La sénescence constitue la dernière étape du cycle de vie de certains organes des plantes. Elle permet leur dégradation tout en réallouant les constituants des tissus sénescents vers d’autres organes. Dans le contexte de la nodulation symbiotique fixatrice d’azote entre certaines plantes légumineuses et des bactéries rhizobia, un processus de sénescence a été décrit. Cependant, les connaissances sur les mécanismes de régulation de la sénescence des nodosités symbiotiques sont limitées. Au sein du laboratoire, le facteur de transcription MtNAC969 a été identifié comme un régulateur de la sénescence des nodosités. L'objectif de cette thèse est d'identifier et de caractériser de nouveaux régulateurs de la sénescence de l'organe symbiotique. Nous avons développé : (i) une approche visant à identifier des facteurs de transcription corégulés avec MtNAC969 ou avec une cystéine protéase MtCP6 utilisée comme marqueur de la sénescence des nodosités ; et (ii), une approche avec "à priori" se focalisant sur la fonction des différentes voies de signalisation des cytokinines. Cette thèse a permis d'identifier deux facteurs de transcription, MtbHLH107 et MtNAC009 et de décrypter le rôle des cytokinines dans la sénescence des nodosités. Cette thèse a permis d'identifier d'une part, deux nouveaux gènes potentiellement régulateurs de la sénescence nodositaire, MtbHLH107 et MtNAC009; et d’autre partde décrypter le rôle des cytokinines dans la sénescence de cet organe symbiotique. / Senescence is the last step of plant organ lifespan and allows their degradation in order to remobilize components from senescent tissues toward others organs. In the nitrogen fixing symbiosis nodulation occurring between legume plants and rhizobia bacteria, a senescence process has been described. However, limited knowledge about regulatory systems controlling senescence in the symbiotic nodule is available. In the laboratory, the MtNAC969 transcription factor was identified as a regulator of nodule senescence. The aim of this PhD project is to identify and characterize new regulatory factors involved in nodule senescence. We developed two independent approaches : (i) the identification of genes coregulated with MtNAC969 or a cystein protease MtCP6 used as nodule senescence marker ; and (ii), targeted approach focused on the role of cytokinin signaling pathways in nodule senescence. This project allowed us to identify two regulator transcription factors, MtbHLH107 and MtNAC009 ; and to decipher the cytokinin role in the senescence of the symbiotic organ. This PhD thesis allowed us to identify two new potential regulators of nodule senescence, MtbHLH107 and MtNAC009; and to decipher the role of cytokinins in the senescence of this symbiotic organ.

The mutualistic associations between Arbuscular mycorrhizal (AM) fungi and plant roots are ancient and ubiquitous across the plant kingdom, where AM fungi provide Phosphorus, Nitrogen, and water to the plant, and receive photosynthetically fixed Carbon in the form of fatty acids and sugars in return. Moreover, AM fungi are associated with increased plant resistance to both abiotic and biotic stressors such as drought and viral pathogens. Frequently used in agriculture, AM fungi are observed to increase crop yields and decrease chemical fertilizer needs for many economically important plant species. The potential to increase AM fungal effectiveness remains a driving force for current research. To determine their role in establishing and/or supporting AM symbiosis, we propose a reverse genetic study of two genes in the model legume Medicago truncatula. Based on RNA sequencing data indicating increased expression during AM symbiosis, we selected one gene that encodes for NAC TF-like protein, which belongs to a large family of plant transcription factors primarily involved in regulating the secretion of defence hormones. The second gene selected, PALM1, was recently discovered to play a role in the regulation of the trifoliate leaf structure of M. truncatula. We hypothesize that the genes under study play mechanistic roles in regulating AM fungal symbiosis and that we will observe a difference between the colonization rates of corresponding gene mutants and control groups. Firstly, we explored the involvement of the PALM1 and NAC TF genes by examining the root developmental phenotype of Medicago truncatula mutants. Secondly, we employed symbiosis assays to investigate the colonization rates of the genes in question. Results indicated that the NAC TF gene had no consistent role in the AM symbiosis, while the PALM1 gene revealed promising results, where significant increases in colonization rates were observed in PALM1 mutants throughout repeated experiments. Future research involves using this study to help pursue more effective ways to use AM fungi symbiosis in sustainable agro ecosystems.