Les microalgues ont développé des approches distinctes pour moduler l'absorption de la lumière et son utilisation par leurs photosystèmes en réponse à des stimuli environnementaux. Dans ce rapport de Thèse je présente les différentes stratégies employées par une algue d'eau douce (Chlamydomonas reinhardtii) et une algue marine (Phaeodactylum tricornutum) pour optimiser leur acclimatation à l'environnement.Dans la première partie de ce rapport, je propose un modèle de cellules entières de la diatomée marine Phaeodactylum tricornutum obtenue par analyses spectroscopiques et biochimiques ainsi que par l’obtention d’images par microscopie électronique et reconstitution 3-D. Ce modèle a été utilisé pour répondre aux questions suivantes i. comment est structuré un chloroplaste secondaire pour faciliter les échanges avec le cytosol à travers les quatre membranes qui le délimitent ii. comment sont structurées les membranes photosynthétiques afin d’optimiser l'absorption de lumière et le flux d'électrons et iii. comment les chloroplastes et les mitochondries sont organisés pour optimiser l'assimilation du CO2 par échange ATP / NADPH.La deuxième partie de ce rapport porte sur la régulation de la collection de la lumière et de sa dissipation chez Chlamydomonas grâce à l'étude d'une part du rôle de la perception de la couleur de la lumière et d'autre part du métabolisme sur la dissipation de l'excès de lumière par quenching non photochimique (NPQ). En utilisant des approches biochimiques et spectroscopiques, j'ai mis en évidence un lien moléculaire entre la photoréception, la photosynthèse et la photoprotection chez Chlamydomonas via le rôle du photorécepteur phototropine, démontrant ainsi que le métabolisme, en plus de la lumière, peut aussi affecter ce processus d'acclimatation.En conclusion, ce travail de thèse révèle l'existence et l'intégration des différentes voies de signalisation dans la régulation des réponses photoprotectrices mises en place chez les microalgues marines et d'eau douce. / Microalgae have developed distinct approaches to modulate light absorption and utilization by their photosystems in response to environmental stimuli. In this Ph.D Thesis, I characterised different strategies employed by freshwater (Chlamydomonas reinhardtii) and marine algae (Phaeodactylum tricornutum) to optimise their acclimation to the environment.In the first part of this work, I used spectroscopic, biochemical, electron microscopy analysis and 3-dimentional reconstitution to generate a model of the entire cell of the marine diatom Phaeodactylum tricornutum. This model has been used to address the following questions: i. how is a secondary chloroplast structured to facilitate exchanges with the cytosol via its four membranes envelope barrier ii. how have diatoms shaped their photosynthetic membranes to optimise light absorption and downstream electron flow and iii. how the cellular organelles interact to optimise CO2 assimilation via ATP/NADPH exchanges.In the second part, I have focused on the regulation of light harvesting and dissipation in Chlamydomonas by studying the role of perception of light colour and metabolism on excess light dissipation via the Non-Photochemical Quenching of energy (NPQ). Using biochemical and spectroscopic approaches, I found a molecular link between photoreception, photosynthesis and photoprotection in Chlamydomonas via the role of the photoreceptor phototropin on excess absorbed energy dissipation (NPQ) and also demonstrated that besides light, downstream metabolism can also affect this acclimation process.Overall this Ph.D work reveals the existence and integration of different signal pathways in the regulation of photoprotective responses by microalgae living in the ocean and in the land.

Previous studies have detected the presence of the capsid protein (CP) and CP RNA of PVX in chloroplasts of transgenic plants expressing the PVX CP and 8kD genes from nuclear transgenes. CP RNA was also found in the chloroplasts of transgenic plants with mutations that eliminated either the CP or the 8kD protein. To further investigate the potential “RNA Tractor” activity of the PVX RNA, various constructs containing limited regions of the PVX 8kD and CP genes were produced and used to transform tobacco plants. RT-PCR analyses of chloroplastic RNA ascertained the presence of the RNA transcript within chloroplasts when as little as 125bp of the PVX sequence was used to transform plants. From this, it was concluded that the PVX proteins did not contribute to the movement of the viral RNA and that a region within the 125bp PVX sequence is acting as a chloroplastic “RNA Tractor”.

Previous studies have detected the presence of the capsid protein (CP) and CP RNA of PVX in chloroplasts of transgenic plants expressing the PVX CP and 8kD genes from nuclear transgenes. CP RNA was also found in the chloroplasts of transgenic plants with mutations that eliminated either the CP or the 8kD protein. To further investigate the potential “RNA Tractor” activity of the PVX RNA, various constructs containing limited regions of the PVX 8kD and CP genes were produced and used to transform tobacco plants. RT-PCR analyses of chloroplastic RNA ascertained the presence of the RNA transcript within chloroplasts when as little as 125bp of the PVX sequence was used to transform plants. From this, it was concluded that the PVX proteins did not contribute to the movement of the viral RNA and that a region within the 125bp PVX sequence is acting as a chloroplastic “RNA Tractor”.

Pure-breeding, early-flowering lines of flax, derived from treatment of germinating seeds with 5-azacytdine in 1990, flower 7-13 days before controls, have fewer leaves, are shorter, and have hypomethylated total DNA, relative to control lines. This thesis examines the changes in DNA methylation levels in the cotyledons and shoot tips of early-flowering Royal flax lines (i.e. RE1 and RE2) and their control (RC) to determine the changes from 24 days to the onset of flowering (approximately 34 days in RE1 and RE2, and 52 days in RC). It also examines the question of whether DNA is methylated in the chloroplast genome of flax. Finally, the thesis looks at the differences in transcript abundance of the flowering genes LEAFY and TERMINAL FLOWER1 in RC and RE2. Methylation levels in RE1 and RE2 were found to be lower than in RC from 24 days of age to the onset of flowering and the levels of all three lines increase with tissue age and/or differentiation. In addition, buds of RE2 were hypomethylated relative to RC. If plants were placed in the dark prior to DNA extraction, hypomethylation was not seen in the total DNA of RE2. The chloroplast DNA of flax was found to be methylated, and RE2 chloroplast DNA was hypomethylated relative to RC. Differences in transcript levels of LFY were seen in RC and RE2 shoot tips, where a higher accumulation of transcript seen in RE2 compared to RC may be related to its earlier flowering time. In leaves, there was no significant difference in the transcript abundance of LFY between RC and RE2. TFL1 was detected in genomic DNA of RC and RE2; it was not detected in the cDNA of the two lines. In summary, compared to RC, hypomethylation was seen in the total DNA of RE2 plants grown under regular light conditions and the methylation levels in the all lines increased with age in shoot tips, cotyledons, and leaves. The chloroplast DNA of RE2 was also hypomethylated relative to that of RC. RE2 accumulated LFY transcript in shoot tips at flowering, which was not the case in RC. Although these ideas cannot be linked at this time, they are all likely related to the early-flowering phenotype.

Pure-breeding, early-flowering lines of flax, derived from treatment of germinating seeds with 5-azacytdine in 1990, flower 7-13 days before controls, have fewer leaves, are shorter, and have hypomethylated total DNA, relative to control lines. This thesis examines the changes in DNA methylation levels in the cotyledons and shoot tips of early-flowering Royal flax lines (i.e. RE1 and RE2) and their control (RC) to determine the changes from 24 days to the onset of flowering (approximately 34 days in RE1 and RE2, and 52 days in RC). It also examines the question of whether DNA is methylated in the chloroplast genome of flax. Finally, the thesis looks at the differences in transcript abundance of the flowering genes LEAFY and TERMINAL FLOWER1 in RC and RE2. Methylation levels in RE1 and RE2 were found to be lower than in RC from 24 days of age to the onset of flowering and the levels of all three lines increase with tissue age and/or differentiation. In addition, buds of RE2 were hypomethylated relative to RC. If plants were placed in the dark prior to DNA extraction, hypomethylation was not seen in the total DNA of RE2. The chloroplast DNA of flax was found to be methylated, and RE2 chloroplast DNA was hypomethylated relative to RC. Differences in transcript levels of LFY were seen in RC and RE2 shoot tips, where a higher accumulation of transcript seen in RE2 compared to RC may be related to its earlier flowering time. In leaves, there was no significant difference in the transcript abundance of LFY between RC and RE2. TFL1 was detected in genomic DNA of RC and RE2; it was not detected in the cDNA of the two lines. In summary, compared to RC, hypomethylation was seen in the total DNA of RE2 plants grown under regular light conditions and the methylation levels in the all lines increased with age in shoot tips, cotyledons, and leaves. The chloroplast DNA of RE2 was also hypomethylated relative to that of RC. RE2 accumulated LFY transcript in shoot tips at flowering, which was not the case in RC. Although these ideas cannot be linked at this time, they are all likely related to the early-flowering phenotype.

Thesis (Ph. D.)--University of Missouri-Columbia, 1999. / Typescript. Vita. Includes bibliographical references (leaves 111-127). Also available on the Internet.

Univ., Diss.--München, 1998.

Thesis (Ph. D.)--University of Texas at Austin, 2004. / Supervisor: Robert K. Jansen. Vita. Includes bibliographical references. Also available from UMI.

Thesis (Ph. D.)--University of Missouri-Columbia, 1999. / Typescript. Vita. Includes bibliographical references (leaves 111-127). Also available on the Internet.

Universiẗat, Diss., 2005--Würzburg.