a quorum sensing (also called cell-cell signaling) system that controls several cellular and biological processes in Xylella fastidiosa, Stenotrophomonas maltophilia and several species of Xanthomonas. The rpf gene cluster is especially used by these pathogenic bacteria to regulate the expression of factors contributing to virulence such as extracellular polysaccharides, extracellular enzymes and flagellum synthesis. This cell-cell signaling is mediated by a signal molecule called DSF (for Diffusible Signaling Factor), and gene rpfF has been shown to be required for production of DSF. Perception of DSF and signal transduction depends on a two-component signal transduction system which comprises the sensor kinase RpfC and the regulator RpfG. The role of these rpf genes in X albilineans is not yet clear. In this study, several mutants of the rpf gene cluster of X albilineans were assessed for their capacity i) to produce DSF and disease symptoms, ii) to colonize the phyllosphere and iii) to spread within the sugarcane stalk. Mutations of rpfF in strain XaFL07-1 of X albilineans from Florida resulted in loss of DSF production and no or only small reduction of disease severity. Single deletion mutants of rpfC and rpfG also produced symptoms that were similar in severity to those caused by the wild type strain of the pathogen. However, when the entire rpfGCF region was deleted, the capacity of the pathogen to cause disease symptoms was slightly reduced and swimming motility was severely affected. Similarly, the capacity of the pathogen to colonize the phyllosphere of the host plant was affected only when the entire rpfGCF region was deleted. Unexpectedly, this latter mutant appeared to produce another signalling molecule, as evidenced by a positive reaction of the DSF reporter strain. Furthermore, all mutations in the DSF signal transduction system resulted in the capacity of the pathogen to colonize sugarcane spatially more efficiently than the wild type strain, whereas mutations in rpfF did not. Therefore, DSF encoded by rpfF does not appear essential for symptom expression, phyllosphere and sugarcane stalk colonization by X. albilineans strain XaFL07-1. However, mutation data in the sensor/regulator system suggest that pathogenicity and cell. cell signalling of X. albilineans may involve another signalling molecule or another sensor/regulator that remain to be identified.

The main objectives of this study were (1) to describe the diversity of mycorrhizal fungal communities associated with Uapaca bojeri, an endemic Euphorbiaceae of Madagascar, and (2) to determine the potential benefits of inoculation with mycorrhizal fungi [ectomycorrhizal and/or arbuscular mycorrhizal (AM) fungi] on the growth of this tree species and on the functional diversity of soil microflora. Ninety-four sporophores were collected from three survey sites. They were identified as belonging to the ectomycorrhizal genera Afroboletus, Amanita, Boletus, Cantharellus, Lactarius, Leccinum, Rubinoboletus, Scleroderma, Tricholoma, and Xerocomus. Russula was the most frequent ectomycorrhizal genus recorded under U. bojeri. AM structures (vesicles and hyphae) were detected from the roots in all surveyed sites. In addition, this study showed that this tree species is highly dependent on both types of mycorrhiza, and controlled ectomycorrhization of this Uapaca species strongly influences soil microbial catabolic diversity. These results showed that the complex symbiotic status of U. bojeri could be managed to optimize its development in degraded areas. The use of selected mycorrhizal fungi such the Scleroderma Sc1 isolate in nursery conditions could be of great interest as (1) this fungal strain is very competitive against native symbiotic microflora, and (2) the fungal inoculation improves the catabolic potentialities of the soil microflora.

International audience; Knowledge of propagation mechanisms is important for understanding the invasion ecology and management of invasive plants in order to restore invaded lands. The identification of recruitment pathways is one of the keys to understanding dispersal mechanisms and determining invasive plant control strategies. The objective of this study was to characterize the recruitment processes of Schinus terebenthifolius, one of the most serious plant invaders in Reunion Island's lowlands. Surprisingly, little attention was given in literature to vegetative propagation by suckering as a dispersal mechanism. Yet, field observations on abandoned farmland and riverbanks combined with germination experiments have shown major differences in recruitment patterns between wet and dry areas. On abandoned dryland farms, seedling emergence is 35 times less than on riverbanks and is replaced by suckering. Birds facilitated seed germination, but their role in seed dispersal (ornithochory) appeared minor and restricted to the formation of satellite invasion foci. Water movement appears to be a more efficient vector than ornithochory because it aids seed transportation (hydrochory), seed germination, and suckering on riverbanks. Control and restoration programs must distinguish the two kinds of recruitment that impact on short- and long-distance dispersal.

Colonization of the sugarcane leaf canopy by Xanthomonas albilineans appears to be an important step in the epidemiological cycle of leaf scald disease in Guadeloupe. Previous studies showed that healthy sugarcane plants can be infected by X. albilineans after aerial transmission of the pathogen. Impact of climatic conditions on variation of colonization of the sugarcane leaf canopy and subsequent stalk infection by the pathogen is, however, unknown. Trials were set up in Guadeloupe in 3 different locations with cultivar B69566, susceptible to leaf scald, but still grown commercially in Guadeloupe. Disease-free tissue-culture propagated sugarcane plants were transferred to the field in 1999. Epiphytic populations of X. albilineans were regularly monitored for 3 crops (plant cane and two ratoons) by measuring bacterial populations in water droplets sampled from the sugarcane leaf surface. Infection of sugarcane stalks by X. albilineans was determined by isolating the pathogen from the stalk sap after 11-12 months of growth in each crop cycle. In plant cane, the first detection of the pathogen on the leaf canopy varied according to proximity of other sugarcane fields and climatic conditions. Additionally, once the leaf canopy was entirely colonized, epiphytic population sizes of the pathogen also varied between crop cycles and locations. Maximum population sizes were observed at the end of the wet season (November-December). These populations ranged from 2 to 107 bacteria per ml of water droplet according to crop location and crop cycle. Bacterial populations on the leaf surface were correlated with total rainfall during the wet season (first half of the crop cycle) and percentage of stalk infection by X. albilineans varied in accordance with epiphytic populations of the pathogen. The amount of rainfall therefore appeared critical for the epiphytic phase of sugarcane leaf scald and subsequent stalk infection. If these results can be confirmed with other sugarcane cultivars, amount of rainfall could be used to predict stalk infection after aerial transmission of the leaf scald pathogen. (Texte intégral)

Bluetongue (BT) disease occurred for the first time in Corsica, France in 2000. Once the presence and the establishment of C. imicola were confirmed in a small area of the southern mainland of France (Var département), the risk of a bluetongue outbreak in this area became higher. In the case of mosquito-borne diseases, the vectorial capacity is a widely and useful adopted estimate used to assess the susceptibility of an area to a particular disease. This integrated approach was applied in the Var context. In this zone, four trapping sites were monitored weekly from mid-April 2005 to mid-November 2005 with a UV light trap. The abundance and the parity of C. imicola were collected and analysed in regard with local meteorological data. Expert opinions and previous published results were used to estimate others parameters of the vectorial capacity. Uncertainty and variability were taken into account by using probabilistic distributions. The vector competence of this C. imicola's population was tested through experimental infections under laboratory conditions. The benefits of such an approach are analysed with the purpose of fording few sensitive parameters which could be monitored in routine. (Texte intégral)

Invaders have often been said to display traits that favor colonisation (r-traits). However, while these traits may allow a species to reach empty habitats before other species do, they are usually associated with low competitive ability. More competitive species, even if they have lower colonisation ability, may prevent the permanent installation of an r-selected invader. Competition rather than colonisation may therefore limit invasion. The best situations to test this hypothesis are "invasive series", when successive invasions by related taxa occur in the same place. Phylogenetic proximity of the taxa and common environment ensure that trade-offs between colonization- and competition-oriented traits can be detected. We expect biological traits to change from colonisation-oriented to increasingly competition-oriented strategies along the invasive series. This prediction was tested using two different model systems, both of which invade tropical insular habitats: freshwater snails of the Thiaridae family in Martinique and Tephritid flies in La Réunion. In both systems, in consistency with our hypothesis, the first invaders were relatively r-selected, and were later replaced by new genotypes and/or taxa with increasingly lower fecundity and higher investment per offspring.