Thèse Interactions Entre la Nutrition en Fer et la Réponse à l'Élévation du Co2 Atmosphérique chez le Peuplier H/F - Doctorat.Gouv.Fr

Établissement : Université Paris-Saclay GS Biosphera - Biologie, Société, Ecologie & Environnement, Ressources, Agriculture & Alimentation École doctorale : Sciences du Végétal : du gène à l'écosystème Laboratoire de recherche : I2BC - Institut de Biologie Intégrative de la Cellule Direction de la thèse : Sébastien THOMINE ORCID 0000000300451701 Début de la thèse : 2026-10-01 Date limite de candidature : 2026-05-05T23:59:59 L'augmentation du CO atmosphérique (eCO) est un élément clé du changement climatique qui a un impact profond sur la physiologie et le métabolisme des plantes. L'eCO entraine une diminution de concentrations en nutriments dans les tissus végétaux, y compris celles de métaux de transition essentiels comme le fer (Fe). Le Fe est un cofacteur important pour les réactions d'oxydoréduction dans les mitochondries et les chloroplastes, et une carence en Fe affecte la photosynthèse et le rendement des plantes. L'eCO entraîne une diminution de la teneur en Fe dans plusieurs espèces cultivées. Les réponses des plantes à des contraintes environnementales combinées ne sont généralement pas prévisibles à partir de leurs effets individuels, ce qui souligne la nécessité de mener des études approfondies pour appréhender les réponses des plantes dans des situations où ces contraintes sont combinées. Les mécanismes sous-jacents à l'interaction entre l'homéostasie des métaux et les réponses à l'eCO sont actuellement étudiés chez Arabidopsis, la tomate et le haricot. Cependant, ils n'ont pas encore été étudiés chez les arbres. Le peuplier est une plante modèle importante sur le plan économique et pour l'étude des réponses des arbres aux stress environnementaux, avec un génome bien caractérisé. Chez le peuplier, deux transporteurs de métaux, IRT1 et NRAMP1, ont été proposés pour médier l'absorption du fer. Les exsudats racinaires, en particulier les composés phénoliques tels que les coumarines, les flavines et les flavonoïdes, jouent également un rôle essentiel dans l'acquisition du fer en chélatant et en réduisant le Fe³dans le sol. Le projet vise à comprendre comment l'eCO affecte les mécanismes d'absorption et d'homéostasie du Fe, et comment la nutrition en Fe module les effets de l'eCO chez le peuplier. Par des approches de physiologie et de génétique moléculaire, le doctorant caractérisera les réponses du peuplier à eCO dans différentes conditions de disponibilité en fer, notamment en analysant la biomasse, l'efficacité photosynthétique, la teneur en minéraux et en métabolites primaires, la composition des exsudats racinaires et les changements dans l'abondance des transcrits. Afin de déterminer le rôle de l'absorption du Fe dans la réponse des peupliers à l'eCO, le doctorant générera des lignées transgéniques surexprimant les gènes IRT1 et NRAMP1 et des mutations « « perte de fonction via CRISPR/Cas9, puis analysera leur réponse à l'eCO. L'ensemble de ces expériences contribuera à une meilleure compréhension des relations entre la nutrition minérale et les réponses à l'eCO chez les arbres. Elevated atmospheric CO (eCO) is a key component of climate change, profoundly affecting plant physiology and metabolism. Generally, eCO enhances photosynthetic rates and biomass production by reducing photorespiration in C3 plants. However, eCO may also decrease tissue nutrient concentrations, including essential micronutrients like iron (Fe) (Lodatze 2014, Marinari et al., 2007, Gojon et al., 2023). Iron is a transition metal that plays a crucial role as a cofactor for essential reactions in mitochondria and chloroplast and is required for photosynthesis and plant growth; Fe deficiency results in chlorosis and decreased plant growth (Connorton et al., 2017). This raise the question of the interaction between Fe nutrition and plant responses to eCO. Plant responses to combined environmental constraints are usually not predictable from their individual effects, highlighting the need for comprehensive studies to apprehend plant responses under such combinations (Rakhmankulova et al., 2024). The genetic mechanisms underlying the impact of eCO on mineral nutrition have been investigated in the model plant Arabidopsis thaliana (Cassan et al., 2024). In Arabidopsis, a putative iron transporter was found to mitigate the decrease in iron content under eCO (Mozzanino 2025). The effects of eCO on Fe nutrition have also been analysed in tomato and bean (Deuchande & vasconcelos 2023; Jin et al. 2009).Trees show the most significant increase in photosynthesis under eCO (Dusenge et al., 2018). Soil fertility was shown to limit the increase in forest productivity induced by eCO (Oren et al. 2001). However, little is known about the interplay between mineral nutrition and eCO responses in trees. Poplar is an ideal model for exploring tree responses to environmental stresses since it is economically significant for wood and biofuel production, it has a well-characterized genome, and it is amenable to genetic engineering. Although poplars are often utilized to restore polluted sites due to their significant tolerance for heavy metals, only a few metal transporters have been identified in this species. The response to Fe deficiency has been characterized in poplar (Masuda et al., 2018). Moreover, two poplar metal transporters, IRT1 (Iron-Regulated Transporter 1) and NRAMP1 (Natural Resistance-Associated Macrophage Protein 1), have been proposed to mediate Fe uptake. In Arabidopsis, IRT1 is primarily responsible for high-affinity Fe² uptake under Fe-deficient conditions, while NRAMP1 contributes to low-affinity uptake (Castaing et al., 2016). As Fe is present as Fe3 in aerated soils, reduction to Fe² by the root surface ferric chelate reductase FRO2 is required prior to uptake by IRT1 or NRAMP1 (Connorton et al., 2017). In poplar, IRT1 is upregulated by iron deficiency (Huang et al., 2015). Notably, NRAMP1 has been suggested to play a significant role in Fe² uptake at the late stages of Fe deficiency and could be involved in sustained Fe acquisition in poplars (Chen et al., 2019). However, these roles have been proposed solely based on PtIRT1 and PtNRAMP1 gene expression patterns without genetic evidence. Root exudation of phenolic compounds like coumarins, flavins, and flavonoids in the rhizosphere is also crucial for Fe acquisition in dicotyledonous plants. These molecules help solubilize Fe by chelating Fe³ and reducing it to Fe² in the soil, making it more available for plant uptake (Tsai & Schmidt,2017). Under iron deficiency, Arabidopsis increase the production of proteins involved and coumarin biosynthesis and upregulate the ABCG-type transporter, PDR9, which releases coumarins in the rhizosphere. On the other hand, eCO influences the accumulation of organic acids, such as citrate and malate that solubilise Fe in plants (Matros et.al., 2005, Deuchande & Vasconcelos 2024). These Fe-mobilizing metabolites could be involved in the interplay between Fe nutrition, eCO. Understanding how poplars regulate Fe acquisition under eCO will be crucial for creating strategies to deal with the damaging effects of climate change on forest ecosystems. The thesis project has 3 main objectives:
1) To provide a better understanding of the mechanisms of Fe acquisition by poplar. Specifically, the PhD student will explore the roles of IRT1 and NRAMP1 transporters in Fe acquisition by generating overexpression lines and loss-of-function mutants via CRISPR/Cas9.
2) To characterize the effect of eCO on Fe nutrition in poplar. This will include measurements of metal content, transcript levels of genes involved in Fe homeostasis and root exudate metabolites involved in Fe mobilization.
3) To investigate the effect of iron nutrition on poplar response to eCO. This will include measurements of biomass, photosynthetic parameters, primary metabolite and mineral contents and C/N ratios under altered iron availability and in genotypes with defective or enhanced Fe uptake
Together the results are expected to shed light on the mechanisms underlying the interplay between iron acquisition and eCO2 response in a model tree.
To reach the objectives, the project will follow 3 parallel lines of research.
1) Functional characterization of poplar transporters involved in Fe acquisition
Poplar genotypes will be generated to assess the role of metal transporters in iron acquisition. These genotypes will include lines overexpressing Populus IRT1 and NRAMP1 and knock out in these 2 genes, which are known to be involved in Fe deficiency responses in poplar. The PhD student will generate vectors to express IRT1 and NRAMP1 under a strong constitutive promoter and use CRISPR-Cas9 technology to create loss-of-function mutants for these genes. The constructs will be transformed into the hybrid poplar clone INRA 717-1-B4 clone (P. tremula x P. alba), using agrobacterium-mediated transformation of stem explants. The transformed calli will be genotyped by PCR and sequencing to screen for mutations in IRT1 and NRAMP1 genes and RT-qPCR to identify overexpressors. The responses of these genotypes to varying Fe nutrition will be characterized.
2) Characterize the impact of eCO on Fe nutrition in poplar
The PhD student will characterize the response to Fe deficiency under ambient or eCO. For this, the hybrid INRA 717-1-B4 clone will be grown under sufficient (20 µM) and limiting (0) Fe supply at ambient or eCO (800 ppm). The PhD student will measure key parameters of Fe nutrition: chlorophyll concentration as marker of deficiency, metal (Fe, Mn, Zn, Cu) content using atomic emission spectrometry, root ferric chelate reductase activity and Fe mobilizing metabolites in root exudates. For this, root exudates will be collected from the growth media of poplars grown under hydropony. The PhD student will quantify organic acids as well as Fe mobilizing coumarins and flavonoids in exudates using LC MS, as previously performed in the team (Cieschi et al. in preparation). In addition, the PhD student will measure the transcript levels of poplar orthologues of genes involved in Fe uptake (IRT1, NRAMP1, FRO2) and in coumarin and organic acid exudation (ALMT, MATE, PDR9) in Arabidopsis thaliana using RT-qPCR (Masuda et al., 2018).
3) Characterize the impact of iron nutrition on poplar response to eCO2
The PhD student will investigate poplar response to eCO under sufficient and limiting Fe supply in the hybrid INRA 717-1-B4 clone as well as in IRT1 and NRAMP1 overexpressing and knockout lines obtained in this background (see 1). The PhD student will measure key parameters of the plant response to eCO : biomass production as marker of CO2 fertilization effect, photosystem efficiency using chlorophyll fluorescence and NIR absorption, leaf gas exchange using LI-COR technology, carbon-to-nitrogen (C/N) ratio as well as the level of metabolites involved in photorespiration. Glycerate, glycolate, glyoxilate, 2-OG, and amino acids will be measured by GC-MS and HPLC. This will allow determining the glycine to serine ratio, which is considered as diagnostic for photorespiration.

Le candidat devra être titulaire d'une Master dans le domaine de la biologie. Une expérience préalable en physiologie végétale et/ou en biologie moléculaire et en génétique sera un atout.
Le candidat doit être fortement motivé pour se tenir au courant de la littérature dans le domaine du projet, qui évolue rapidement. On attend de lui qu'il devienne rapidement autonome et qu'il soit prêt à interagir avec les autres membres de l'équipe et de l'institut, ainsi qu'avec les collaborateurs, afin de mettre en oeuvre le projet de manière efficace.
Une bonne maîtrise de l'anglais est nécessaire.