Retrograde signaling in plants: A critical review focusing on the GUN pathway and beyond

Recent advance on the physiological functions of proteases in chloroplast

Chloroplast proteases play an essential role in orchestrating chloroplast biogenesis and maintaining the homeostasis of diverse metabolic pathways within these organelles, including photosynthesis, amino acid biosynthesis, and stress response regulation. Recent advances in chloroplast proteostasis research have systematically elucidated the physiological functions of key protease families (e.g., FtsH, Deg, and CLP complexes) within chloroplast. This review systematically integrates cutting-edge advances in the physiological functions of chloroplast proteolytic systems, including protein maturation, protein quantity control, protein quality control, and amino acid recovery, and provide a fresh perspective to understand proteases in chloroplasts. According to the latest research progress, the key remaining problems and future research directions in this field are highlighted.

Physiological and Molecular Mechanisms of Light-Induced Greening in Potatoes: A Path to Food Safety

The potato (Solanum tuberosum L.) ranks among the most consumed agricultural products globally and is nutrient-rich. Exposure of potato tubers' epidermal and subcutaneous tissues to light results in greening and the production of neurotoxic steroidal glycoalkaloids, which significantly reduces tuber quality, increases food safety risks, and leads to rejection by consumers and the processing industry. This review conducts an in-depth analysis of three key aspects: (1) the differences in various light conditions, namely, the disparities in light sources, wavelengths, intensities, and durations; (2) the characteristics of potato cultivars resistant to greening; and (3) the molecular mechanisms of light-induced biosynthesis of chlorophyll and steroidal glycoalkaloids (SGAs). This review is expected to provide technical support for potato food safety measures and a theoretical foundation for the molecular breeding of green-resistant potato varieties.

A Holistic Investigation of Arabidopsis Proteomes Altered in Chloroplast Biogenesis and Retrograde Signalling Identifies PsbO as a Key Regulator of Chloroplast Quality Control

Communication between the diverse compartments of plant cells relies on an intricate network of molecular interactions that orchestrate organellar development and adaptation to environmental conditions. Plastid-to-nucleus signalling pathways play a key role in relaying information from developing, mature, and damaged or disintegrating chloroplasts to the nucleus, which serves to coordinate gene expression between the two genomes. To shed light on these mechanisms, we performed a comprehensive analysis of the response of the Arabidopsis thaliana proteomes to perturbation of chloroplast biogenesis by the antibiotic lincomycin (Lin) in the absence of GENOMES UNCOUPLED 1 (GUN1), a key player in plastid-to-nucleus signalling. The topological analysis of protein-protein interactions (PPIs) and co-expression networks enabled the identification of protein hubs in each genotype and condition tested, and highlighted whole-cell adaptive responses to the disruption of chloroplast biogenesis. Our findings reveal a novel role for PsbO, a subunit of the oxygen-evolving complex (OEC), which behaves as an atypical photosynthetic protein upon inhibition of plastid protein synthesis. Notably, and unlike all other subunits of the thylakoid electron transport chain, PsbO accumulates in non-photosynthetic plastids, and is crucial for the breakdown of damaged chloroplasts.

Plastidial metabolites and retrograde signaling: A case study of MEP pathway intermediate MEcPP that orchestrates plant growth and stress responses

Plants are frequently exposed to environmental stresses. In a plant cell, chloroplast acts as machinery that rapidly senses changing environmental conditions and coordinates with the nucleus and other subcellular organelles by exchanging plastidial metabolites, proteins/peptides, or lipid derivatives, some of which may act as retrograde signals. These specific plastidial metabolites include carotenoid derivatives, isoprenes, phosphoadenosines, tetrapyrroles, phytohormone (like salicylic acid), and reactive electrophile species (RES), which mediate retrograde communications to sustain stress conditions. The methylerythritol phosphate (MEP) pathway is an essential and evolutionarily conserved isoprenoid biosynthetic pathway operating in bacteria and plastids, synthesizing metabolites such as terpenoids, gibberellins, abscisic acid, phytol chain of chlorophyll, carotenoids, tocopherols, and glycosides. The MEP pathway is susceptible to oxidative stress, which results in the overaccumulation of its intermediates, such as methylerythritol cyclodiphosphate (MEcPP). Recent studies revealed that under stress conditions, leading to its accumulation, MEcPP mediates retrograde signaling that alters the nuclear gene expression, leading to growth inhibition and acclimation. This review covers aspects of its generation, signaling, mechanism of action, and interplay with other factors to acquire adaptive responses during stress conditions. The review highlights the importance of plastids as sensors of stress and plastidial metabolites as retrograde signals communicating with nucleus and other sub-cellular organelles to regulate plants' response to different stress conditions.

Regulatory and retrograde signaling networks in the chlorophyll biosynthetic pathway

Plants, algae and photosynthetic bacteria convert light into chemical energy by means of photosynthesis, thus providing food and energy for most organisms on Earth. Photosynthetic pigments, including chlorophylls (Chls) and carotenoids, are essential components that absorb the light energy necessary to drive electron transport in photosynthesis. The biosynthesis of Chl shares several steps in common with the biosynthesis of other tetrapyrroles, including siroheme, heme and phycobilins. Given that many tetrapyrrole precursors possess photo-oxidative properties that are deleterious to macromolecules and can lead to cell death, tetrapyrrole biosynthesis (TBS) requires stringent regulation under various developmental and environmental conditions. Thanks to decades of research on model plants and algae, we now have a deeper understanding of the regulatory mechanisms that underlie Chl synthesis, including (i) the many factors that control the activity and stability of TBS enzymes, (ii) the transcriptional and post-translational regulation of the TBS pathway, and (iii) the complex roles of tetrapyrrole-mediated retrograde signaling from chloroplasts to the cytoplasm and the nucleus. Based on these new findings, Chls and their derivatives will find broad applications in synthetic biology and agriculture in the future.

Characterization of a tomato chlh mis-sense mutant reveals a new function of ChlH in fruit ripening

Tomato fruit ripening is a complex developmental process that is important for fruit quality and shelf life. Many factors, including ethylene and several key transcription factors, have been shown to play important roles in the regulation of tomato fruit ripening. However, our understanding of the regulation of tomato fruit ripening is still limited. Here, we describe mut26, an EMS-induced tomato (Solanum lycopersicum) mutant that exhibits chlorophyll-deficient phenotypes in various organs, including fruits. Genetic mapping and functional analyses revealed that a single-nucleotide substitution and a corresponding Pro398->Ser mis-sense mutation in SlChlH (GENOMES UNCOUPLED 5, GUN5), which encodes the H subunit of magnesium chelatase, are responsible for the defects in the mut26 strain. Transcript analyses towards the expression of many SlPhANGs revealed that mut26 is defective in plastid retrograde signalling during tomato fruit ripening initiation, namely the transition from mature green to breaker stage. mut26 exhibits delayed progression of fruit ripening characterized by reduced fruit ethylene emission, increased fruit firmness, reduced carotenoid content and delayed plastid conversion from chloroplast to chromoplast. Given that fruit ripening requires signalling from plastids to nucleus, these data support the hypothesis that GUN5-mediated plastid retrograde signalling promotes tomato fruit ripening. We further showed that the delayed fruit ripening of mut26 is not likely caused by reduced chlorophyll content. Taken together, we identified a new function of SlChlH in the promotion of tomato fruit ripening and ethylene biosynthesis, suggesting that GUN5-mediated plastid retrograde signalling plays a promotive role in tomato fruit ripening.

Proteome and Metabolome Analyses of Albino Bracts in Davidia involucrata

Although the mechanisms underlying albino phenotypes have been examined in model plants and major crops, our knowledge of bract albinism is still in its infancy. Davidia involucrata, a relic plant called dove tree, is best known for the intriguing trait with a pair of white bracts covering the capitula. Here, comparative physiological, cytological, proteomic, and metabolomic analyses were performed to dissect the albinism mechanism of D. involucrata bracts. The bracts exhibited low chlorophyll and carotenoid contents, reduced photosynthetic efficiency, and impaired chloroplast structure. The severe deficiency of photosynthetic pigments and the substantial decrease in cuticle thickness made the bracts light-sensitive. In total, 1134 differentially expressed proteins (DEPs) were obtained between bracts and leaves. Pathway enrichment analysis of DEPs revealed that photosynthetic pigment biosynthesis and photosynthesis were suppressed, whereas protein processing in endoplasmic reticulum, flavonoid biosynthesis, and the ubiquitin-proteasome system (UPS) were activated in bracts. Strikingly, DEPs implicated in chloroplast development, including PPR and AARS proteins, were mainly down-regulated in bracts. We further investigated albinism-induced metabolic changes and detected 412 differentially abundant metabolites (DAMs). Among them, enhanced flavonoids accumulation can plausibly explain the role of bracts in pollinator attraction. Amino acids and their derivatives in bracts showed remarkably increased abundance, which might be causally linked to enhanced UPS function. Our work could lay foundations for understanding albinism mechanisms and adaptive significance of plant bracts and facilitate future utilization of D. involucrata resources.

The interplay of singlet oxygen and ABI4 in plant growth regulation

Abscisic acid (ABA) and the AP2/ERF (APETALA 2/ETHYLENE-RESPONSIVE FACTOR)-type transcription factor ABA INSENSITIVE 4 (ABI4) control plant growth and development. We review how singlet oxygen, which is produced in chloroplasts of the fluorescent mutant of Arabidopsis thaliana (arabidopsis), and ABI4 may cooperate in transcriptional and translational reprogramming to cause plants to halt growth or demise. Key elements of singlet oxygen- and ABI4-dependent chloroplast-to-nucleus retrograde signaling involve the chloroplast EXECUTER (EX) 1 and EX2 proteins as well as nuclear WRKY transcription factors. Mutants designed to study singlet oxygen signaling, that lack either ABI4 or the EX1 and EX2 proteins, do not show most of the growth effects of singlet oxygen. We propose a model that positions ABI4 downstream of WRKY transcription factors and EX1 and EX2.

Stress on the Endoplasmic Reticulum Impairs the Photosynthetic Efficiency of Chlamydomonas

Stress on the Endoplasmic reticulum (ER) can severely disrupt cellular function by impairing protein folding and post-translational modifications, thereby leading to the accumulation of poor-quality proteins. However, research on its impact on photosynthesis remains limited. In this study, we investigated the impact of ER stress on the photosynthetic efficiency of Chlamydomonas reinhardtii using pharmacological inducers, tunicamycin (TM) and brefeldin A (BFA), which specifically target the ER. Our measurements of photosynthetic parameters showed that these ER stress-inducing compounds caused a significant decline in photosynthetic efficiency. A proteomic analysis confirmed that TM and BFA effectively induce ER stress, as evidenced by the upregulation of ER stress-related proteins. Furthermore, we observed a widespread downregulation of photosynthesis-related proteins, which is consistent with the results obtained from our measurements of photosynthetic parameters. These findings suggest that the stress on ER has a profound impact on chloroplast function, disrupting photosynthetic processes. This study highlights the critical interdependence between the ER and chloroplasts, and it underscores the broader implications of ER stress on the cellular metabolism and energy efficiency of photosynthetic organisms.

GENOMES UNCOUPLED PROTEIN1 binds to plastid RNAs and promotes their maturation

Plastid biogenesis and the coordination of plastid and nuclear genome expression through anterograde and retrograde signaling are essential for plant development. GENOMES UNCOUPLED1 (GUN1) plays a central role in retrograde signaling during early plant development. The putative function of GUN1 has been extensively studied, but its molecular function remains controversial. Here, we evaluate published transcriptome data and generate our own data from gun1 mutants grown under signaling-relevant conditions to show that editing and splicing are not relevant for GUN1-dependent retrograde signaling. Our study of the plastid (post)transcriptome of gun1 seedlings with white and pale cotyledons demonstrates that GUN1 deficiency significantly alters the entire plastid transcriptome. By combining this result with a pentatricopeptide repeat code-based prediction and experimental validation by RNA immunoprecipitation experiments, we identified several putative targets of GUN1, including tRNAs and RNAs derived from ycf1.2, rpoC1, and rpoC2 and the ndhH-ndhA-ndhI-ndhG-ndhE-psaC-ndhD gene cluster. The absence of plastid rRNAs and the significant reduction of almost all plastid transcripts in white gun1 mutants account for the cotyledon phenotype. Our study provides evidence for RNA binding and maturation as the long-sought molecular function of GUN1 and resolves long-standing controversies. We anticipate that our findings will serve as a basis for subsequent studies on mechanisms of plastid gene expression and will help to elucidate the function of GUN1 in retrograde signaling.

Function of plastid translation in plant temperature acclimation: Retrograde signalling or extraribosomal 'moonlighting' functions?

Specific components of the plastid ribosome could act as pivotal limiting factors in plant temperature acclimation. We endeavour to elucidate the molecular nexus between plastid translation and temperature acclimation by incorporating the concept of extraribosomal ‘moonlighting’ functions of plastid ribosome proteins.

The chloroplast protease system degrades stromal DUF760-1 and DUF760-2 domain-containing proteins at different rates

The chloroplast chaperone CLPC1 aids to select, unfold, and deliver hundreds of proteins to the CLP protease for degradation. Through in vivo CLPC1, trapping we previously identified dozens of proteins that are (potential) substrate adaptors or substrates for the CLP chaperone-protease system. In this study, we show that two of these highly trapped proteins, DUF760-1 and DUF760-2, are substrates for the CLP protease in Arabidopsis (Arabidopsis thaliana). Loss-of-function mutants and transgenic plants were created for phenotyping, protein expression, and localization using immunoblotting and confocal microscopy. In planta BiFC, cycloheximide chase assays, and yeast 2-hybrid analyses were conducted to determine protein interactions and protein half-life. Both DUF760 proteins directly interacted with the N-domain of CLPC1 and both were highly enriched in clpc1-1 and clpr2-1 mutants. Accordingly, in vivo cycloheximide chase assays demonstrated that both DUF760 proteins are degraded by the CLP protease. The half-life of DUF760-1 was 4 to 6 h, whereas DUF760-2 was highly unstable and difficult to detect unless CLP proteolysis was inhibited. Null mutants for DUF760-1 and DUF760-2 showed weak but differential pigment phenotypes and differential sensitivity to protein translation inhibitors. This study demonstrates that DUF760-1 and DUF760-2 are substrates of the CLP chaperone-protease system and excellent candidates for the determination of CLP substrate degrons.

Plastid retrograde signaling: A developmental perspective

Chloroplast activities influence nuclear gene expression, a phenomenon referred to as retrograde signaling. Biogenic retrograde signals have been revealed by changes in nuclear gene expression when chloroplast development is disrupted. Research on biogenic signaling has focused on repression of Photosynthesis-Associated Nuclear Genes (PhANGs), but this is just one component of a syndrome involving altered expression of thousands of genes involved in diverse processes, many of which are upregulated. We discuss evidence for a framework that accounts for most of this syndrome. Disruption of chloroplast biogenesis prevents the production of signals required to progress through discrete steps in the program of photosynthetic differentiation, causing retention of juvenile states. As a result, expression of PhANGs and other genes that act late during photosynthetic differentiation is not initiated, while expression of genes that act early is retained. The extent of juvenility, and thus the transcriptome, reflects the disrupted process: lack of plastid translation blocks development very early, whereas disruption of photosynthesis without compromising plastid translation blocks development at a later stage. We discuss implications of these and other recent observations for the nature of the plastid-derived signals that regulate photosynthetic differentiation and the role of GUN1, an enigmatic protein involved in biogenic signaling.

Overexpression of the transcription factor ANAC017 results in a genomes uncoupled phenotype under lincomycin

Over-expression (OE) lines for the ER-tethered NAC transcription factor ANAC017 displayed de-repression of gun marker genes when grown on lincomycin (lin). RNA-seq revealed that ANAC017OE2 plants constitutively expressed greater than 40% of the genes induced in wild-type with lin treatment, including plastid encoded genes ycf1.2 and the gene cluster ndhH-ndhA-ndhI-ndhG-ndhE-psaC-ndhD, documented as direct RNA targets of GUN1. Genes encoding components involved in organelle translation were enriched in constitutively expressed genes in ANAC017OE2. ANAC017OE resulted in constitutive location in the nucleus and significant constitutive binding of ANAC017 was detected by ChIP-Seq to target genes. ANAC017OE2 lines maintained the ability to green on lin, were more ABA sensitive, did not show photo-oxidative damage after exposure of de-etiolated seedlings to continuous light and the transcriptome response to lin were as much as 80% unique compared to gun1-1. Both double mutants, gun1-1:ANAC017OE and bzip60:ANAC017OE (but not single bzip60), have a gun molecular gene expression pattern and result in variegated and green plants, suggesting that ANAC017OE may act through an independent pathway compared to gun1. Over-expression of ANAC013 or rcd1 did not produce a GUN phenotype or green plants on lin. Thus, constitutive ANAC017OE2 establishes an alternative transcriptional program that likely acts through a number of pathways, that is, maintains plastid gene expression, and induction of a variety of transcription factors involved in reactive oxygen species metabolism, priming plants for lin tolerance to give a gun phenotype.

A genetically encoded fluorescent heme sensor detects free heme in plants

Heme is produced in plants via a plastid-localized metabolic pathway and is subsequently distributed to all cellular compartments. In addition to covalently and noncovalently bound heme, a comparatively small amount of free heme that is not associated with protein is available for incorporation into heme-dependent proteins in all subcellular compartments and for regulatory purposes. This "labile" fraction may also be toxic. To date, the distribution of the free heme pool in plant cells remains poorly understood. Several fluorescence-based methods for the quantification of intracellular free heme have been described. For this study, we used the previously described genetically encoded heme sensor 1 (HS1) to measure the relative amounts of heme in different plant subcellular compartments. In a proof of concept, we manipulated heme content using a range of biochemical and genetic approaches and verified the utility of HS1 in different cellular compartments of Arabidopsis (Arabidopsis thaliana) and tobacco (Nicotiana tabacum and Nicotiana benthamiana) plants transformed either transiently or stably with HS1 and HS1(M7A), a variant with lower affinity for heme. This approach makes it possible to trace the distribution and dynamics of free heme and provides relevant information about its mobilization. The application of these heme sensors will create opportunities to explore and validate the importance of free heme in plant cells and to identify mutants that alter the subcellular allocation of free heme.

Intra-chloroplast proteases: A holistic network view of chloroplast proteolysis

Different proteases and peptidases are present within chloroplasts and nonphotosynthetic plastids to process precursor proteins and to degrade cleaved chloroplast transit peptides and damaged, misfolded, or otherwise unwanted proteins. Collectively, these proteases and peptidases form a proteolysis network, with complementary activities and hierarchies, and build-in redundancies. Furthermore, this network is distributed across the different intra-chloroplast compartments (lumen, thylakoid, stroma, envelope). The challenge is to determine the contributions of each peptidase (system) to this network in chloroplasts and nonphotosynthetic plastids. This will require an understanding of substrate recognition mechanisms, degrons, substrate, and product size limitations, as well as the capacity and degradation kinetics of each protease. Multiple extra-plastidial degradation pathways complement these intra-chloroplast proteases. This review summarizes our current understanding of these intra-chloroplast proteases in Arabidopsis and crop plants with an emphasis on considerations for building a qualitative and quantitative network view.

Plastid HSP90C C-terminal extension region plays a regulatory role in chaperone activity and client binding

HSP90Cs are essential molecular chaperones localized in the plastid stroma that maintain protein homeostasis and assist the import and thylakoid transport of chloroplast proteins. While HSP90C contains all conserved domains as an HSP90 family protein, it also possesses a unique feature in its variable C-terminal extension (CTE) region. This study elucidated the specific function of this HSP90C CTE region. Our phylogenetic analyses revealed that this intrinsically disordered region contains a highly conserved DPW motif in the green lineages. With biochemical assays, we showed that the CTE is required for the chaperone to effectively interact with client proteins PsbO1 and LHCB2 to regulate ATP-independent chaperone activity and to effectuate its ATP hydrolysis. The CTE truncation mutants could support plant growth and development reminiscing the wild type under normal conditions except for a minor phenotype in cotyledon when expressed at a level comparable to wild type. However, higher HSP90C expression was observed to correlate with a stronger response to specific photosystem II inhibitor DCMU, and CTE truncations dampened the response. Additionally, when treated with lincomycin to inhibit chloroplast protein translation, CTE truncation mutants showed a delayed response to PsbO1 expression repression, suggesting its role in chloroplast retrograde signaling. Our study therefore provides insights into the mechanism of HSP90C in client protein binding and the regulation of green chloroplast maturation and function, especially under stress conditions.

Tuber quality enhancement via grafting potato onto a wooden goji rootstock through vitalizing multi-pathways

Grafting is applied in Solanaceae to improve growth and quality traits. However, grafting potato onto a wooden goji rootstock is rare. Our study introduces a novel distant grafting technique to investigate potato scion responses, specifically regarding photosynthetic and tuber nutritional quality. The physiological and transcriptomic findings reveal an increase in photosynthesis ratio and carbon fixation in potato leaves after 45 days of grafting due to the upregulation of pivotal genes (PsbA, PPC1, rbcl, and GAPDH). After 95 days of long-term growth, the leaf redox balance was maintained with intensified chlorophyll synthesis, facilitated by the enrichment of crucial genes (GUN4, CHLH, CHLP, CAO) and several light-harvesting proteins (Lhca and Lhcb) in potato leaves. The tubers of grafted plants showed a 6.5% increase in crude protein, 51% in anthocyanin, and lower carbohydrate content. Goji altered the expression of tubers genes involved in assimilatory sulfate reduction, which subsequently affects cysteine-methionine biosynthesis. Furthermore, the tuber transcriptome shows ABA signaling and transcription factors regulate the expression of key biosynthetic genes involved in inducing the secondary metabolites, such as scopoletin and anthocyanin accumulation, which are primary polyphenols in goji. Our innovative grafting approach offers valuable insights into the interactions between woody and herbaceous plants for developing future strategies to modulate growth efficiency and tuber quality in the face of climate challenges and to meet the demand for nutritious food.

Proteomic profiling of Arabidopsis nuclei reveals distinct protein accumulation kinetics upon heat stress

Heat stress (HS) impacts the nuclear proteome and, subsequently, protein activities in different nuclear compartments. In Arabidopsis thaliana, a short exposure to 37 °C leads to loss of the standard tripartite architecture of the nucleolus, the most prominent nuclear substructure, and, consequently, affects the assembly of ribosomes. Here, we report a quantitative label-free LC‒MS/MS (Liquid Chromatography coupled to tandem Mass Spectrometry) analysis to determine the nuclear proteome of Arabidopsis at 22 °C, HS (37 °C for 4 and 24 h), and a recovery phase. This analysis identified ten distinct groups of proteins based on relative abundance changes in the nucleus before, during and after HS: Early, Late, Transient, Early Persistent, Late Persistent, Recovery, Early-Like, Late-Like, Transient-Like and Continuous Groups (EG, LG, TG, EPG, LPG, RG, ELG, LLG, TLG and CG, respectively). Interestingly, the RNA polymerase I subunit NRPA3 and other main nucleolar proteins, including NUCLEOLIN 1 and FIBRILLARIN 1 and 2, were detected in RG and CG, suggesting that plants require increased nucleolar activity and likely ribosome assembly to restore protein synthesis after HS.

A prion-like domain is required for phase separation and chloroplast RNA processing during cold acclimation in Arabidopsis

Arabidopsis (Arabidopsis thaliana) plants can produce photosynthetic tissue with active chloroplasts at temperatures as low as 4°C, and this process depends on the presence of the nuclear-encoded, chloroplast-localized RNA-binding protein CP29A. In this study, we demonstrate that CP29A undergoes phase separation in vitro and in vivo in a temperature-dependent manner, which is mediated by a prion-like domain (PLD) located between the two RNA recognition motif domains of CP29A. The resulting droplets display liquid-like properties and are found near chloroplast nucleoids. The PLD is required to support chloroplast RNA splicing and translation in cold-treated tissue. Together, our findings suggest that plant chloroplast gene expression is compartmentalized by inducible condensation of CP29A at low temperatures, a mechanism that could play a crucial role in plant cold resistance.

The uS10c-BPG2 module mediates ribosomal RNA processing in chloroplast nucleoids

In plant chloroplasts, certain ribosomal proteins (RPs) and ribosome biogenesis factors (RBFs) are present in nucleoids, implying an association between nucleoids and ribosome biogenesis. In Arabidopsis, the YqeH-type GTPase Brassinazole-Insensitive Pale Green2 (BPG2) is a chloroplast nucleoid-associated RBF. Here, we investigated the relationship between nucleoids and BPG2-involved ribosome biogenesis steps by exploring how BPG2 targets ribosomes. Our findings demonstrate that BPG2 interacts with an essential plastid RP, uS10c, in chloroplast nucleoids in a ribosomal RNA (rRNA)-independent manner. We also discovered that uS10c is a haploinsufficient gene, as the heterozygous deletion of this gene leads to variegated shoots and chlorophyll aggregation. uS10c is integrated into 30S ribosomal particles when rRNA is relatively exposed and also exists in polysome fractions. In contrast, BPG2 exclusively associates with 30S ribosomal particles. Notably, the interaction between BPG2 and 30S particles is influenced by the absence of uS10c, resulting in BPG2 diffusing in chloroplasts instead of targeting nucleoids. Further, our results reveal that the loss of BPG2 function and the heterozygous deletion of uS10c impair the processing of 16S and 23S-4.5S rRNAs, reduce plastid protein accumulation, and trigger the plastid signaling response. Together, these findings indicate that the uS10c-BPG2 module mediates ribosome biogenesis in chloroplast nucleoids.

Spliceosomal complex components are critical for adjusting the C:N balance during high-light acclimation

Plant acclimation to an ever-changing environment is decisive for growth, reproduction, and survival. Light availability limits biomass production on both ends of the intensity spectrum. Therefore, the adjustment of plant metabolism is central to high-light (HL) acclimation, and the accumulation of photoprotective anthocyanins is commonly observed. However, mechanisms and factors regulating the HL acclimation response are less clear. Two Arabidopsis mutants of spliceosome components exhibiting a pronounced anthocyanin overaccumulation in HL were isolated from a forward genetic screen for new factors crucial for plant acclimation. Time-resolved physiological, transcriptome, and metabolome analysis revealed a vital function of the spliceosome components for rapidly adjusting gene expression and metabolism. Deficiency of INCREASED LEVEL OF POLYPLOIDY1 (ILP1), NTC-RELATED PROTEIN1 (NTR1), and PLEIOTROPIC REGULATORY LOCUS1 (PRL1) resulted in a marked overaccumulation of carbohydrates and strongly diminished amino acid biosynthesis in HL. While not generally limited in N-assimilation, ilp1, ntr1, and prl1 showed higher glutamate levels and reduced amino acid biosynthesis in HL. The comprehensive analysis reveals a function of the spliceosome components in the conditional regulation of the carbon:nitrogen balance and the accumulation of anthocyanins during HL acclimation. The importance of gene expression, metabolic regulation, and re-direction of carbon towards anthocyanin biosynthesis for HL acclimation are discussed.

Reviving resilience: MEcPP-mediated ASK1-IMPα-9-TRP2 stress-responsive module

Chloroplasts are not only critical photosynthesis sites in plants, but they also participate in plastidial retrograde signaling in response to developmental and environmental signals. MEcPP (2-C-Methyl-D-erythritol-2,4-cyclopyrophosphate) is an intermediary in the methylerythritol phosphate (MEP) pathway in chloroplasts. It is a critical precursor for the synthesis of isoprenoids and terpenoid derivatives, which play crucial roles in plant growth and development, photosynthesis, reproduction, and defense against environmental constraints. Accumulation of MEcPP under stressful conditions triggers the expression of IMPα-9 and TPR2, contributing to the activation of abiotic stress-responsive genes. In this correspondence, we discuss plastidial retrograde signaling in support of a recently published paper in Molecular Plant (Zeng et al. 2024). We hope that it can shed more insight on the retrograde signaling cascade.

GhCTSF1, a short PPR protein with a conserved role in chloroplast development and photosynthesis, participates in intron splicing of rpoC1 and ycf3-2 transcripts in cotton

Cotton is one of the most important textile fibers worldwide. As crucial agronomic traits, leaves play an essential role in the growth, disease resistance, fiber quality, and yield of cotton plants. Pentatricopeptide repeat (PPR) proteins are a large family of nuclear-encoded proteins involved in organellar or nuclear RNA metabolism. Using a virus-induced gene silencing assay, we found that cotton plants displayed variegated yellow leaf phenotypes with decreased chlorophyll content when expression of the PPR gene GhCTSF1 was silenced. GhCTSF1 encodes a chloroplast-localized protein that contains only two PPR motifs. Disruption of GhCTSF1 substantially reduces the splicing efficiency of rpoC1 intron 1 and ycf3 intron 2. Loss of function of the GhCTSF1 ortholog EMB1417 causes splicing defects in rpoC1 and ycf3-2, leading to impaired chloroplast structure and decreased photosynthetic rates in Arabidopsis. We also found that GhCTSF1 interacts with two splicing factors, GhCRS2 and GhWTF1. Defects in GhCRS2 and GhWTF1 severely affect intron splicing of rpoC1 and ycf3-2 in cotton, leading to defects in chloroplast development and a reduction in photosynthesis. Our results suggest that GhCTSF1 is specifically required for splicing rpoC1 and ycf3-2 in cooperation with GhCRS2 and GhWTF1.

Arabidopsis BBX14 is involved in high light acclimation and seedling development

The development of photosynthetically competent seedlings requires both light and retrograde biogenic signaling pathways. The transcription factor GLK1 functions at the interface between these pathways and receives input from the biogenic signal integrator GUN1. BBX14 was previously identified, together with GLK1, in a core module that mediates the response to high light (HL) levels and biogenic signals, which was studied by using inhibitors of chloroplast development. Our chromatin immunoprecipitation-Seq experiments revealed that BBX14 is a direct target of GLK1, and RNA-Seq analysis suggests that BBX14 may function as a regulator of the circadian clock. In addition, BBX14 plays a role in chlorophyll biosynthesis during early onset of light. Knockout of BBX14 results in a long hypocotyl phenotype dependent on a retrograde signal. Furthermore, the expression of BBX14 and BBX15 during biogenic signaling requires GUN1. Investigation of the role of BBX14 and BBX15 in GUN-type biogenic (gun) signaling showed that the overexpression of BBX14 or BBX15 caused de-repression of CA1 mRNA levels, when seedlings were grown on norflurazon. Notably, transcripts of the LHCB1.2 marker are not de-repressed. Furthermore, BBX14 is required to acclimate plants to HL stress. We propose that BBX14 is an integrator of biogenic signals and that BBX14 is a nuclear target of retrograde signals downstream of the GUN1/GLK1 module. However, we do not classify BBX14 or BBX15 overexpressors as gun mutants based on a critical evaluation of our results and those reported in the literature. Finally, we discuss a classification system necessary for the declaration of new gun mutants.

Natural genetic variation in GLK1-mediated photosynthetic acclimation in response to light

BACKGROUND

GOLDEN-like (GLK) transcription factors are central regulators of chloroplast biogenesis in Arabidopsis and other species. Findings from Arabidopsis show that these factors also contribute to photosynthetic acclimation, e.g. to variation in light intensity, and are controlled by retrograde signals emanating from the chloroplast. However, the natural variation of GLK1-centered gene-regulatory networks in Arabidopsis is largely unexplored.

RESULTS

By evaluating the activities of GLK1 target genes and GLK1 itself in vegetative leaves of natural Arabidopsis accessions grown under standard conditions, we uncovered variation in the activity of GLK1 centered regulatory networks. This is linked with the ecogeographic origin of the accessions, and can be associated with a complex genetic variation across loci acting in different functional pathways, including photosynthesis, ROS and brassinosteroid pathways. Our results identify candidate upstream regulators that contribute to a basal level of GLK1 activity in rosette leaves, which can then impact the capacity to acclimate to different environmental conditions. Indeed, accessions with higher GLK1 activity, arising from habitats with a high monthly variation in solar radiation levels, may show lower levels of photoinhibition at higher light intensities.

CONCLUSIONS

Our results provide evidence for natural variation in GLK1 regulatory activities in vegetative leaves. This variation is associated with ecogeographic origin and can contribute to acclimation to high light conditions.

Regulation of chloroplast biogenesis, development, and signaling by endogenous and exogenous cues

Chloroplasts are one of the defining features in most plants, primarily known for their unique property to carry out photosynthesis. Besides this, chloroplasts are also associated with hormone and metabolite productions. For this, biogenesis and development of chloroplast are required to be synchronized with the seedling growth to corroborate the maximum rate of photosynthesis following the emergence of seedlings. Chloroplast biogenesis and development are dependent on the signaling to and from the chloroplast, which are in turn regulated by several endogenous and exogenous cues. Light and hormones play a crucial role in chloroplast maturation and development. Chloroplast signaling involves a coordinated two-way connection between the chloroplast and nucleus, termed retrograde and anterograde signaling, respectively. Anterograde and retrograde signaling are involved in regulation at the transcriptional level and downstream modifications and are modulated by several metabolic and external cues. The communication between chloroplast and nucleus is essential for plants to develop strategies to cope with various stresses including high light or high heat. In this review, we have summarized several aspects of chloroplast development and its regulation through the interplay of various external and internal factors. We have also discussed the involvement of chloroplasts as sensors of various external environment stress factors including high light and temperature, and communicate via a series of retrograde signals to the nucleus, thus playing an essential role in plants' abiotic stress response.

Integrating the multiple functions of CHLH into chloroplast-derived signaling fundamental to plant development and adaptation as well as fruit ripening

Chlorophyll (Chl)-mediated oxygenic photosynthesis sustains life on Earth. Greening leaves play fundamental roles in plant growth and crop yield, correlating with the idea that more Chls lead to better adaptation. However, they face significant challenges from various unfavorable environments. Chl biosynthesis hinges on the first committed step, which involves inserting Mg2+ into protoporphyrin. This step is facilitated by the H subunit of magnesium chelatase (CHLH) and features a conserved mechanism from cyanobacteria to plants. For better adaptation to fluctuating land environments, especially drought, CHLH evolves multiple biological functions, including Chl biosynthesis, retrograde signaling, and abscisic acid (ABA) responses. Additionally, it integrates into various chloroplast-derived signaling pathways, encompassing both retrograde signaling and hormonal signaling. The former comprises ROS (reactive oxygen species), heme, GUN (genomes uncoupled), MEcPP (methylerythritol cyclodiphosphate), β-CC (β-cyclocitral), and PAP (3'-phosphoadenosine-5'-phosphate). The latter involves phytohormones like ABA, ethylene, auxin, cytokinin, gibberellin, strigolactone, brassinolide, salicylic acid, and jasmonic acid. Together, these elements create a coordinated regulatory network tailored to plant development and adaptation. An intriguing example is how drought-mediated improvement of fruit quality provides insights into chloroplast-derived signaling, aiding the shift from vegetative to reproductive growth. In this context, we explore the integration of CHLH's multifaceted roles into chloroplast-derived signaling, which lays the foundation for plant development and adaptation, as well as fruit ripening and quality. In the future, manipulating chloroplast-derived signaling may offer a promising avenue to enhance crop yield and quality through the homeostasis, function, and regulation of Chls.

Relocation of chloroplast proteins from cytosols into chloroplasts

The chloroplasts in terrestrial plants play a functional role as a major sensor for perceiving physiological changes under normal and stressful conditions. Despite the fact that the plant chloroplast genome encodes around 120 genes, which are mainly essential for photosynthesis and chloroplast biogenesis, the functional roles of the genes remain to be determined in plant's response to environmental stresses. Photosynthetic electron transfer D (PETD) is a key component of the chloroplast cytochrome b6f complex. Chloroplast ndhA (NADH dehydrogenase A) and ndhB (NADH dehydrogenase B) interact with photosystem I (PSI), forming NDH-PSI supercomplex. Notably, artificial targeting of chloroplasts-encoded proteins, PETD, NDHA, or NDHB, was successfully relocated from cytosols into chloroplasts. The result suggests that artificial targeting of proteins to chloroplasts is potentially open to the possibility of chloroplast biotechnology in engineering of plant tolerance against biotic and abiotic stresses.

PSI Photoinhibition and Changing CO2 Levels Initiate Retrograde Signals to Modify Nuclear Gene Expression

Photosystem I (PSI) is a critical component of the photosynthetic machinery in plants. Under conditions of environmental stress, PSI becomes photoinhibited, leading to a redox imbalance in the chloroplast. PSI photoinhibition is caused by an increase in electron pressure within PSI, which damages the iron-sulfur clusters. In this study, we investigated the susceptibility of PSI to photoinhibition in plants at different concentrations of CO2, followed by global gene expression analyses of the differentially treated plants. PSI photoinhibition was induced using a specific illumination protocol that inhibited PSI with minimal effects on PSII. Unexpectedly, the varying CO2 levels combined with the PSI-PI treatment neither increased nor decreased the likelihood of PSI photodamage. All PSI photoinhibition treatments, independent of CO2 levels, upregulated genes generally involved in plant responses to excess iron and downregulated genes involved in iron deficiency. PSI photoinhibition also induced genes encoding photosynthetic proteins that act as electron acceptors from PSI. We propose that PSI photoinhibition causes a release of iron from damaged iron-sulfur clusters, which initiates a retrograde signal from the chloroplast to the nucleus to modify gene expression. In addition, the deprivation of CO2 from the air initiated a signal that induced flavonoid biosynthesis genes, probably via jasmonate production.

The molecular basis of heat stress responses in plants

Global warming impacts crop production and threatens food security. Elevated temperatures are sensed by different cell components. Temperature increases are classified as either mild warm temperatures or excessively hot temperatures, which are perceived by distinct signaling pathways in plants. Warm temperatures induce thermomorphogenesis, while high-temperature stress triggers heat acclimation and has destructive effects on plant growth and development. In this review, we systematically summarize the heat-responsive genetic networks in Arabidopsis and crop plants based on recent studies. In addition, we highlight the strategies used to improve grain yield under heat stress from a source-sink perspective. We also discuss the remaining issues regarding the characteristics of thermosensors and the urgency required to explore the basis of acclimation under multifactorial stress combination.

Salicylate and jasmonate intertwine in ROS-triggered chloroplast-to-nucleus retrograde signaling

Plants, being sessile, are frequently exposed to environmental perturbations, affecting their sustenance and survival. In response, distinct inherent mechanisms emerged during plant evolution to deal with environmental stresses. Among various organelles, chloroplast plays an indispensable role in plant cells. Besides providing the site for photosynthesis and biosynthesis of many important primary and secondary metabolites, including hormones, chloroplasts also act as environmental sensors. Any environmental perturbation directly influences the photosynthetic electron transport chain, leading to excess accumulation of reactive oxygen species (ROS), causing oxidative damages to biomolecules in the vicinity. To prevent excess ROS accumulation and the consequent oxidative damages, the chloroplast activates retrograde signaling (RS) pathways to reprogramme nuclear gene expression, defining plant's response to stress. Based on levels and site of ROS accumulation, distinct biomolecules are oxidized, generating specific derivatives that act as genuine signaling molecules, triggering specific RS pathways to instigate distinctive responses, including growth inhibition, acclimation, and programmed cell death. Though various RS pathways independently modulate nuclear gene expression, they also implicate the defense hormone salicylic acid (SA) and oxylipins, including 12-oxo-phytodienoic acid (OPDA) and jasmonic acid (JA), by promoting their biosynthesis and utilizing them for intra- and intercellular communications. Several studies reported the involvement of both hormones in individual RS pathways, but the precise dissection of their activation and participation in a given RS pathway remains an enigma. The present review describes the current understanding of how SA and JA intertwine in ROS-triggered RS pathways. We have also emphasized the future perspectives for elucidating stress specificity and spatiotemporal accumulation of respective hormones in a given RS pathway.

Supplementing the Nuclear-Encoded PSII Subunit D1 Induces Dramatic Metabolic Reprogramming in Flag Leaves during Grain Filling in Rice

Our previous study has demonstrated that the nuclear-origin supplementation of the PSII core subunit D1 protein stimulates growth and increases grain yields in transgenic rice plants by enhancing photosynthetic efficiency. In this study, the underlying mechanisms have been explored regarding how the enhanced photosynthetic capacity affects metabolic activities in the transgenic plants of rice harboring the integrated transgene RbcSPTP-OspsbA cDNA, cloned from rice, under control of the AtHsfA2 promoter and N-terminal fused with the plastid-transit peptide sequence (PTP) cloned from the AtRbcS. Here, a comparative metabolomic analysis was performed using LC-MS in flag leaves of the transgenic rice plants during the grain-filling stage. Critically, the dramatic reduction in the quantities of nucleotides and certain free amino acids was detected, suggesting that the increased photosynthetic assimilation and grain yield in the transgenic plants correlates with the reduced contents of free nucleotides and the amino acids such as glutamine and glutamic acid, which are cellular nitrogen sources. These results suggest that enhanced photosynthesis needs consuming more free nucleotides and nitrogen sources to support the increase in biomass and yields, as exhibited in transgenic rice plants. Unexpectedly, dramatic changes were measured in the contents of flavonoids in the flag leaves, suggesting that a tight and coordinated relationship exists between increasing photosynthetic assimilation and flavonoid biosynthesis. Consistent with the enhanced photosynthetic efficiency, the substantial increase was measured in the content of starch, which is the primary product of the Calvin-Benson cycle, in the transgenic rice plants under field growth conditions.

Iron uptake of etioplasts is independent from photosynthesis but applies the reduction-based strategy

Introduction

Iron (Fe) is one of themost important cofactors in the photosynthetic apparatus, and its uptake by chloroplasts has also been associated with the operation of the photosynthetic electron transport chain during reduction-based plastidial Fe uptake. Therefore, plastidial Fe uptake was considered not to be operational in the absence of the photosynthetic activity. Nevertheless, Fe is also required for enzymatic functions unrelated to photosynthesis, highlighting the importance of Fe acquisition by non-photosynthetic plastids. Yet, it remains unclear how these plastids acquire Fe in the absence of photosynthetic function. Furthermore, plastids of etiolated tissues should already possess the ability to acquire Fe, since the biosynthesis of thylakoid membrane complexes requires a massive amount of readily available Fe. Thus, we aimed to investigate whether the reduction-based plastidial Fe uptake solely relies on the functioning photosynthetic apparatus.

Methods

In our combined structure, iron content and transcript amount analysis studies, we used Savoy cabbage plant as a model, which develops natural etiolation in the inner leaves of the heads due to the shading of the outer leaf layers.

Results

Foliar and plastidial Fe content of Savoy cabbage leaves decreased towards the inner leaf layers. The leaves of the innermost leaf layers proved to be etiolated, containing etioplasts that lacked the photosynthetic machinery and thus were photosynthetically inactive. However, we discovered that these etioplasts contained, and were able to take up, Fe. Although the relative transcript abundance of genes associated with plastidial Fe uptake and homeostasis decreased towards the inner leaf layers, both ferric chelate reductase FRO7 transcripts and activity were detected in the innermost leaf layer. Additionally, a significant NADP(H) pool and NAD(P)H dehydrogenase activity was detected in the etioplasts of the innermost leaf layer, indicating the presence of the reducing capacity that likely supports the reduction-based Fe uptake of etioplasts.

Discussion

Based on these findings, the reduction-based plastidial Fe acquisition should not be considered exclusively dependent on the photosynthetic functions.

An Efficient Transformation System for Fast Production of VcCHS Transgenic Blueberry Callus and Its Expressional Analysis

The Agrobacterium tumefaciens-mediated transformation for blueberries remains less efficient than is desirable. A new leaf callus regeneration and genetic transformation system was investigated in blueberries in this study. The leaf explants of cv. 'Legacy' and 'Northland' were used to establish the stable callus induction system when placed on the woody plant medium (WPM) supplemented with 1.0 mg·L-1 2, 4-D, 0.4 mg·L-1 6-BA for 30 d; then, the callus was sub-cultured in the proliferation medium supplemented with 1.5 mg·L-1 2, 4-D, 0.4 mg·L-1 6-BA in the darkness at 25 °C every 30 days. The co-cultivation of callus with A. tumefaciens was operated on WPM plus 100 μM acetosyringone for 4 days; then, the transferred callus was grown in WPM supplemented with 1.5 mg·L-1 2,4-D, 0.4 mg·L-1 6-BA, 50 mg·L-1 hygromycin, and 200 mg·L-1 cefotaxime. The VcCHS transgenic blueberry callus with both GFP signal and Hyg resistance was obtained from the transformed callus of cv. 'Northland'. The rate of GFP signal detected in the transformed callus was as high as 49.02%, which was consistent with the PCR assay. Collectively, this study provides a highly efficient genetic transformation system in blueberry callus and a powerful approach for the molecular breeding of blueberries.

Components and processes involved in retrograde signaling from chloroplast to nucleus

Retrograde signaling conceptually means the transfer of signals from semi-autonomous cell organelles to the nucleus to modulate nuclear gene expression. A generalized explanation is that chloroplasts are highly sensitive to environmental stimuli and quickly generate signaling molecules (retrograde signals) and transport them to the nucleus through the cytosol to reprogram nuclear gene expression for cellular/metabolic adjustments to cope with environmental fluctuations. During the past decade, substantial advancements have been made in the area of retrograde signaling, including information on putative retrograde signals. Researchers have also proposed possible mechanisms for generating retrograde signals and their transmission. However, the exact mechanisms and processes responsible for transmitting retrograde signaling from the chloroplast to the nucleus remain elusive, demanding substantial attention. This review highlights strategies employed to detect retrograde signals, their possible modes of signaling to the nucleus, and their implications for cellular processes during stress conditions. The present review also summarizes the role of ROS-mediated retrograde signaling in plastid-nucleus communication and its functional significance in co-coordinating the physiological profile of plant cells.

Shading-Dependent Greening Process of the Leaves in the Light-Sensitive Albino Tea Plant 'Huangjinya': Possible Involvement of the Light-Harvesting Complex II Subunit of Photosystem II in the Phenotypic Characteristic

The light-sensitive albino tea plant can produce pale-yellow shoots with high levels of amino acids which are suitable to process high-quality tea. In order to understand the mechanism of the albino phenotype formation, the changes in the physio-chemical characteristics, chloroplast ultrastructure, chlorophyll-binding proteins, and the relevant gene expression were comprehensively investigated in the leaves of the light-sensitive albino cultivar 'Huangjinya' ('HJY') during short-term shading treatment. In the content of photosynthetic pigments, the ultrastructure of the chloroplast, and parameters of the photosynthesis in the leaves of 'HJY' could be gradually normalized along with the extension of the shading time, resulting in the leaf color transformed from pale yellow to green. BN-PAGE and SDS-PAGE revealed that function restoration of the photosynthetic apparatus was attributed to the proper formation of the pigment-protein complexes on the thylakoid membrane that benefited from the increased levels of the LHCII subunits in the shaded leaves of 'HJY', indicating the low level of LHCII subunits, especially the lack of the Lhcb1 might be responsible for the albino phenotype of the 'HJY' under natural light condition. The deficiency of the Lhcb1 was mainly subject to the strongly suppressed expression of the Lhcb1.x which might be modulated by the chloroplast retrograde signaling pathway GUN1 (GENOMES UNCOUPLED 1)-PTM (PHD type transcription factor with transmembrane domains)-ABI4 (ABSCISIC ACID INSENSITIVE 4).

The Sapria himalayana genome provides new insights into the lifestyle of endoparasitic plants

BACKGROUND

Sapria himalayana (Rafflesiaceae) is an endoparasitic plant characterized by a greatly reduced vegetative body and giant flowers; however, the mechanisms underlying its special lifestyle and greatly altered plant form remain unknown. To illustrate the evolution and adaptation of S. himalayasna, we report its de novo assembled genome and key insights into the molecular basis of its floral development, flowering time, fatty acid biosynthesis, and defense responses.

RESULTS

The genome of S. himalayana is ~ 1.92 Gb with 13,670 protein-coding genes, indicating remarkable gene loss (~ 54%), especially genes involved in photosynthesis, plant body, nutrients, and defense response. Genes specifying floral organ identity and controlling organ size were identified in S. himalayana and Rafflesia cantleyi, and showed analogous spatiotemporal expression patterns in both plant species. Although the plastid genome had been lost, plastids likely biosynthesize essential fatty acids and amino acids (aromatic amino acids and lysine). A set of credible and functional horizontal gene transfer (HGT) events (involving genes and mRNAs) were identified in the nuclear and mitochondrial genomes of S. himalayana, most of which were under purifying selection. Convergent HGTs in Cuscuta, Orobanchaceae, and S. himalayana were mainly expressed at the parasite-host interface. Together, these results suggest that HGTs act as a bridge between the parasite and host, assisting the parasite in acquiring nutrients from the host.

CONCLUSIONS

Our results provide new insights into the flower development process and endoparasitic lifestyle of Rafflesiaceae plants. The amount of gene loss in S. himalayana is consistent with the degree of reduction in its body plan. HGT events are common among endoparasites and play an important role in their lifestyle adaptation.

NPR1 Translocation from Chloroplast to Nucleus Activates Plant Tolerance to Salt Stress

Chloroplasts play crucial roles in biotic and abiotic stress responses, regulated by nuclear gene expression through changes in the cellular redox state. Despite lacking the N-terminal chloroplast transit peptide (cTP), nonexpressor of pathogenesis-related genes 1 (NPR1), a redox-sensitive transcriptional coactivator was consistently found in the tobacco chloroplasts. Under salt stress and after exogenous application of H₂O₂ or aminocyclopropane-1-carboxylic acid, an ethylene precursor, transgenic tobacco plants expressing green fluorescent protein (GFP)-tagged NPR1 (NPR1-GFP) showed significant accumulation of monomeric nuclear NPR1, irrespective of the presence of cTP. Immunoblotting and fluorescence image analyses indicated that NPR1-GFP, with and without cTP, had similar molecular weights, suggesting that the chloroplast-targeted NPR1-GFP is likely translocated from the chloroplasts to the nucleus after processing in the stroma. Translation in the chloroplast is essential for nuclear NPR1 accumulation and stress-related expression of nuclear genes. An overexpression of chloroplast-targeted NPR1 enhanced stress tolerance and photosynthetic capacity. In addition, compared to the wild-type lines, several genes encoding retrograde signaling-related proteins were severely impaired in the Arabidopsis npr1-1 mutant, but were enhanced in NPR1 overexpression (NPR1-Ox) transgenic tobacco line. Taken together, chloroplast NPR1 acts as a retrograding signal that enhances the adaptability of plants to adverse environments.