Plastid Signals Research Articles

The discovery of plastid-to-nucleus retrograde signaling\u2014a personal perspective

DNA and machinery for gene expression have been discovered in chloroplasts during the 1960s. It was soon evident that the chloroplast genome is relatively small, that most genes for chloroplast-localized proteins reside in the nucleus and that chloroplast membranes, ribosomes, and protein complexes are composed of proteins encoded in both the chloroplast and the nuclear genome. This situation has made the existence of mechanisms highly probable that coordinate the gene expression in plastids and nucleus. In the 1970s, the first evidence for plastid signals controlling nuclear gene expression was provided by studies on plastid ribosome deficient mutants with reduced amounts and/or activities of nuclear-encoded chloroplast proteins including the small subunit of Rubisco, ferredoxin NADP+ reductase, and enzymes of the Calvin cycle. This review describes first models of plastid-to-nucleus signaling and their discovery. Today, many plastid signals are known. They do not only balance gene expression in chloroplasts and nucleus during developmental processes but are also generated in response to environmental changes sensed by the organelles.

Identification of core subunits of photosystemII as action sites of HSP21, which is activated by the GUN5-mediated retrograde pathway in Arabidopsis.

PhotosystemII (PSII) is the most thermolabile photosynthetic complex. Physiological evidence suggests that the small chloroplast heat-shock protein21 (HSP21) is involved in plant thermotolerance, but the molecular mechanism of its action remains largely unknown. Here, we have provided genetic and biochemical evidence that HSP21 is activated by the GUN5-dependent retrograde signaling pathway, and stabilizes PSII by directly binding to its core subunits such as D1 and D2 proteins under heat stress. We further demonstrate that the constitutive expression of HSP21 sufficiently rescues the thermosensitive stability of PSII and survival defects of the gun5 mutant with dramatically improving granal stacking under heat stress, indicating that HSP21 is a key chaperone protein in maintaining the integrity of the thylakoid membrane system under heat stress. In line with our interpretation based on several lines of invitro and invivo protein-interaction evidence that HSP21 interacts with core subunits of PSII, the kinetics of HSP21 binding to the D1 and D2 proteins was determined by performing an analysis of microscale thermophoresis. Considering the major role of HSP21 in protecting the core subunits of PSII from thermal damage, its heat-responsive activation via the heat-shock transcription factor HsfA2 is critical for the survival of plants under heat stress. Our findings reveal an auto-adaptation loop pathway that plant cells optimize particular needs of chloroplasts in stabilizing photosynthetic complexes by relaying the GUN5-dependent plastid signal(s) to activate the heat-responsive expression of HSP21 in the nucleus during adaptation to heat stress in plants.

Perturbations of carotenoid and tetrapyrrole biosynthetic pathways result in differential alterations in chloroplast function and plastid signaling

In this study, we used the biosynthetic inhibitors of carotenoid and tetrapyrrole biosynthetic pathways, norflurazon (NF) and oxyfluorfen (OF), as tools to gain insight into mechanisms of photooxidation in rice plants. NF resulted in bleaching symptom on leaves of the treated plants, whereas OF treatment developed a fast symptom of an apparent necrotic phenotype. Both plants exhibited decreases in photosynthetic efficiency, as indicated by Fv/Fm. NF caused severe disruption in thylakoid membranes, whereas OF-treated plants exhibited disruption of chloroplast envelope and plasma membrane. Levels of Lhca and Lhcb proteins in photosystem I (PSI) and PSII were reduced by photooxidative stress in NF- and OF-treated plants, with a greater decrease in NF plants. The down-regulation of nuclear-encoded photosynthesis genes Lhcb and rbcS was also found in both NF- and OF-treated plants, whereas plastid-encoded photosynthetic genes including RbcL, PsaC, and PsbD accumulated normally in NF plants but decreased drastically in OF plants. This proposes that the plastids in NF plants retain their potential to develop thylakoid membranes and that photobleaching is mainly controlled by nuclear genes. Distinct photooxidation patterns between NF- and OF-treated plants developed differential signaling, which might enable the plant to coordinate the expression of photosynthetic genes from the nuclear and plastidic genomes.

CHLH/GUN5 Function in Tetrapyrrole Metabolism Is Correlated with Plastid Signaling but not ABA Responses in Guard Cells.

Expression of Photosynthesis-Associated Nuclear Genes (PhANGs) is controlled by environmental stimuli and plastid-derived signals (“plastid signals”) transmitting the developmental and functional status of plastids to the nucleus. Arabidopsis genomes uncoupled (gun) mutants exhibit defects in plastid signaling, leading to ectopic expression of PhANGs in the absence of chloroplast development. GUN5 encodes the plastid-localized Mg-chelatase enzyme subunit (CHLH), and recent studies suggest that CHLH is a multifunctional protein involved in tetrapyrrole biosynthesis, plastid signaling and ABA responses in guard cells. To understand the basis of CHLH multifunctionality, we investigated 15 gun5 missense mutant alleles and transgenic lines expressing a series of truncated CHLH proteins in a severe gun5 allele (cch) background (tCHLHs, 10 different versions). Here, we show that Mg-chelatase function and plastid signaling are generally correlated; in contrast, based on the analysis of the gun5 missense mutant alleles, ABA-regulated stomatal control is distinct from these two other functions. We found that none of the tCHLHs could restore plastid-signaling or Mg-chelatase functions. Additionally, we found that both the C-terminal half and N-terminal half of CHLH function in ABA-induced stomatal movement.

Ubiquitin-Proteasome Dependent Regulation of the GOLDEN2-LIKE 1 Transcription Factor in Response to Plastid Signals.

Arabidopsis (Arabidopsis thaliana) GOLDEN2-LIKE (GLK) transcription factors promote chloroplast biogenesis by regulating the expression of photosynthesis-related genes. Arabidopsis GLK1 is also known to participate in retrograde signaling from chloroplasts to the nucleus. To elucidate the mechanism by which GLK1 is regulated in response to plastid signals, we biochemically characterized Arabidopsis GLK1 protein. Expression analysis of GLK1 protein indicated that GLK1 accumulates in aerial tissues. Both tissue-specific and Suc-dependent accumulation of GLK1 were regulated primarily at the transcriptional level. In contrast, norflurazon- or lincomycin-treated gun1-101 mutant expressing normal levels of GLK1 mRNA failed to accumulate GLK1 protein, suggesting that plastid signals directly regulate the accumulation of GLK1 protein in a GUN1-independent manner. Treatment of the glk1glk2 mutant expressing functional GFP-GLK1 with a proteasome inhibitor, MG-132, induced the accumulation of polyubiquitinated GFP-GLK1. Furthermore, the level of endogenous GLK1 in plants with damaged plastids was partially restored when those plants were treated with MG-132. Collectively, these data indicate that the ubiquitin-proteasome system participates in the degradation of Arabidopsis GLK1 in response to plastid signals.

Tetrapyrrole Signaling in Plants

Tetrapyrroles make critical contributions to a number of important processes in diverse organisms. In plants, tetrapyrroles are essential for light signaling, the detoxification of reactive oxygen species, the assimilation of nitrate and sulfate, respiration, photosynthesis, and programed cell death. The misregulation of tetrapyrrole metabolism can produce toxic reactive oxygen species. Thus, it is not surprising that tetrapyrrole metabolism is strictly regulated and that tetrapyrrole metabolism affects signaling mechanisms that regulate gene expression. In plants and algae, tetrapyrroles are synthesized in plastids and were some of the first plastid signals demonstrated to regulate nuclear gene expression. In plants, the mechanism of tetrapyrrole-dependent plastid-to-nucleus signaling remains poorly understood. Additionally, some of experiments that tested ideas for possible signaling mechanisms appeared to produce conflicting data. In some instances, these conflicts are potentially explained by different experimental conditions. Although the biological function of tetrapyrrole signaling is poorly understood, there is compelling evidence that this signaling is significant. Specifically, this signaling appears to affect the accumulation of starch and may promote abiotic stress tolerance. Tetrapyrrole-dependent plastid-to-nucleus signaling interacts with a distinct plastid-to-nucleus signaling mechanism that depends on GENOMES UNCUOPLED1 (GUN1). GUN1 contributes to a variety of processes, such as chloroplast biogenesis, the circadian rhythm, abiotic stress tolerance, and development. Thus, the contribution of tetrapyrrole signaling to plant function is potentially broader than we currently appreciate. In this review, I discuss these aspects of tetrapyrrole signaling.

Singlet oxygen initiates a plastid signal controlling photosynthetic gene expression.

Summary Retrograde signals from the plastid regulate photosynthesis‐associated nuclear genes and are essential to successful chloroplast biogenesis. One model is that a positive haem‐related signal promotes photosynthetic gene expression in a pathway that is abolished by the herbicide norflurazon. Far‐red light (FR) pretreatment and transfer to white light also results in plastid damage and loss of photosynthetic gene expression. Here, we investigated whether norflurazon and FR pretreatment affect the same retrograde signal.We used transcriptome analysis and real‐time reverse transcription−polymerase chain reaction (RT‐PCR) to analyse the effects of these treatments on nuclear gene expression in various Arabidopsis (Arabidopsis thaliana) retrograde signalling mutants.Results showed that the two treatments inhibited largely different nuclear gene sets, suggesting that they affected different retrograde signals. Moreover, FR pretreatment resulted in singlet oxygen (1O2) production and a rapid inhibition of photosynthetic gene expression. This inhibition was partially blocked in the executer1executer2 mutant, which is impaired in 1O2 signalling.Our data support a new model in which a 1O2 retrograde signal, generated by chlorophyll precursors, inhibits expression of key photosynthetic and chlorophyll synthesis genes to prevent photo‐oxidative damage during de‐etiolation. Such a signal would provide a counterbalance to the positive haem‐related signal to fine tune regulation of chloroplast biogenesis.

Plastoglobules in algae: A comprehensive comparative study of the presence of major structural and functional components in complex plastids

Plastoglobules (PG) are lipophilic droplets attached to thylakoid membranes in higher plants and green algae and are implicated in prenyl lipid biosynthesis. They might also represent a central hub for integration of plastid signals under stress and therefore the adaptation of the thylakoid membrane under such conditions. In Arabidopsis thaliana, PG contain around 30 specific proteins of which Fibrillins (FBN) and Activity of bc1 complex kinases (ABC1K) represent the majority with respect to both number and protein mass. However, nothing is known about the presence of PG in most algal species, which are responsible for about 50% of global primary production. Therefore, we searched the genomes of publicly available algal genomes for components of PG and the associated functional network in order to predict their presence and potential evolutionary conservation of physiological functions. We could identify homologous sequences for core components of PG, like FBN and ABC1K, in most investigated algal species. Furthermore, proteins at central and interesting positions within the PG functional coexpression network were identified. Phylogenetic sequence analysis revealed diversity within FBN and ABC1K sequences among algal species with complex plastids of the red lineage and large differences compared with green lineage species. Two types of FBN were detected that differ in their isoelectric point which seems to correlate with subcellular localization. Subgroups of FBN were shared between many investigated species and modeling of their 3D-structure implied a conserved structure. FBN and ABC1K are essential structural and functional components of PG. Their occurrence in investigated algal species suggests presence of PG therein and functions in prenyl lipid metabolism and adaptation of the thylakoid membrane that are conserved during evolution.

Circadian and Plastid Signaling Pathways Are Integrated to Ensure Correct Expression of the CBF and COR Genes during Photoperiodic Growth.

The circadian clock synchronizes a wide range of biological processes with the day/night cycle, and correct circadian regulation is essential for photosynthetic activity and plant growth. We describe here a mechanism where a plastid signal converges with the circadian clock to fine-tune the regulation of nuclear gene expression in Arabidopsis (Arabidopsis thaliana). Diurnal oscillations of tetrapyrrole levels in the chloroplasts contribute to the regulation of the nucleus-encoded transcription factors C-REPEAT BINDING FACTORS (CBFs). The plastid signal triggered by tetrapyrrole accumulation inhibits the activity of cytosolic HEAT SHOCK PROTEIN90 and, as a consequence, the maturation and stability of the clock component ZEITLUPE (ZTL). ZTL negatively regulates the transcription factor LONG HYPOCOTYL5 (HY5) and PSEUDO-RESPONSE REGULATOR5 (PRR5). Thus, low levels of ZTL result in a HY5- and PRR5-mediated repression of CBF3 and PRR5-mediated repression of CBF1 and CBF2 expression. The plastid signal thereby contributes to the rhythm of CBF expression and the downstream COLD RESPONSIVE expression during day/night cycles. These findings provide insight into how plastid signals converge with, and impact upon, the activity of well-defined clock components involved in circadian regulation.

Functional relationship between mTERF4 and GUN1 in retrograde signaling.

Plastid-to-nucleus retrograde signaling plays an important role in regulating the expression of photosynthesis-associated nuclear genes (PhANGs) in accordance with physiological demands on chloroplast biogenesis and function. Despite its fundamental importance, little is known about the molecular nature of the plastid gene expression (PGE)-dependent type of retrograde signaling. PGE is a multifaceted process, and several factors, including pentatricopeptide repeat (PPR) proteins, are involved in its regulation. The PPR protein GUN1 plays a central role in PGE-dependent retrograde signaling. In this study, we isolated a mutant exhibiting up-regulation of CHLOROPHYLL A/B-BINDING PROTEIN (CAB) under normal growth conditions (named coe1 for CAB overexpression 1). The coe1 mutant has a single-base mutation in the gene for mitochondrial transcription termination factor 4 (mTERF4)/BSM/RUG2, which plays a role in regulating the processing of certain plastid transcripts. Defects in GUN1 or mTERF4 de-repressed the expression of specific plastid mRNAs in the presence of lincomycin (LIN). In wild-type plants, treatment with LIN or spectinomycin (SPE) inhibited processing of plastid transcripts. Comparative analysis revealed that in gun1 and coe1/mterf4, but not in wild-type, gun4, or gun5 plants, the processing of plastid transcripts and expression levels of Lhcb1 mRNA were affected in opposite ways when plants were grown in the presence of LIN or SPE. In addition, the coe1 mutation affected the intracellular accumulation and distribution of GUN1, as well as its plastid signaling activity. Taken together, these results suggest that GUN1 and COE1 cooperate in PGE and retrograde signaling.

Chloroplast Biogenesis-Associated Nuclear Genes: Control by Plastid Signals Evolved Prior to Their Regulation as Part of Photomorphogenesis.

The assembly of photosynthetically competent chloroplasts occurs in angiosperm seedlings when first exposed to light, and is due to the control by light of photosynthesis-associated nuclear genes (PhANGs), also dependent upon plastid-to-nucleus “biogenic” communication signals. The relationship between light- and plastid signal-regulation of PhANGs is close but poorly understood. In contrast, many conifers green in the dark and the promoter of a pine PhANG, Lhcb, is active in the dark in tobacco. Here, we show that the activity of this promoter in tobacco is sensitive to plastid photobleaching, or to the inhibition of plastid translation in the light or the dark, and the same interventions reduce expression of the native gene in pine seedlings, demonstrating classic plastid biogenic signaling in gymnosperms. Furthermore, Arabidopsis mutations causing defective plastid biogenesis suppress the effect in darkness of mutations in COP1 and DET1, repressors of photomorphogenesis, for the expression of several PhANGs but not a photosynthesis-unrelated, light-regulated gene. GLK transcriptional regulators mediate the response of LHCB but not of other tested PhANGs. We propose the ability to suppress PhANG response to positive plastid biogenic signals in the dark may have contributed to the evolution of light-controlled chloroplast biogenesis.

Chloroplast signaling within, between and beyond cells.

The most conspicuous function of plastids is the oxygenic photosynthesis of chloroplasts, yet plastids are super-factories that produce a plethora of compounds that are indispensable for proper plant physiology and development. Given their origins as free-living prokaryotes, it is not surprising that plastids possess their own genomes whose expression is essential to plastid function. This semi-autonomous character of plastids requires the existence of sophisticated regulatory mechanisms that provide reliable communication between them and other cellular compartments. Such intracellular signaling is necessary for coordinating whole-cell responses to constantly varying environmental cues and cellular metabolic needs. This is achieved by plastids acting as receivers and transmitters of specific signals that coordinate expression of the nuclear and plastid genomes according to particular needs. In this review we will consider the so-called retrograde signaling occurring between plastids and nuclei, and between plastids and other organelles. Another important role of the plastid we will discuss is the involvement of plastid signaling in biotic and abiotic stress that, in addition to influencing retrograde signaling, has direct effects on several cellular compartments including the cell wall. We will also review recent evidence pointing to an intriguing function of chloroplasts in regulating intercellular symplasmic transport. Finally, we consider an intriguing yet less widely known aspect of plant biology, chloroplast signaling from the perspective of the entire plant. Thus, accumulating evidence highlights that chloroplasts, with their complex signaling pathways, provide a mechanism for exquisite regulation of plant development, metabolism and responses to the environment. As chloroplast processes are targeted for engineering for improved productivity the effect of such modifications on chloroplast signaling will have to be carefully considered in order to avoid unintended consequences on plant growth and development.

Role of plastid signals in modulating Arabidopsis responses to Cucumber mosaic virus

Chloroplasts can serve as sensors for detecting perturbations in the subcellular environment and actively communicate these signals to other organelles. They also play a critical role in plant immunity. Previous studies have indicated that some plastid-signaling deficient mutants have impaired abiotic stresses tolerance. However, whether plastid signals are involved in biotic stress response is rarely reported. In this study, the relationship between plastid signals and Cucumber mosaic virus (CMV) stress response was investigated in Arabidopsis thaliana. Our results showed that plastid-signaling defective mutants were more susceptible to CMV infection and showed more serious stress damages [higher oxidative damages, more compromised in PSII photochemistry and more reactive oxygen species accumulation] compared with the wild-type plants. Furthermore, the induction of defense-associated genes and antioxidant enzymes activity in plastid-signaling defective mutants gun1 and abi4 were lesser than that observed in the wild-type plants after CMV infection. Collectively, our reports provide evidences that plastid signals are necessary for defense responses to CMV in A. thaliana. Moreover, the present study contributes to the understanding of the signaling pathway mediated by chloroplast in response to virus pathogens.

Transcriptional regulation of thylakoid galactolipid biosynthesis coordinated with chlorophyll biosynthesis during the development of chloroplasts in Arabidopsis.

Biogenesis of thylakoid membranes in chloroplasts requires the coordinated synthesis of chlorophyll and photosynthetic proteins with the galactolipids monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG), which constitute the bulk of the thylakoid lipid matrix. MGD1 and DGD1 are the key enzymes of MGDG and DGDG synthesis, respectively. We investigated the expression profiles of MGD1 and DGD1 in Arabidopsis to identify the transcriptional regulation that coordinates galactolipid synthesis with the synthesis of chlorophyll and photosynthetic proteins during chloroplast biogenesis. The expression of both MGD1 and DGD1 was repressed in response to defects in chlorophyll synthesis. Moreover, these genes were downregulated by norflurazon-induced chloroplast malfunction via the GENOMES-UNCOUPLED1-mediated plastid signaling pathway. Similar to other photosynthesis-associated nuclear genes, the expression of MGD1 and DGD1 was induced by light, in which both cytokinin signaling and LONG HYPOCOTYL5-mediated light signaling played crucial roles. The expression of these galactolipid-synthesis genes, and particularly that of DGD1 under continuous light, was strongly affected by the activities of the GOLDEN2-LIKE transcription factors, which are potent regulators of chlorophyll synthesis and chloroplast biogenesis. These results suggest tight transcriptional coordination of galactolipid synthesis with the formation of the photosynthetic chlorophyll–protein complexes during leaf development. Meanwhile, unlike the photosynthetic genes, the galactolipid synthesis genes were not upregulated during chloroplast biogenesis in the roots, even though the galactolipids accumulated with chlorophylls, indicating the importance of post-transcriptional regulation of galactolipid synthesis during root greening. Our data suggest that plants utilize complex regulatory mechanisms to modify galactolipid synthesis with chloroplast development during plant growth.

Influence of plastids on light signalling and development.

In addition to their contribution to metabolism, chloroplasts emit signals that influence the expression of nuclear genes that contribute to numerous plastidic and extraplastidic processes. Plastid-to-nucleus signalling optimizes chloroplast function, regulates growth and development, and affects responses to environmental cues. An incomplete list of plastid signals is available and particular plastid-to-nucleus signalling mechanisms are partially understood. The plastid-to-nucleus signalling that depends on the GENOMES UNCOUPLED (GUN) genes couples the expression of nuclear genes to the functional state of the chloroplast. Analyses of gun mutants provided insight into the mechanisms and biological functions of plastid-to-nucleus signalling. GUN genes contribute to chloroplast biogenesis, the circadian rhythm, stress tolerance, light signalling and development. Some have criticized the gun mutant screen for employing inhibitors of chloroplast biogenesis and suggested that gun alleles do not disrupt significant plastid-to-nucleus signalling mechanisms. Here, I briefly review GUN-dependent plastid-to-nucleus signalling, explain the flaws in the major criticisms of the gun mutant screen and review the influence of plastids on light signalling and development.

Plastid signals induce ALTERNATIVE OXIDASE expression to enhance the cold stress tolerance in Arabidopsis thaliana

The external environment affects the metabolic activities of plants, and plastids play an important role in sensing the environmental situation and executing adaptive responses to environmental stresses. Previous studies have indicated that some plastid-signaling deficient mutants have impaired basal thermo-tolerance, drought tolerance, and high-light tolerance. In this study, we investigated the role of plastid signals in alleviating cold stress damage in Arabidopsis (Arabidopsis thaliana). Thermographic analysis revealed that wide-type (Col-0) plants maintained a higher temperature at about 22 °C, whereas the leaf temperature of plastid-signaling deficient mutants was about 18 °C upon 4 °C cold stress. The relative water content of the mutants significantly decreased after cold treatment, whereas H2O2 content, electrolyte leakage, and malonaldehyde content increased compared with Col-0. The cyanide-resistant respiration rates of the mutants were lower than those of wild-type plants. Quantitative real-time PCR analysis revealed that AOX1a (encoding alternative oxidase, a key enzyme for cyanide-resistant respiration) transcripts in the mutants were much lower than wild-type after cold stress. Moreover, investigations with aox1a mutant further indicated that plastid signals enhance plant cold stress tolerance mainly through the induction of AOX1a.

Functional characterization of the two ferrochelatases in Arabidopsis thaliana

The enzyme ferrochelatase catalyses the formation of protoheme by inserting Fe(2+) into protoporphyrin IX. Although most organisms express only one ferrochelatase, all land plants analysed so far possess at least two ferrochelatase proteins. Analysis of publicly available expression data suggests that the two Arabidopsis thaliana ferrochelatases, FC1 and FC2, serve different functions, corroborating previous assumptions. Co-expression analysis of FC1 and FC2, together with microarray analyses, implies that fc1 and fc2 trigger different modes of plastid signalling in roots and leaves, respectively, and indicates that FC2 might be involved in stress responses. Thus, loss of FC2 increases resistance to salt and flagellin treatment. Whereas fc1 plants showed no obvious mutant phenotype, fc2 mutants formed abnormally small, pale green rosette leaves; were low in chlorophylls, carotenoids and several photosynthetic proteins; and their photosynthetic performance was impaired. These phenotypes are attenuated by growth in continuous light, in agreement with the finding that fc2 plants accumulate protochlorophyllide and display a fluorescent (flu) phenotype in the dark. In consequence we show that, contrary to earlier suggestions, FC2 produces heme not only for photosynthetic cytochromes, but also for proteins involved in stress responses, whereas the impairment of FC1 apparently interferes only marginally with stress responses.

Chemicals to induce plastid signals in Arabidopsis seedlings

Tetrapyrroles and abscisic acid (ABA) were considered as plastid signal molecules for plant cells. However, they attract hot controversies. High levels of sugars also trigger plastid signals to regulate nuclear gene expression. However, the effective concentrations of sugar are extremely high (100–10,000 times of tetrapyrroles). To resolve these controversies and inconsistencies, we investigated which chemicals at what concentrations may work as plastid signal molecules, and summarize their common chemical structures. We found that they can be classified into two categories: (1) tetrapyrroles or other carbon–nitrogen heterocyclic compounds with an adjacent three-membered ring (two of them may be incomplete), with a signaling impact order: Mg–protoporphyrin IX/ABA>Fe–protoporphyrin IX>Zn–protoporphyrin IX>protoporphyrin IX>tryptophan>histidine. The completeness of the rings with a chelated metal ion has a positive effect on the effectiveness of plastid signaling stimulation. (2) Sugar or other carbon–oxygen (sulfur) five- or six-membered ring compounds, such as cyclohexane and thiophene. The effective concentrations of the second category of signal molecules are very high (about 400 mM for the monosaccharide). The two types of signal molecules are possibly perceived by different receptors in the plastid.

The roles of two transcription factors, ABI4 and CBFA, in ABA and plastid signalling and stress responses

Genetic and physiological studies have revealed evidences for multiple signaling pathways by which the plastid exerts retrograde control over photosynthesis-associated-nuclear-genes. In this study we have examined the mechanisms of control of transcription by plastid signals, focusing on transcription factors. We have also further addressed the physical nature of plastid signals and the physiological role, in stress acclimation of this regulatory pathway. ABI4, a master Apetala 2 (AP2)-type transcription factor (TF), is targeted by multiple signalling pathways in plant cells, such as abscisic acid (ABA) signals, sugar signals and plastid signals derived from reactive oxygen species (ROS) and chlorophyll intermediates. ABI4 binds the promoter of target genes to prevent their transcription by competing with other competitive TFs. However, we found that once ABI4 bound the element (CCACGT), it may not be bound by other TFs, therefore making the signalling long-lasting. Downstream of ABI4, CBFA (CCAAT binding factor A) is a subunit of the HAP2/HAP3/HAP5 (Heme activator protein) trimeric transcription complex. CBFA however is a redundant HAP3 subunit. When emergency occurs (such as herbicide treatments or environmental stresses followed by ABA and ROS accumulation), the master transcription factor ABI4 down-regulates some TFs, like CBFA, and then some other TF subunits enter the transcription complex and transcriptional efficiency of stress-responsive genes (including the transcription co-factor CBP) is improved instantaneously. abi4, cbfA and cbp mutants showed weaker drought-tolerance after a herbicide norflurazon treatment, which indicated the physiological role of these key transcription factors.

Loss of plastoglobule kinases ABC1K1 and ABC1K3 causes conditional degreening, modified prenyl-lipids, and recruitment of the jasmonic acid pathway.

Plastoglobules (PGs) are plastid lipid-protein particles. This study examines the function of PG-localized kinases ABC1K1 and ABC1K3 in Arabidopsis thaliana. Several lines of evidence suggested that ABC1K1 and ABC1K3 form a protein complex. Null mutants for both genes (abc1k1 and abc1k3) and the double mutant (k1 k3) displayed rapid chlorosis upon high light stress. Also, k1 k3 showed a slower, but irreversible, senescence-like phenotype during moderate light stress that was phenocopied by drought and nitrogen limitation, but not cold stress. This senescence-like phenotype involved degradation of the photosystem II core and upregulation of chlorophyll degradation. The senescence-like phenotype was independent of the EXECUTER pathway that mediates genetically controlled cell death from the chloroplast and correlated with increased levels of the singlet oxygen-derived carotenoid β-cyclocitral, a retrograde plastid signal. Total PG volume increased during light stress in wild type and k1 k3 plants, but with different size distributions. Isolated PGs from k1 k3 showed a modified prenyl-lipid composition, suggesting reduced activity of PG-localized tocopherol cyclase (VTE1), and was consistent with loss of carotenoid cleavage dioxygenase 4. Plastid jasmonate biosynthesis enzymes were recruited to the k1 k3 PGs but not wild-type PGs, while pheophytinase, which is involved in chlorophyll degradation, was induced in k1 k3 and not wild-type plants and was localized to PGs. Thus, the ABC1K1/3 complex contributes to PG function in prenyl-lipid metabolism, stress response, and thylakoid remodeling.