Transcriptome analysis of the Populus trichocarpa-Rhizophagus irregularis Mycorrhizal Symbiosis: Regulation of Plant and Fungal Transportomes under Nitrogen Starvation

Nutrient transfer is a key feature of the arbuscular mycorrhizal (AM) symbiosis. Valuable mineral nutrients are transferred from the AM fungus to the plant, increasing its fitness and productivity, and, in exchange, the AM fungus receives carbohydrates as an energy source from the plant. Here, we analyzed the transcriptome of the Populus trichocarpa-Rhizophagus irregularis symbiosis using RNA-sequencing of non-mycorrhizal or mycorrhizal fine roots, with a focus on the effect of nitrogen (N) starvation. In R. irregularis, we identified 1,015 differentially expressed genes, whereby N starvation led to a general induction of gene expression. Genes of the functional classes of cell growth, membrane biogenesis and cell structural components were highly abundant. Interestingly, N starvation also led to a general induction of fungal transporters, indicating increased nutrient demand upon N starvation. In non-mycorrhizal P. trichocarpa roots, 1,341 genes were differentially expressed under N starvation. Among the 953 down-regulated genes in N starvation, most were involved in metabolic processes including amino acids, carbohydrate and inorganic ion transport, while the 342 up-regulated genes included many defense-related genes. Mycorrhization led to the up-regulation of 549 genes mainly involved in secondary metabolite biosynthesis and transport; only 24 genes were down-regulated. Mycorrhization specifically induced expression of three ammonium transporters and one phosphate transporter, independently of the N conditions, corroborating the hypothesis that these transporters are important for symbiotic nutrient exchange. In conclusion, our data establish a framework of gene expression in the two symbiotic partners under high-N and low-N conditions

The small secreted effector protein MiSSP7.6 of Laccaria bicolor is required for the establishment of ectomycorrhizal symbiosis

To establish and maintain a symbiotic relationship, the ectomycorrhizal fungus Laccaria bicolor releases mycorrhiza-induced small secreted proteins (MiSSPs) into host roots. Here, we have functionally characterized the MYCORRHIZA-iNDUCED SMALL SECRETED PROTEIN OF 7.6 kDa (MiSSP7.6) from L. bicolor by assessing its induced expression in ectomycorrhizae, silencing its expression by RNAi, and tracking in planta subcellular localization of its protein product. We also carried out yeast two-hybrid assays and bimolecular fluorescence complementation analysis to identify possible protein targets of the MiSSP7.6 effector in Populus roots. We showed that MiSSP7.6 expression is upregulated in ectomycorrhizal rootlets and associated extramatrical mycelium during the late stage of symbiosis development. RNAi mutants with a decreased MiSSP7.6 expression have a lower mycorrhization rate, suggesting a key role in the establishment of the symbiosis with plants. MiSSP7.6 is secreted, and it localizes both to the nuclei and cytoplasm in plant cells. MiSSP7.6 protein was shown to interact with two Populus Trihelix transcription factors. Furthermore, when coexpressed with one of the Trihelix transcription factors, MiSSP7.6 is localized to plant nuclei only. Our data suggest that MiSSP7.6 is a novel secreted symbiotic effector and is a potential determinant for ectomycorrhiza formation.Fil: Kang, Heng. Institut National de la Recherche Agronomique; FranciaFil: Chen, Xin. Institut National de la Recherche Agronomique; FranciaFil: Kemppainen, Minna Johanna. Universidad Nacional de Quilmes. Departamento de Ciencia y Tecnologia. Instituto de Microbiologia Basica y Aplicada. Laboratorio de Micologia Molecular.; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Pardo, Alejandro Guillermo. Universidad Nacional de Quilmes. Departamento de Ciencia y Tecnologia. Instituto de Microbiologia Basica y Aplicada. Laboratorio de Micologia Molecular.; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Veneault Fourrey, Claire. Institut National de la Recherche Agronomique; FranciaFil: Kohler, Annegret. Institut National de la Recherche Agronomique; FranciaFil: Martin, Francis M.. Institut National de la Recherche Agronomique; Franci

The mutualism effector MiSSP7 of Laccaria bicolor alters the interactions between the poplar JAZ6 protein and its associated proteins

Despite the pivotal role of jasmonic acid in the outcome of plant-microorganism interactions, JA-signaling components in roots of perennial trees like western balsam poplar (Populus trichocarpa) are poorly characterized. Here we decipher the poplar-root JA-perception complex centered on PtJAZ6, a co-repressor of JA-signaling targeted by the effector protein MiSSP7 from the ectomycorrhizal basidiomycete Laccaria bicolor during symbiotic development. Through protein–protein interaction studies in yeast we determined the poplar root proteins interacting with PtJAZ6. Moreover, we assessed via yeast triple-hybrid how the mutualistic effector MiSSP7 reshapes the association between PtJAZ6 and its partner proteins. In the absence of the symbiotic effector, PtJAZ6 interacts with the transcription factors PtMYC2s and PtJAM1.1. In addition, PtJAZ6 interacts with it-self and with other Populus JAZ proteins. Finally, MiSSP7 strengthens the binding of PtJAZ6 to PtMYC2.1 and antagonizes PtJAZ6 homo-/heterodimerization. We conclude that a symbiotic effector secreted by a mutualistic fungus may promote the symbiotic interaction through altered dynamics of a JA-signaling-associated protein–protein interaction network, maintaining the repression of PtMYC2.1-regulated genes

10 New Insights into Ectomycorrhizal Symbiosis Evolution and Function

International audienc

Who is Controlling whom within the Ectomycorrhizal Symbiosis: Insights from Genomic and Functional Analyses

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Signaling pathways driving the development of ectomycorrhizal symbiosis

In forest ecosystems, the roots of trees are continously in contact with beneficial, commensal and pathogenic soil microbes. These belowground microbial communities, called the microbiome, are also responsible for nutrient (C, N, P) recycling and nutrient exchanges, and have an impact on soil fertility and carbon sequestration. Consequently, the root microbiome drives forest health, productivity and sustainability. Within the rhizospheric zoo, the mutualistic ectomycorrhizal (ECM) fungi occupy a unique niche, with a shift from extramatrical or free‐living mycelium in soil to hyphae in intimate con­ tact with the apoplast of root cells. ECM interactions contribute to better tree growth and health via improving mineral nutrition, strengthening plant defenses and direct contribution to the exclusion of competitive microbes (Smith and Read, 2008). Despite their ecological importance, ECM symbiosis is still not well understood at the molecular level, partly because of the complexity of eukaryotic cells and their multi‐cellularity. During ECM establishment, soil‐borne fungal hyphae first grow towards host root cells and encompass short lateral roots to form the mantle. Mycelia then colonize the apoplastic space forming the Hartig net – the symbiotic interface where a molecular dialogue and an efficient nutrient exchanges take place (Peterson and Massicote, 2004; Martin, 2007). The tree supplies the ECM fungus with up to 20% of its photosynthesis‐derived carbohydrates, in return for up to 70% of its nitrogen and phosphorus needs, received from the ECM hyphal net­ works that extend deep within the soil. During the course of root colonization by an ECM fungus, the plant root undergoes a number of morphological changes, from the cessation of growth, to the alteration of plant cell wall properties and, finally, the alternate control of membrane‐bound trans­ porters to accommodate the new paradigm of nutrient fluxes inherent in mutualistic interactions. Therefore, establishment of a mutualistic interaction firstly requires recognition between both partners, and secondly a coordination of microbial and plant responses. Thanks to enormous efforts in fungal genome sequencing and, in particular, ECM fungal genomes, molecular mechanisms driving ECM development and functioning are receiving renewed attention (Kuo et al., 2014; Martin et al., 2008, 2010). In this chapter, we will first present our current knowledge on signal molecules and putative receptors that promote and mediate the very early steps of ECM establishment. We will then emphasize both the hormone‐based and symbiosis effector‐based dialogues, partly explaining how ECM hyphae can proliferate in host roots without eliciting plant defenses. Metabolic responses of colonized ECM roots will be also presented