The role of phosphorylation in redox regulation of photosynthesis genes psaA and psbA during photosynthetic acclimation of mustard

The Rice YL4 Gene Encoding a Ribosome Maturation Domain Protein Is Essential for Chloroplast Development

Chloroplast RNA splicing and ribosome maturation (CRM) domain proteins are a family of plant-specific proteins associated with RNA binding. In this study, we have conducted a detailed characterization of a novel rice CRM gene (LOC_Os04g39060) mutant, yl4, which showed yellow-green leaves at all the stages, had fewer tillers, and had a decreased plant height. Map-based cloning and CRISPR/Cas9 editing techniques all showed that YL4 encoded a CRM domain protein in rice. In addition, subcellular localization revealed that YL4 was in chloroplasts. YL4 transcripts were highly expressed in all leaves and undetectable in roots and stems, and the mutation of YL4 affected the transcription of chloroplast-development-related genes. This study indicated that YL4 is essential for chloroplast development and affects some agronomic traits.

Effect of Common ER Stress-Inducing Drugs on the Growth and Lipid Phenotypes of Chlamydomonas and Arabidopsis

Endoplasmic reticulum (ER) stress is caused by the stress-induced accumulation of unfolded proteins in the ER. Several compounds are used to induce the unfolded protein response (UPR) in animals, with different modes of action, but which ER stress-inducing drugs induce ER stress in microalgae or land plants is unclear. In this study, we examined the effects of seven chemicals that were reported to induce ER stress in animals on the growth, UPR gene expression and fatty acid profiles of Chlamydomonas reinhardtii (Chlamydomonas) and Arabidopsis thaliana (Arabidopsis): 2-deoxyglucose, dithiothreitol (DTT), tunicamycin (TM), thapsigargin, brefeldin A (BFA), monensin (MON) and eeyarestatin I. In both model photosynthetic organisms, DTT, TM, BFA and MON treatment induced ER stress, as indicated by the induction of spliced bZIP1 and bZIP60, respectively. In Chlamydomonas, DTT, TM and BFA treatment induced the production of transcripts related to lipid biosynthesis, but MON treatment did not. In Arabidopsis, DTT, TM, BFA and MON inhibited seed germination and seedling growth with the activation of bZIP60. These findings lay the foundation for using four types of ER stress-inducing drugs in photosynthetic organisms, and they help uncover the mode of action of each compound.

Rice TCD8 Encoding a Multi-Domain GTPase Is Crucial for Chloroplast Development of Early Leaf Stage at Low Temperatures

The multi-domain GTPase (MnmE) is conservative from bacteria to human and participates in tRNA modified synthesis. However, our understanding of how the MnmE is involved in plant chloroplast development is scarce, let alone in rice. A novel rice mutant, thermo-sensitive chlorophyll-deficient mutant 8 (tcd8) was identified in this study, which apparently presented an albino phenotype at 20 °C but a normal green over 24 °C, coincided with chloroplast development and chlorophyll content. Map-based cloning and complementary test revealed the TCD8 encoded a multi-domain GTPase localized in chloroplasts. In addition, the disturbance of TCD8 suppressed the transcripts of certain chloroplast-related genes at low temperature, although the genes were recoverable to nearly normal levels at high temperature (32 °C), indicating that TCD8 governs chloroplast development at low temperature. The multi-domain GTPase gene in rice is first reported in this study, which endorses the importance in exploring chloroplast development in rice.

Retrograde and anterograde signaling in the crosstalk between chloroplast and nucleus

The chloroplast is a complex cellular organelle that not only performs photosynthesis but also synthesizes amino acids, lipids, and phytohormones. Nuclear and chloroplast genetic activity are closely coordinated through signaling chains from the nucleus to chloroplast, referred to as anterograde signaling, and from chloroplast to the nucleus, named retrograde signaling. The chloroplast can act as an environmental sensor and communicates with other cell compartments during its biogenesis and in response to stress, notably with the nucleus through retrograde signaling to regulate nuclear gene expression in response to developmental cues and stresses that affect photosynthesis and growth. Although several components involved in the generation and transmission of plastid-derived retrograde signals and in the regulation of the responsive nuclear genes have been identified, the plastid retrograde signaling network is still poorly understood. Here, we review the current knowledge on multiple plastid retrograde signaling pathways, and on potential plastid signaling molecules. We also discuss the retrograde signaling-dependent regulation of nuclear gene expression within the frame of a multilayered network of transcription factors.

Photosystem stoichiometry adjustment is a photoreceptor-mediated process in Arabidopsis

Plant growth under spectrally-enriched low light conditions leads to adjustment in the relative abundance of the two photosystems in an acclimatory response known as photosystem stoichiometry adjustment. Adjustment of photosystem stoichiometry improves the quantum efficiency of photosynthesis but how this process perceives light quality changes and how photosystem amount is regulated remain largely unknown. By using a label-free quantitative mass spectrometry approach in Arabidopsis here we show that photosystem stoichiometry adjustment is primarily driven by the regulation of photosystem I content and that this forms the major thylakoid proteomic response under light quality. Using light and redox signaling mutants, we further show that the light quality-responsive accumulation of photosystem I gene transcripts and proteins requires phytochrome B photoreceptor but not plastoquinone redox signaling as previously suggested. In far-red light, the increased acceptor side limitation might deplete active photosystem I pool, further contributing to the adjustment of photosystem stoichiometry.

A fully assembled plastid-encoded RNA polymerase complex detected in etioplasts and proplastids in Arabidopsis

The plastid-encoded genes of higher plants are transcribed by at least two types of RNA polymerases, the nuclear-encoded RNA polymerase (NEP) and the plastid-encoded RNA polymerase (PEP). In mature photosynthesizing leaves, the vast majority of the genes are transcribed by PEP. However, the regulatory mechanisms controlling plastid transcription during early light response is unclear. Chloroplast development is suggested to be associated with a shift in the usage of the primary RNA polymerase from NEP to PEP as the expression of the plastid-encoded photosynthesis genes is induced upon light exposure. Assembly of the PEP complex has been suggested as a rate-limiting step for full activation of plastid-encoded photosynthesis gene expression. However, two sigma factor mutants, sig2 and sig6, with reduced PEP activity, showed significantly lower expression of the plastid-encoded photosynthesis genes already in the dark and during the first hours of light exposure indicating that PEP activity is required for basal expression of plastid-encoded photosynthesis genes in the dark and during early light response. Furthermore, in etioplasts and proplastids a fully assembled PEP complex was revealed on Blue Native PAGE. Our results indicate that a full assembly of the PEP complex is possible in the dark and that PEP drives basal transcriptional activity of plastid-encoded photosynthesis genes in the dark. Assembly of the complex is most likely not a rate-limiting step for full activation of plastid-encoded photosynthesis gene expression which is rather achieved either by the abundance of the PEP complex or by some posttranslational regulation of the individual PEP components.

Effect of light on the rearrangements of PSI super-and megacomplexes in the non-appressed thylakoid domains of maize mesophyll chloroplasts

We demonstrated the existence of PSI-LHCI-LHCII-Lhcb4 supercomplexes and PSI-LHCI-PSII-LHCII megacomplexes in the stroma lamellae and grana margins of maize mesophyll chloroplasts; these complexes consist of different LHCII trimers and monomer antenna proteins per PSI photocentre. These complexes are formed in both low (LL) and high (HL) light growth conditions, but with different contents. We attempted to identify the components and structure of these complexes in maize chloroplasts isolated from the leaves of low and high light-grown plants after darkness and transition to far red (FR) light of high intensity. Exposition of plants from high and low light growth condition on FR light induces different rearrangements in the composition of super- and megacomplexes. During FR light exposure, in plants from LL, the PSI-LHCI-LHCII-Lhcb4 supercomplex dissociates into free LHCII-Lhcb4 and PSI-LHCI complexes, and these complexes associate with the PSII monomer. This process occurs differently in plants from HL. Exposition to FR light causes dissociation of both PSI-LHCI-LHCII-Lhcb4 supercomplexes and PSI-PSII megacomplexes. These results suggest a different function of super- and megacomplex organization than the classic state transitions model, which assumes that the movement of LHCII trimers in the thylakoid membraneis considered as a mechanism for balancing light absorption between the two photosystems in light stress. The behavior of the complexes described in this article does not seem to be well explained by this model, i.e., it does not seem likely that the primary purpose of these megacomplexes dynamics is to balance excitation pressure. Rather, as stated in this article, it seems to indicate a role of these complexes for PSI in excitation quenching and for PSII in turnover.

Comparative analysis of thylakoid protein complexes in state transition mutants nsi and stn7: focus on PSI and LHCII

The photosynthetic machinery of plants can acclimate to changes in light conditions by balancing light-harvesting between the two photosystems (PS). This acclimation response is induced by the change in the redox state of the plastoquinone pool, which triggers state transitions through activation of the STN7 kinase and subsequent phosphorylation of light-harvesting complex II (LHCII) proteins. Phosphorylation of LHCII results in its association with PSI (state 2), whereas dephosphorylation restores energy allocation to PSII (state 1). In addition to state transition regulation by phosphorylation, we have recently discovered that plants lacking the chloroplast acetyltransferase NSI are also locked in state 1, even though they possess normal LHCII phosphorylation. This defect may result from decreased lysine acetylation of several chloroplast proteins. Here, we compared the composition of wild type (wt), stn7 and nsi thylakoid protein complexes involved in state transitions separated by Blue Native gel electrophoresis. Protein complex composition and relative protein abundances were determined by LC-MS/MS analyses using iBAQ quantification. We show that despite obvious mechanistic differences leading to defects in state transitions, no major differences were detected in the composition of PSI and LHCII between the mutants. Moreover, both stn7 and nsi plants show retarded growth and decreased PSII capacity under fluctuating light as compared to wt, while the induction of non-photochemical quenching under fluctuating light was much lower in both nsi mutants than in stn7.

A DEAD-box RNA helicase TCD33 that confers chloroplast development in rice at seedling stage under cold stress

Cold stress is one of the most common unfavorable environmental factors affecting the growth, development, and survival of plants. The DEAD-box RNA helicases play important roles in all types of processes of RNA metabolism. However, the function of DEAD-box RNA helicase under cold stress is poorly explored in plants, especially in rice. This study reported the identification of a novel rice thermo-sensitive chlorophyll-deficient mutant, tcd33, which displayed an albino phenotype before the four-leaf stage, then withered and eventually died at 20 °C, while wild-type plants exhibited normal green coloration at 32 °C. The tcd33 seedlings also exhibited less chlorophyll contents and severe defects of chloroplast structure under 20 °C condition. Map-based cloning and complementation experiments suggested that TCD33 encodes a chloroplast-located DEAD-box RNA helicase protein. The transcript expression level of TCD33 indicated that the genes related to chlorophyll (Chl) biosynthesis, photosynthesis, and chloroplast development in tcd33 mutants were down-regulated at 20 °C, while the down-regulated genes were nearly recovered to or slightly higher than the WT level at 32 °C. Together, our results suggest that the cold-inducible TCD33 is essential for early chloroplast development and is important for cold-responsive gene regulation and cold tolerance in rice.

Mutation of the rice TCM12 gene encoding 2,3-bisphosphoglycerate-independent phosphoglycerate mutase affects chlorophyll synthesis, photosynthesis and chloroplast development at seedling stage at low temperatures

Glycolysis is a central metabolic pathway that provides energy and products of primary metabolites. 2,3-Biphosphoglycerate-independent phosphoglycerate mutase (iPGAM) is a key enzyme that catalyses the reversible interconversion of 3-phosphoglycerate (3-PGA) to 2-phosphoglycerate (2-PGA) in glycolysis. Low temperature is a common abiotic stress in rice production. However, the mechanism for rice iPGAM genes is not fully understood at low temperature. In this study, the rice mutant tcm12, with chlorosis, malformed chloroplasts and impaired photosynthesis, was grown at a low temperature (<20 °C) to the three-leaf stage, while the normal phenotype at 32 °C was used. Chlorophyll fluorescence analysis and transmission electron microscopy were used to examine features of the tcm12 mutant. The inheritance behaviour and function of TCM12 were then analysed thorough map-based cloning, transgenic complementation and subcellular localisation. The thermo-sensitive chlorosis phenotype was caused by a single nucleotide mutation (T→C) on the fifth exon of TCM12 (LOC_Os12g35040) encoding iPGAM, localised to both nucleus and membranes. In addition, TCM12 was constitutively expressed, and its disruption resulted in down-regulation of some genes associated with chlorophyll biosynthesis and photosynthesis at low temperatures (20 °C). This is the first report of the involvement of rice iPGAM gene in chlorophyll synthesis, photosynthesis and chloroplast development, providing new insights into the mechanisms underlying early growth of rice at low temperatures.

Quantitative Phosphoproteomic and Physiological Analyses Provide Insights into the Formation of the Variegated Leaf in Catalpa fargesii

Variegated plants are valuable materials for investigating leaf color regulated mechanisms. To unveil the role of posttranslational modification in the variegated phenotype, we conducted global quantitative phosphoproteomic analysis on different leaf color sectors of Maiyuanjinqiu and the corresponding of Catalpa fargesii using Ti⁴⁺-IMAC phosphopeptide enrichment. A total of 3778 phosphorylated sites assigned to 1646 phosphoproteins were identified, and 3221 in 1434 proteins were quantified. Differential phosphoproteins (above 1.5 or below 1/1.5) in various leaf color sectors were selected for functional enrichment analyses. Gene ontology (GO) enrichment revealed that processes of photosynthesis, regulation of the generation of precursor metabolites, response to stress, homeostasis, amino acid metabolism, transport–related processes, and most of the energy metabolisms might contribute to leaf color. KEGG pathway enrichment analysis was performed based on differential phosphoproteins (DPs) in different organelles. The result showed that most enriched pathways were located in the chloroplasts and cytosol. The phosphorylation levels of glycometabolism enzymes might greatly affect leaf variegation. Measurements of fluorescence parameters and enzyme activities confirmed that protein phosphorylation could affect plant physiology by regulating enzyme activity. These results provide new clues for further study the formation mechanisms of naturally variegated phenotype.

Differential impact of heat stress on the expression of chloroplast-encoded genes

Heat shock is one of the major abiotic factors that causes severe retardation in plant growth and development. To dissect the principal effects of hyperthermia on chloroplast gene expression, we studied the temporal dynamics of transcript accumulation for chloroplast-encoded genes in Arabidopsis thaliana and genes for the chloroplast transcription machinery against a background of changes in physiological parameters. A marked reduction in the transcript amounts of the majority of the genes at the early phases of heat shock (HS) was followed by a return to the baseline levels of rbcL and the housekeeping genes clpP, accD, rps14 and rrn16. The decline in the mRNA levels of trnE (for tRNA^glu) and the PSI genes psaA and psaB was opposed by the transient increase in the transcript accumulation of ndhF and the PSII genes psbA, psbD, and psbN and their subsequent reduction with the development of stress. However, the up-regulation of PSII genes in response to elevated temperature was absent in the heat stress-sensitive mutants abi1 and abi2 with the impaired degradation of D2 protein. The expression of rpoA and rpoB, which encode subunits of PEP, was strongly down-regulated throughout the duration of the heat treatment. In addition, heat stress-induced PEP deficiency caused the compensatory up-regulation of the genes for the nuclear-encoded RNA polymerases RPOTp and RPOTmp, the PEP-associated proteins PAP6 and PAP8, the Ser/Thr protein kinase cPCK2, and the stress-inducible sigma factor gene SIG5. Thus, heat stress differentially modulates the transcript accumulation of plastid-encoded genes in A. thaliana at least in part via the expression of HS-responsive nuclear genes for the plastid transcription machinery.

Redox regulation of PEP activity during seedling establishment in Arabidopsis thaliana

Activation of the plastid-encoded RNA polymerase is tightly controlled and involves a network of phosphorylation and, as yet unidentified, thiol-mediated events. Here, we characterize PLASTID REDOX INSENSITIVE2, a redox-regulated protein required for full PEP-driven transcription. PRIN2 dimers can be reduced into the active monomeric form by thioredoxins through reduction of a disulfide bond. Exposure to light increases the ratio between the monomeric and dimeric forms of PRIN2. Complementation of prin2-2 with different PRIN2 protein variants demonstrates that the monomer is required for light-activated PEP-dependent transcription and that expression of the nuclear-encoded photosynthesis genes is linked to the activity of PEP. Activation of PEP during chloroplast development likely is the source of a retrograde signal that promotes nuclear LHCB expression. Thus, regulation of PRIN2 is the thiol-mediated mechanism required for full PEP activity, with PRIN2 monomerization via reduction by TRXs providing a mechanistic link between photosynthetic electron transport and activation of photosynthetic gene expression.

The plastid-encoded RNA polymerase PEP is regulated according to plastid redox state. Here, the authors show that the redox-regulated PRIN2 protein is reduced to monomeric form in a thiol-dependent manner in response to light and that PRIN2 monomers are required for PEP activity and retrograde signaling.

The rice TCD11 encoding plastid ribosomal protein S6 is essential for chloroplast development at low temperature

Plastid ribosome proteins (PRPs) are important components for chloroplast biogenesis and early chloroplast development. Although it has been known that chloroplast ribosomes are similar to bacterial ones, the precise molecular function of ribosomal proteins remains to be elucidated in rice. Here, we identified a novel rice mutant, designated tcd11 (thermo-sensitive chlorophyll-deficient mutant 11), characterized by the albino phenotype until it died at 20°C, while displaying normal phenotype at 32°C. The alteration of leaf color in tcd11 mutants was aligned with chlorophyll (Chl) content and chloroplast development. The map-based cloning and molecular complementation showed that TCD11 encodes the ribosomal small subunit protein S6 in chloroplasts (RPS6). TCD11 was abundantly expressed in leaves, suggesting its different expressions in tissues. In addition, the disruption of TCD11 greatly reduced the transcript levels of certain chloroplasts-associated genes and prevented the assembly of ribosome in chloroplasts at low temperature (20°C), whereas they recovered to nearly normal levels at high temperature (32°C). Thus, our data indicate that TCD11 plays an important role in chloroplast development at low temperature. Upon our knowledge, the observations from this study provide a first glimpse into the importance of RPS6 function in rice chloroplast development.

Retrograde signaling between plastid and nucleus: A review

Retrograde signaling, defined as the signaling events leading from the plastids to the nucleus, coordinates the expression of plastid and nuclear genes and is crucial for metabolic as well as developmental processes of the plastids. In the recent past, the identification of various components that are involved in the generation and transmission of plastid-originated retrograde signals and the regulation of nuclear gene expression has only provided a glimpse of the plastid retrograde signaling network, which remains poorly understood. The basic assumptions underlying our current understanding of retrograde signaling stayed untouched for many years. Therefore, an attempt has been made in this review article to summarize established facts and recent advances regarding various retrograde signaling pathways derived from different sources, the identification of key elements mediating retrograde signal transduction and also to give an overview of possible signaling molecules that remain to be investigated.

How sugars might coordinate chloroplast and nuclear gene expression during acclimation to high light intensities

The concept of retrograde control of nuclear gene expression assumes the generation of signals inside the chloroplasts, which are either released from or sensed inside of the organelle. In both cases, downstream signaling pathways lead eventually to a differential regulation of nuclear gene expression and the production of proteins required in the chloroplast. This concept appears reasonable as the majority of the over 3000 predicted plastidial proteins are encoded by nuclear genes. Hence, the nucleus needs information on the status of the chloroplasts, such as during acclimation responses, which trigger massive changes in the protein composition of the thylakoid membrane and in the stroma. Here, we propose an additional control mechanism of nuclear- and plastome-encoded photosynthesis genes, taking advantage of pathways involved in sugar- or hormonal signaling. Sugars are major end products of photosynthesis and their contents respond very sensitively to changes in light intensities. Based on recent findings, we ask the question as to whether the carbohydrate status outside the chloroplast can be directly sensed within the chloroplast stroma. Sugars might synchronize the responsiveness of both genomes and thereby help to coordinate the expression of plastome- and nuclear-encoded photosynthesis genes in concert with other, more specific retrograde signals.

Redox activity of thioredoxin z and fructokinase-like protein 1 is dispensable for autotrophic growth of Arabidopsis thaliana

Redox modulation of protein activity by thioredoxins (TRXs) plays a key role in cellular regulation. Thioredoxin z (TRX z) and its interaction partner fructokinase-like protein 1 (FLN1) represent subunits of the plastid-encoded RNA polymerase (PEP), suggesting a role of both proteins in redox regulation of chloroplast gene expression. Loss of TRX z or FLN1 expression generates a PEP-deficient phenotype and renders the plants incapable to grow autotrophically. This study shows that PEP function in trx z and fln1 plants can be restored by complementation with redox-inactive TRX z C106S and FLN1 C105/106A protein variants, respectively. The complemented plants showed wild-type levels of chloroplast gene expression and were restored in photosynthetic capacity, indicating that redox regulation of PEP through TRX z/FLN1 per se is not essential for autotrophic growth. Promoter-reporter gene studies indicate that TRX z and FLN1 are expressed during early phases of leaf development while expression ceases at maturation. Taken together, our data support a model in which TRX z and FLN1 are essential structural components of the PEP complex and their redox activity might only play a role in the fine tuning of PEP function.

The functions of WHIRLY1 and REDOX-RESPONSIVE TRANSCRIPTION FACTOR 1 in cross tolerance responses in plants: a hypothesis

Chloroplasts are important sensors of environment change, fulfilling key roles in the regulation of plant growth and development in relation to environmental cues. Photosynthesis produces a repertoire of reductive and oxidative (redox) signals that provide information to the nucleus facilitating appropriate acclimation to a changing light environment. Redox signals are also recognized by the cellular innate immune system allowing activation of non-specific, stress-responsive pathways that underpin cross tolerance to biotic-abiotic stresses. While these pathways have been intensively studied in recent years, little is known about the different components that mediate chloroplast-to-nucleus signalling and facilitate cross tolerance phenomena. Here, we consider the properties of the WHIRLY family of proteins and the REDOX-RESPONSIVE TRANSCRIPTION FACTOR 1 (RRTF1) in relation to chloroplast redox signals that facilitate the synergistic co-activation of gene expression pathways and confer cross tolerance to abiotic and biotic stresses. We propose a new hypothesis for the role of WHIRLY1 as a redox sensor in chloroplast-to-nucleus retrograde signalling leading to cross tolerance, including acclimation and immunity responses. By virtue of its association with chloroplast nucleoids and with nuclear DNA, WHIRLY1 is an attractive candidate coordinator of the expression of photosynthetic genes in the nucleus and chloroplasts. We propose that the redox state of the photosynthetic electron transport chain triggers the movement of WHIRLY1 from the chloroplasts to the nucleus, and draw parallels with the regulation of NONEXPRESSOR OF PATHOGENESIS-RELATED GENES 1 (NPR1).

AtECB1/MRL7, a thioredoxin-like fold protein with disulfide reductase activity, regulates chloroplast gene expression and chloroplast biogenesis in Arabidopsis thaliana

Plastid-encoded RNA polymerase (PEP) is closely associated with numerous factors to form PEP complex for plastid gene expression and chloroplast development. However, it is not clear how PEP complex are regulated in chloroplast. Here, one thioredoxin-like fold protein, Arabidopsis early chloroplast biogenesis 1 (AtECB1), an allele of MRL7, was identified to regulate PEP function and chloroplast biogenesis. The knockout lines for AtECB1 displayed albino phenotype and impaired chloroplast development. The transcripts of PEP-dependent plastid genes were barely detected, suggesting that the PEP activity is almost lost in atecb1-1. Although AtECB1 was not identified in PEP complex, a yeast two-hybrid assay and pull-down experiments demonstrated that it can interact with Trx Z and FSD3, two intrinsic subunits of PEP complex, respectively. This indicates that AtECB1 may play a regulatory role for PEP-dependent plastid gene expression through these two subunits. AtECB1 contains a βαβαββα structure in the thioredoxin-like fold domain and lacks the typical C-X-X-C active site motif. Insulin assay demonstrated that AtECB1 harbors disulfide reductase activity in vitro using the purified recombinant AtECB1 protein. This showed that this thioredoxin-like fold protein, AtECB1 also has the thioredoxin activity. AtECB1 may play a role in thioredoxin signaling to regulate plastid gene expression and chloroplast development.

A purification strategy for analysis of the DNA/RNA-associated sub-proteome from chloroplasts of mustard cotyledons

Plant cotyledons are a tissue that is particularly active in plastid gene expression in order to develop functional chloroplasts from pro-plastids, the plastid precursor stage in plant embryos. Cotyledons, therefore, represent a material being ideal for the study of composition, function and regulation of protein complexes involved in plastid gene expression. Here, we present a pilot study that uses heparin-Sepharose and phospho-cellulose chromatography in combination with isoelectric focussing and denaturing SDS gel electrophoresis (two-dimensional gel electrophoresis) for investigating the nucleic acids binding sub-proteome of mustard chloroplasts purified from cotyledons. We describe the technical requirements for a highly resolved biochemical purification of several hundreds of protein spots obtained from such samples. Subsequent mass spectrometry of peptides isolated out of cut spots that had been treated with trypsin identified 58 different proteins within 180 distinct spots. Our analyses indicate a high enrichment of proteins involved in transcription and translation and, in addition, the presence of massive post-translational modification of this plastid protein sub-fraction. The study provides an extended catalog of plastid proteins from mustard being involved in gene expression and its regulation and describes a suitable purification strategy for further analysis of low abundant gene expression related proteins.

Nuclear-encoded factors associated with the chloroplast transcription machinery of higher plants

Plastid transcription is crucial for plant growth and development. There exist two types of RNA polymerases in plastids: a nuclear-encoded RNA polymerase (NEP) and plastid-encoded RNA polymerase (PEP). PEP is the major RNA polymerase activity in chloroplast. Its core subunits are encoded by the plastid genome, and these are embedded into a larger complex of nuclear-encoded subunits. Biochemical and genetics analysis identified at least 12 proteins are tightly associated with the core subunit, while about 34 further proteins are associated more loosely generating larger complexes such as the transcriptionally active chromosome (TAC) or a part of the nucleoid. Domain analyses and functional investigations suggested that these nuclear-encoded factors may form several functional modules that mediate regulation of plastid gene expression by light, redox, phosphorylation, and heat stress. Genetic analyses also identified that some nuclear-encoded proteins in the chloroplast that are important for plastid gene expression, although a physical association with the transcriptional machinery is not observed. This covers several PPR proteins including CLB19, PDM1/SEL1, OTP70, and YS1 which are involved in the processing of transcripts for PEP core subunit as well as AtECB2, Prin2, SVR4-Like, and NARA5 that are also important for plastid gene expression, although their functions are unclear.

Photosynthetic gene expression in higher plants

Within the chloroplasts of higher plants and algae, photosynthesis converts light into biological energy, fueling the assimilation of atmospheric carbon dioxide into biologically useful molecules. Two major steps, photosynthetic electron transport and the Calvin-Benson cycle, require many gene products encoded from chloroplast as well as nuclear genomes. The expression of genes in both cellular compartments is highly dynamic and influenced by a diverse range of factors. Light is the primary environmental determinant of photosynthetic gene expression. Working through photoreceptors such as phytochrome, light regulates photosynthetic genes at transcriptional and posttranscriptional levels. Other processes that affect photosynthetic gene expression include photosynthetic activity, development, and biotic and abiotic stress. Anterograde (from nucleus to chloroplast) and retrograde (from chloroplast to nucleus) signaling insures the highly coordinated expression of the many photosynthetic genes between these different compartments. Anterograde signaling incorporates nuclear-encoded transcriptional and posttranscriptional regulators, such as sigma factors and RNA-binding proteins, respectively. Retrograde signaling utilizes photosynthetic processes such as photosynthetic electron transport and redox signaling to influence the expression of photosynthetic genes in the nucleus. The basic C3 photosynthetic pathway serves as the default form used by most of the plant species on earth. High temperature and water stress associated with arid environments have led to the development of specialized C4 and CAM photosynthesis, which evolved as modifications of the basic default expression program. The goal of this article is to explain and summarize the many gene expression and regulatory processes that work together to support photosynthetic function in plants.

Transcript patterns of chloroplast-encoded genes in cultured Symbiodinium spp. (Dinophyceae): testing the influence of a light shift and diel periodicity

Microalgae possess numerous cellular mechanisms specifically employed for acclimating the photosynthetic pathways to changes in the physical environment. Despite the importance of coral-dinoflagellate symbioses, little focus has been given as to how the symbiotic algae (Symbiodinium spp.) regulate the expression of their photosynthetic genes. This study used real-time PCR to investigate the transcript abundance of the plastid-encoded genes, psbA (encoding the D1 protein of photosystem II) and psaA (encoding the P700 protein in photosystem I), within the cultured Symbiodinium ITS-2 (internal transcribed spacer region) types A20 and A13. Transcript abundance was monitored during a low to high-light shift, as well as over a full diel light cycle. In addition, psaA was characterized in three isolates (A20, A13, and D4-5) and noted as another example of a dinoflagellate plastid gene encoded on a minicircle. In general, the overall incongruence of transcript patterns for both psbA and psaA between the Symbiodinium isolates and other models of transcriptionally controlled chloroplast gene expression (e.g., Pisum sativum [pea], Sinapis alba [mustard seedling], and Synechocystis sp. PCC 6803 [cyanobacteria]) suggests that Symbiodinium is reliant on posttranscriptional mechanisms for homeostatic regulation of its photosynthetic proteins.

TAC7, an essential component of the plastid transcriptionally active chromosome complex, interacts with FLN1, TAC10, TAC12 and TAC14 to regulate chloroplast gene expression in Arabidopsis thaliana

Transcriptionally active chromosome (TAC) is a fraction of protein/DNA complexes with RNA polymerase activity in the plastid. However, the function of most TAC proteins remains unknown. Here, we isolated two allelic mutants of the gene for a TAC component, TAC7, and performed functional analysis in plastid gene expression and chloroplast development in Arabidopsis. tac7-1 is a mutant with a premature translation termination isolated from a population treated with ethyl methane sulfonate, and tac7-2 is a transfer-DNA tagging mutant. Both of them showed an albino phenotype when grown under normal light conditions, and a few appressed membranes were observed inside the defective chloroplasts. These data indicate that TAC7 is important for thylakoid biogenesis. The TAC7 gene encodes an uncharacterized 161 amino acids polypeptide localized in chloroplast. The transcriptional levels of plastid-encoded polymerase (PEP)-dependent genes were downregulated in tac7-2, suggesting that PEP activity was decreased in the mutant. Yeast two-hybrid assay shows that TAC7 can interact with the four TAC components including FLN1, TAC10, TAC12 and TAC14 which are involved in redox state changes, phosphorylation processes and phytochrome-dependent light signaling, respectively, These data indicate that TAC7 plays an important role for TAC to regulate PEP-dependent chloroplast gene expression and chloroplast development.

Evolutionary rewiring: a modified prokaryotic gene-regulatory pathway in chloroplasts

Photosynthetic electron transport regulates chloroplast gene transcription through the action of a bacterial-type sensor kinase known as chloroplast sensor kinase (CSK). CSK represses photosystem I (PS I) gene transcription in PS I light and thus initiates photosystem stoichiometry adjustment. In cyanobacteria and in non-green algae, CSK homologues co-exist with their response regulator partners in canonical bacterial two-component systems. In green algae and plants, however, no response regulator partner of CSK is found. Yeast two-hybrid analysis has revealed interaction of CSK with sigma factor 1 (SIG1) of chloroplast RNA polymerase. Here we present further evidence for the interaction between CSK and SIG1. We also show that CSK interacts with quinone. Arabidopsis SIG1 becomes phosphorylated in PS I light, which then specifically represses transcription of PS I genes. In view of the identical signalling properties of CSK and SIG1 and of their interactions, we suggest that CSK is a SIG1 kinase. We propose that the selective repression of PS I genes arises from the operation of a gene-regulatory phosphoswitch in SIG1. The CSK-SIG1 system represents a novel, rewired chloroplast-signalling pathway created by evolutionary tinkering. This regulatory system supports a proposal for the selection pressure behind the evolutionary stasis of chloroplast genes.

Redox regulation of thylakoid protein kinases and photosynthetic gene expression

SIGNIFICANCE

Photosynthetic organisms are subjected to frequent changes in their environment that include fluctuations in light quality and quantity, temperature, CO(2) concentration, and nutrient availability. They have evolved complex responses to these changes that allow them to protect themselves against photo-oxidative damage and to optimize their growth under these adverse conditions. In the case of light changes, these acclimatory processes can occur in either the short or the long term and are mainly mediated through the redox state of the plastoquinone pool and the ferredoxin/thioredoxin system.

RECENT ADVANCES

Short-term responses involve a dynamic reorganization of photosynthetic complexes, and long-term responses (LTRs) modulate the chloroplast and nuclear gene expression in such a way that the levels of the photosystems and their antennae are rebalanced for an optimal photosynthetic performance. These changes are mediated through a complex signaling network with several protein kinases and phosphatases that are conserved in land plants and algae. The phosphorylation status of the light-harvesting proteins of photosystem II and its core proteins is mainly determined by two complementary kinase-phosphatase pairs corresponding to STN7/PPH1 and STN8/PBCP, respectively.

CRITICAL ISSUES

The activity of the Stt7 kinase is principally regulated by the redox state of the plastoquinone pool, which in turn depends on the light irradiance, ambient CO(2) concentration, and cellular energy status. In addition, this kinase is also involved in the LTR.

FUTURE DIRECTIONS

Other chloroplast kinases modulate the activity of the plastid transcriptional machinery, but the global signaling network that connects all of the identified kinases and phosphatases is still largely unknown.

Plastid-to-nucleus communication, signals controlling the running of the plant cell

The presence of genes encoding organellar proteins in both the nucleus and the organelle necessitates tight coordination of expression by the different genomes, and this has led to the evolution of sophisticated intracellular signaling networks. Organelle-to-nucleus signaling, or retrograde control, coordinates the expression of nuclear genes encoding organellar proteins with the metabolic and developmental state of the organelle. Complex networks of retrograde signals orchestrate major changes in nuclear gene expression and coordinate cellular activities and assist the cell during plant development and stress responses. It has become clear that, even though the chloroplast depends on the nucleus for its function, plastid signals play important roles in an array of different cellular processes vital to the plant. Hence, the chloroplast exerts significant control over the running of the cell. This article is part of a Special Issue entitled: Protein Import and Quality Control in Mitochondria and Plastids.

Photosynthetic acclimation responses of maize seedlings grown under artificial laboratory light gradients mimicking natural canopy conditions

In this study we assessed the ability of the C4 plant maize to perform long-term photosynthetic acclimation in an artificial light quality system previously used for analyzing short-term and long-term acclimation responses (LTR) in C3 plants. We aimed to test if this light system could be used as a tool for analyzing redox-regulated acclimation processes in maize seedlings. Photosynthetic parameters obtained from maize samples harvested in the field were used as control. The results indicated that field grown maize performed a pronounced LTR with significant differences between the top and the bottom levels of the plant stand corresponding to the strong light gradients occurring in it. We compared these data to results obtained from maize seedlings grown under artificial light sources preferentially exciting either photosystem II or photosystem I. In C3 plants, this light system induces redox signals within the photosynthetic electron transport chain which trigger state transitions and differential phosphorylation of LHCII (light harvesting complexes of photosystem II). The LTR to these redox signals induces changes in the accumulation of plastid psaA transcripts, in chlorophyll (Chl) fluorescence values F \rm s/F \rm m, in Chl a/b ratios and in transient starch accumulation in C3 plants. Maize seedlings grown in this light system exhibited a pronounced ability to perform both short-term and long-term acclimation at the level of psaA transcripts, Chl fluorescence values F \rm s/F \rm m and Chl a/b ratios. Interestingly, maize seedlings did not exhibit redox-controlled variations of starch accumulation probably because of its specific differences in energy metabolism. In summary, the artificial laboratory light system was found to be well-suited to mimic field light conditions and provides a physiological tool for studying the molecular regulation of the LTR of maize in more detail.

Plastid-to-nucleus communication, signals controlling the running of the plant cell

The presence of genes encoding organellar proteins in both the nucleus and the organelle necessitates tight coordination of expression by the different genomes, and this has led to the evolution of sophisticated intracellular signaling networks. Organelle-to-nucleus signaling, or retrograde control, coordinates the expression of nuclear genes encoding organellar proteins with the metabolic and developmental state of the organelle. Complex networks of retrograde signals orchestrate major changes in nuclear gene expression and coordinate cellular activities and assist the cell during plant development and stress responses. It has become clear that, even though the chloroplast depends on the nucleus for its function, plastid signals play important roles in an array of different cellular processes vital to the plant. Hence, the chloroplast exerts significant control over the running of the cell. This article is part of a Special Issue entitled: Protein Import and Quality Control in Mitochondria and Plastids.

The hidden function of photosynthesis: a sensing system for environmental conditions that regulates plant acclimation responses

Plants convert light energy from the sun into chemical energy by photosynthesis. Since they are sessile, they have to deal with a wide range of conditions in their immediate environment. Many abiotic and biotic parameters exhibit considerable fluctuations which can have detrimental effects especially on the efficiency of photosynthetic light harvesting. During evolution, plants, therefore, evolved a number of acclimation processes which help them to adapt photosynthesis to such environmental changes. This includes protective mechanisms such as excess energy dissipation and processes supporting energy redistribution, e.g. state transitions or photosystem stoichiometry adjustment. Intriguingly, all these responses are triggered by photosynthesis itself via the interplay of its light reaction and the Calvin-Benson cycle with the residing environmental condition. Thus, besides its primary function in harnessing and converting light energy, photosynthesis acts as a sensing system for environmental changes that controls molecular acclimation responses which adapt the photosynthetic function to the environmental change. Important signalling parameters directly or indirectly affected by the environment are the pH gradient across the thylakoid membrane and the redox states of components of the photosynthetic electron transport chain and/or electron end acceptors coupled to it. Recent advances demonstrate that these signals control post-translational modifications of the photosynthetic protein complexes and also affect plastid and nuclear gene expression machineries as well as metabolic pathways providing a regulatory framework for an integrated response of the plant to the environment at all cellular levels.

The plastid redox insensitive 2 mutant of Arabidopsis is impaired in PEP activity and high light-dependent plastid redox signalling to the nucleus

The photosynthetic apparatus is composed of proteins encoded by genes from both the nuclear and the chloroplastic genomes. The activities of the nuclear and chloroplast genomes must therefore be closely coordinated through intracellular signalling. The plastids produce multiple retrograde signals at different times of their development, and in response to changes in the environment. These signals regulate the expression of nuclear-encoded photosynthesis genes to match the current status of the plastids. Using forward genetics we identified PLASTID REDOX INSENSITIVE 2 (PRIN2), a chloroplast component involved in redox-mediated retrograde signalling. The allelic mutants prin2-1 and prin2-2 demonstrated a misregulation of photosynthesis-associated nuclear gene expression in response to excess light, and an inhibition of photosynthetic electron transport. As a consequence of the misregulation of LHCB1.1 and LHCB2.4, the prin2 mutants displayed a high irradiance-sensitive phenotype with significant photoinactivation of photosystem II, indicated by a reduced variable to maximal fluorescence ratio (F(v) /F(m) ). PRIN2 is localized to the nucleoids, and plastid transcriptome analyses demonstrated that PRIN2 is required for full expression of genes transcribed by the plastid-encoded RNA polymerase (PEP). Similarly to the prin2 mutants, the ys1 mutant with impaired PEP activity also demonstrated a misregulation of LHCB1.1 and LHCB2.4 expression in response to excess light, suggesting a direct role for PEP activity in redox-mediated retrograde signalling. Taken together, our results indicate that PRIN2 is part of the PEP machinery, and that the PEP complex responds to photosynthetic electron transport and generates a retrograde signal, enabling the plant to synchronize the expression of photosynthetic genes from both the nuclear and plastidic genomes.

Environmental control of plant nuclear gene expression by chloroplast redox signals

Plant photosynthesis takes place in specialized cell organelles, the chloroplasts, which perform all essential steps of this process. The proteins involved in photosynthesis are encoded by genes located on the plastid and nuclear genomes. Proper function and regulation of light harvesting and energy fixation thus requires a tight coordination of the gene expression machineries in the two genetic compartments. This is achieved by a bi-directional exchange of information between nucleus and plastids. Signals emerging from plastids report the functional and developmental state of the organelle to the nucleus and initiate distinct nuclear gene expression profiles, which trigger responses that support or improve plastid functions. Recent research indicated that this signaling is absolutely essential for plant growth and development. Reduction/oxidation (redox) signals from photosynthesis are key players in this information network since they do report functional disturbances in photosynthesis, the primary energy source of plants. Such disturbances are caused by environmental fluctuations for instance in illumination, temperature, or water availability. These environmental changes affect the linear electron flow of photosynthesis and result in changes of the redox state of the components involved [e.g., the plastoquinone (PQ) pool] or coupled to it (e.g., the thioredoxin pool). Thus, the changes in redox state directly reflect the environmental impact and serve as immediate plastidial signals to the nucleus. The triggered responses range from counterbalancing reactions within the physiological range up to severe stress responses including cell death. This review focuses on physiological redox signals from photosynthetic electron transport (PET), their relation to the environment, potential transduction pathways to the nucleus and their impact on nuclear gene expression.

Nucleoid-enriched proteomes in developing plastids and chloroplasts from maize leaves: a new conceptual framework for nucleoid functions

Plastids contain multiple copies of the plastid chromosome, folded together with proteins and RNA into nucleoids. The degree to which components of the plastid gene expression and protein biogenesis machineries are nucleoid associated, and the factors involved in plastid DNA organization, repair, and replication, are poorly understood. To provide a conceptual framework for nucleoid function, we characterized the proteomes of highly enriched nucleoid fractions of proplastids and mature chloroplasts isolated from the maize (Zea mays) leaf base and tip, respectively, using mass spectrometry. Quantitative comparisons with proteomes of unfractionated proplastids and chloroplasts facilitated the determination of nucleoid-enriched proteins. This nucleoid-enriched proteome included proteins involved in DNA replication, organization, and repair as well as transcription, mRNA processing, splicing, and editing. Many proteins of unknown function, including pentatricopeptide repeat (PPR), tetratricopeptide repeat (TPR), DnaJ, and mitochondrial transcription factor (mTERF) domain proteins, were identified. Strikingly, 70S ribosome and ribosome assembly factors were strongly overrepresented in nucleoid fractions, but protein chaperones were not. Our analysis strongly suggests that mRNA processing, splicing, and editing, as well as ribosome assembly, take place in association with the nucleoid, suggesting that these processes occur cotranscriptionally. The plastid developmental state did not dramatically change the nucleoid-enriched proteome but did quantitatively shift the predominating function from RNA metabolism in undeveloped plastids to translation and homeostasis in chloroplasts. This study extends the known maize plastid proteome by hundreds of proteins, including more than 40 PPR and mTERF domain proteins, and provides a resource for targeted studies on plastid gene expression. Details of protein identification and annotation are provided in the Plant Proteome Database.

Identification of essential subunits in the plastid-encoded RNA polymerase complex reveals building blocks for proper plastid development

The major RNA polymerase activity in mature chloroplasts is a multisubunit, Escherichia coli-like protein complex called PEP (for plastid-encoded RNA polymerase). Its subunit structure has been extensively investigated by biochemical means. Beside the "prokaryotic" subunits encoded by the plastome-located RNA polymerase genes, a number of additional nucleus-encoded subunits of eukaryotic origin have been identified in the PEP complex. These subunits appear to provide additional functions and regulation modes necessary to adapt transcription to the varying functional situations in chloroplasts. However, despite the enormous progress in genomic data and mass spectrometry techniques, it is still under debate which of these subunits belong to the core complex of PEP and which ones represent rather transient or peripheral components. Here, we present a catalog of true PEP subunits that is based on comparative analyses from biochemical purifications, protein mass spectrometry, and phenotypic analyses. We regard reproducibly identified protein subunits of the basic PEP complex as essential when the corresponding knockout mutants reveal an albino or pale-green phenotype. Our study provides a clearly defined subunit catalog of the basic PEP complex, generating the basis for a better understanding of chloroplast transcription regulation. In addition, the data support a model that links PEP complex assembly and chloroplast buildup during early seedling development in vascular plants.

The transcription machineries of plant mitochondria and chloroplasts: Composition, function, and regulation

Although genomes of mitochondria and plastids are very small compared to those of their bacterial ancestors, the transcription machineries of these organelles are of surprising complexity. With respect to the number of different RNA polymerases per organelle, the extremes are represented on one hand by chloroplasts of eudicots which use one bacterial-type RNA polymerase and two phage-type RNA polymerases to transcribe their genes, and on the other hand by Physcomitrella possessing three mitochondrial RNA polymerases of the phage type. Transcription of genes/operons is often driven by multiple promoters in both organelles. This review describes the principle components of the transcription machineries (RNA polymerases, transcription factors, promoters) and the division of labor between the different RNA polymerases. While regulation of transcription in mitochondria seems to be only of limited importance, the plastid genes of higher plants respond to exogenous and endogenous cues rather individually by altering their transcriptional activities.

Strategies for psbA gene expression in cyanobacteria, green algae and higher plants: from transcription to PSII repair

The Photosystem (PS) II of cyanobacteria, green algae and higher plants is prone to light-induced inactivation, the D1 protein being the primary target of such damage. As a consequence, the D1 protein, encoded by the psbA gene, is degraded and re-synthesized in a multistep process called PSII repair cycle. In cyanobacteria, a small gene family codes for the various, functionally distinct D1 isoforms. In these organisms, the regulation of the psbA gene expression occurs mainly at the level of transcription, but the expression is fine-tuned by regulation of translation elongation. In plants and green algae, the D1 protein is encoded by a single psbA gene located in the chloroplast genome. In chloroplasts of Chlamydomonas reinhardtii the psbA gene expression is strongly regulated by mRNA processing, and particularly at the level of translation initiation. In chloroplasts of higher plants, translation elongation is the prevalent mechanism for regulation of the psbA gene expression. The pre-existing pool of psbA transcripts forms translation initiation complexes in plant chloroplasts even in darkness, while the D1 synthesis can be completed only in the light. Replacement of damaged D1 protein requires also the assistance by a number of auxiliary proteins, which are encoded by the nuclear genome in green algae and higher plants. Nevertheless, many of these chaperones are conserved between prokaryotes and eukaryotes. Here, we describe the specific features and fundamental differences of the psbA gene expression and the regeneration of the PSII reaction center protein D1 in cyanobacteria, green algae and higher plants. This article is part of a Special Issue entitled Photosystem II.

Update on chloroplast research: new tools, new topics, and new trends

Chloroplasts, the green differentiation form of plastids, are the sites of photosynthesis and other important plant functions. Genetic and genomic technologies have greatly boosted the rate of discovery and functional characterization of chloroplast proteins during the past decade. Indeed, data obtained using high-throughput methodologies, in particular proteomics and transcriptomics, are now routinely used to assign functions to chloroplast proteins. Our knowledge of many chloroplast processes, notably photosynthesis and photorespiration, has reached such an advanced state that biotechnological approaches to crop improvement now seem feasible. Meanwhile, efforts to identify the entire complement of chloroplast proteins and their interactions are progressing rapidly, making the organelle a prime target for systems biology research in plants.

Plastidial retrograde signalling--a true "plastid factor" or just metabolite signatures?

The genetic compartments of plant cells, nuclei, plastids and mitochondria exchange information by anterograde (nucleus-to-organelle) and retrograde (organelle-to-nucleus) signalling. These avenues of communication coordinate activities during the organelles' development and function. Despite extensive research retrograde signalling remains poorly understood. The proposed cytosolic signalling pathways and the putative organellar signalling molecules remain elusive, and a clear functional distinction from the signalling cascades of other cellular perception systems (i.e. photoreceptors or phytohormones) is difficult to obtain. Notwithstanding the stagnant progress, some basic assumptions about the process have remained virtually unchanged for many years, potentially obstructing the view on alternative routes for retrograde communication. Here, I critically assess the current models of retrograde signalling and discuss novel ideas and potential connections.

Plastidial thioredoxin z interacts with two fructokinase-like proteins in a thiol-dependent manner: evidence for an essential role in chloroplast development in Arabidopsis and Nicotiana benthamiana

Here, we characterize a plastidial thioredoxin (TRX) isoform from Arabidopsis thaliana that defines a previously unknown branch of plastidial TRXs lying between x- and y-type TRXs and thus was named TRX z. An Arabidopsis knockout mutant of TRX z had a severe albino phenotype and was inhibited in chloroplast development. Quantitative real-time RT-PCR analysis of the mutant suggested that the expressions of genes that depend on a plastid-encoded RNA polymerase (PEP) were specifically decreased. Similar results were obtained upon virus-induced gene silencing (VIGS) of the TRX z ortholog in Nicotiana benthamiana. We found that two fructokinase-like proteins (FLN1 and FLN2), members of the pfkB-carbohydrate kinase family, were potential TRX z target proteins and identified conserved Cys residues mediating the FLN-TRX z interaction. VIGS in N. benthamiana and inducible RNA interference in Arabidopsis of FLNs also led to a repression of PEP-dependent gene transcription. Remarkably, recombinant FLNs displayed no detectable sugar-phosphorylating activity, and amino acid substitutions within the predicted active site imply that the FLNs have acquired a new function, which might be regulatory rather than metabolic. We were able to show that the FLN2 redox state changes in vivo during light/dark transitions and that this change is mediated by TRX z. Taken together, our data strongly suggest an important role for TRX z and both FLNs in the regulation of PEP-dependent transcription in chloroplasts.

AtSIG6, a plastid sigma factor from Arabidopsis, reveals functional impact of cpCK2 phosphorylation

Plastids contain sigma factors, i.e. gene-regulatory proteins for promoter binding and transcription initiation. Despite the physical and functional similarity shared with their prokaryotic counterparts, the plant sigma factors have distinguishing features: most notably the existence of a variable extra sequence comprising their N-terminal portions. This distinct architecture is reflected by functional differences, including phosphorylation control by organellar protein kinase(s) closely related to nucleocytosolic, rather than bacterial-type, enzymes. In particular, cpCK2, a nuclear-coded plastid-targeted casein kinase 2, has been implicated as a key component in plant sigma factor phosphorylation and transcriptional regulation (Eur. J. Biochem. 269, 2002, 3329; Planta, 219, 2004, 298). Although this notion is based mainly on biochemical evidence and in vitro systems, the recent availability of Arabidopsis sigma knock-out lines for complementation by intact and mutant sigma cDNAs has opened up new strategies for the study of transcription regulatory mechanisms in vivo. Using Arabidopsis sigma factor 6 (AtSIG6) as a paradigm, we present data suggesting that: (i) this factor is a substrate for regulatory phosphorylation by cpCK2 both in vitro and in vivo; (ii) cpCK2 phosphorylation of SIG6 occurs at multiple sites, which can widely differ in their effect on the visual and/or molecular phenotype; (iii) in vivo usage of the perhaps most critical cpCK2 site defined by Ser174 requires (pre-)phosphorylation at the n + 3 serine residue Ser177, pointing to 'pathfinder' kinase activity capable of generating a functional cpCK2 substrate site.

Hypothesis: A binary redox control mode as universal regulator of photosynthetic light acclimation

In nature, plants experience considerable changes in the prevailing illumination, which can drastically reduce photosynthetic efficiency and yield. Such adverse effects are counterbalanced by acclimation responses which ensure high photosynthetic productivity by structural reconfiguration of the photosynthetic apparatus. Those acclimation responses are controlled by reduction-oxidation (redox) signals from two pools of redox compounds, the plastoquinone and the thioredoxin pools. The relative impact of these two redox signaling systems on this process, however, remains controversial. Recently, we showed that photosynthesis controls nuclear gene expression and cellular metabolite states in an integrated manner, thus, stabilizing the varying energetic demands of the plant. Here, we propose a novel model based on a binary redox control mode to explain adaptation of plant primary productivity to the light environment. Plastoquinone and thioredoxin pools are proposed to define specific environmental situations cooperatively and to initiate appropriate acclimation responses controlled by four binary combinations of their redox states. Our model indicates a hierarchical redox regulation network that controls plant primary productivity and supports the notion that photosynthesis is an environmental sensor affecting plant growth and development.

Dynamic plastid redox signals integrate gene expression and metabolism to induce distinct metabolic states in photosynthetic acclimation in Arabidopsis

Plants possess acclimation responses in which structural reconfigurations adapt the photosynthetic apparatus to fluctuating illumination. Long-term acclimation involves changes in plastid and nuclear gene expression and is controlled by redox signals from photosynthesis. The kinetics of these signals and the adjustments of energetic and metabolic demands to the changes in the photosynthetic apparatus are currently poorly understood. Using a redox signaling system that preferentially excites either photosystem I or II, we measured the time-dependent impact of redox signals on the transcriptome and metabolome of Arabidopsis thaliana. We observed rapid and dynamic changes in nuclear transcript accumulation resulting in differential and specific expression patterns for genes associated with photosynthesis and metabolism. Metabolite pools also exhibited dynamic changes and indicate readjustments between distinct metabolic states depending on the respective illumination. These states reflect reallocation of energy resources in a defined and reversible manner, indicating that structural changes in the photosynthetic apparatus during long-term acclimation are additionally supported at the level of metabolism. We propose that photosynthesis can act as an environmental sensor, producing retrograde redox signals that trigger two parallel adjustment loops that coordinate photosynthesis and metabolism to adapt plant primary productivity to the environment.