Apomixis, asexual or clonal seed production in plants, can decrease the cost of producing hybrid seed and enable currently open pollinated crops to be converted to more vigorous and higher yielding hybrids that can reproduce themselves through their own seed. Sexual reproduction may be triggered by a programmed stress signaling event that occurs in both the meiocyte, just prior to meiosis, and later in the egg just prior to embryo sac maturation. The prevention of stress signaling and the activation of a pro-growth signal prior to meiosis triggered apomeiosis, the first half of apomixis. The same approach was used prior to embryo sac maturation to trigger parthenogenesis, the second half of apomixis. This discovery suggests that apomixis exists as a program that can be activated by the appropriate metabolic signal at the appropriate developmental stages. Therefore, apomixis may be alternative mode of reproduction rather a ‘broken’ form of sexual reproduction.

La croissance d’un organisme multicellulaire pour atteindre une taille bien définie, nécessite une coordination de la prolifération cellulaire, de l’expansion et de la différentiation cellulaire ainsi que de la mort cellulaire. Ces processus sont sous l’influence de l’état nutritionnel de l’organisme, les conditions de son environnement et des signaux hormonaux. Translationally controlled tumor protein (TCTP) est un facteur essentiel à la croissance des plantes et des animaux. La protéine TCTP de plante contrôle la croissance mitotique, tandis que la protéine TCTP animale contrôle la croissance mitotique et post-mitotique. Une voie importante dans la régulation de la croissance en réponse aux nutriments est la voie Target of Rapamycin (TOR). Chez la Drosophile, il a été montré que dTCTP serait un régulateur positif en amont de TOR. Au cours de ma thèse, j’ai étudié le lien entre TCTP et la voie TOR, afin de savoir si, comme chez les animaux, AtTCTP agit en amont de la voie TOR pour contrôler la croissance des organes. Afin de savoir si la voie TCTP était liée à l’état nutritionnel, j’ai recherché l’impact du milieu de culture sur la létalité de la mutation tctp. J’ai ensuite caractérisé l’impact de la mutation tctp sur le transport et l’homéostasie de l’hormone auxine. J’ai enfin analysé pourquoi TCTP de plante ne contrôle pas la croissance post-mitotique par expansion cellulaire, contrairement à TCTP animale. Les données de la littérature montrent que chez les animaux TCTP est un activateur positif en amont de la voie TOR. Chez la plante Arabidopsis thaliana, mes données d’interactions génétiques sont en faveur d’un modèle dans lequel AtTCTP agit indépendamment de la voie TOR, contrairement de ce qu’il a été proposé chez les animaux. Chez les plantes, la perte de fonction de TCTP est associée à un retard du développement embryonnaire et à la mort. Cette létalité peut être complémentée par sauvetage des embryons sur du milieu riche en nutriments. J’ai montré que l’ajout de sucrose ou de glutamine dans le milieu de sauvetage des embryons tctp est nécessaire à leur développement. Ces données suggèrent qu’in vitro, AtTCTP n’est pas nécessaire à l’approvisionnement et à l’utilisation des nutriments sucrose, glucose ou glutamine. Dans leur ensemble, ces résultats réévaluent le rôle du régulateur de croissance TCTP en montrant que le gène AtTCTP régule la croissance mitotique indépendamment de la voie TOR et des voies de signalisation liées aux nutriments. L’observation des flux d’auxine en suivant la localisation de PIN1-GFP dans les embryons et les inflorescences du mutant tctp ne montre aucune altération par rapport au phénotype sauvage. De même, l’homeostasie de l’auxine, suivie à l’aide du rapporteur DR5::GFP n’est pas altérée dans les embryons tctp. Ceci suggère que le défaut de croissance du mutant tctp n’est pas lié à une altération du flux ou de l’homéostasie de l’auxine. La protéine TCTP de plante ne contrôle pas la croissance post-mitotique, contrairement à la protéine TCTP animale. J’ai réalisé un échange de domaines protéiques entre AtTCTP et Drosophila dTCTP. Le but était d’identifier les domaines protéiques de la protéine TCTP animale qui permettent la croissance post-mitotique. La plupart des protéines chimères étaient instables dans la Drosophile. Afin de comprendre pourquoi, j’ai réalisé du modelage par homologie et j’ai discuté la structure des chimères dans ma thèse.L’ensemble de mes résultats permet de mieux comprendre la fonction de TCTP chez les végétaux, en montrant que cette fonction s’exerce indépendamment de la voie TOR. / The growth of a multicellular organism and its size determination require the tight regulation of cell proliferation, cell differentiation, cell growth and apoptosis. These processes are influenced by the nutritional state of the organism, its environmental conditions and hormonal signals. Translationally controlled tumor protein (TCTP) is an essential regulator of growth in plants and animals. In plants it controls mitotic growth, whereas in animals, it controls mitotic and post-mitotic growth. One of the important pathways involved in the control of growth in response to nutrients is the Target of Rapamycin (TOR) pathway. In Drosophila, dTCTP was proposed to act a positive regulator upstream of TOR, although this role remains a matter of debate in the animal field.During the past 3 years of my PhD. thesis, I addressed the question whether plant TCTP acts upstream of TOR to control organ growth. I studied the impact of nutrient availability and hormones on TCTP role to control growth in plants and vice versa. Finally, I examined why plant TCTP does not control post-mitotic cell expansion growth, conversely to animal TCTP using a structure-function approach.In animals, TCTP was proposed to act as a positive activator upstream of the TOR pathway. In plants, my data support a model in which AtTCTP acts independently from the plant TOR pathway, thus in contrast to what has been proposed in animals. TCTP loss of function leads to delay of embryo development and death. Nutrient supplement rescues this embryos lethality. First, I demonstrate that embryos grown on nutrients lacking sucrose or glutamine fail to develop correctly. My data demonstrate that in vitro AtTCTP is not essential to the uptake, the use of and the response to the nutrients glucose, sucrose or glutamine. Taken together, these results reevaluate the role of AtTCTP as a growth regulator controlling mitotic growth independently from the TOR pathway and likely from nutrient related signaling pathways. Interestingly, my data also show that AtTCTP controls growth independently from auxin flux or homeostasis and that auxin-induced growth can occur without TCTP. To address why plant TCTP do not control post-mitotic growth conversely to animal counterpart, I performed protein domain swaps and created chimera proteins between Arabidopsis AtTCTP and Drosophila dTCTP. The rational was to identify protein domains that differentiate plant and animal TCTPs with regard to post-mitotic growth control. Most of chimera proteins were instable and I was unable to complement tctp loss of function in Drosophila. I performed a structure based modeling to understand this phenotype and the outcome is discussed in my PhD thesis.Altogether my results improve the understanding of plant morphogenesis by reevaluating the role of the central growth regulator TCTP.

Pbp1 is a component of glucose deprivation induced stress granules and is involved in P-body-dependent granule assembly. We have recently shown that Pbp1 plays an important role in the interplay between three sensory protein kinases in yeast: AMP-regulated kinase (Snf1 in yeast), PAS kinase 1 (Psk1 in yeast), and the target of rapamycin complex 1 (TORC1), to regulate glucose allocation during nutrient depletion. This signaling cascade occurs through the SNF1-dependent phosphorylation and activation of Psk1, which phosphorylates and activates poly(A)- binding protein binding protein 1 (Pbp1), which then inhibits TORC1 through sequestration at stress granules. In this study we further characterized the regulation of Pbp1 by PAS kinase through the characterization of the role of the Psk1 homolog (Psk2) in Pbp1 regulation, and the identification of functional Pbp1 binding partners. Human ataxin-2 (ATXN2) is the homolog of yeast Pbp1 and has been shown to play an important role in the development of several ataxias. In this study we have also provided the evidence that human ataxin-2 can complement Pbp1 in yeast, and that human PAS kinase can phosphorylate human ataxin-2. Further characterizing this interplay between PAS kinase and Pbp1/ATXN2 aid in understanding pathways required for proper glucose allocation during nutrient depletion, including reducing cell growth and proliferation when energy is low. In addition, it yields valuable insights into the role of ataxin-2 in the development of devastating ataxias.

Nutrient sensing kinases sense available nutrients and regulate cell activity accordingly. Three of these enzymes are AMP regulated kinase (AMPK, or Snf1 in yeast), PAS kinase, and target of rapamycin (TOR), are conserved from yeast to man and have overlapping function. AMPK and Snf1 are important in sensing when nutrient status in the cell is low and down regulating energy consuming pathways. PAS kinase is required for glucose homeostasis in the cell, and responds to glucose levels. TOR senses nutrients such as amino acids and upregulates cell growth pathways primarily through protein synthesis. Due to the varying nature of these enzymes, cross talk is expected in order for the cell to properly regulate cellular metabolism and growth in response to energy and nutrient availability. Previous studies have shown that activation of yeast PAS kinase under nutrient stress conditions requires the presence of Snf1. The aim of this thesis is to determine whether Snf1 directly phosphorylates and activates PAS kinase through both in vivo and in vitro approaches. PAS kinase was found to require Snf1 for both activation and phosphorylation in vivo. In vitro kinase assays were also performed to confirm a direct phosphorylation event. The results from this study support the direct phosphorylation and activation of PAS kinase by Snf1, linking cellular energy status to glucose allocation.

Among abiotic stresses, salinity is considered the main limiting stress that negatively affects plant growth and reduces productivity worldwide. To overcome this challenge, a sustainable solution such as plant growth-promoting bacteria (PGPB) can be used to meet the increasing demand for food. The desert microbe Enterobacter sp SA187, an endophytic PGPB, induces salt tolerance in both model plant and crops. The interaction between SA187 and the host plant triggers the sulfur pathway in the bacteria which then provides multiple sulfur-containing compounds to its host plant. However, the molecular sensor of these compounds in the host plant is not known. Here, we show that SA187 activates the plant target of rapamycin (TOR) pathway. The beneficial effect of SA187 was lost in TOR mutants like raptor, and by the application of TOR inhibitor AZD8055. Next, we show that SA187 modulates the one- carbon (1C) metabolism of the host plant consisting of methionine and folate cycles. The beneficial effect of SA187 was compromised by using chemical inhibitors of folate cycle like Methotrexate (MTX) and Sulfadiazine (SDZ). The intermediates of the 1C metabolism like Homocysteine and S-adenosyl methionine (SAM) showed similar beneficial effects as SA187 colonized plants. Finally, we showed that SA187 enhances 1C metabolism activity by increasing methylation index (SAM/SAH ratio) in the plants. Taken together, we could show that host TOR-1C axis is essential for plant salt tolerance by SA187.