L’oxygène singulet ($^1O_2$) est la principale espèce réactive de l’oxygène produite dans le chloroplaste lors d'un stress photo-oxydant. L’$^1O_2$ est hautement cytotoxique s'attaquant aux protéines et lipides membranaires. L’$^1O_2$ est une molécule signal impliquée dans une signalisation rétrograde entre les chloroplastes et le noyau via des médiateurs, menant à la mort cellulaire programmée (MCP), ou à l’acclimatation. Nous avons caractérisé l’implication de la phytohormone salicylate, agissant en aval de la signalisation de la MCP dépendante d’OXI1 (OXIDATIVE SIGNAL INDUCIBLE 1) et du jasmonate. Nous avons mis en évidence une voie de régulation négative faisant intervenir des protéines inhibitrices de la MCP : DAD1 et DAD2. La sur-expression de ces protéines permet à la plante d’éviter de la MCP en inhibant la signalisation contrôlée par OXI1. Dans le Réticulum Endoplasmique (RE) a lieu la conformation d’une grande partie des protéines. Une perturbation dans ce compartiment induit l’activation d’une réponse adaptative appelée l’UPR (Unfolded Protein Response). Nous avons monté que la production d’$^1O_2$ active l’UPR. L’acclimatation à la forte lumière (FL) fait intervenir la branche bZIP28/BiP3 de l’UPR. Un pré-traitement modéré d’un inducteur de stress RE (tunicamycine), induisant l'UPR, déclenche une réponse d’acclimatation à l’$^1O_2$ permettant l’évitement de la MCP lors de l’exposition à la FL. Nous avons réalisé un crible génétique pour rechercher des révertants du mutant \textit{ch1} (un surproducteur d’1$^1O_2$) où l’inhibition de la croissance par l’$^1O_2$ est partiellement levée. Les gènes candidats identifiés feront l’objet d’études complémentaires. / Singlet oxygen ($^1O_2$) is a major reactive oxygen species produced within the chloroplasts during high light (HL) stress. $^1O_2$ has a cytotoxic effect due to its high reactivity towards macromolecules including proteins and membrane lipids. $^1O_2$ also acts as a signal molecule that plays a role in chloroplast-to-nucleus retrograde signaling involving mediators and leading either to programmed cell death (PCD) or to stress acclimation.We have shown the involvement of the phytohormone salicylic acid in HL-induced cell death, acting downstream of the OXI1 kinase and jasmonate. We have also shown a negative regulation of this signaling pathway by PCD inhibitory proteins: DAD1 and DAD2 (DEFENDER AGAINST CELL DEATH 1 and 2). Overexpressing those proteins inhibits OXI1-mediated PCD. Protein folding of most secreted proteins takes place in the Endoplasmic Reticulum (ER). Perturbations in this compartment lead to the activation of an adaptive response called UPR (Unfolded Protein Response). When ER stress is too intense, NRPs-mediated ER stress-induced cell death is activated. We have shown that 1O2 production activates UPR. In particular, the bZIP28/BiP3 UPR branch is activated during acclimation to HL. The induction of UPR by a chemical inducer of ER stress (Tunicamycin) can induce acclimation to $^1O_2$ production and can avoid HL-induced PCD.We performed a genetic screen to search for revertants of the $^1O_2$ overproducing \textit{ch1} mutants in which growth inhibition by$^1O_2$2 is partially released. The candidate genes will have to be confirmed by further phenotypic studies.

Among the major phytohormones, the cytokinin exhibits unique features for its ability to positively affect the developmental status of plastids. Even early on in its research, cytokinins were known to promote plastid differentiation and to reduce the loss of chlorophyll in detached leaves. Since the discovery of the components of cytokinin perception and primary signaling, the genes involved in photosynthesis and plastid differentiation have been identified as those directly targeted by type-B response regulators. Furthermore, cytokinins are known to modulate versatile cellular processes such as promoting the division and differentiation of cells and, in concert with auxin, initiating the de novo formation of shoot apical meristem (SAM) in tissue cultures. Yet how cytokinins precisely participate in such diverse cellular phenomena, and how the associated cellular processes are coordinated as a whole, remains unclear. A plausible presumption that would account for the coordinated gene expression is the tight and reciprocal communication between the nucleus and plastid. The fact that cytokinins affect plastid developmental status via gene expression in both the nucleus and plastid is interpreted here to suggest that cytokinin functions as an initiator of anterograde (nucleus-to-plastid) signaling. Based on this viewpoint, we first summarize the physiological relevance of cytokinins to the coordination of plastid differentiation with de novo shoot organogenesis in tissue culture systems. Next, the role of endogenous cytokinins in influencing plastid differentiation within the SAM of intact plants is discussed. Finally, a presumed plastid-derived signal in response to cytokinins for coupled nuclear gene expression is proposed.

Die Endprodukte der Tetrapyrrolbiosynthese sind essentiell für die Schwefel- und Stickstoffassimilation (Sirohäm), der von Photorezeptoren abhängigen Genexpression (Phytochromobilin), Elektronenübertragungsreaktionen (Häm) und der Photosynthese (Chlorophyll). Die Synthese von Chlorophyllen wird durch eine Mg-Chelatase (MgCh) eingeleitet, die durch das GENOMES UNCOUPLED 4 (GUN4) Protein stimuliert wird. GUN4 ist essentiell für die Aktivierung der MgCh und die Synthese von Chlorophyllen. Das GUN4 aus Arabidopsis thaliana wird ausschließlich an der vorletzten Aminosäure (S264) des C-Terminus phosphoryliert. Die in vitro und in vivo MgCh-Aktivität wird hingegen durch phosphoryliertes GUN4 nicht mehr stimuliert. De-phosphoryliertes GUN4 bewirkt die lichtabhängige Aktivierung der MgCh im Übergang von der Nacht zum Tag in Angiospermen. Im Laufe der Evolution photosynthetisch aktiver Organismen hat sich die in den Angiospermen hochkonservierte Phosphorylierungsstelle entwickelt. GUN4-Homologe aus Synechocystis oder Chlamydomonas werden nicht phosphoryliert. Im Rahmen der Suche nach der GUN4-spezifischen Proteinkinase wurden vier in den Plastiden lokalisierte PLASTID PROTEIN KINASE WITH UNKNOWN FUNCTION identifiziert. In dieser Arbeit wurden zusätzlich Experimente zum durch die GUN-Proteine vermittelten retrograden Signalweg durchgeführt. gun Mutanten sind durch eine defizitäre cytosolische Anthocyan-/Flavonoidbiosynthese charakterisiert. Auf der Suche nach Hinweisen für einen Zusammenhang zwischen Anthocyanen und der De-repression von PHOTOSYNTHESIS-ASSOCIATED NUCLEAR GENES wurde eine neue gun Mutante identifiziert. Der knockout der durch TRANSPARENT TESTA 4 (TT4) kodierten CHALCON SYNTHASE führte zu einer mit den gun Mutanten vergleichbaren De-repression der PHANGs nach Norflurazon-Behandlung. Pharmakologische Experimente belegen eine mögliche Funktion der Phenylpropanoidbiosynthese in der durch die GUN-Proteine vermittelten retrograden Kommunikation. / Endproducts of the tetrapyrrole biosynthesis pathway are essential for the assimilation of sulfur and nitrogen (siroheme), photoreceptor mediated control of nuclear gene expression (phytochromobilin), electron transfer reactions (heme) and photosynthesis (chlorophyll). The synthesis of chlorophyll is initiated by a Mg-chelatase (MgCh) which is stimulated by the GUN4 protein. GUN4 is essential for the activation of MgCh and synthesis of chlorophyll. GUN4 from Arabidopsis thaliana is exclusively phosphorylated at the next-to-last amino acid of the C-terminus (S264). The stimulatory impact towards MgCh is reduced upon GUN4 phosphorylation. De-phosphorylated GUN4 stimulates MgCh activity during the transition from night to daytime. The phosphorylation site of GUN4 has evolved in the clade of angiosperms. GUN4 homologs of Synechocystis or Chlamydomonas are not phosphorylated. In an attempt to isolate the GUN4-kinase four formerly unknown PLASTID PROTEIN KINASE WITH UNKNOWN FUNCTION were identified. In addition to the elucidation of the post-translational GUN4 modifications, experiments concerning the GUN-dependent retrograde signaling pathway were performed. Under conditions which lead to a block of chloroplast development the de-repression of PHOTOSYNTHESIS-ASSOCIATED NUCLEAR GENES is paralleled by a reduced accumulation of anthocyanins in the gun mutants. When searching for a correlation between anthocyanin biosynthesis and expression of PHANGs a new gun mutant was identified. The knockout of CHALCONE SYNTHASE encoded by TRANSPARENT TESTA 4 (TT4) leads to a comparable de-repression of PHANGs after norflurazon treatment as it was observed for the gun mutants. Pharmacological modification of phenylpropanoid biosynthesis revealed that an intermediate of the pathway is a component of chloroplast-to-nucleus communication. Hence, first evidences for a function of the phenylpropanoid biosynthesis pathway in mediating the GUN-dependent retrograde signal were obtained.