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International audience; In the soil-dwelling organism Pseudomonas putida KT2440, the rare earth element (REE)-utilizing, and pyrroloquinoline quinone (PQQ)-dependent ethanol dehydrogenase PedH is part of a periplasmic oxidation system that is vital for growth on various alcoholic volatiles. Production of PedH and its Ca 2+-dependent counterpart PedE is inversely regulated in response to lanthanide (Ln 3+) bioavailability, a mechanism termed the REEswitch. In the present study, we demonstrate that copper, zinc, and in particular, iron availability influences this regulation in a pyoverdine-independent manner by increasing the minimal Ln 3+ concentration required for the REE-switch to occur by several orders of magnitude. A combined genetic and physiological approach reveals that an ABC-type transporter system encoded by the gene cluster pedA1A2BC is essential for efficient growth on 2-phenylethanol with low (nanomolar) Ln 3+ concentrations. In the absence of pedA1A2BC, a ∼100-fold higher La 3+-concentration is needed for PedH-dependent growth but not for the ability to repress growth based on PedE activity. From these results, we conclude that cytoplasmic uptake of lanthanides through PedA1A2BC is essential to facilitate REE-dependent growth on 2-phenylethanol under environmental conditions with poor REE bioavailability. Our data further suggest that the La 3+ /Fe 2+/3+ ratio impacts the REE-switch through the mismetallation of putative La 3+-binding proteins, such as the sensor histidine kinase PedS2, in the presence of high iron concentrations. As such, this study provides an example for the complexity of bacteria-metal interactions and highlights the importance of medium compositions when studying physiological traits in vitro in particular in regard to REE-dependent phenomena.

International audience

International audience

Phytomanagement is a handling method of contaminated sites based on the ability of plants to sequester pollutants. Fungi could favor plant growth in contaminated sites but were far less studied in the context of phytomanagement. Dark septate endophytes (DSE) were reported to improve plant tolerance against metallic trace elements (TE). The present work is part of the LORVER project and aims to understand the “plant-DSE-TE”. DSE were isolated from poplar roots growing on TE-contaminated soils and characterized. They belong to Leptodontidium, Phialophora, Cadophora and Phialocephala genera. Three strains were inoculated to birch and poplar in TE-contaminated soils. Leptodontidium sp. improved plant growth with an increase in chlorophyll, P and K concentrations, whereas Cadophora sp. decreased Cd concentration in shoots of birch. Then, Cadophora sp. Fe06, was used with an endomycorrhizal fungus, to co-inoculate ryegrass in a TE-polluted soil. The co-inoculation leads to the decrease of Cd concentration in shoots. Finally, the mechanisms involved in TE-tolerance by DSE were studied through the screening of a library of Leptondontidium sp. insertional random-mutants and an evaluation of the role of melanin against TE stress. The present work highlights the complexity of the interactions between DSE and plants under TE stress. These interactions were influenced by different parameters including plant species, DSE strains, and soil properties. Nevertheless, the ability of DSE to colonize a broad spectrum of plants in different soils and to promote plant growth, suggest that DSE could represent a substantial benefit for the fungus-assisted phytomanagement of polluted soils, Le phytomanagement est une méthode de gestion de sites pollués basée sur la capacité des plantes à séquestrer les polluants. L’utilisation de symbiotes fongiques peut représenter un atout afin d’augmenter la production de biomasse. Les endophytes bruns septés (DSE) pourraient stimuler la croissance végétale et augmenter leur protection contre les éléments-traces-métalliques (ETM). Cette thèse se déroule dans le cadre du projet LORVER et vise à comprendre l’interaction « plantes-DSE-sols pollués ». A ces fins, des champignons DSE isolés de racines de peupliers issues de sols pollués par des ETM ont été caractérisés. Ils appartiennent aux genres Leptodontidium, Phialophora, Cadophora et Phialocephala. Trois de ces souches ont ensuite été inoculées à des plantes en présence d’ETM. Dans certains cas, la croissance végétale a été stimulée et une augmentation de la teneur en chlorophylles, en P et en K, ou une réduction de la teneur en Cd dans les parties aériennes ont été mises en évidence. Puis, la souche la plus efficace sur la croissance végétale, Cadophora sp. Fe06, a été utilisée dans un double inoculum avec une souche endomycorhizienne, chez du ray-grass en présence d’ETM. Cette association diminue la teneur en Cd dans les feuilles. Enfin, les mécanismes impliqués dans la tolérance aux ETM chez les DSE ont été examinés via le criblage d’une banque de mutants d’insertions aléatoires, et via l’étude du rôle de la mélanine. Ce travail met en évidence que l’utilisation de DSE, de part leur capacité à coloniser de nombreuses plantes dans différents sols et à stimuler leur croissance, pourrait représenter un atout considérable pour des opérations de phytomanagement

International audience