Author Archives: eric

Post-doctoral position IAM

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Open position: Two-year postdoctoral position in the team “Stress response and redox regulation” in Nancy

A two-year postdoctoral position is open in the team “Stress response and redox regulation” (UMR1136 Université de Lorraine/INRA) located at the faculty of sciences in Vandoeuvre-lès-Nancy, France. The research project, funded by the national funding agency (ANR), aims at dissecting the mechanisms of maturation of iron-sulfur (Fe-S) proteins in plants using a functional genomics approach, focusing on A-type carrier proteins, which are present both in chloroplast and mitochondria. Several roles have been proposed for these proteins, but their exact function is unclear. We want to develop a multidisciplinary project associating plant genetics and physiology as well as molecular, biochemical and structural approaches. This approach will allow delineating the biochemical and spectroscopic properties of recombinant proteins and should help identifying the interaction partners and understanding the transcriptional or post-transcriptional regulation mechanisms necessary to coordinate the expression and activity of the different components of the Fe-S cluster assembly machineries. For the latter aspect, we would like to develop fluorescent probes. Applicants should have a solid background in biochemistry and structural biology, especially concerning metalloproteins if possible. It would also be appreciated that the candidates possess competence in cell imaging.

The position will be open in the first months of 2014. Applicants should send their CV with the names of referees to Nicolas Rouhier (Nicolas.Rouhier@univ-lorraine.fr).

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Article: New Phytologist

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Ethylene and jasmonic acid act as negative modulators during mutualistic symbiosis between Laccaria bicolor and Populus roots. J. Plett, A. Khatchane, M. Ouassou, B. Sundberg, A. Kohler, F. Martin. New Phytologist

Summary

  • The plant hormones ethylene, jasmonic acid and salicylic acid have interconnecting roles during the response of plant tissues to mutualistic and pathogenic symbionts.
  • We used morphological studies of transgenic- or hormone-treated Populus roots as well as whole-genome oligoarrays to examine how these hormones affect root colonization by the mutualistic ectomycorrhizal fungus Laccaria bicolor S238N.
  • We found that genes regulated by ethylene, jasmonic acid and salicylic acid were regulated in the late stages of the interaction between L. bicolor and poplar. Both ethylene and jasmonic acid treatments were found to impede fungal colonization of roots, and this effect was correlated to an increase in the expression of certain transcription factors (e.g. ETHYLENE RESPONSE FACTOR1) and a decrease in the expression of genes associated with microbial perception and cell wall modification. Further, we found that ethylene and jasmonic acid showed extensive transcriptional cross-talk, cross-talk that was opposed by salicylic acid signaling.
  • We conclude that ethylene and jasmonic acid pathways are induced late in the colonization of root tissues in order to limit fungal growth within roots. This induction is probably an adaptive response by the plant such that its growth and vigor are not compromised by the fungus.

PhD Defense : A. Vayssière

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The defense will be held the 13th January 2014 at 13H30 in Amphitheater 7, Faculté des Sciences, Boulevard des Aiguillettes, Vandoeuvre

Auxin control in poplar root development in response to the ectomycorrhizal fungus Laccaria bicolor

Root systems of host trees are known to establish the ectomycorrhizal (ECM) symbiosis with rhizospheric fungi. This mutualistic association leads to modifications of root development that including a stimulation of lateral host roots, and a modification in root growth. The phytohormone auxin (Indole-3-acetic acid, IAA) is known to regulate LRs formation and root growth. Our research focussed on auxin pathways in poplar root in response to L. bicolor.

In this study, our data showed that the poplar-Laccaria bicolor interaction leads to the arrest of LRs and adventitious root growth after two weeks of interaction. We also showed that this arrest is not regulated by the Hartig net. Differential auxin responses were analyzed by using an auxin-responsive DR5::GUS marker line and revealed a loss of auxin response in ECM roots. An oligoarray-based transcript profiling of poplar roots in contact with L. bicolor highlights a differential expression of auxin asociated genes in ECM. Measurement of auxin metabolite in ECM and in the free living partners revealed an IAA accumulation, an activation of the IPyA (Indol-3-Pyruvic Acid) dependant IAA biosynthesis pathway in both partners, as well as changes in IAA conjugation pathways in poplar and in IAA degradation pathways in L. bicolor. Our findings illustrate the impact of L. bicolor colonization on root auxin metabolism and response, and also suggest a role of auxin as! a signal in the formation of ECM and in the regulation of ECM function.

In parallel, PtaPIN9 function analysis in response to L. bicolor has been performed. PtaPIN9 immunolocalization in poplar roots showed similar localization to AtPIN2 in epidermis cells. Transgenic lines having a modification in PtaPIN9 expression, did not formed new LRs in respond to L. bicolor. In ECM roots, the loss of PtaPIN9 signal is observed but modifications of PtaPIN9 expression did not modify the root growth arrest and the Hartig net formation.

These results show major changes in auxin associate pathways in poplar root by the symbiotic fungus L. bicolor, during the formation of the mycorrhiza root. Our results offer perspectives on the role of auxin in root development and in the context plants-microbes interactions.

Keywords: Root architecture, Auxin, Mycorrhiza, Lateral root, Root growth, Poplar, Laccaria bicolor

Contrôle de l’auxine dans les modifications du développement racinaire du peuplier en réponse au champignon ectomycorhizien, Laccaria bicolor

Le système racinaire des arbres peut établir des symbioses ectomycorhiziennes (ECM) avec des champignons rhizosphériques. La mise en place de la symbiose est accompagnée d’une stimulation de la formation des racines latérales (RLs), et d’une modification de la croissance racinaire. Ces processus développementaux conduisent à la formation de racines courtes typiques des ECMs. Il a été montré que l’auxine est une phytohormone clef dans la formation des RLs ainsi que dans la croissance racinaire. Notre projet s’est focalisé sur l’étude de la régulation des voies de l’auxine dans la racine de peuplier en réponse à L. bicolor.

Dans cette étude, nous avons mis en évidence un arrêt de croissance des RLs et des racines adventives du peuplier Populus tremula x P. alba, après deux semaines de co-culture avec L. bicolor. De plus, nous avons aussi montré que cet arrêt n’est pas conditionné par la présence du réseau de Hartig. Une analyse de l’expression globale des gènes de peuplier dans la mycorhize a été réalisée au cours de la formation de la mycorhize. Cette analyse, couplée à des observations du gradient auxinique via le patron d’expression du promoteur DR5, montre que la signalisation auxinique est affectée dans l’organe symbiotique. La quantification de l’auxine (acide indole 3-acétique, AIA) et des métabolites associés a permis de mettre en évidence un envir! onnement symbiotique riche en auxine dans la mycorhize, qui pourrait expliquer les modifications de la signalisation auxinique. De plus, un changement de la conjugaison et de la dégradation de l’AIA est détecté dans la racine, ainsi qu’une dégradation de l’AIA dans les hyphes de L. bicolor.

En parallèle, une analyse fonctionnelle de PtaPIN9, un orthologue de AtPIN2, responsable du transport basipète de l’auxine à l’apex racinaire chez Arabidopsis thaliana, a été réalisée au cours de la mycorhization avec L. bicolor. L’immunolocalisation de PtPIN9 dans les racines de peuplier a montré une localisation similaire à AtPIN2, dans les cellules épidermiques. Les lignées transgéniques ayant une modification de l’expression de ce gène ne répondent pas à L. bicolor en terme de stimulation de RLs. Dans les racines mycorhizées, PtaPIN9 n’est plus observée, mais les modifications de l’expression de PtaPIN9 ne modifient ni l’arrêt de croissance racinaire, ni la formation du réseau de Hartig.

Ces résultats montrent des modifications majeures des voies de l’auxine du peuplier par le champignon symbiotique L. bicolor. Cette étude ouvre des perspectives sur la compréhension du rôle de l’auxine dans le développement racinaire ainsi que dans le contexte des interactions plantes-microorganismes.

Mots-clés : Architecture racinaire, Auxine, Mycorhize, Racines latérales, croissance racinaire, Peuplier, Laccaria bicolor

Article: Frontiers in Plant Science

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Towards a refined classification of class I dithiol glutaredoxins from poplar: biochemical basis for the definition of two subclasses
J Couturier, JP Jacquot, N Rouhier
Frontiers in Plant Science 4, 518

Glutaredoxins (Grxs) are small oxidoreductases particularly specialized in the reduction of protein-glutathione adducts. Compared to other eukaryotic organisms, higher plants present an increased diversity of Grxs which are organized into four classes. This work presents a thorough comparative analysis of the biochemical and catalytic properties of dithiol class I Grxs from poplar, namely GrxC1, GrxC2, GrxC3 and GrxC4. By evaluating the in vitro oxidoreductase activity of wild type and cysteine mutated variants and by determining their dithiol-disulfide redox potentials, pKa values of the catalytic cysteine, redox state changes in response to oxidative treatments, two subgroups can be distinguished. In accordance with their probable quite recent duplication, GrxC1 and GrxC2 are less efficient catalysts for the reduction of dehydroascorbate and hydroxyethyldisulfide compared to GrxC3 and GrxC4, and they can form covalent dimers owing to the presence of an additional C-terminal cysteine (CysC). Interestingly, the second active site cysteine (CysB) influences the reactivity of the catalytic cysteine (CysA) in GrxC1 and GrxC2 as already observed with GrxC5 (restricted to A. thaliana), but not in GrxC3 and C4. However, all proteins can form an intramolecular disulfide between the two active site cysteines (CysA-CysB) which could represent either a protective mechanism considering that this second cysteine is dispensable for deglutathionylation reaction or a true catalytic intermediate occurring during the reduction of particular disulfide substrates or in specific conditions or compartments where glutathione levels are insufficient to support Grx regeneration. Overall, in addition to their different sub-cellular localization and expression pattern, the duplication and maintenance along evolution of several class I Grxs in higher plants can be explained by the existence of differential biochemical and catalytic properties.

Article: Environmental Microbiology

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Diversity and structure of bacterial communities associated with Phanerochaete chrysosporium during wood decay. V Hervé, X Le Roux, S Uroz, E Gelhaye, P Frey‐Klett
Environmental Microbiology

Abstract

Wood recycling is key to forest biogeochemical cycles, largely driven by microorganisms such as white-rot fungi which naturally coexist with bacteria in the environment. We have tested whether and to what extent the diversity of the bacterial community associated with wood decay is determined by wood and/or by white-rot fungus Phanerochaete chrysosporium. We combined a microcosm approach with an enrichment procedure, using beech sawdust inoculated with or without P. chrysosporium. During eighteen weeks, we used 16S rRNA gene-based pyrosequencing to monitor the forest bacterial community inoculated into these microcosms. We found bacterial communities associated with wood to be substantially less diverse than the initial forest soil inoculum. The presence of most bacterial OTUs varied over time and between replicates, regardless of their treatment, suggestive of the stochastic processes. However, we observed two OTUs belonging to Xanthomonadaceae and Rhizobium, together representing 50% of the relative bacterial abundance, as consistently associated with the wood substrate, regardless of fungal presence. Moreover, after twelve weeks, the bacterial community composition based on relative abundance was significantly modified by the presence of the white-rot fungus. Effectively, members of the Burkholderiagenus were always associated with P. chrysoporium, representing potential taxonomic bioindicators of the white-rot mycosphere.