Results

Description of the work performed and main results achieved so far

Thylakoid protein kinases and their regulation and role in signalling of plant acclimation to environmental changes is studied by the groups of Eva-Mari Aro in Turku and Jean-David Rochaix in Geneva. The STN7 kinase was shown to be essential for providing equal distribution of excitation energy between both photosystems in Arabidopsis enabling fluent photosynthesis despite of fluctuations in light intensity. Indeed, the growth of both the stn7 and stn7 stn8 kinase mutants is strongly compromised under fluctuating light conditions, mimicking natural growth environments. BN gel analysis revealed that Chlamydomonas Stt7 forms a core complex with the cytochrome b6f complex, and that upon activation of the kinase in state 2 large Stt7 supercomplexes are assembled, appearing to be involved in the reversible transfer of LHCII from PSII to PSI. The Leister group in Munich identified the photosynthesis altered mutant 48 (pam48), in which chloroplast-to-nucleus communication is perturbed. The PAM48 protein shares sequence similarity with human mitochondrial transcription termination factors but was localized to chloroplasts. Currently it is analyzed whether PAM48 regulates chloroplastic gene expression and could act as chloroplastic transcription termination factor.

Calcium-dependent signalling pathways and the identification of involved components is the focus of the work in the Vothknecht group in Munich and the Teige group in Vienna. Different proteomics approaches and phosphorylation assays identified several targets showing Ca2+-dependent phosphorylation. These factors involve components of photosynthesis and related metabolic pathways and putative metabolite transporters in the chloroplast envelope. Ca2+-dependent signalling pathways induced in response to the allelopathic signal decadienal (DD) in diatoms are studied in the Bowler group in Paris. DD induces cell death via calcium and nitric-oxide and the putative nitric-oxide generating enzyme NOA is localized in chloroplasts. Overexpressing (OE) and silenced (RNAi) lines were produced to characterize chloroplast-derived nitric-oxide and calcium signals, which are visualized using GFP-AEQ targeted to the chloroplast or mitochondria.

Functional links between stress signalling and metabolic adaptation are studied by the Jonak group in Vienna, the Noctor group in Orsay, the Foyer group in Leeds, and by Bayer BioScience in Gent. A plastid-localised protein kinase, which associates with starch granules has been studied, mutant lines for this kinase have been established and potential interaction partners have been identified. Sensitivity of its kinase activity to hydrogen peroxide points towards a redox-regulation. Arabidopsis lines for specific photoreceptors in an oxidative stress genetic background were analyzed at the transcriptomic, biochemical, metabolomic, and phenotypic levels in order to uncover new links between light signalling and reactive oxygen species. Glutathione (GSH) was found to co-localise with nuclear DNA at the early stages of proliferation in plant cells and GSH sequestration in the nucleus during the G1 phase of the cell cycle has a profound impact on cellular redox homeostasis, gene expression and glutathione synthesis. The nuclear GSH pool might act as regulator of nuclear proteins such as poly (ADP-ribose) polymerases (PARP). The influence of chemical inhibition of PARP under different abiotic stress treatments is studied by Bayer BioScience in Gent on a wide spectrum of physiological, biochemical and molecular phenotypes. PARP and Anthocyanin accumulation during stress was found to be associated with an enhanced growth of the plants under stress conditions and that these changes are further associated with photosynthetic, redox and gene expression changes.

Light signals are studied by the Falciatore group in Naples (diatoms) and by the Gabrys group in Krakow (higher plants). Analysis of mutants generated in the diatome Phaeodactylum tricornutum indicates that the recently identified putative red light phytochrome (DPh) photoreceptor is involved in the cellular aggregation control, likely due to a key role in the neighbour perception and/or regulation of photo-protection mechanisms. The Gabrys group in Krakow studies signalling pathways, which control blue light-induced chloroplast redistribution in the mesophyll of higher plants to identify a link between light and stress/redox signalling, and to define the exact role of calcium in the movements. It was found that infiltration of Arabidopsis leaves with GSH resulted in an enhancement of chloroplast movement response. Using transgenic Aequorin-GFP reporter lines (generated by the Vothknecht group) it was found that chloroplasts might have an input to blue-light-mediated calcium regulation of their movement.

Expected final results and their potential impact and use
We aim at obtaining detailed knowledge of the signalling mechanisms governing chloroplast function and its coordination with the metabolic network of the entire cell. This includes involved kinases, potentially also phosphatases, and also factors involved in Ca2+-signalling and decoding of those signals. Understanding of these molecular mechanisms enabling acclimation to stress conditions is vital for sustainable agriculture in a changing environment.