Illumination changes elicit modifications of thylakoid proteins and reorganization of the photosynthetic machinery. This involves, in the short term, phosphorylation of photosystem II (PSII) and light-harvesting (LHCII) proteins. PSII phosphorylation is thought to be relevant for PSII turnover 1,2 , whereas LHCII phosphorylation is associated with the relocation of LHCII and the redistribution of excitation energy (state transitions) between photosystems 3,4 . In the long term, imbalances in energy distribution between photosystems are counteracted by adjusting photosystem stoichiometry 5,6 . In the green alga Chlamydomonas and the plant Arabidopsis, state transitions require the orthologous protein kinases STT7 and STN7, respectively 7,8 . Here we show that in Arabidopsis a second protein kinase, STN8, is required for the quantitative phosphorylation of PSII core proteins. However, PSII activity under high-intensity light is affected only slightly in stn8 mutants, and D1 turnover is indistinguishable from the wild type, implying that reversible protein phosphorylation is not essential for PSII repair. Acclimation to changes in light quality is defective in stn7 but not in stn8 mutants, indicating that short-term and long-term photosynthetic adaptations are coupled. Therefore the phosphorylation of LHCII, or of an unknown substrate of STN7, is also crucial for the control of photosynthetic gene expression.STT7 and STN7 are orthologous protein kinases required for LHCII phosphorylation and for state transitions in Chlamydomonas and Arabidopsis, respectively 7,8 . In Arabidopsis, another STT7/STN7-like protein (STN8) exists that is not required for state transitions 8 . STN8 is located in the chloroplast, as shown by in vivo subcellular localization of its amino-terminal region fused to the dsRED protein and by the import of, and transit peptide removal from, STN8 translated in vitro (Fig. 1a, b). Chloroplast subfractionation after import revealed that the protein is associated, like STT7 and STN7, with thylakoids ( Fig. 1c) (refs 7, 8).Insertion mutants for STN8 and STN7 were obtained from the Salk collection 9 , and for each gene two independent mutant alleles lacking the respective transcript were identified (Supplementary Fig. S1). The stn7 stn8 double mutant was generated by crossing stn7 and stn8 single knockouts and screening the resulting F 2 generation for homozygous double mutants. All mutants were indistinguishable from the wild type with regard to the timing of seed germination and growth rate in the greenhouse ( Supplementary Fig. S1). In stn7 and stn7 stn8 mutants, a slight decrease in the levels of neoxanthin, lutein and total chlorophyll was found (Supplementary Table S1). These subtle changes can be attributed to a minor decrease in LHCII content, not detectable by polyacrylamide-gel electrophoresis (PAGE) analysis ( Supplementary Fig. S2).Photosynthetic electron flow, measured on the basis of chlorophyll fluorescence, was not altered in the mutants (Supplementary Table S2). State transitions w...

Flowering plants control energy allocation to their photosystems in response to light quality changes. This includes the phosphorylation and migration of light-harvesting complex II (LHCII) proteins (state transitions or short-term response) as well as long-term alterations in thylakoid composition (long-term response or LTR). Both responses require the thylakoid protein kinase STN7. Here, we show that the signaling pathways triggering state transitions and LTR diverge at, or immediately downstream from, STN7. Both responses require STN7 activity that can be regulated according to the plastoquinone pool redox state. However, LTR signaling does not involve LHCII phosphorylation or any other state transition step. State transitions appear to play a prominent role in flowering plants, and the ability to perform state transitions becomes critical for photosynthesis in Arabidopsis thaliana mutants that are impaired in thylakoid electron transport but retain a functional LTR. Our data imply that STN7-dependent phosphorylation of an as yet unknown thylakoid protein triggers LTR signaling events, whereby an involvement of the TSP9 protein in the signaling pathway could be excluded. The LTR signaling events then ultimately regulate in chloroplasts the expression of photosynthesis-related genes on the transcript level, whereas expression of nuclear-encoded proteins is regulated at multiple levels, as indicated by transcript and protein profiling in LTR mutants.

Excitation imbalances between photosystem I and II generate redox signals in the thylakoid membrane of higher plants which induce acclimatory changes in the structure of the photosynthetic apparatus. They affect the accumulation of reaction center and light-harvesting proteins as well as chlorophylls a and b. In Arabidopsis thaliana the re-adjustment of photosystem stoichiometry is mainly mediated by changes in the number of photosystem I complexes, which are accompanied by corresponding changes in transcripts for plastid reaction center genes. Because chloroplast protein complexes contain also many nuclear encoded components we analyzed the impact of such photosynthetic redox signals on nuclear genes. Light shift experiments combined with application of the electron transport inhibi- The light environment of plants is highly variable. This is of particular importance for photosynthesis, because changes in incident light intensity or quality can reduce the efficiency of photosynthetic electron transport and therefore the net energy fixation. Plants have developed many acclimatory mechanisms at the molecular level that enable them to cope with such changes. Most prominent responses are dynamic changes in the structure and composition of the photosynthetic apparatus (1-3).Light quality and quantity gradients that occur e.g. in dense plant populations induce an imbalance in excitation energy distribution between the two photosystems (which work electrochemically in series) and therefore reduce photosynthetic efficiency. To counteract such imbalances plants re-distribute light energy in a short term by state transitions (4, 5) and in a long term by a re-adjustment of photosystem stoichiometry. This results in a supply of more light quanta to the less active side of the electron transport chain (6 -8). Both processes are regulated by light-induced changes in the redox state of photosynthetic components (9 -11). While the short term response acts via post-translational phosphorylation of existing antenna proteins, the long term response (LTR) 1 requires the synthesis of new components and hence has to affect gene expression. This implies signaling routes that connect photosynthetic electron transport/efficiency with the expression machinery. Studies in the last decade show that such functional connections exist at multiple levels and in virtually all classes of photosynthetic organisms. In higher plants photosynthetic redox control has been found at the levels of transcription (12-19), transcript stability (20 -23), ribosome loading (24 -26), translation initiation (27), and protein accumulation (28).The origin of the respective signal transduction pathways can be very different. To date three classes of redox signals can be distinguished: the first one is generated directly within the electron transport chain, the second is represented by photosynthesis-coupled redox-active compounds such as thioredoxin or glutathione, and the third is constituted by reactive oxygen species, which are unavoidable by-products of photo...

The photosynthetic function of chloroplasts represents an important sensor that integrates various abiotic changes in the environment into corresponding molecular signals, which, in turn, regulate cellular activities to counterbalance the environmental changes or stresses.

The long-term response (LTR) of higher plants to varying light qualities increases the photosynthetic yield; however, the benefit of this improvement for physiology and survival of plants is largely unknown, and its functional relation to other light acclimation responses has never been investigated. To unravel positive effects of the LTR we acclimated Arabidopsis thaliana for several days to light sources, which preferentially excite photosystem I (PSI) or photosystem II (PSII). After acclimation, plants revealed characteristic differences in chlorophyll fluorescence, thylakoid membrane stacking, phosphorylation state of PSII subunits and photosynthetic yield of PSII and PSI. These LTR-induced changes in the structure, function and efficiency of the photosynthetic machinery are true effects by light quality acclimation, which could not be induced by light intensity variations in the low light range. In addition, high light stress experiments indicated that the LTR is not involved in photoinhibition; however, it lowers non-photochemical quenching (NPQ) by directing more absorbed light energy into photochemical work. NPQ in turn is not essential for the LTR, since npq mutants performed a normal acclimation. We quantified the beneficial potential of the LTR by comparing wild-type plants with the LTR-deficient mutant stn7. The mutant exhibited a decreased effective quantum yield and produced only half of seeds when grown under fluctuating light quality conditions. Thus, the LTR represents a distinct acclimation response in addition to other already known responses that clearly improves plant physiology under low light conditions resulting in a pronounced positive effect on plant fitness.