Singlet Oxygen-Induced Cell Death in Arabidopsis under High-Light Stress Is Controlled by OXI1 Kinase

Investigating the mechanism of chloroplast singlet oxygen signaling in the Arabidopsis thaliana accelerated cell death 2 mutant

As sessile organisms, plants have evolved complex signaling mechanisms to sense stress and acclimate. This includes the use of reactive oxygen species (ROS) generated during dysfunctional photosynthesis to initiate signaling. One such ROS, singlet oxygen (1O2), can trigger retrograde signaling, chloroplast degradation, and programmed cell death. However, the signaling mechanisms are largely unknown. Several proteins (e.g. PUB4, OXI1, EX1) are proposed to play signaling roles across three Arabidopsis thaliana mutants that conditionally accumulate chloroplast 1O2 (fluorescent in blue light (flu), chlorina 1 (ch1), and plastid ferrochelatase 2 (fc2)). We previously demonstrated that these mutants reveal at least two chloroplast 1O2 signaling pathways (represented by flu and fc2/ch1). Here, we test if the 1O2-accumulating lesion mimic mutant, accelerated cell death 2 (acd2), also utilizes these pathways. The pub4-6 allele delayed lesion formation in acd2 and restored photosynthetic efficiency and biomass. Conversely, an oxi1 mutation had no measurable effect on these phenotypes. acd2 mutants were not sensitive to excess light (EL) stress, yet pub4-6 and oxi1 both conferred EL tolerance within the acd2 background, suggesting that EL-induced 1O2 signaling pathways are independent from spontaneous lesion formation. Thus, 1O2 signaling in acd2 may represent a third (partially overlapping) pathway to control cellular degradation.

What can reactive oxygen species (ROS) tell us about the action mechanism of herbicides and other phytotoxins?

Reactive oxygen species (ROS) are formed in plant cells continuously. When ROS production exceeds the antioxidant capacity of the cells, oxidative stress develops which causes damage of cell components and may even lead to the induction of programmed cell death (PCD). The levels of ROS production increase upon abiotic stress, but also during pathogen attack in response to elicitors, and upon application of toxic compounds such as synthetic herbicides or natural phytotoxins. The commercial value of many synthetic herbicides is based on weed death as result of oxidative stress, and for a number of them, the site and the mechanism of ROS production have been characterized. This review summarizes the current knowledge on ROS production in plants subjected to different groups of synthetic herbicides and natural phytotoxins. We suggest that the use of ROS-specific fluorescent probes and of ROS-specific marker genes can provide important information on the mechanism of action of these toxins. Furthermore, we propose that, apart from oxidative damage, elicitation of ROS-induced PCD is emerging as one of the important processes underlying the action of herbicides and phytotoxins.

On-Site Quantitative Visualization of Singlet Oxygen in Crops via an Organic Small Molecule-Based Ratiometric Fluorescent Probe and a Mobile Fluorescence Analysis Device

Singlet oxygen (1O2) plays imperative roles in a variety of biotic or abiotic stresses in crops. The change of its concentration within a crop is closely related to the crop growth and development. Accordingly, there is an urgent need to develop an efficient analytical method for on-site quantitative detection of 1O2 in crops. Here, we judiciously constructed a novel ratiometric fluorescent probe, SX-2, for the detection of 1O2 in crops. Upon treating with 1O2, probe SX-2 displayed highly selective ratiometric fluorescence response, which is favorable for the quantitative detection of 1O2. Concurrently, the fluorescence solution color of probe SX-2 was varied, obviously from blue to yellow, indicating that the probe is beneficial for on-site detection by the naked eye. Sensing reaction mechanism studies showed that the 2,3-diphenyl imidazole group in SX-2 could function as a new selective recognition group for 1O2. Probe SX-2 was utilized for the detection of photoirradiation-induced 1O2 and endogenous 1O2 in living cells. The changes in the 1O2 level in zebrafish were also tracked by fluorescence imaging. In addition, the production of 1O2 in crop leaves under a light source of different wavelengths was studied. The results demonstrated more 1O2 were produced under a light source of 365 nm. Furthermore, to achieve on-site quantitative detection, a mobile fluorescence analysis device has been made. Probe SX-2 and mobile fluorescence analysis device were capable of on-site quantitative detecting of 1O2 in crops. The method developed herein will be convenient for the on-site quantitative measurement of 1O2 in distinct crops.

Singlet oxygen signalling and its potential roles in plant biotic interactions

Singlet oxygen (SO) is among the most potent reactive oxygen species, and readily oxidizes proteins, lipids and DNA. It can be generated at the plant surface by phototoxins in the epidermis, acting as a direct defense against pathogens and herbivores (including humans). SO can also accumulate within mitochondria, peroxisomes, cytosol and the nucleus through multiple enzymatic and nonenzymatic processes. However, the majority of research on intracellular SO generation in plants has focused on transfer of light energy to triplet oxygen by photopigments from the chloroplast. SO accumulates in response to diverse stresses that perturb chloroplast metabolism, and while its high reactivity limits diffusion distances, it participates in retrograde signalling through the EXECUTER1 sensor, generation of carotenoid metabolites and possibly other unknown pathways. SO thereby reprogrammes nuclear gene expression and modulates hormone signalling and programmed cell death. While SO signalling has long been known to regulate plant responses to high-light stress, recent literature also suggests a role in plant interactions with insects, bacteria and fungi. The goals of this review are to provide a brief overview of SO, summarize evidence for its involvement in biotic stress responses and discuss future directions for the study of SO in defense signalling.

A genetic screen for dominant chloroplast reactive oxygen species signaling mutants reveals life stage-specific singlet oxygen signaling networks

Introduction

Plants employ intricate molecular mechanisms to respond to abiotic stresses, which often lead to the accumulation of reactive oxygen species (ROS) within organelles such as chloroplasts. Such ROS can produce stress signals that regulate cellular response mechanisms. One ROS, singlet oxygen (1O2), is predominantly produced in the chloroplast during photosynthesis and can trigger chloroplast degradation, programmed cell death (PCD), and retrograde (organelle-to-nucleus) signaling. However, little is known about the molecular mechanisms involved in these signaling pathways or how many different signaling 1O2 pathways may exist.

Methods

The Arabidopsis thaliana plastid ferrochelatase two (fc2) mutant conditionally accumulates chloroplast 1O2, making fc2 a valuable genetic system for studying chloroplast 1O2-initiated signaling. Here, we have used activation tagging in a new forward genetic screen to identify eight dominant fc2 activation-tagged (fas) mutations that suppress chloroplast 1O2-initiated PCD.

Results

While 1O2-triggered PCD is blocked in all fc2 fas mutants in the adult stage, such cellular degradation in the seedling stage is blocked in only two mutants. This differential blocking of PCD suggests that life-stage-specific 1O2-response pathways exist. In addition to PCD, fas mutations generally reduce 1O2-induced retrograde signals. Furthermore, fas mutants have enhanced tolerance to excess light, a natural mechanism to produce chloroplast 1O2. However, general abiotic stress tolerance was only observed in one fc2 fas mutant (fc2 fas2). Together, this suggests that plants can employ general stress tolerance mechanisms to overcome 1O2 production but that this screen was mostly specific to 1O2 signaling. We also observed that salicylic acid (SA) and jasmonate (JA) stress hormone response marker genes were induced in 1O2-stressed fc2 and generally reduced by fas mutations, suggesting that SA and JA signaling is correlated with active 1O2 signaling and PCD.

Discussion

Together, this work highlights the complexity of 1O2 signaling by demonstrating that multiple pathways may exist and introduces a suite of new 1O2 signaling mutants to investigate the mechanisms controlling chloroplast-initiated degradation, PCD, and retrograde signaling.

Singlet Oxygen Detection by Chemiluminescence Probes in Living Cells

Singlet oxygen is a reactive oxygen species that causes oxidative damage to plant cells, but intriguingly it can also act as a signalling molecule to reprogram gene expression required to induce plant physiological/cellular responses. Singlet oxygen photosensitization in plants mainly occurs in chloroplasts after the molecular collision of ground-state molecular oxygen with triplet-excited-state chlorophyll. Singlet oxygen direct detection through phosphorescence emission in chloroplasts is a herculean task due to its extremely low luminescence quantum yield. Because of this, indirect alternative methods have been developed for its detection in biological systems, for example, by measuring the changes in the EPR signal or fluorescence intensity of singlet oxygen reaction-based probes. The singlet oxygen chemiluminescence (SOCL) is a chemiluminescence probe with high sensitivity and selectivity towards singlet oxygen and promising use to detect it in living cells without the inconvenience of low stability of the EPR signal of spin probes in the presence of redox compounds, spurious light scattering coming from the light source required for the excitation of fluorescence probes or the light emission of endogenous fluorescent molecules like chlorophyll in chloroplasts. The protocol presented in this chapter describes the first steps to characterizing singlet oxygen production within the biological system under study; this is accomplished through monitoring molecular oxygen consumption by SOCL using a Clark-type oxygen electrode and measuring the chemiluminescence generated by SOCL 1,2-dioxetane using a spectrofluorometer. For singlet oxygen detection within living cells, a version of SOCL with increased membrane permeability (SOCL-CPP) is described.

Redox regulation, thioredoxins, and glutaredoxins in retrograde signalling and gene transcription

Integration of reactive oxygen species (ROS)-mediated signal transduction pathways via redox sensors and the thiol-dependent signalling network is of increasing interest in cell biology for their implications in plant growth and productivity. Redox regulation is an important point of control in protein structure, interactions, cellular location, and function, with thioredoxins (TRXs) and glutaredoxins (GRXs) being key players in the maintenance of cellular redox homeostasis. The crosstalk between second messengers, ROS, thiol redox signalling, and redox homeostasis-related genes controls almost every aspect of plant development and stress response. We review the emerging roles of TRXs and GRXs in redox-regulated processes interacting with other cell signalling systems such as organellar retrograde communication and gene expression, especially in plants during their development and under stressful environments. This approach will cast light on the specific role of these proteins as redox signalling components, and their importance in different developmental processes during abiotic stress.

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.

SnRK2.10 kinase differentially modulates expression of hub WRKY transcription factors genes under salinity and oxidative stress in Arabidopsis thaliana

In nature, all living organisms must continuously sense their surroundings and react to the occurring changes. In the cell, the information about these changes is transmitted to all cellular compartments, including the nucleus, by multiple phosphorylation cascades. Sucrose Non-Fermenting 1 Related Protein Kinases (SnRK2s) are plant-specific enzymes widely distributed across the plant kingdom and key players controlling abscisic acid (ABA)-dependent and ABA-independent signaling pathways in the plant response to osmotic stress and salinity. The main deleterious effects of salinity comprise water deficiency stress, disturbances in ion balance, and the accompanying appearance of oxidative stress. The reactive oxygen species (ROS) generated at the early stages of salt stress are involved in triggering intracellular signaling required for the fast stress response and modulation of gene expression. Here we established in Arabidopsis thaliana that salt stress or induction of ROS accumulation by treatment of plants with H2O2 or methyl viologen (MV) induces the expression of several genes encoding transcription factors (TFs) from the WRKY DNA-Binding Protein (WRKY) family. Their induction by salinity was dependent on SnRK2.10, an ABA non-activated kinase, as it was strongly reduced in snrk2.10 mutants. The effect of ROS was clearly dependent on their source. Following the H2O2 treatment, SnRK2.10 was activated in wild-type (wt) plants and the induction of the WRKY TFs expression was only moderate and was enhanced in snrk2.10 lines. In contrast, MV did not activate SnRK2.10 and the WRKY induction was very strong and was similar in wt and snrk2.10 plants. A bioinformatic analysis indicated that the WRKY33, WRKY40, WRKY46, and WRKY75 transcription factors have a similar target range comprising numerous stress-responsive protein kinases. Our results indicate that the stress-related functioning of SnRK2.10 is fine-tuned by the source and intracellular distribution of ROS and the co-occurrence of other stress factors.

Nitrile-Specific Protein NSP2 and Its Interacting Protein MPK3 Synergistically Regulate Plant Disease Resistance in Arabidopsis

Glucosinolates and their degradation products have a wide range of actions and are important components of plant defense. NSP2 (nitrile-specific protein 2) is a key regulator in the breakdown process of glucosinolates. However, the precise function of NSP2 in plant disease resistance beyond its role in glucosinolate degradation is still unclear. In this study, we discovered that NSP2 which was induced by Pst DC3000, influenced PR genes expression and reactive oxygen burst. Additionally, omics analysis revealed that NSP2 was engaged in plant-pathogen interaction and several hormone signal transduction pathways. Furthermore, immunoprecipitation-tandem mass spectrometry analysis (IP-MS), bimolecular fluorescence complementation (BiFC), and co-immunoprecipitation demonstrated that NSP2 interacts with MPK3. Genetic analysis shows that NSP2 may be a function downstream of MPK3. Upon pathogen inoculation, NSP2 protein levels increase while MPK3 protein levels decrease. Moreover, the level of phosphorylated NSP2 decreases. Taken together, this study sheds light on a new mode of synergistic action between NSP2 and MPK3 in the disease resistance process.

PROGRAMMED CELL DEATH8 interacts with tetrapyrrole biosynthesis enzymes and ClpC1 to maintain homeostasis of tetrapyrrole metabolites in Arabidopsis

Tetrapyrrole biosynthesis (TBS) is a dynamically and strictly regulated process. Disruptions in tetrapyrrole metabolism influence many aspects of plant physiology, including photosynthesis, programmed cell death (PCD), and retrograde signaling, thus affecting plant growth and development at multiple levels. However, the genetic and molecular basis of TBS is not fully understood. We report here PCD8, a newly identified thylakoid-localized protein encoded by an essential gene in Arabidopsis. PCD8 knockdown causes a necrotic phenotype due to excessive chloroplast damage. A burst of singlet oxygen that results from overaccumulated tetrapyrrole intermediates upon illumination is suggested to be responsible for cell death in the knockdown mutants. Genetic and biochemical analyses revealed that PCD8 interacts with ClpC1 and a number of TBS enzymes, such as HEMC, CHLD, and PORC of TBS. Taken together, our findings uncover the function of chloroplast-localized PCD8 and provide a new perspective to elucidate molecular mechanism of how TBS is finely regulated in plants.

Light-Dependent Regulatory Interactions between the Redox System and miRNAs and Their Biochemical and Physiological Effects in Plants

Light intensity and spectrum play a major role in the regulation of the growth, development, and stress response of plants. Changes in the light conditions affect the formation of reactive oxygen species, the activity of the antioxidants, and, consequently, the redox environment in the plant tissues. Many metabolic processes, thus the biogenesis and function of miRNAs, are redox-responsive. The miRNAs, in turn, can modulate various components of the redox system, and this process is also associated with the alteration in the intensity and spectrum of the light. In this review, we would like to summarise the possible regulatory mechanisms by which the alterations in the light conditions can influence miRNAs in a redox-dependent manner. Daily and seasonal fluctuations in the intensity and spectral composition of the light can affect the expression of miRNAs, which can fine-tune the various physiological and biochemical processes due to their effect on their target genes. The interactions between the redox system and miRNAs may be modulated by light conditions, and the proposed function of this regulatory network and its effect on the various biochemical and physiological processes will be introduced in plants.

Multiple pathways mediate chloroplast singlet oxygen stress signaling

Chloroplast singlet oxygen initiates multiple pathways to control chloroplast degradation, cell death, and nuclear gene expression. Chloroplasts can respond to stress and changes in the environment by producing reactive oxygen species (ROS). Aside from being cytotoxic, ROS also have signaling capabilities. For example, the ROS singlet oxygen (¹O₂) can initiate nuclear gene expression, chloroplast degradation, and cell death. To unveil the signaling mechanisms involved, researchers have used several ¹O₂-producing Arabidopsis thaliana mutants as genetic model systems, including plastid ferrochelatase two (fc2), fluorescent in blue light (flu), chlorina 1 (ch1), and accelerated cell death 2 (acd2). Here, we compare these ¹O₂-producing mutants to elucidate if they utilize one or more signaling pathways to control cell death and nuclear gene expression. Using publicly available transcriptomic data, we demonstrate fc2, flu, and ch1 share a core response to ¹O₂ accumulation, but maintain unique responses, potentially tailored to respond to their specific stresses. Subsequently, we used a genetic approach to determine if these mutants share ¹O₂ signaling pathways by testing the ability of genetic suppressors of one ¹O₂ producing mutant to suppress signaling in a different ¹O₂ producing mutant. Our genetic analyses revealed at least two different chloroplast ¹O₂ signaling pathways control cellular degradation: one specific to the flu mutant and one shared by fc2, ch1, and acd2 mutants, but with life-stage-specific (seedling vs. adult) features. Overall, this work reveals chloroplast stress signaling involving ¹O₂ is complex and may allow cells to finely tune their physiology to environmental inputs.

The CRK5 and WRKY53 Are Conditional Regulators of Senescence and Stomatal Conductance in Arabidopsis

In Arabidopsis thaliana, cysteine-rich receptor-like kinases (CRKs) constitute a large group of membrane-localized proteins which perceive external stimuli and transduce the signal into the cell. Previous reports based on their loss-of-function phenotypes and expression profile support their role in many developmental and stress-responsive pathways. Our study revealed that one member of this family, CRK5, acts as a negative regulator of leaf aging. Enrichment of the CRK5 promoter region in W-box cis-elements demonstrated that WRKY transcription factors control it. We observed significantly enhanced WRKY53 expression in crk5 and reversion of its early-senescence phenotype in the crk5 wrky53 line, suggesting a negative feedback loop between these proteins antagonistically regulating chlorophyll a and b contents. Yeast-two hybrid assay showed further that CRK5 interacts with several proteins involved in response to water deprivation or calcium signaling, while gas exchange analysis revealed a positive effect of CRK5 on water use efficiency. Consistent with that, the crk5 plants showed disturbed foliar temperature, stomatal conductance, transpiration, and increased susceptibility to osmotic stress. These traits were fully or partially reverted to wild-type phenotype in crk5 wrky53 double mutant. Obtained results suggest that WRKY53 and CRK5 are antagonistic regulators of chlorophyll synthesis/degradation, senescence, and stomatal conductance.

Plastid and cytoplasmic origins of 1O2-mediated transcriptomic responses

The reactive oxygen species singlet oxygen, ¹O₂, has an extremely short half-life, yet is intimately involved with stress signalling in the cell. We previously showed that the effects of ¹O₂ on the transcriptome are highly correlated with 80S ribosomal arrest due to oxidation of guanosine residues in mRNA. Here, we show that dysregulation of chlorophyll biosynthesis in the flu mutant or through feeding by δ-aminolevulinic acid can lead to accumulation of photoactive chlorophyll intermediates in the cytoplasm, which generates ¹O₂ upon exposure to light and causes the oxidation of RNA, eliciting ¹O₂-responsive genes. In contrast, transcriptomes derived from DCMU treatment, or the Ch1 mutant under moderate light conditions display commonalties with each other but do not induce ¹O₂ gene signatures. Comparing ¹O₂ related transcriptomes to an index transcriptome induced by cycloheximide inhibition enables distinction between ¹O₂ of cytosolic or of plastid origin. These comparisons provide biological insight to cases of mutants or environmental conditions that produce ¹O₂.

Genome-wide identification and expression analysis of BrAGC genes in Brassica rapa reveal their potential roles in sexual reproduction and abiotic stress tolerance

AGC protein kinases play important roles in regulating plant growth, immunity, and cell death. However, the function of AGC in Brassica rapa has not yet been clarified. In this study, 62 BrAGC genes were identified, and these genes were distributed on 10 chromosomes and divided into six subfamilies. Analysis of gene structure and conserved motifs showed that the activation segment of BrAGC genes was highly conserved, and genes of the same subfamily showed higher sequence and structural similarity. Collinearity analysis revealed that BrAGCs were more closely related to AtAGCs than to OsAGCs. Expression profile analysis revealed that BrAGCs were preferentially expressed in flowers and BrAGC26, BrAGC33, and BrAGC04 were preferentially expressed in the stigma; the expression of these genes was significantly upregulated after self-incompatibility pollination, and the expression of BrAGC13 and BrAGC32 was significantly upregulated after cross-pollination. In addition, several typical cis-elements involved in the stress response were identified in BrAGC promoters. The expression levels of BrAGC37 and BrAGC44 significantly varied under different types of abiotic stress. Collectively, we identified that BrAGC26, BrAGC33, and BrAGC44 have the greatest potential in regulating pollen-pistil interaction and abiotic stress tolerance, respectively. Our findings will aid future functional investigations of BrAGCs in B. rapa.

Gene expression and phytohormone levels in the asymptomatic and symptomatic phases of infection in potato tubers inoculated with Dickeya solani

Dickeya solani is a soft rot bacterium with high virulence. In potato, D. solani, like the other potato-infecting soft rot bacteria, causes rotting and wilting of the stems and rotting of tubers in the field and in storage. Latent, asymptomatic infections of potato tubers are common in harvested tubers, and if the storage conditions are not optimal, the latent infection turns into active rotting. We characterized potato gene expression in artificially inoculated tubers in nonsymptomatic, early infections 1 and 24 hours post-inoculation (hpi) and compared the results to the response in symptomatic tuber tissue 1 week (168 hpi) later with RNA-Seq. In the beginning of the infection, potato tubers expressed genes involved in the detection of the bacterium through pathogen-associated molecular patterns (PAMPs), which induced genes involved in PAMPs-triggered immunity, resistance, production of pathogenesis-related proteins, ROS, secondary metabolites and salicylic acid (SA) and jasmonic acid (JA) biosynthesis and signaling genes. In the symptomatic tuber tissue one week later, the PAMPs-triggered gene expression was downregulated, whereas primary metabolism was affected, most likely leading to free sugars fueling plant defense but possibly also aiding the growth of the pathogen. In the symptomatic tubers, pectic enzymes and cell wall-based defenses were activated. Measurement of hormone production revealed increased SA concentration and almost no JA in the asymptomatic tubers at the beginning of the infection and high level of JA and reduced SA in the symptomatic tubers one week later. These findings suggest that potato tubers rely on different defense strategies in the different phases of D. solani infection even when the infection takes place in fully susceptible plants incubated in conditions leading to rotting. These results support the idea that D. solani is a biotroph rather than a true necrotroph.

Transcriptomic analysis of CO2-treated strawberries (Fragaria vesca) with enhanced resistance to softening and oxidative stress at consumption

One of the greatest threats to wild strawberries (Fragaria vesca Mara des Bois) after harvest is the highly perishability at ambient temperature. Breeders have successfully met the quality demands of consumers, but the prevention of waste after harvest in fleshy fruits is still pending. Most of the waste is due to the accelerated progress of senescence-like process after harvest linked to a rapid loss of water and firmness at ambient temperature. The storage life of strawberries increases at low temperature, but their quality is limited by the loss of cell structure. The application of high CO₂ concentrations increased firmness during cold storage. However, the key genes related to resistance to softening and cell wall disassembly following transference from cold storage at 20°C remain unclear. Therefore, we performed RNA-seq analysis, constructing a weighted gene co-expression network analysis (WGCNA) to identify which molecular determinants play a role in cell wall integrity, using strawberries with contrasting storage conditions, CO₂-cold stored (CCS), air-cold stored (ACS), non-cold stored (NCS) kept at ambient temperature, and intact fruit at harvest (AH). The hub genes associated with the cell wall structural architecture of firmer CO₂-treated strawberries revealed xyloglucans stabilization attributed mainly to a down-regulation of Csl E1, XTH 15, Exp-like B1 and the maintenance of expression levels of nucleotide sugars transferases such as GMP and FUT as well as improved lamella integrity linked to a down-regulation of RG-lyase, PL-like and PME. The preservation of cell wall elasticity together with the up-regulation of LEA, EXPA4, and MATE, required to maintain cell turgor, is the mechanisms controlled by high CO₂. In stressed air-cold stored strawberries, in addition to an acute softening, there is a preferential transcript accumulation of genes involved in lignin and raffinose pathways. Non-cold stored strawberries kept at 20°C after harvest are characterized by an enrichment in genes mainly involved in oxidative stress and up-expression of genes involved in jasmonate biosynthesis. The present results on transcriptomic analysis of CO₂-treated strawberries with enhanced resistance to softening and oxidative stress at consumption will help to improve breeding strategies of both wild and cultivated strawberries.

Phenylalanine suppresses cell death caused by loss of fumarylacetoacetate hydrolase in Arabidopsis

Fumarylacetoacetate hydrolase (FAH) catalyzes the final step of Tyrosine (Tyr) degradation pathway essential to animals and the deficiency of FAH causes an inborn lethal disease. In plants, a role of this pathway was unknown until we found that mutation of Short-day Sensitive Cell Death1 (SSCD1), encoding Arabidopsis FAH, results in cell death under short day. Phenylalanine (Phe) could be converted to Tyr and then degraded in both animals and plants. Phe ingestion in animals worsens the disease caused by FAH defect. However, in this study we found that Phe represses cell death caused by FAH defect in plants. Phe treatment promoted chlorophyll biosynthesis and suppressed the up-regulation of reactive oxygen species marker genes in the sscd1 mutant. Furthermore, the repression of sscd1 cell death by Phe could be reduced by α-aminooxi-β-phenylpropionic acid but increased by methyl jasmonate, which inhibits or activates Phe ammonia-lyase catalyzing the first step of phenylpropanoid pathway, respectively. In addition, we found that jasmonate signaling up-regulates Phe ammonia-lyase 1 and mediates the methyl jasmonate enhanced repression of sscd1 cell death by Phe. These results uncovered the relation between chlorophyll biosynthesis, phenylpropanoid pathway and jasmonate signaling in regulating the cell death resulting from loss of FAH in plants.

ROS production and signalling in chloroplasts: cornerstones and evolving concepts

Reactive oxygen species (ROS) such as singlet oxygen, superoxide (O₂^●- ) and hydrogen peroxide (H₂ O₂ ) are the markers of living cells. Oxygenic photosynthesis produces ROS in abundance, which act as a readout of a functional electron transport system and metabolism. The concept that photosynthetic ROS production is a major driving force in chloroplast to nucleus retrograde signalling is embedded in the literature, as is the role of chloroplasts as environmental sensors. The different complexes and components of the photosynthetic electron transport chain (PETC) regulate O₂^●- production in relation to light energy availability and the redox state of the stromal Cys-based redox systems. All of the ROS generated in chloroplasts have the potential to act as signals and there are many sulphhydryl-containing proteins and peptides in chloroplasts that have the potential to act as H₂ O₂ sensors and function in signal transduction. While ROS may directly move out of the chloroplasts to other cellular compartments, ROS signalling pathways can only be triggered if appropriate ROS-sensing proteins are present at or near the site of ROS production. Chloroplast antioxidant systems serve either to propagate these signals or to remove excess ROS that cannot effectively be harnessed in signalling. The key challenge is to understand how regulated ROS delivery from the PETC to the Cys-based redox machinery is organised to transmit redox signals from the environment to the nucleus. Redox changes associated with stromal carbohydrate metabolism also play a key role in chloroplast signalling pathways.

Plant Kinases in the Perception and Signaling Networks Associated With Arthropod Herbivory

The success in the response of plants to environmental stressors depends on the regulatory networks that connect plant perception and plant response. In these networks, phosphorylation is a key mechanism to activate or deactivate the proteins involved. Protein kinases are responsible for phosphorylations and play a very relevant role in transmitting the signals. Here, we review the present knowledge on the contribution of protein kinases to herbivore-triggered responses in plants, with a focus on the information related to the regulated kinases accompanying herbivory in Arabidopsis. A meta-analysis of transcriptomic responses revealed the importance of several kinase groups directly involved in the perception of the attacker or typically associated with the transmission of stress-related signals. To highlight the importance of these protein kinase families in the response to arthropod herbivores, a compilation of previous knowledge on their members is offered. When available, this information is compared with previous findings on their role against pathogens. Besides, knowledge of their homologous counterparts in other plant-herbivore interactions is provided. Altogether, these observations resemble the complexity of the kinase-related mechanisms involved in the plant response. Understanding how kinase-based pathways coordinate in response to a specific threat remains a major challenge for future research.

Mutations that alter Arabidopsis flavonoid metabolism affect the circadian clock

Flavonoids are a well-known class of specialized metabolites that play key roles in plant development, reproduction, and survival. Flavonoids are also of considerable interest from the perspective of human health, as both phytonutrients and pharmaceuticals. RNA sequencing analysis of an Arabidopsis null allele for chalcone synthase (CHS), which catalyzes the first step in flavonoid metabolism, has uncovered evidence that these compounds influence the expression of genes associated with the plant circadian clock. Analysis of promoter-luciferase constructs further showed that the transcriptional activity of CCA1 and TOC1, two key clock genes, is altered in CHS-deficient seedlings across the day/night cycle. Similar findings for a mutant line lacking flavonoid 3'-hydroxylase (F3'H) activity, and thus able to synthesize mono- but not dihydroxylated B-ring flavonoids, suggests that the latter are at least partially responsible; this was further supported by the ability of quercetin to enhance CCA1 promoter activity in wild-type and CHS-deficient seedlings. The effects of flavonoids on circadian function were also reflected in photosynthetic activity, with chlorophyll cycling abolished in CHS- and F3'H-deficient plants. Remarkably, the same phenotype was exhibited by plants with artificially high flavonoid levels, indicating that neither the antioxidant potential nor the light-screening properties of flavonoids contribute to optimal clock function, as has recently also been demonstrated in animal systems. Collectively, the current experiments point to a previously unknown connection between flavonoids and circadian cycling in plants and open the way to better understanding of the molecular basis of flavonoid action.

Function of Protein Kinases in Leaf Senescence of Plants

Leaf senescence is an evolutionarily acquired process and it is critical for plant fitness. During senescence, macromolecules and nutrients are disassembled and relocated to actively growing organs. Plant leaf senescence process can be triggered by developmental cues and environmental factors, proper regulation of this process is essential to improve crop yield. Protein kinases are enzymes that modify their substrates activities by changing the conformation, stability, and localization of those proteins, to play a crucial role in the leaf senescence process. Impressive progress has been made in understanding the role of different protein kinases in leaf senescence recently. This review focuses on the recent progresses in plant leaf senescence-related kinases. We summarize the current understanding of the function of kinases on senescence signal perception and transduction, to help us better understand how the orderly senescence degeneration process is regulated by kinases, and how the kinase functions in the intricate integration of environmental signals and leaf age information.

Plastid retrograde regulation of miRNA expression in response to light stress

BACKGROUND

MicroRNAs (miRNAs) are a class of endogenous noncoding RNAs that play a pivotal role in the regulation of plant development and responses to the surrounding environment. Despite the efforts made to elucidate their function in the adaptation of plants to many abiotic and biotic stresses, their role in high light (HL) stress is still vague. HL stress often arises upon plant exposure to full sunlight. Subsequent changes in nuclear gene expression are triggered by chloroplast-derived retrograde signals.

RESULTS

In this study, we show that HL is involved in miRNA-dependent regulation in Arabidopsis thaliana rosettes. Microtranscriptomic screening revealed a limited number of miRNAs reacting to HL. To explain the miRNA regulation mechanisms at the different biogenesis stages, chemical and genetic approaches were applied. First, we tested the possible role of plastoquinone (PQ) redox changes using photosynthetic electron transport chain inhibitors. The results suggest that increased primary transcript abundance (pri-miRNAs) of HL-regulated miRNAs is dependent on signals upstream of PQ. This indicates that such signals may originate from photosystem II, which is the main singlet oxygen (¹O₂) source. Nevertheless, no changes in pri-miRNA expression upon a dark-light shift in the conditional fluorescent (flu) mutant producing ¹O₂ were observed when compared to wild-type plants. Thus, we explored the ¹O₂ signaling pathway, which is initiated independently in HL and is related to β-carotene oxidation and production of volatile derivatives, such as β-cyclocitral (β-CC). Pri-miRNA induction by β-CC, which is a component of this ¹O₂ pathway, as well as an altered response in the methylene blue sensitivity 1 (mbs1) mutant support the role of ¹O₂ signaling in miRNA regulation.

CONCLUSIONS

We show that light stress triggers changes in miRNA expression. This stress response may be regulated by reactive oxygen species (ROS)-related signaling. In conclusion, our results link ROS action to miRNA biogenesis, suggesting its contribution to inconsistent pri- and mature miRNA dynamics.

The jasmonate biosynthesis Gene OsOPR7 can mitigate salinity induced mitochondrial oxidative stress

Salinity poses a serious threat to global agriculture and human food security. A better understanding of plant adaptation to salt stress is, therefore, mandatory. In the non-photosynthetic cells of the root, salinity perturbs oxidative balance in mitochondria, leading to cell death. In parallel, plastids accumulate the jasmonate precursor cis (+)12-Oxo-Phyto-Dienoic Acid (OPDA) that is then translocated to peroxisomes and has been identified as promoting factor for salt-induced cell death as well. In the current study, we probed for a potential interaction between these three organelles that are primarily dealing with oxidative metabolism. We made use of two tools: (i) Rice OPDA Reductase 7 (OsOPR7), an enzyme localised in peroxisomes converting OPDA into the precursors of the stress hormone JA-Ile. (ii) A Trojan Peptoid, Plant PeptoQ, which can specifically target to mitochondria and scavenge excessive superoxide accumulating in response to salt stress. We show that overexpression of OsOPR7 as GFP fusion in tobacco (Nicotiana tabacum L. cv. Bright Yellow 2, BY-2) cells, as well as a pretreatment with Plant PeptoQ can mitigate salt stress with respect to numerous aspects including proliferation, expansion, ionic balance, redox homeostasis, and mortality. This mitigation correlates with a more robust oxidative balance, evident from a higher activity of superoxide dismutase (SOD), lower levels of superoxide and lipid peroxidation damage, and a conspicuous and specific upregulation of mitochondrial SOD transcripts. Although both, Plant PeptoQ and ectopic OsOPR7, were acting in parallel and mostly additive, there are two specific differences: (i) OsOPR7 is strictly localised to the peroxisomes, while Plant PeptoQ found in mitochondria. (ii) Plant PeptoQ activates transcripts of NAC, a factor involved in retrograde signalling from mitochondria to the nucleus, while these transcripts are suppressed significantly in the cells overexpressing OsOPR7. The fact that overexpression of a peroxisomal enzyme shifting the jasmonate pathway from the cell-death signal OPDA towards JA-Ile, a hormone linked with salt adaptation, is accompanied by more robust redox homeostasis in a different organelle, the mitochondrion, indicates that cross-talk between peroxisome and mitochondrion is a crucial factor for efficient adaptation to salt stress.

Phytochrome B Positively Regulates Red Light-Mediated ER Stress Response in Arabidopsis

Light plays a crucial role in plant growth and development, and light signaling is integrated with various stress responses to adapt to different environmental changes. During this process, excessive protein synthesis overwhelms the protein-folding ability of the endoplasmic reticulum (ER), causing ER stress. Although crosstalk between light signaling and ER stress response has been reported in plants, the molecular mechanisms underlying this crosstalk are poorly understood. Here, we demonstrate that the photoreceptor phytochrome B (phyB) induces the expression of ER luminal protein chaperones as well as that of unfolded protein response (UPR) genes. The phyB-5 mutant was less sensitive to tunicamycin (TM)-induced ER stress than were the wild-type plants, whereas phyB-overexpressing plants displayed a more sensitive phenotype under white light conditions. ER stress response genes (BiP2 and BiP3), UPR-related bZIP transcription factors (bZIP17, bZIP28, and bZIP60), and programmed cell death (PCD)-associated genes (OXI1, NRP1, and MC8) were upregulated in phyB-overexpressing plants, but not in phyB-5, under ER stress conditions. The ER stress-sensitive phenotype of phyB-5 under red light conditions was eliminated with a reduction in photo-equilibrium by far-red light and darkness. The N-terminal domain of phyB is essential for signal transduction of the ER stress response in the nucleus, which is similar to light signaling. Taken together, our results suggest that phyB integrates light signaling with the UPR to relieve ER stress and maintain proper plant growth.

MdUGT88F1-mediated phloridzin biosynthesis coordinates carbon and nitrogen accumulation in apple

The high accumulation of phloridzin makes apple (Malus domestica) unique in the plant kingdom, which suggests a vital role of its biosynthesis in physiological processes. In our previous study, silencing MdUGT88F1 (a key UDP-GLUCOSE: PHLORETIN 2'-O-GLUCOSYLTRANSFERASE gene) revealed the importance of phloridzin biosynthesis in apple development and Valsa canker resistance. Here, results from MdUGT88F1-silenced lines showed that phloridzin biosynthesis was indispensable for normal chloroplast development and photosynthetic carbon fixation by maintaining MdGLK1/2 (GOLDEN2-like1/2) expression. Interestingly, increased phloridzin biosynthesis did not affect plant (or chloroplast) development, but reduced nitrogen accumulation, leading to chlorophyll deficiency, light sensitivity, and sugar accumulation in MdUGT88F1-overexpressing apple lines. Further analysis revealed that MdUGT88F1-mediated phloridzin biosynthesis negatively regulated the cytosolic glutamine synthetase1-asparagine synthetase-asparaginase (GS1-AS-ASPG) pathway of ammonium assimilation and limited chlorophyll synthesis in apple shoots. The interference of phloridzin biosynthesis in the GS1-AS-ASPG pathway was also assumed to be associated with its limitation of the carbon skeleton of ammonium assimilation through metabolic competition with the tricarboxylic acid cycle. Taken together, our findings shed light on the role of MdUGT88F1-mediated phloridzin biosynthesis in the coordination between carbon and nitrogen accumulation in apple trees.

Chloroplast ROS and stress signaling

Chloroplasts overproduce reactive oxygen species (ROS) under unfavorable environmental conditions, and these ROS are implicated in both signaling and oxidative damage. There is mounting evidence for their roles in translating environmental fluctuations into distinct physiological responses, but their targets, signaling cascades, and mutualism and antagonism with other stress signaling cascades and within ROS signaling remain poorly understood. Great efforts made in recent years have shed new light on chloroplast ROS-directed plant stress responses, from ROS perception to plant responses, in conditional mutants of Arabidopsis thaliana or under various stress conditions. Some articles have also reported the mechanisms underlying the complexity of ROS signaling pathways, with an emphasis on spatiotemporal regulation. ROS and oxidative modification of affected target proteins appear to induce retrograde signaling pathways to maintain chloroplast protein quality control and signaling at a whole-cell level using stress hormones. This review focuses on these seemingly interconnected chloroplast-to-nucleus retrograde signaling pathways initiated by ROS and ROS-modified target molecules. We also discuss future directions in chloroplast stress research to pave the way for discovering new signaling molecules and identifying intersectional signaling components that interact in multiple chloroplast signaling pathways.

Determination of ROS-Induced Lipid Peroxidation by HPLC-Based Quantification of Hydroxy Polyunsaturated Fatty Acids

Because they are highly unsaturated, plant lipids are sensitive to oxidation and constitute a primary target of reactive oxygen species. Therefore, quantification of lipid peroxidation provides a pertinent approach to evaluating oxidative stress in plants. Here, we describe a simple method to measure upstream products of the peroxidation of the major polyunsaturated fatty acids in plants, namely, linolenic acid (C18:3) and linoleic acid (C18:2). The method uses conventional HPLC with UV detection to measure hydroxy C18:3 and C18:2 after reduction of their respective hydroperoxides. The described experimental approach requires low amounts of plant material (a few hundred milligrams), monitors oxidation of both membrane and free fatty acids, and can discriminate between enzymatic and non-enzymatic lipid peroxidation.

Plastid-mediated singlet oxygen in regulated cell death

Reactive oxygen species (ROS) generation within a cell is a natural process of specific subcellular components involved in redox reactions. Within a plant cell, chloroplasts are one of the major sources of ROS generation. Plastid-generated ROS molecules include singlet oxygen (¹ O₂ ), superoxide radical (O₂ ^- ), hydroxyl radical (OH^• ) and hydrogen peroxide (H₂ O₂ ), which are produced mainly during photochemical reactions of photosynthesis and chlorophyll biosynthetic process. Under normal growth and developmental, generated ROS molecules act as a secondary messenger controlling several metabolic reactions; however, perturbed environmental conditions lead to multi-fold amplification of cellular ROS that eventually kill the target cell. To maintain homeostasis between production and scavenging of ROS, the cell has instituted several enzymatic and non-enzymatic antioxidant machineries to maintain ROS at a physiological level. Among chloroplastic ROS molecules, excess generation of singlet oxygen (¹ O₂ ) is highly deleterious to the cell metabolic functions and survival. Interestingly, within cellular antioxidant machinery, enzymes involved in detoxification of ¹ O₂ are lacking. Recent studies suggest that under optimal concentrations, ¹ O₂ acts as a signalling molecule and drives the cell to either the acclimation pathway or regulated cell death (RCD). Stress-induced RCD is a survival mechanism for the whole plant, while the involvement of chloroplasts and chloroplast-localized molecules that execute RCD are not well understood. In this review, we advocate for participation of chloroplasts-generated ¹ O₂ in signalling and RCD in plants.

Chloroplast quality control pathways are dependent on plastid DNA synthesis and nucleotides provided by cytidine triphosphate synthase two

Reactive oxygen species (ROS) produced in chloroplasts cause oxidative damage, but also signal to initiate chloroplast quality control pathways, cell death, and gene expression. The Arabidopsis thaliana plastid ferrochelatase two (fc2) mutant produces the ROS singlet oxygen in chloroplasts that activates such signaling pathways, but the mechanisms are largely unknown. Here we characterize one fc2 suppressor mutation and map it to CYTIDINE TRIPHOSPHATE SYNTHASE TWO (CTPS2), which encodes one of five enzymes in Arabidopsis necessary for de novo cytoplasmic CTP (and dCTP) synthesis. The ctps2 mutation reduces chloroplast transcripts and DNA content without similarly affecting mitochondria. Chloroplast nucleic acid content and singlet oxygen signaling are restored by exogenous feeding of the dCTP precursor deoxycytidine, suggesting ctps2 blocks signaling by limiting nucleotides for chloroplast genome maintenance. An investigation of CTPS orthologs in Brassicaceae showed CTPS2 is a member of an ancient lineage distinct from CTPS3. Complementation studies confirmed this analysis; CTPS3 was unable to compensate for CTPS2 function in providing nucleotides for chloroplast DNA and signaling. Our studies link cytoplasmic nucleotide metabolism with chloroplast quality control pathways. Such a connection is achieved by a conserved clade of CTPS enzymes that provide nucleotides for chloroplast function, thereby allowing stress signaling to occur.

Oxygen and reactive oxygen species-dependent regulation of plant growth and development

Oxygen and reactive oxygen species (ROS) have been co-opted during evolution into the regulation of plant growth, development, and differentiation. ROS and oxidative signals arising from metabolism or phytohormone-mediated processes control almost every aspect of plant development from seed and bud dormancy, liberation of meristematic cells from the quiescent state, root and shoot growth, and architecture, to flowering and seed production. Moreover, the phytochrome and phytohormone-dependent transmissions of ROS waves are central to the systemic whole plant signaling pathways that integrate root and shoot growth. The sensing of oxygen availability through the PROTEOLYSIS 6 (PRT6) N-degron pathway functions alongside ROS production and signaling but how these pathways interact in developing organs remains poorly understood. Considerable progress has been made in our understanding of the nature of hydrogen peroxide sensors and the role of thiol-dependent signaling networks in the transmission of ROS signals. Reduction/oxidation (redox) changes in the glutathione (GSH) pool, glutaredoxins (GRXs), and thioredoxins (TRXs) are important in the control of growth mediated by phytohormone pathways. Although, it is clear that the redox states of proteins involved in plant growth and development are controlled by the NAD(P)H thioredoxin reductase (NTR)/TRX and reduced GSH/GRX systems of the cytosol, chloroplasts, mitochondria, and nucleus, we have only scratched the surface of this multilayered control and how redox-regulated processes interact with other cell signaling systems.

Hormonal impact on photosynthesis and photoprotection in plants

Photosynthesis is not only essential for plants, but it also sustains life on Earth. Phytohormones play crucial roles in developmental processes, from organ initiation to senescence, due to their role as growth and developmental regulators, as well as their central role in the regulation of photosynthesis. Furthermore, phytohormones play a major role in photoprotection of the photosynthetic apparatus under stress conditions. Here, in addition to discussing our current knowledge on the role of the phytohormones auxin, cytokinins, gibberellins, and strigolactones in promoting photosynthesis, we will also highlight the role of abscisic acid beyond stomatal closure in modulating photosynthesis and photoprotection under various stress conditions through crosstalk with ethylene, salicylates, jasmonates, and brassinosteroids. Furthermore, the role of phytohormones in controlling the production and scavenging of photosynthesis-derived reactive oxygen species, the duration and extent of photo-oxidative stress and redox signaling under stress conditions will be discussed in detail. Hormones have a significant impact on the regulation of photosynthetic processes in plants under both optimal and stress conditions, with hormonal interactions, complementation, and crosstalk being important in the spatiotemporal and integrative regulation of photosynthetic processes during organ development at the whole-plant level.

Cellular oxidative stress in programmed cell death: focusing on chloroplastic 1O2 and mitochondrial cytochrome-c release

The programmed cell death (PCD) occurs when the targeted cells have fulfilled their task or under conditions as oxidative stress generated by ROS species. Thus, plants have to deal with the singlet oxygen ¹O₂ produced in chloroplasts. ¹O₂ is unlikely to act as a primary retrograde signal owing to its high reactivity and short half-life. In addition to its high toxicity, the ¹O₂ generated under an excess or low excitation energy might also act as a highly versatile signal triggering chloroplast-to-nucleus retrograde signaling (ChNRS) and nuclear reprogramming or cell death. Molecular and biochemical studies with the flu mutant, which accumulates protochlorophyllide in the dark, demonstrated that chloroplastic ¹O₂-driven EXECUTER-1 (EX1) and EX2 proteins are involved in the ¹O₂-dependent response. Both EX1 and EX2 are necessary for full suppression of ¹O₂-induced gene expression. That is, EXECUTER proteolysis via the ATP-dependent zinc protease (FtsH) is an integral part of ¹O₂-triggered retrograde signaling. The existence of at least two independent ChNRS involving EX1 and β-cyclocitral, and dihydroactinidiolide and OXI1, respectively, seem clear. Besides, this update also focuses on plant PCD and its relation with mitochondrial cytochrome-c (Cytc) release to cytosol. Changes in the dynamics and morphology of mitochondria were shown during the onset of cell death. The mitochondrial damage and translocation of Cytc may be one of the major causes of PCD triggering. Together, this current overview illustrates the complexity of the cellular response to oxidative stress development. A puzzle with the majority of its pieces still not placed.

The light and dark sides of nitric oxide: multifaceted roles of nitric oxide in plant responses to light

Light drives photosynthesis and informs plants about their surroundings. Regarded as a multifunctional signaling molecule in plants, nitric oxide (NO) has been repeatedly demonstrated to interact with light signaling cascades to control plant growth, development and metabolism. During early plant development, light-triggered NO accumulation counteracts negative regulators of photomorphogenesis and modulates the abundance of, and sensitivity to, plant hormones to promote seed germination and de-etiolation. In photosynthetically active tissues, NO is generated at distinct rates under light or dark conditions and acts at multiple target sites within chloroplasts to regulate photosynthetic reactions. Moreover, changes in NO concentrations in response to light stress promote plant defenses against oxidative stress under high light or ultraviolet-B radiation. Here we review the literature on the interaction of NO with the complicated light and hormonal signaling cascades controlling plant photomorphogenesis and light stress responses, focusing on the recently identified molecular partners and action mechanisms of NO in these events. We also discuss the versatile role of NO in regulating both photosynthesis and light-dependent stomatal movements, two key determinants of plant carbon gain. The regulation of nitrate reductase (NR) by light is highlighted as vital to adjust NO production in plants living under natural light conditions.

Luminescence imaging of leaf damage induced by lipid peroxidation products and its modulation by β-cyclocitral

Lipid peroxidation is a primary event associated with oxidative stress in plants. This phenomenon secondarily generates bioactive and/or toxic compounds such as reactive carbonyl species (RCS), phytoprostanes, and phytofurans, as confirmed here in Arabidopsis plants exposed to photo-oxidative stress conditions. We analyzed the effects of exogenous applications of secondary lipid oxidation products on Arabidopsis plants by luminescence techniques. Oxidative damage to attached leaves was measured by autoluminescence imaging, using a highly sensitive CCD camera, and the activity of the detoxification pathway, dependent on the transcription regulator SCARECROW-LIKE 14 (SCL14), was monitored with a bioluminescent line expressing the firefly LUCIFERASE (LUC) gene under the control of the ALKENAL REDUCTASE (AER) gene promoter. We identified 4-hydroxynonenal (HNE), and to a lesser extent 4-hydroxyhexenal (HHE), as highly reactive compounds that are harmful to leaves and can trigger AER gene expression, contrary to other RCS (pentenal, hexenal) and to isoprostanoids. Although the levels of HNE and other RCS were enhanced in the SCL14-deficient mutant (scl14), exogenously applied HNE was similarly damaging to this mutant, its wild-type parent and a SCL14-overexpressing transgenic line (OE:SCL14). However, strongly boosting the SCL14 detoxification pathway and AER expression by a pre-treatment of OE:SCL14 with the signaling apocarotenoid β-cyclocitral canceled the damaging effects of HNE. Conversely, in the scl14 mutant, the effects of β-cyclocitral and HNE were additive, leading to enhanced leaf damage. These results indicate that the cellular detoxification pathway induced by the low-toxicity β-cyclocitral targets highly toxic compounds produced during lipid peroxidation, reminiscent of a safener-type mode of action.

Interplay between antioxidants in response to photooxidative stress in Arabidopsis

Tocochromanols (tocopherols, tocotrienols and plastochromanol-8), isoprenoid quinone (plastoquinone-9 and plastoquinol-9) and carotenoids (carotenes and xanthophylls), are lipid-soluble antioxidants in the chloroplasts, which play an important defensive role against photooxidative stress in plants. In this study, the interplay between the antioxidant activities of those compounds in excess light stress was analyzed in wild-type (WT) Arabidopsis thaliana and in a tocopherol cyclase mutant (vte1), a homogentisate phytyl transferase mutant (vte2) and a tocopherol cyclase overexpressor (VTE1oex). The results reveal a strategy of cooperation and replacement between α-tocopherol, plastochromanol-8, plastoquinone-9/plastoquinol-9 and zeaxanthin. In the first line of defense (non-radical mechanism), singlet oxygen is either physically or chemically quenched by α-tocopherol; however, when α-tocopherol is consumed, zeaxanthin and plastoquinone-9/plastoquinol-9 can provide alternative protection against singlet oxygen toxicity by functional replacement of α-tocopherol either by zeaxanthin for the physical quenching or by plastoquinone-9/plastoquinol-9 for the chemical quenching. When singlet oxygen escapes this first line of defense, it oxidizes lipids and forms lipid hydroperoxides, which are oxidized to lipid peroxyl radicals by ferric iron. In the second line of defense (radical mechanism), lipid peroxyl radicals are scavenged by α-tocopherol. After its consumption, plastochromanol-8 overtakes this function. We provide a comprehensive description of the reaction pathways underlying the non-radical and radical antioxidant activities of α-tocopherol, carotenoids, plastoquinone-9/plastoquinol-9 and plastochromanol-8. The interplay between the different plastid lipid-soluble antioxidants in the non-radical and the radical mechanism provides step by step insights into protection against photooxidative stress in higher plants.

Tanned or Sunburned: How Excessive Light Triggers Plant Cell Death

Plants often encounter light intensities exceeding the capacity of photosynthesis (excessive light) mainly due to biotic and abiotic factors, which lower CO₂ fixation and reduce light energy sinks. Under excessive light, the photosynthetic electron transport chain generates damaging molecules, hence leading to photooxidative stress and eventually to cell death. In this review, we summarize the mechanisms linking the excessive absorption of light energy in chloroplasts to programmed cell death in plant leaves. We highlight the importance of reactive carbonyl species generated by lipid photooxidation, their detoxification, and the integrating role of the endoplasmic reticulum in the adoption of phototolerance or cell-death pathways. Finally, we invite the scientific community to standardize the conditions of excessive light treatments.

FATTY ACID DESATURASE5 Is Required to Induce Autoimmune Responses in Gigantic Chloroplast Mutants of Arabidopsis

Chloroplasts mediate genetically controlled cell death via chloroplast-to-nucleus retrograde signaling. To decipher the mechanism, we examined chloroplast-linked lesion-mimic mutants of Arabidopsis (Arabidopsis thaliana) deficient in plastid division, thereby developing gigantic chloroplasts (GCs). These GC mutants, including crumpled leaf (crl), constitutively express immune-related genes and show light-dependent localized cell death (LCD), mirroring typical autoimmune responses. Our reverse genetic approach excludes any potential role of immune/stress hormones in triggering LCD. Instead, transcriptome and in silico analyses suggest that reactive electrophile species (RES) generated via oxidation of polyunsaturated fatty acids (PUFAs) or lipid peroxidation-driven signaling may induce LCD. Consistent with these results, the one of the suppressors of crl, dubbed spcrl4, contains a causative mutation in the nuclear gene encoding chloroplast-localized FATTY ACID DESATURASE5 (FAD5) that catalyzes the conversion of palmitic acid (16:0) to palmitoleic acid (16:1). The loss of FAD5 in the crl mutant might attenuate the levels of RES and/or lipid peroxidation due to the reduced levels of palmitic acid-driven PUFAs, which are prime targets of reactive oxygen species. The fact that fad5 also compromises the expression of immune-related genes and the development of LCD in other GC mutants substantiates the presence of an intrinsic retrograde signaling pathway, priming the autoimmune responses in a FAD5-dependent manner.

Retrograde Signaling: Understanding the Communication between Organelles

Understanding how cell organelles and compartments communicate with each other has always been an important field of knowledge widely explored by many researchers. However, despite years of investigations, one point-and perhaps the only point that many agree on-is that our knowledge about cellular-signaling pathways still requires expanding. Chloroplasts and mitochondria (because of their primary functions in energy conversion) are important cellular sensors of environmental fluctuations and feedback they provide back to the nucleus is important for acclimatory responses. Under stressful conditions, it is important to manage cellular resources more efficiently in order to maintain a proper balance between development, growth and stress responses. For example, it can be achieved through regulation of nuclear and organellar gene expression. If plants are unable to adapt to stressful conditions, they will be unable to efficiently produce energy for growth and development-and ultimately die. In this review, we show the importance of retrograde signaling in stress responses, including the induction of cell death and in organelle biogenesis. The complexity of these pathways demonstrates how challenging it is to expand the existing knowledge. However, understanding this sophisticated communication may be important to develop new strategies of how to improve adaptability of plants in rapidly changing environments.

Cell death resulted from loss of fumarylacetoacetate hydrolase in Arabidopsis is related to phytohormone jasmonate but not salicylic acid

Fumarylacetoacetate hydrolase (FAH) catalyzes the final step in Tyr degradation pathway essential to animals but not well understood in plants. Previously, we found that mutation of SSCD1 encoding Arabidopsis FAH causes cell death under short day, which uncovered an important role of Tyr degradation pathway in plants. Since phytohormones salicylic acid (SA) and jasmonate (JA) are involved in programmed cell death, in this study, we investigated whether sscd1 cell death is related to SA and JA, and found that (1) it is accompanied by up-regulation of JA- and SA-inducible genes as well as accumulation of JA but not SA; (2) it is repressed by breakdown of JA signaling but not SA signaling; (3) the up-regulation of reactive oxygen species marker genes in sscd1 is repressed by breakdown of JA signaling; (4) treatment of wild-type Arabidopsis with succinylacetone, an abnormal metabolite caused by loss of FAH, induces expression of JA-inducible genes whereas treatment with JA induces expression of some Tyr degradation genes with dependence of JA signaling. These results demonstrated that cell death resulted from loss of FAH in Arabidopsis is related to JA but not SA, and suggested that JA signaling positively regulates sscd1 cell death by up-regulating Tyr degradation.

Endoplasmic reticulum-mediated unfolded protein response is an integral part of singlet oxygen signalling in plants

Singlet oxygen (¹ O₂ ) is a by-product of photosynthesis that triggers a signalling pathway leading to stress acclimation or to cell death. By analyzing gene expressions in a ¹ O₂ -overproducing Arabidopsis mutant (ch1) under different light regimes, we show here that the ¹ O₂ signalling pathway involves the endoplasmic reticulum (ER)-mediated unfolded protein response (UPR). ch1 plants in low light exhibited a moderate activation of UPR genes, in particular bZIP60, and low concentrations of the UPR-inducer tunicamycin enhanced tolerance to photooxidative stress, together suggesting a role for UPR in plant acclimation to low ¹ O₂ levels. Exposure of ch1 to high light stress ultimately leading to cell death resulted in a marked upregulation of the two UPR branches (bZIP60/IRE1 and bZIP28/bZIP17). Accordingly, mutational suppression of bZIP60 and bZIP28 increased plant phototolerance, and a strong UPR activation by high tunicamycin concentrations promoted high light-induced cell death. Conversely, light acclimation of ch1 to ¹ O₂ stress put a limitation in the high light-induced expression of UPR genes, except for the gene encoding the BIP3 chaperone, which was selectively upregulated. BIP3 deletion enhanced Arabidopsis photosensitivity while plants treated with a chemical chaperone exhibited enhanced phototolerance. In conclusion, ¹ O₂ induces the ER-mediated UPR response that fulfils a dual role in high light stress: a moderate UPR, with selective induction of BIP3, is part of the acclimatory response to ¹ O₂ , and a strong activation of the whole UPR is associated with cell death.

Chloroplast-associated molecular patterns as concept for fine-tuned operational retrograde signalling

Chloroplasts compose about one-quarter of the mesophyll cell volume and contain about 60% of the cell protein. Photosynthetic carbon assimilation is the dominating metabolism in illuminated leaves. To optimize the resource expenditure in these costly organelles and to control and adjust chloroplast metabolism, an intensive transfer of information between nucleus-cytoplasm and chloroplasts occurs in both directions as anterograde and retrograde signalling. Recent research identified multiple retrograde pathways that use metabolite transfer and include reaction products of lipids and carotenoids with reactive oxygen species (ROS). Other pathways use metabolites of carbon, sulfur and nitrogen metabolism, low molecular weight antioxidants and hormone precursors to carry information between the cell compartments. This review focuses on redox- and ROS-related retrograde signalling pathways. In analogy to the microbe-associated molecular pattern, we propose the term 'chloroplast-associated molecular pattern' which connects chloroplast performance to extrachloroplast processes such as nuclear gene transcription, posttranscriptional processing, including translation, and RNA and protein fate. This article is part of the theme issue 'Retrograde signalling from endosymbiotic organelles'.

Photosynthetic signalling during high light stress and recovery: targets and dynamics

The photosynthetic apparatus is one of the major primary sensors of the plant's external environment. Changes in environmental conditions affect the balance between harvested light energy and the capacity to deal with excited electrons in the stroma, which alters the redox homeostasis of the photosynthetic electron transport chain. Disturbances to redox balance activate photosynthetic regulation mechanisms and trigger signalling cascades that can modify the transcription of nuclear genes. H₂O₂ and oxylipins have been identified as especially prominent regulators of gene expression in response to excess light stress. This paper explores the hypothesis that photosynthetic imbalance triggers specific signals that target discrete gene profiles and biological processes. Analysis of the major retrograde signalling pathways engaged during high light stress and recovery demonstrates both specificity and overlap in gene targets. This work reveals distinct, time-resolved profiles of gene expression that suggest a regulatory interaction between rapidly activated abiotic stress response and induction of secondary metabolism and detoxification processes during recovery. The findings of this study show that photosynthetic electron transport provides a finely tuned sensor for detecting and responding to the environment through chloroplast retrograde signalling. This article is part of the theme issue 'Retrograde signalling from endosymbiotic organelles'.

Linking mitochondrial and chloroplast retrograde signalling in plants

Retrograde signalling refers to the regulation of nuclear gene expression in response to functional changes in organelles. In plants, the two energy-converting organelles, mitochondria and chloroplasts, are tightly coordinated to balance their activities. Although our understanding of components involved in retrograde signalling has greatly increased in the last decade, studies on the regulation of the two organelle signalling pathways have been largely independent. Thus, the mechanism of how mitochondrial and chloroplastic retrograde signals are integrated is largely unknown. Here, we summarize recent findings on the function of mitochondrial signalling components and their links to chloroplast retrograde responses. From this, a picture emerges showing that the major regulators are integrators of both organellar retrograde signalling pathways. This article is part of the theme issue 'Retrograde signalling from endosymbiotic organelles'.

Singlet Oxygen in Plants: Generation, Detection, and Signaling Roles

Singlet oxygen (1O2) refers to the lowest excited electronic state of molecular oxygen. It easily oxidizes biological molecules and, therefore, is cytotoxic. In plant cells, 1O2 is formed mostly in the light in thylakoid membranes by reaction centers of photosystem II. In high concentrations, 1O2 destroys membranes, proteins and DNA, inhibits protein synthesis in chloroplasts leading to photoinhibition of photosynthesis, and can result in cell death. However, 1O2 also acts as a signal relaying information from chloroplasts to the nucleus, regulating expression of nuclear genes. In spite of its extremely short lifetime, 1O2 can diffuse from the chloroplasts into the cytoplasm and the apoplast. As shown by recent studies, 1O2-activated signaling pathways depend not only on the levels but also on the sites of 1O2 production in chloroplasts, and can activate two types of responses, either acclimation to high light or programmed cell death. 1O2 can be produced in high amounts also in root cells during drought stress. This review summarizes recent advances in research on mechanisms and sites of 1O2 generation in plants, on 1O2-activated pathways of retrograde- and cellular signaling, and on the methods to study 1O2 production in plants.

Execution of programmed cell death by singlet oxygen generated inside the chloroplasts of Arabidopsis thaliana

Absorption of excess excitation energy induces overproduction of singlet oxygen (¹O₂) in plants. The major sources of singlet oxygen production are chlorophyll and its intermediates located in the chloroplast. Over-accumulation of the chlorophyll biosynthetic intermediate protochlorophyllide by the exogenous application of 5-aminolevulinic acid (ALA), the precursor of tetrapyrrole, induced singlet oxygen production in the plastidic membranes. Over-expression of protochlorophyllide oxidoreductase C (PORC) in Arabidopsis thaliana resulted in efficient light-induced photo-transformation of protochlorophyllide to chlorophyllide that limited the accumulation of protochlorophyllide. Consequently, the ¹O₂ generation decreased in the PORC overexpressors (PORCx) and their cell death was minimal. Conversely, porC-2 over-accumulated protochlorophyllide in response to ALA treatment and generated higher amounts of ¹O₂ in light and had highest cell death as monitored by Evans blue staining. The protoplasts isolated from PORCx plants, when treated with ALA, generated minimal amounts of ¹O₂ as revealed by singlet oxygen sensor green (SOSG) fluorescence emission from chloroplasts. Conversely, the protoplasts of porC-2 mutants under identical conditions generated the maximum SOSG fluorescence in their chloroplasts and cytosol surrounding the chloroplasts most likely due to the leakage from the organelle. The membrane blebbing, a hallmark of programmed cell death, was clearly visible in WT and porC-2 protoplasts. Similarly, the nick end labelling (TUNEL) assay revealed nicks in the DNA. The TUNEL-positive nuclei after 30 min of light exposure were highest in porC-2 and lowest in PORCx protoplasts. The results demonstrate that higher amounts of singlet oxygen produced in the chloroplasts play an important role in programmed cell death.

The Arabidopsis SAFEGUARD1 suppresses singlet oxygen-induced stress responses by protecting grana margins

Singlet oxygen (¹O₂), the major reactive oxygen species (ROS) produced in chloroplasts, has been demonstrated recently to be a highly versatile signal that induces various stress responses. In the fluorescent (flu) mutant, its release causes seedling lethality and inhibits mature plant growth. However, these drastic phenotypes are suppressed when EXECUTER1 (EX1) is absent in the flu ex1 double mutant. We identified SAFEGUARD1 (SAFE1) in a screen of ethyl methanesulfonate (EMS) mutagenized flu ex1 plants for suppressor mutants with a flu-like phenotype. In flu ex1 safe1, all ¹O₂-induced responses, including transcriptional rewiring of nuclear gene expression, return to levels, such as, or even higher than, those in flu Without SAFE1, grana margins (GMs) of chloroplast thylakoids (Thys) are specifically damaged upon ¹O₂ generation and associate with plastoglobules (PGs). SAFE1 is localized in the chloroplast stroma, and release of ¹O₂ induces SAFE1 degradation via chloroplast-originated vesicles. Our paper demonstrates that flu-produced ¹O₂ triggers an EX1-independent signaling pathway and proves that SAFE1 suppresses this signaling pathway by protecting GMs.