Acacia spirorbis is a legume tree species distributed all over the Melanesian arc and mainly in New Caledonia where it is ubiquitous over a wide range of variety of soils. Indeed, from North to South of the territory and up to an altitude of 400m, this species is very dynamic in terms of implantation, growth and renewal capacity, demonstrating an amazingly wide range of adaptation, probably unique among vascular plants. New Caledonia offers an extreme diversity of soils, inducing different types of stresses (for exemple: water stress in metalliferous, calcareous and silty soils, heavy metal toxicity in metalliferous soils and aluminium toxicity in bauxite soils). Another important feature of A. spirorbis is its ability to develop nitrogen-fixing nodules, arbuscular mycorrhizas and ectomycorrhizas. This ability to establish a triple symbiosis is known only for a very limited number of angiosperms: some species among australian Acacia and some actinorhizal plants. Each of these symbioses has been characterized as important actors in adaptation of plants to edaphic stresses and this triple symbiosis is often mentioned to explain the high adaptive plasticity of their hosts. However, few of the “trisymbiotic” acacias are naturally facing such contrasted edaphic situations on a such reduced areas as A. spirorbis in New Caledonia. One plant species, three different symbioses naturally living in a contrasted range of more or less toxic edaphic conditions : such is the context of ADASPIR. The main objective of ADASPIR is to analyze the weight of each symbiosis in the adaptation of A. spirorbis to its different environments from the community to the individual and gene levels. As part of a multidisciplinary project ADASPIR proposes to study the mechanisms of adaptation of A. spirorbis and its associated microorganisms face to different abiotic stresses. The project is organized in three work packages: Coordination, dissemination and exploitation of results (WP1), A. spirorbis and its environments (WP2) and Role of symbiotic microorganisms in A. spirorbis adaptation to abiotic constrainsts (WP3) through 9 tasks: (1) coordination and development of work, (2) selection of study sites: 7 A. spirorbis stands will be selected on their contrasted edaphic characteristics, (3) pedological, geochemical and mineralogical characterization of the soils in the 7 studied sites, (4) genetypic and phenotypic characterisation of A. spirorbis : being in a context of high endemism, in dispersed geographical locations, these points will be analyzed, (5) In situ quantification of nitrogen fixation in A. spirorbis and live traits history: the impact of environment on A. spirorbis nitrogen fixation ability will be studied in the 7 sites through 15N natural abundance determinations, (6) diversity and functionality of the nitrogen-fixing bacteria associated with A. spirorbis, (7) diversity and functionality of arbuscular mycorrhizal fungi of A. spirorbis, (8) diversity and functionality of ectomycorrhizal fungi of A. spirorbis, (9) functioning of the Pisolithus albus/ A. spirorbis symbiosis under abiotic stresses. In conclusion, the ADASPIR project proposes to conduct basic research to characterize the mechanisms of adaptation of a species, A. spirorbis to various abiotic soil tresses: polymetallic toxicities (Ni, Cr, Mn, Co), unbalanced Ca/Mg ratio (1/40), aluminum toxicity, excess of Ca carbonate, poverty in major elements, in metalliferous, bauxite, limestone and siliceous or acid soils. It should be noted that this species is able to grow and form very dynamic stands on a very diverse range of soils: limestone, siliceous (acid and neutral), aluminum, ferralitic, serpentine... On a industrial point of view, A. spirorbis is a widely used species in revegetation programs of former mining sites in New Caledonia. The ADASPIR project aims to better target the use of this species and to optimize the management of sustainable restoration of degraded ecosystems.

Legumes play a major agronomical and ecological role due to their ability to fix atmospheric nitrogen during symbiosis with rhizobia. The main legume crops are tropical species (soybean, peanut, mungbean, …) that represent more than 85% of the grain legume production. These species are all nodulated by Bradyrhizobium strains which contain nodulation genes (nod genes) necessary for the synthesis of key symbiotic signals, named Nod factors (NFs), but also T3SS genes that encode the Type 3 Secretion System. This secretory machinery, initially identified in animal and plant bacterial pathogens, permits the delivery of effector proteins inside the host cells where they interfere with various host processes including suppression of immune responses and favour the infection. For a long time, it was assumed that nodulation absolutely required NFs to trigger nodule organogenesis and infection. The T3SS machinery on the other hand was viewed as an accessory equipment, which modulates the efficiency and the host spectrum of the bacteria. However, it has been shown that some legume species of the Aeschynomene genus but also the cultivar Glycine max cv. Enrie are nodulated by Bradyrhizobium strains even if NF synthesis is disrupted. In this case, the establishment of the interaction requires that the bacteria has a functional T3SS indicating that specific Type 3 effectors can directly activate the nodulation signalling pathway in legumes, bypassing the perception of NFs. Recently, we have demonstrated that in the Bradyrhizobium strain ORS3257 this T3SS-dependent symbiosis relies on a cocktail of at least five effectors playing synergistic and complementary roles in nodule organogenesis, infection and repression of plant immune responses. Among them, we identified the nuclear-targeted ErnA effector, which is highly conserved among bradyrhizobia, as a key actor for nodule organogenesis. Furthermore, preliminary data indicate that other Bradyrhizobium strains can use other Type 3 effectors, distinct of ErnA, to trigger nodulation in legumes. Our discovery that a single effector protein is sufficient to induce nodule organogenesis without the need of NFs is a paradigm shift in the field and indicates that legume nodulation programs are not exclusively controlled by NFs. Our main goals in the current ET-Nod project are: i) to decipher the molecular mechanisms by which ErnA activates nodulation in Aeschynomene, ii) to identify new effectors (named ET-Nods) behaving like ErnA in the triggering of nodulation and iii) to characterize the importance of this effector family in the symbiotic efficiency of agronomically important legumes. For these purposes, our consortium, involving specialists in plant symbiosis and pathogenesis will i) combine biochemical, genetic and omic approaches to characterize the molecular target(s) and interactome of ErnA, ii) develop at the level of the Bradyrhizobium genus a comparative genomic analysis coupled with a mutagenesis approach to identify new ET-Nod effectors and iii) investigate, using bacterial and plant genetics, the role played by ErnA and ET-Nod effectors in various Bradyrhizobium strains during symbioses with legume crops (soybean, peanut, cowpea …). The knowledge acquired during this project could be exploited in agronomy to improve yield of several legume crops and to design new strategies aimed at transferring nitrogen-fixing symbiosis to cereals.

The soils of Martinique and Guadeloupe are contaminated by chlordecone (CLD), an organochlorine pesticide used in the French West Indies until 1993 to control the banana weevil. Despite a cessation of use for almost 30 years, soil still represents a continuous source of CLD that can be transferred to other environmental compartments such as surface and groundwater. This impact on ecosystems has environmental but also health consequences. Reducing soil contamination by CLD is therefore a major challenge in order to reduce exposure and, consequently, reduce the impacts on health and the environment. The objective of DéMETer is to implement an efficient, economically viable, operational and acceptable method for soil treatment with regard to CLD and its degradation products. If there are several promising remediation solutions, such as those combining chemical reduction and phytoremediation, their operational implementation requires resolving technical and societal obstacles. These are the objectives of DéMETer: 1) optimize remediation processes regarding both efficiency and cost, 2) ensure that these remediation processes are socially acceptable and acquired by the stakeholders, and 3) validate the change of scale by implementing demonstrators on site and evaluating the technical and economic constraints for their eventual implementation on a very large scale. The later (obj 4) also includes ensuring the transferability of this integrated approach to similar contexts, i.e., Guadeloupe. To achieve these objectives, DéMETer, which relies on a transdisciplinary consortium, answers the challenges of axis 1 (prevention of exposure) and axis 2 (Science and society) of the call for projects and responds to several expected outcomes of the Chlordecone IV plan.

Mediterranean terrestrial ecosystems are facing increasing desertification because of the worsening of environmental pressures due to global change. The desertification processes lead to plant cover degradation, soil erosion, nutrient depletion and a decrease of microbial activity. The establishment of global political strategies aiming at a better management of terrestrial ecosystems is thus crucial for their conservation. In this context, Ceratonia siliqua L. (carob tree), a xerophilous tree adapted to Mediterranean climate, appears as a key model for afforestation/restoration programs because of its resistance and adaptation to extreme environmental conditions and its high socio-economic added value. Carob is a non-nodulated legume highly dependent of arbuscular mycorrhizal (AM) symbiosis for its survival and productivity. Its biological nitrogen fixation status remains uncertain but AM fungi have been hypothesized as an "obligatory vector" of nitrogen-fixing endophytic bacteria into the carob intracellular compartment. The management of carob populations is therefore closely linked to a better understanding and use (ecological engineering strategies) of the symbiotic community associated with carob. The main hypothesis of DYNAMIC is that infra-specific plant evolutionary differentiation is a determinant, but overlooked, driver of the diversity and structure of the symbiotic community, optimizing symbiotic efficiency. However, the evolutionary history and genetic diversity structure of carob is mostly unknown at the Mediterranean scale. Geographical isolation, long term vicariance and selection for agriculture are expected to have caused extensive genetic and physiological modifications in carob, conducing to potential changes/adaptations of its associated symbiotic microbiome. The overall objective of DYNAMIC is to decipher the symbiotic network in Mediterranean carob-based (agro)ecosytems to develop innovative ecological strategies based on efficient symbiotic interactions. The project is tackling this issue by (i) revealing the evolutionary significant units and genetic structure of carob at the Mediterranean scale, (ii) characterizing the alpha and beta taxonomic and phylogenetic diversity of carob symbiotic microbiome, (iii) exploring the links between these genetic parameters and environmental data to determine symbiotic networks and their drivers (genetic x ecological) and finally by (iv) testing experimentally the results to optimize the host-symbiont efficiency in carob tree cultures. Field investigations will be done through the carob dissemination history (native and exotic areas) and in contrasting ecological contexts (shrublands, agroforestry systems, pure stands). The symbiotic networks will be characterized by combining high-throughput molecular approaches, bioinformatic analyses based on ecological network theory, and then applied to develop innovative ecological engineering strategies. The perspectives are a better understanding of plant-microbiome genetic relationships driving ecosystem functioning and the identification of a core and an accessory "SymbiOme" in carob populations. More generally, DYNAMIC should give new insights on the ecological drivers governing host-symbiont specificity and efficiency and should propose new avenues for the development of efficient ecological engineering strategies applied to ecosystem restoration and ecological intensification of (agro)ecosystems.

Plants, as sessile organisms, are highly impacted by environmental conditions. The soil is a highly heterogeneous environment for plant nutrition composed of a mosaic of local favorable versus stressful areas. Different stresses can be experienced locally by root systems, such as nutrient limitation, water deficit, and pathogen infections. Legume plants have also a unique capacity to associate with soil nitrogen fixing bacteria (rhizobia) to form specific root endosymbiotic organs called nodules. This plant-bacteria symbiosis allows legumes, in contrast to other crops, to acquire nitrogen from an unlimited source of gaseous atmospheric nitrogen (N2). This way, legume crops can escape from soil mineral nitrogen shortage that frequently limits plant growth in unfertilized soils. As root and nodule organ development and function are highly sensitive to abiotic and biotic stresses, this currently leads to unstable yields in many legume crops. There is therefore an urgent need to improve the adaptation of legumes to environmental stresses to allow generalizing the use of efficient symbiotic nitrogen-fixing plants in agro-ecological practices. Integrative developmental strategies allow plants to adapt to heterogeneous and fluctuating soil environments. Signals induced by local stresses in different part of the root system are integrated at the whole plant level to generate long distance systemic responses to promote root and symbiotic nodule development in non-stressed soil areas. These so-called foraging compensatory responses allow the plant to improve its nutrient acquisition capacity in non-stressed areas to compensate the effect of stresses. A better understanding of these systemic regulatory mechanisms may provide a biological basis to improve plant stress tolerance through innovative selection strategies. The PSYCHE project thus aims to compare systemic responses to various local stresses, namely Nitrogen (N)-deficiency, water-deficiency (drought), or pathogen infection, to identify gene co-expression networks involved in wide-ranging developmental compensatory responses induced by these different stresses to the local suppression of root and/or nodule formation and/or function. To discriminate unambiguously between direct responses associated to local stresses and responses related to systemic signaling, the PSYCHE project will use split-root experimental systems in the Medicago truncatula model legume. Preliminary data have already shown that systemic responses to a local N-deficit or to a local drought induce similar developmental compensatory responses, suggesting that systemic regulatory pathways related to the whole plant nutritional status may regulate the development of both roots and symbiotic nodules in response to different environmental stresses. These systemic responses to a local N- or water-deficiency will then be investigated at physiological and transcriptomic levels to identify common gene co-expression networks. In addition, to test if the concept of a shared systemic regulation of root and/or nodule compensatory responses can be extended to a biotic stress, we will use a local Aphanomyces euteiches root infection. The common systemic stress-regulated gene networks identified will then be functionally tested in mutants or transgenic plants altered in systemic pathways regulating both root and nodule responses. In addition, using a collection of natural stress-contrasting M. truncatula genotypes, we will identify evolutionary adaptive gene networks, whose genetic variation is both targeted by natural selection and associated with drought- and/or Aphanomyces-tolerance. Finally, we will determine the conservation of the systemic signaling pathways identified from the M. truncatula model to the pea legume crop, with the aim of developing molecular breeding strategies in different crops to improve plant compensatory responses to various heterogeneous soil stresses.