@phdthesis{Uflewski2021,
  author    = {Uflewski, Michal},
  title     = {Characterizing the regulation of proton antiport across the thylakoid membrane},
  school      = {Universit{\"a}t Potsdam},
  pages     = {122},
  year      = {2021},
  abstract  = {Die Energie, die zum Antrieb photochemischer Reaktionen ben{\"o}tigt wird, stammt aus der Ladungstrennung an der Thylakoidmembran. Aufrgrund des Unterschieds in der Protonenkonzentration zwischen dem Stroma der Chloroplasten und dem Thylakoidlumen wird eine Protonenmotorische Kraft (pmf) erzeugt. Die pmf setzt sich aus dem Protonengradienten (ΔpH) und dem Membranpotential (ΔΨ) zusammen, die gemeinsam die ATP-Synthese antreiben. In der Natur schwankt die Energiemenge, die die Photosynthese antreibt, aufgrund h{\"a}ufiger {\"A}nderungen der Lichtintensit{\"a}t. Der Thylakoid-Ionentransport kann den Energiefluss durch einen Photosyntheseapparat an die Lichtverf{\"u}gbarkeit anpassen, indem er die pmf-Zusammensetzung ver{\"a}ndert. Die Dissipation von ΔΨ verringert die Ladungsrekombination am Photosystem II, so dass ein Anstieg der ΔpH-Komponente eine R{\"u}ckkopplung zur Herabregulierung der Photosynthese ausl{\"o}sen kann. Der durch den K+-Austausch-Antiporter 3 (KEA3) gesteuerte K+/H+-Antiport reduziert den ΔpH-Anteil von pmf und d{\"a}mpft dadurch das nicht-photochemische Quenching (NPQ). Infolgedessen erh{\"o}ht sich die Photosyntheseeffizienz beim {\"U}bergang zu geringerer Lichtintensit{\"a}t. Ziel dieser Arbeit war es, Antworten auf Fragen zur Regulierung der KEA3-Aktivit{\"a}t und ihrer Rolle in der Pflanzenentwicklung zu finden. Die vorgestellten Daten zeigen, dass KEA3 in Pflanzen, denen der Chloroplasten-ATP-Synthase-Assembly-Faktor CGL160 fehlt und die eine verminderte ATP-Synthase-Aktivit{\"a}t aufweisen, eine zentrale Rolle bei der Regulierung der Photosynthese und des Pflanzenwachstums unter station{\"a}ren Bedingungen spielt. Das Fehlen von KEA3 in der cgl160-Mutante f{\"u}hrt zu einer starken Beeintr{\"a}chtigung des Wachstums, da die Photosynthese aufgrund des erh{\"o}hten pH-abh{\"a}ngigen NPQs und des verringerten Elektronenflusses durch den Cytochrom b6f-Komplex eingeschr{\"a}nkt ist. Die {\"U}berexpression von KEA3 in der cgl160-Mutante erh{\"o}ht die Ladungsrekombination im Photosystem II und f{\"o}rdert die Photosynthese. In Zeiten geringer ATP-Synthase-Aktivit{\"a}t profitieren die Pflanzen also von der KEA3-Aktivit{\"a}t. KEA3 unterliegt einer Dimerisierung {\"u}ber seinen regulatorischen C-Terminus (RCT). Der RCT reagiert auf Ver{\"a}nderungen der Lichtintensit{\"a}t, da die Pflanzen, die KEA3 ohne diese Dom{\"a}ne exprimieren, einen reduzierten Lichtschutzmechanismus bei Lichtintensit{\"a}tsschwankungen aufweisen. Allerdings fixieren diese Pflanzen w{\"a}hrend der Photosynthese-Induktionsphase mehr Kohlenstoff als Gegenleistung f{\"u}r einen langfristigen Photoprotektor, was die regulierende Rolle von KEA3 in der Pflanzenentwicklung zeigt. Der KEA3-RCT ist dem Thylakoidstroma zugewandt, so dass seine Regulierung von lichtinduzierten Ver{\"a}nderungen in der Stroma-Umgebung abh{\"a}ngt. Die Regulierung der KEA3-Aktivit{\"a}t {\"u}berschneidet sich mit den pH-{\"A}nderungen im Stroma, die bei Lichtschwankungen auftreten. Es hat sich gezeigt, dass ATP und ADP eine Affinit{\"a}t zum heterolog exprimierten KEA3 RCT haben. Eine solche Wechselwirkung verursacht Konformations{\"a}nderungen in der RCT-Struktur. Die Faltung der RCT-Liganden-Interaktion h{\"a}ngt vom pH-Wert der Umgebung ab. Mit einer Kombination aus Bioinformatik und In-vitro-Ansatz wurde die ATP-Bindungsstelle am RCT lokalisiert. Das Einf{\"u}gen einer Punktmutation in der KEA3-RCT Bindungsstelle in planta f{\"u}hrte zu einer Deregulierung der Antiporteraktivit{\"a}t beim {\"U}bergang zu wenig Licht. Die in dieser Arbeit vorgestellten Daten erm{\"o}glichten es uns, die Rolle von KEA3 bei der Anpassung der Photosynthese umfassender zu bewerten und Modelle zur Regulierung der KEA3-Aktivit{\"a}t w{\"a}hrend des {\"U}bergangs zwischen verschiedenen Lichtintensit{\"a}ten vorzuschlagen.},
  language  = {en}
}
@phdthesis{vonBismarck2023,
  author    = {von Bismarck, Thekla},
  title     = {The influence of long-term light acclimation on photosynthesis in dynamic light},
  school      = {Universit{\"a}t Potsdam},
  pages     = {x, 163},
  year      = {2023},
  abstract  = {Photosynthesis converts light into metabolic energy which fuels plant growth. In nature, many factors influence light availability for photosynthesis on different time scales, from shading by leaves within seconds up to seasonal changes over months. Variability of light energy supply for photosynthesis can limit a plant´s biomass accumulation. Plants have evolved multiple strategies to cope with strongly fluctuation light (FL). These range from long-term optimization of leaf morphology and physiology and levels of pigments and proteins in a process called light acclimation, to rapid changes in protein activity within seconds. Therefore, uncovering how plants deal with FL on different time scales may provide key ideas for improving crop yield. Photosynthesis is not an isolated process but tightly integrates with metabolism through mutual regulatory interactions. We thus require mechanistic understanding of how long-term light acclimation shapes both, dynamic photosynthesis and its interactions with downstream metabolism. To approach this, we analyzed the influence of growth light on i) the function of known rapid photosynthesis regulators KEA3 and VCCN1 in dynamic photosynthesis (Chapter 2-3) and ii) the interconnection of photosynthesis with photorespiration (PR; Chapter 4). We approached topic (i) by quantifying the effect of different growth light regimes on photosynthesis and photoprotection by using kea3 and vccn1 mutants. Firstly, we found that, besides photosynthetic capacity, the activities of VCCN1 and KEA3 during a sudden high light phase also correlated with growth light intensity. This finding suggests regulation of both proteins by the capacity of downstream metabolism. Secondly, we showed that KEA3 accelerated photoprotective non-photochemical quenching (NPQ) kinetics in two ways: Directly via downregulating the lumen proton concentration and thereby de-activating pH-dependent NPQ, and indirectly via suppressing accumulation of the photoprotective pigment zeaxanthin. For topic (ii), we analyzed the role of PR, a process which recycles a toxic byproduct of the carbon fixation reactions, in metabolic flexibility in a dynamically changing light environment. For this we employed the mutants hpr1 and ggt1 with a partial block in PR. We characterized the function of PR during light acclimation by tracking molecular and physiological changes of the two mutants. Our data, in contrast to previous reports, disprove a generally stronger physiological relevance of PR under dynamic light conditions. Additionally, the two different mutants showed pronounced and distinct metabolic changes during acclimation to a condition inducing higher photosynthetic activity. This underlines that PR cannot be regarded purely as a cyclic detoxification pathway for 2PG. Instead, PR is highly interconnected with plant metabolism, with GGT1 and HPR1 representing distinct metabolic modulators. In summary, the presented work provides further insight into how energetic and metabolic flexibility is ensured by short-term regulators and PR during long-term light acclimation.},
  language  = {en}
}
@phdthesis{Kappel2023,
  author    = {Kappel, Sandrine},
  title     = {Photosynthesis in fluctuating light},
  school      = {Universit{\"a}t Potsdam},
  pages     = {172},
  year      = {2023},
  abstract  = {Light is the essential energy source for plants to drive photosynthesis. In nature, light availability is highly variable and often fluctuates on very short time scales. As a result, plants developed mechanisms to cope with these fluctuations. Understanding how to improve light use efficiency in natural fluctuating light (FL) conditions is a major target for agronomy. In the first project, we identified an Arabidopsis thaliana plant that showed reduced levels of rapidly inducible non-photochemical quenching (NPQ). This plant was devoid of any T-DNA insertion. Using a mapping-by-sequencing approach, we successfully located the causal genomic region near the end of chromosome 4. Through variant investigations in that region, we identified a deletion of about 20 kb encompassing 9 genes. By complementation analysis, we confirmed that one of the deleted genes, VTC2, is the causal gene responsible for the low NPQ. Loss of VTC2 decreased NPQ particularly in old leaves, with young leaves being only slightly affected. Additionally, ascorbate levels were almost abolished in old leaves, likely causing the NPQ decrease by reducing the activity of the xanthophyll cycle. Although ascorbate levels in younger leaves were reduced compared to wild-type plants, they remained at a comparably higher level. This difference may be due to the VTC2 paralog VTC5, which is expressed at a higher level in young leaves than in old ones. Plants require the PROTON GRADIENT REGULATION 5 (PGR5) protein for survival in FL. pgr5 mutants die because they fail to increase the luminal proton concentration in response to high light (HL) phases. A rapid elevation in ∆pH is needed to slow down electron transport through the Cytochrome b6 f complex (photosynthetic control). In FL, such lack of control in the pgr5 mutants results in photosystem I (PSI) overreduction, reactive oxygen species (ROS) production, and cell death. Decreases in photosystem II (PSII) activity introduced by crossing pgr5 with PSII deficient mutants rescued the lethality of pgr5 in FL. PGR5 was suggested to act as part of the ferredoxin-plastoquinone reductase (FQR), involved in cyclic electron transfer around PSI. However, the proposed molecular role of PGR5 remains highly debated. To learn more about PGR5 function, we performed a forward genetic screen in Arabidopsis thaliana to identify EMS-induced suppressor mutants surviving longer when grown in FL compared to pgr5 mutants (referred to as "suppressor of pgr5 lethality in fluctuating light", splf ). 11 different candidate genes were identified in a total of 22 splf plants. Mutants of seven of these genes in the pgr5 background showed low Fv/Fm values when grown in non-fluctuating low light (LL). Five of these 4genes were previously reported to have a role in PSII biogenesis or function. Two others, RPH1 and a DEAD/DEAH box helicase (AT3G02060), have not been linked to PSII function before. Three of splf candidate genes link to primary metabolism, fructose-2,6-bisphosphatase (F2KP ), udp-glucose pyrophosphorylase 1 (UGP1 ) and ferredoxin-dependent glutamate synthase (Fd-GOGAT ). They are characterized by the fact that they survive longer in FL than pgr5 mutants but do not procede beyond the early vegetative phase and then die.},
  language  = {en}
}