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Environmental Acclimation of Photosynthesis

Final Report Summary - ACCLIPHOT (Environmental Acclimation of Photosynthesis)

The overall research goal of the FP7 Marie Curie Initial Training Network project, AccliPhot, is to establish a systems-wide understanding of the acclimation processes of the photosynthetic machinery in algae and higher plants. The AccliPhot Consortium was composed of 26 researchers (13 PhD students, 2 Post-Doctoral researchers, and 11 principal investigators) based in 11 institutions in Germany, Italy, Switzerland, France, United Kingdom and Ireland. The interdisciplinary project investigated signalling pathways that respond to environmental changes, electron transport chain activity, photosynthetic metabolism, and growth of plants and algal populations. The project was split into five interlinking work packages with specific objectives: (1) identification of signalling pathways for acclimation; (2) regulation of the photosynthetic electron transport chain; (3) environmental control of metabolism, carbon partitioning and feedback to light reactions; (4) whole organism response; and (5) population-level response of algal cultures.

The interdisciplinary research projects undertaken by AccliPhot underline how by increasing our understanding on the different processes linked to photosynthesis (light absorption, dissipation, electron flow and carbon assimilation for metabolism) we are successfully unravelling the mysteries of photosynthetic acclimation.

WP1 gave insight into short- and long- term acclimation processes in Arabidopsis thaliana and Chlamydomonas reinhardtii. Results that indicate that the substrate specificity of the algal phosphatases are different from their A.thaliana orthologues suggest a different regulation of light acclimation processes by protein phosphorylation in C. reinhardtii. Further investigation of thylakoid-associated kinases Stn7 and Stn8 uncovered potential players connecting photosynthetic electron transport activity with the regulation of downstream sinks and long–term adaptation processes that affect the regulation of RNA metabolism. Furthermore, a model to predict the effect of heat-shock responses on photosynthetic parameters and on photoinhibition was developed.

WP2 furthered our understanding of the mechanism of enhanced thermal dissipation of excess light (NPQ – qE) in different organisms. The successful collaboration of several institutions dissected the mechanisms of NPQ-qE with a multiorganism approach. Focusing on the LHCSR proteins, they pinpointed the role of the different LHCX isoforms in diatoms, revealed a link between photoperception and photoprotection in C. reinhardtii, and addressed the role of LHCSR3 in Physcomitrella patens, exploiting the occurrence of nuclear homologous recombination in this moss. We also successfully developed a comprehensive model that recapitulates light acclimation in plants and microalgae. This includes the “memory” of NPQ as well as the chromatic adaptation in C. reinhardtii. Moreover, the successful collaboration of industry and academia revealed the energetic bases for mixotrophy in diatoms by revealing mitochondrial-chloroplast energetic interactions. Tomography of diatom cells has provided the structural bases for these functional interactions. Overall, the knowledge generated by WP2 will deeply influence the understanding of light acclimation by the international photosynthesis community.

WP3 yielded significant new insight into the interactions between photosynthesis and downstream metabolism that underpins growth. The plastid redox poise, which is an important regulator of gene expression and metabolic activities, was investigated by generating redox-insensitive A. thaliana lines. Results showed no phenotypical differences to the wild-type, as well as no significant imbalance of Calvin-Benson cycle intermediate metabolites. Mixotrophic growth of Phaeodactylum tricornutum with glycerol was investigated by one of our industrial partners. Transcriptomic and metabolomic analyses, aided by genome-scale metabolic model of P. tricornutum, indicated that the glycerol stimulates central carbon metabolism and identified candidate genes that could be used for metabolic engineering.

WP4 constructed and analysed metabolic models of Arabidopsis thaliana, Chlamydomonas reinhardtii and Phaeodactylum tricornutum. A novel “constraint scanning” technique, whereby a linear program is repeatedly solved with an incremental increase in some constraint was developed to identify small cycles hydrolysing ATP or oxidising NAD(P)H , that would be expected to form components of larger energy dissipating modes. 55 such short cycles have been identified in the A. thaliana network, and 24 in the C. reinhardtii network. A similar constraint scanning approach was applied to P. tricornutum identified two independent groups of reactions that are predicted to respond to increased demand for triacylglyceride. Interestingly this suggests a link between lipid and photorespiratory metabolism. Furthermore the concentration of reactants of many of these reactions are seen to change significantly under different light regimes in the metabolite data from other AccliPhot projects.

WP5 gained insight into the performance of algal populations in large-scale cultures. Using both experimental and theoretical approaches, a biochemically-based structured model for the autotrophic growth of C. reinhardtii in photobioreactors (PBRs) using knowledge of the detailed underlying metabolic network previously determined was developed. Mixotrophic growth was also investigated in the work package, leading to the elucidation of the main pathways involved in the mixotrophic growth. This allowed the gene targets for the metabolic engineering of P. tricornutum in order to optimise the efficiency of mixotrophic metabolism to be pinpointed. A novel medium composition that optimises mixotrophic growth of P. tricornutum was developed, and the optimised growth conditions tested in laboratory-scale 2L PBRs. Experimentation on algal cultures over 2L posed a few hurdles, particularly biological contamination of the cultures. By investigating the bacterial community associated with P. tricornutum, we revealed a dynamic bacterial community that changed over time and in differing media conditions. A proposed network of putative interactions between P. tricornutum and the main bacterial factions was translated into a set of ordinary differential equations constituting a computational dynamic model. The proposed mathematical model is able to capture the population dynamics and therefore represents a simple yet important proof of concept of the hypothesised community-level interactions.

One of the fundamental goals of AccliPhot was to illustrate the importance of an interdisciplinary approach to scientific research. In general, experimental researchers tend to become sceptical at the usability of findings provided by mathematical models and therefore undermine the importance of this in silico approaches. AccliPhot shows the successful marriage of theoretical and experimental approaches, where developed models have assisted in experimental design, and where experimental data has allowed for the development of a model as a predictive tool. The results of AccliPhot will certainly impact future strategies to boost plant and algal biomass productivity for biotechnology applications.

Contact Information:

Project Coordinator:

Jun.-Prof. Dr. Oliver Ebenhöh
Heinrich-Heine-Universität Düsseldorf,
Building: 25.32.03.24
Universitätsstraße 1,
0225 Düsseldorf, Germany

Email: oliver.ebenhoeh@hhu.de
Phone: (+49) 211 81 02923
Project website: http://accliphot.eu/