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Adaptation of mechanisms for regulation of photosynthetic electron transport upon land colonization
Full text linkIn eukaryotic organisms, oxygenic photosynthesis is catalyzed by 4 multiprotein complexes present on the thylakoid membranes inside the chloroplasts: Photosystem (PS) II, Cytochrome b6f (Cyt b6f), PSI and chloroplast ATP synthase (ATPase). The three former mediate a Linear Electron Flow (LEF) from H2O to NADP+ to synthetize NADPH. LEF also generate a proton gradient across the thylakoid membrane, supporting ATP synthesis. NADPH and ATP are exploited by Calvin Benson cycle to fix inorganic CO2 in sugars available for the metabolism.
Photosynthetic process is accompanied by the formation of unstable molecules that can generate reactive oxygen species (ROS) harmful for the cell. Particular regulatory mechanisms prevent the formation of these molecules. Among these, the Alternative Electron Flow (AEF) divert electrons from photosynthetic processes preventing over-reduction of electron transporters and ROS production. AEF mechanisms can be divided in Cyclic Electron Flow (CEF) and Pseudo-CEF (PCEF) according to the acceptors of the pathways, respectively the plastoquinone and O2. Two major CEF are known and mediated by PGR5/PGRL1 or by NDH-1 complex. PCEF is instead mediated by Flavodiiron proteins (FLV). In this thesis I focused on the early land plant Physcomitrella patens, this moss presents all the main AEF identified in algae and vascular plants and is thus a suitable model to directly compared their activity in the same organisms.
FLV proteins were lost in Angiosperms, one of the plausible explanations to this loss is that another AEF mechanism replaces their function, making FLV dispensable. FLV were shown to be fundamental to respond to light fluctuation that occurs in the natural environments, a function that in angiosperms was replaced by PGR5/PGRL1.
To evaluate the hypothesis that PGRL1/PGR5 CEF mediated pathway can replace FLV, in chapter 3 we isolated a P. patens mutant defective for both FLV and PGRL1. Our experiments highlighted a strong synergy between the 2 pathways especially when plants were exposed to stressful light regimes, suggesting a functional overlap and the possible replacement of one pathway over the other during evolution. Further analysis allowed us to establish that FLV and PGRL1 are particularly important for protection of PSI from over-reduction, thus preventing photodamage.
In chapter 4, we characterized NDH-1 complex in P. patens isolating a line defective for NDH-1 function. NDH-1 defective mutants showed a small defect in PSI functionality in the first moment after light exposure. Reiterating cycle of dark and light, the defect observed increased significantly in NDH-1 defective plants suggesting that this complex may operates in first seconds after light exposure to avoid PSI over-reduction. However, this defect did not impact the plant growth not even in fluctuating light conditions, probably because of the compensation from other mechanisms present in the chloroplast.
In order to entangle this possible compensation in chapter 5 NDH-1 was depleted in plants knock out for both PGRL1 and FLV, by isolating a double mutant lacking both CEF pathways (pgrl1-ndhm) and a triple mutant defective for both CEF and PCEF (flva-pgrl1-ndhm). Both exhibited a drastic decrease in growth and photosynthetic performances showing that NDH-1 depletion has a much larger effect if other mechanisms for AEF are also absent, eliminating compensating effects. The triple mutant showed growth defects even in non-stressing conditions suggesting these mechanisms are important also for steady state photosynthesis. Spectroscopic and biochemical characterization detected extensive damage to PSI, correlated with the growth defects.Negli organismi eucarioti la fotosintesi ossigenica è catalizzata da 4 complessi localizzati sulle membrane tilacoidali dei cloroplasti: il fotosistema (PS) II, il citocromo b6f, il PSI e l’ATPasi cloroplastica. I primi tre mediano un trasporto lineare degli elettroni (LEF) dall’H2O al NADP+ per formare NADPH. L’ATPasi invece sfrutta il gradiente protonico generato dal LEF per sintetizzare ATP. NADPH e ATP sono utilizzati dal ciclo di Calvin per fissare la CO2 in zuccheri disponibili per il metabolismo.
Durante la fotosintesi si formano molecole instabili che possono generare molecole dannose per la cellula quali le specie reattive dell’ossigeno (ROS). Particolari meccanismi regolatori prevengono la formazione di queste molecole, tra questi i trasporti elettronici alternativi (AEF) rimuovono elettroni dal LEF prevenendo l’over-riduzione dei traportatori e a formazione di ROS. Gli AEF possono essere divisi in trasporti ciclici (CEF) e pseudo-ciclici (PCEF) a seconda dell’accettore finale della via, rispettivamente il plastochinone o l’O2. I due principali CEF conosciuti sono mediati dalle proteine PGR5/PGRL1 e dal complesso NDH-1. Il PCEF è invece mediato dalle proteine FLV. In questa tesi è stato studiato Physcomitrella patens, un muschio che presenta tutti i principali AEF identificati in alghe e piante vascolari ed è quindi un buon modello dove poter comparare l’attività dei diversi AEF presenti.
Le proteine FLV non sono presenti nelle angiosperme, una possibile spiegazione alla loro perdita è che un altro AEF abbia sostituito il loro ruolo rendendole dispensabili. Le FLV sono importanti nella risposta alla luce fluttuante, ruolo che nelle angiosperme è ricoperto da PGR5/PGRL1. Per valutare l’ipotesi che PGR5/PGRL1 abbia rimpiazzato le FLV, nel capitolo 3 di questa tesi abbiamo isolato un mutante di P. patens difettivo per entrambe le vie. Abbiamo individuato una forte sinergia tra le 2 vie, in particolare quando i mutanti sono stati esposti a regimi luminosi restrittivi, suggerendo una sovrapposizione funzionale e un possibile sostituzione di uno con l’altro nel corso dell’evoluzione. Inoltre, abbiamo stabilito che le FLV e PGR5/PGRL1 sono importanti per proteggere il PSI dalla sovra-riduzione, prevendendo cos’ danni ossidativi.
Nel capitolo 4, abbiamo caratterizzato il complesso NDH-1 di P. patens, grazie all’isolamento di una linea difettiva per la sua funzione. Questi mutanti hanno evidenziato solo una piccola differenza nella funzionalità del PSI al momento in cui le piante sono state esposte ad una luce. Reiterando cicli di buio è luce, il difetto osservato aumentava significativamente suggerendo che NDH-1 possa essere attivo nei primi secondo dopo l’esposizione ad una fonte luminosa, proteggendo il PSI. Tuttavia, anche cresciute in una condizione di luce fluttuante le piante difettive di NDH-1 non hanno mostrato fenotipi di crescita, probabilmente a causa di un meccanismo di compensazione durante il processo di acclimatazione a questa condizione.
Per identificare questa possibile compensazione, nel capitolo 5 abbiamo rimosso la funzione NDH-1 in piante già mancanti di PGRL1 e FLV, isolando un doppio mutante difettivo per entrambi CEF e un triplo mutante mancante sia di CEF che di PCEF. Per Entrambi abbiamo riscontrato ridotta crescita e capacità fotosintetica, evidenziando che la funzione di NDH-1 è molto più evidente quando gli altri AEF sono assenti, eliminando l’effetto di compensazione. Il triplo mutante inoltre ha mostrato una ridotta crescita anche in condizioni non stressanti dimostrando che i AEF sono indispensabili per una corretta attività fotosintetica. Analisi spettroscopiche e biochimiche hanno mostrato che i difetti individuati sono dovuti a evidenti danni a carico del PSI in queste piante
Balancing protection and efficiency in the regulation of photosynthetic electron transport across plant evolution
No full textThe chloroplast NADH dehydrogenase-like complex influences the photosynthetic activity of the moss Physcomitrella patens
Full text linkAlternative electron pathways contribute to regulation of photosynthetic light reactions to adjust to metabolic demands in dynamic environments. The chloroplast NADH dehydrogenase-like (NDH) complex mediates the cyclic electron transport pathway around photosystem I (PSI) in different cyanobacteria, algae and plant species, but it is not fully conserved in all photosynthetic organisms. In order to assess how this complex's physiological role changed during plant evolution, we isolated Physcomitrella patens lines knocked out of the NDHM gene that encodes for a subunit fundamental for the activity of the complex. ndhm knock-out (KO) mosses indicated high PSI acceptor side limitation upon abrupt changes in illumination. In P. patens, pseudo-cyclic electron transport mediated by Flavodiiron proteins (FLVs) was also shown to prevent PSI overreduction in plants exposed to light fluctuations. flva ndhm double KO mosses had altered photosynthetic performance and growth defects under fluctuating light compared to wild-type and single KO mutants. The results evidenced that while NDH contribution to electron transport is minor compared to FLV, NDH still participates in modulating photosynthetic activity, and it is critical to avoid PSI photoinhibition, especially when FLVs are inactive. The functional overlap between NDH- and FLV-dependent electron transport supports PSI activity and prevents its photoinhibition under light variations
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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