H2O2-triggered retrograde signaling from chloroplasts to nucleus plays specific role in response to stress

Unveiling the role of epigenetic mechanisms and redox signaling in alleviating multiple abiotic stress in plants

Anthropogenic activities and subsequent global climate change instigate drastic crop productivity and yield changes. These changes comprise a rise in the number and severity of plant stress factors, which can arise simultaneously or sequentially. When abiotic stress factors are combined, their impact on plants is more substantial than that of a singleton stress factor. One such impact is the alteration of redox cellular homeostasis, which, in turn, can regulate downstream stress-responsive gene expression and resistance response. The epigenetic regulation of gene expression in response to varied stress factors is an interesting phenomenon, which, conversely, can be stable and heritable. The epigenetic control in plants in response to abiotic stress combinations and their interactions with cellular redox alteration is an emerging field to commemorate crop yield management under climate change. The article highlights the integration of the redox signaling pathways and epigenetic regulations as pivotal components in the complex network of plant responses against multi-combinatorial stresses across time and space. This review aims to lay the foundation for developing novel approaches to mitigate the impact of environmental stresses on crop productivity, bridging the gap between theoretical understanding and practical solutions in the face of a changing climate and anthropogenic disturbances.

Overexpression of EgrZFP6 from Eucalyptus grandis increases ROS levels by downregulating photosynthesis in plants

In plants, abiotic stressors are frequently encountered during growth and development. To counteract these challenges, zinc finger proteins play a critical role as transcriptional regulators. The EgrZFP6 gene, which codes for a zinc finger protein of the C2H2 type, was shown to be considerably elevated in the leaves of Eucalyptus grandis seedlings in the current study when they were subjected to a variety of abiotic stimuli, including heat, salinity, cold, and drought. Analysis conducted later showed that in EgrZFP6 transgenic Arabidopsis thaliana, EgrZFP6 was essential for causing hyponastic leaves and controlling the stress response. Furthermore, the transgenic plants showed elevated levels of reactive oxygen species (ROS), such as superoxide and hydrogen peroxide (H2O2). Additionally, in EgrZFP6-overexpressing plants, transcriptome sequencing analysis demonstrated a considerable downregulation of many genes involved in photosynthesis, decreasing electron transport efficiency and perhaps promoting the buildup of ROS. Auxin levels were higher and auxin signal transduction was compromised in the transgenic plants. Stress-related genes were also upregulated in Arabidopsis as a result of EgrZFP6 overexpression. It is hypothesized that EgrZFP6 can downregulate photosynthesis, which would cause the production of ROS in chloroplasts. As a result, this protein may alter plant stress responses and leaf morphology via a retrograde mechanism driven by ROS. These results highlight the significance of zinc finger proteins in this sophisticated process and advance our understanding of the complex link between gene regulation, ROS signaling, and plant stress responses.

Emerging concern of nano-pollution in agro-ecosystem: Flip side of nanotechnology

Nanomaterials (NMs) have proven to be a game-changer in agriculture, showcasing their potential to boost plant growth and safeguarding crops. The agricultural sector has widely adopted NMs, benefiting from their small size, high surface area, and optical properties to augment crop productivity and provide protection against various stressors. This is attributed to their unique characteristics, contributing to their widespread use in agriculture. Human exposure from various components of agro-environmental sectors (soil, crops) NMs residues are likely to upsurge with exposure paths may stimulates bioaccumulation in food chain. With the aim to achieve sustainability, nanotechnology (NTs) do exhibit its potentials in various domains of agriculture also have its flip side too. In this review article we have opted a fusion approach using bibliometric based analysis of global research trend followed by a holistic assessment of pros and cons i.e. toxicological aspect too. Moreover, we have also tried to analyse the current scenario of policy associated with the application of NMs in agro-environment.

Complete Loss of RelA and SpoT Homologs in Arabidopsis Reveals the Importance of the Plastidial Stringent Response in the Interplay between Chloroplast Metabolism and Plant Defense Response

The highly phosphorylated nucleotide, guanosine tetraphosphate (ppGpp), functions as a secondary messenger in bacteria and chloroplasts. The accumulation of ppGpp alters plastidial gene expression and metabolism, which are required for proper photosynthetic regulation and robust plant growth. However, because four plastid-localized ppGpp synthases/hydrolases function redundantly, the impact of the loss of ppGpp-dependent stringent response on plant physiology remains unclear. We used CRISPR/Cas9 technology to generate an Arabidopsis thaliana mutant lacking all four ppGpp synthases/hydrolases and characterized its phenotype. The mutant showed over 20-fold less ppGpp levels than the wild type under normal growth conditions and exhibited leaf chlorosis and increased expression of defense-related genes as well as salicylic acid and jasmonate levels upon transition to nitrogen-starvation conditions. These results demonstrate that proper levels of ppGpp in plastids are required for controlling not only plastid metabolism but also phytohormone signaling, which is essential for plant defense.

Physiological function and regulation of ascorbate peroxidase isoforms

Ascorbate peroxidase (APX) reduces H2O2 to H2O by utilizing ascorbate as a specific electron donor and constitutes the ascorbate-glutathione cycle in organelles of plants including chloroplasts, cytosol, mitochondria, and peroxisomes. It has been almost 40 years since APX was discovered as an important plant-specific H2O2-scavenging enzyme, during which time many research groups have conducted molecular physiological analyses. It is now clear that APX isoforms function not only just as antioxidant enzymes but also as important factors in intracellular redox regulation through the metabolism of reactive oxygen species. The function of APX isoforms is regulated at multiple steps, from the transcriptional level to post-translational modifications of enzymes, thereby allowing them to respond flexibly to ever-changing environmental factors and physiological phenomena such as cell growth and signal transduction. In this review, we summarize the physiological functions and regulation mechanisms of expression of each APX isoform.

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.

J, Hase T, Jensen P, Hanke GT. Ferredoxin C2 is required for chlorophyll biosynthesis and accumulation of photosynthetic antennae in Arabidopsis

Ferredoxins (Fd) are small iron-sulphur proteins, with sub-types that have evolved for specific redox functions. Ferredoxin C2 (FdC2) proteins are essential Fd homologues conserved in all photosynthetic organisms and a number of different FdC2 functions have been proposed in angiosperms. Here we use RNAi silencing in Arabidopsis thaliana to generate a viable fdC2 mutant line with near-depleted FdC2 protein levels. Mutant leaves have ~50% less chlorophyll a and b, and chloroplasts have poorly developed thylakoid membrane structure. Transcriptomics indicates upregulation of genes involved in stress responses. Although fdC2 antisense plants show increased damage at photosystem II (PSII) when exposed to high light, PSII recovers at the same rate as wild type in the dark. This contradicts literature proposing that FdC2 regulates translation of the D1 subunit of PSII, by binding to psbA transcript. Measurement of chlorophyll biosynthesis intermediates revealed a build-up of Mg-protoporphyrin IX, the substrate of the aerobic cyclase. We localise FdC2 to the inner chloroplast envelope and show that the FdC2 RNAi line has a disproportionately lower protein abundance of antennae proteins, which are nuclear-encoded and must be refolded at the envelope after import.

Potential role of tocopherol in protecting crop plants against abiotic stresses

The changing global climate have given rise to abiotic stresses that adversely affect the metabolic activities of plants, limit their growth, and agricultural output posing a serious threat to food production. The abiotic stresses commonly lead to production of reactive oxygen species (ROS) that results in cellular oxidation. Over the course of evolution, plants have devised efficient enzymatic and non-enzymatic anti-oxidative strategies to counteract harmful effects of ROS. Among the emerging non-enzymatic anti-oxidative technologies, the chloroplast lipophilic antioxidant vitamin A (Tocopherol) shows great promise. Working in coordination with the other cellular antioxidant machinery, it scavenges ROS, prevents lipid peroxidation, regulates stable cellular redox conditions, simulates signal cascades, improves membrane stability, confers photoprotection and enhances resistance against abiotic stresses. The amount of tocopherol production varies based on the severity of stress and its proposed mechanism of action involves arresting lipid peroxidation while quenching singlet oxygen species and lipid peroxyl radicals. Additionally, studies have demonstrated its coordination with other cellular antioxidants and phytohormones. Despite its significance, the precise mechanism of tocopherol action and signaling coordination are not yet fully understood. To bridge this knowledge gap, the present review aims to explore and understand the biosynthesis and antioxidant functions of Vitamin E, along with its signal transduction and stress regulation capacities and responses. Furthermore, the review delves into the light harvesting and photoprotection capabilities of tocopherol. By providing insights into these domains, this review offers new opportunities and avenues for using tocopherol in the management of abiotic stresses in agriculture.

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.

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.

Novel insight into functions of ascorbate peroxidase in higher plants: More than a simple antioxidant enzyme

As plants are sessile organisms, they are inevitably exposed to a variety of environmental stimuli that trigger rapid changes in the generation and disposal of reactive oxygen species such as hydrogen peroxide (H₂O₂). A major H₂O₂ scavenging system in plant cells is the ascorbate-glutathione cycle, in which ascorbate peroxidase (APX) catalyzes the conversion of H₂O₂ into water employing ascorbate as specific electron donor. In higher plants, distinct APX isoforms can occur in multiple subcellular compartments, including chloroplasts, mitochondria, and peroxisomes and the cytosol, to modulate organellar and cellular levels of H₂O₂. It is well established that APX plays crucial roles in protecting plant cells against diverse environmental stresses, as well as in plant growth and development. Apart from ascorbate, recently, APXs have been found to have a broader substrate specificity and possess chaperone activity, hence participating various biological processes. In this review, we describe the antioxidant properties of APXs and highlight their novel roles beyond 'ascorbate peroxidases'.

Design of an Arabidopsis thaliana reporter line to detect heat-sensing and signaling mutants

BACKGROUND

Global warming is a major challenge for plant survival and growth. Understanding the molecular mechanisms by which higher plants sense and adapt to upsurges in the ambient temperature is essential for developing strategies to enhance plant tolerance to heat stress. Here, we designed a heat-responsive Arabidopsis thaliana reporter line that allows an in-depth investigation of the mechanisms underlying the accumulation of protective heat-shock proteins (HSPs) in response to high temperature.

METHODS

A transgenic Arabidopsis thaliana reporter line named "Heat-Inducible Bioluminescence And Toxicity" (HIBAT) was designed to express from a conditional heat-inducible promoter, a fusion gene encoding for nanoluciferase and D-amino acid oxidase, whose expression is toxic in the presence of D-valine. HIBAT seedlings were exposed to different heat treatments in presence or absence of D-valine and analyzed for survival rate, bioluminescence and HSP gene expression.

RESULTS

Whereas at 22 °C, HIBAT seedlings grew unaffected by D-valine, and all survived iterative heat treatments without D-valine, 98% died following heat treatments on D-valine. The HSP17.3B promoter was highly specific to heat as it remained unresponsive to various plant hormones, Flagellin, H₂O₂, osmotic stress and high salt. RNAseq analysis of heat-treated HIBAT seedlings showed a strong correlation with expression profiles of two wild type lines, confirming that HIBAT does not significantly differ from its Col-0 parent. Using HIBAT, a forward genetic screen revealed candidate loss-of-function mutants, apparently defective either at accumulating HSPs at high temperature or at repressing HSP accumulation at non-heat-shock temperatures.

CONCLUSION

HIBAT is a valuable candidate tool to identify Arabidopsis mutants defective in the response to high temperature stress. It opens new avenues for future research on the regulation of HSP expression and for understanding the mechanisms of plant acquired thermotolerance.

Hormesis Responses of Photosystem II in Arabidopsis thaliana under Water Deficit Stress

Since drought stress is one of the key risks for the future of agriculture, exploring the molecular mechanisms of photosynthetic responses to water deficit stress is, therefore, fundamental. By using chlorophyll fluorescence imaging analysis, we evaluated the responses of photosystem II (PSII) photochemistry in young and mature leaves of Arabidopsis thaliana Col-0 (cv Columbia-0) at the onset of water deficit stress (OnWDS) and under mild water deficit stress (MiWDS) and moderate water deficit stress (MoWDS). Moreover, we tried to illuminate the underlying mechanisms in the differential response of PSII in young and mature leaves to water deficit stress in the model plant A. thaliana. Water deficit stress induced a hormetic dose response of PSII function in both leaf types. A U-shaped biphasic response curve of the effective quantum yield of PSII photochemistry (Φ_PSII) in A. thaliana young and mature leaves was observed, with an inhibition at MiWDS that was followed by an increase in Φ_PSII at MoWDS. Young leaves exhibited lower oxidative stress, evaluated by malondialdehyde (MDA), and higher levels of anthocyanin content compared to mature leaves under both MiWDS (+16%) and MoWDS (+20%). The higher Φ_PSII of young leaves resulted in a decreased quantum yield of non-regulated energy loss in PSII (Φ_NO), under both MiWDS (-13%) and MoWDS (-19%), compared to mature leaves. Since Φ_NO represents singlet-excited oxygen (¹O₂) generation, this decrease resulted in lower excess excitation energy at PSII, in young leaves under both MiWDS (-10%) and MoWDS (-23%), compared to mature leaves. The hormetic response of PSII function in both young and mature leaves is suggested to be triggered, under MiWDS, by the intensified reactive oxygen species (ROS) generation, which is considered to be beneficial for activating stress defense responses. This stress defense response that was induced at MiWDS triggered an acclimation response in A. thaliana young leaves and provided tolerance to PSII when water deficit stress became more severe (MoWDS). We concluded that the hormesis responses of PSII in A. thaliana under water deficit stress are regulated by the leaf developmental stage that modulates anthocyanin accumulation in a stress-dependent dose.

Genetic Research Progress: Heat Tolerance in Rice

Heat stress (HS) caused by high-temperature weather seriously threatens international food security. Indeed, as an important food crop in the world, the yield and quality of rice are frequently affected by HS. Therefore, clarifying the molecular mechanism of heat tolerance and cultivating heat-tolerant rice varieties is urgent. Here, we summarized the identified quantitative trait loci (Quantitative Trait Loci, QTL) and cloned rice heat tolerance genes in recent years. We described the plasma membrane (PM) response mechanisms, protein homeostasis, reactive oxygen species (ROS) accumulation, and photosynthesis under HS in rice. We also explained some regulatory mechanisms related to heat tolerance genes. Taken together, we put forward ways to improve heat tolerance in rice, thereby providing new ideas and insights for future research.

Role of reactive oxygen species in the modulation of auxin flux and root development in Arabidopsis thaliana

Reactive oxygen species (ROS) play a dual role in plant biology, acting as important signal transduction molecules and as toxic byproducts of aerobic metabolism that accumulate in cells upon exposure to different stressors and lead to cell death. In plants, root architecture is regulated by the distribution and intercellular flow of the phytohormone auxin. In this study, we identified ROS as an important modulator of auxin distribution and response in the root. ROS production is necessary for root growth, proper tissue patterning, cell growth, and lateral root (LR) induction. Alterations in ROS balance led to altered auxin distribution and response in SOD and RHD2 loss-of-function mutants. Treatment of Arabidopsis seedlings with additional sources of ROS (hydrogen peroxide) or an ROS production inhibitor (diphenylene iodonium) induced phenocopies of the mutants studied. Simultaneous application of auxin and ROS increased LR primordia induction, and PIN-FORMED protein immunolocalization further demonstrated the existing link between auxin and ROS in orchestrating cell division and auxin flux during root development. In Arabidopsis roots, genetic alterations in ROS balance led to defective auxin distribution and growth-related responses in roots. Exogenous hydrogen peroxide alters the establishment of the endogenous auxin gradient in the root meristem through regulation of PIN-FORMED polarity, while the simultaneous application of hydrogen peroxide and auxin enhanced LR induction in a dose- and position-dependent manner through activation of cell division.

Shaping the landscape of N6-methyladenosine RNA methylation in Arabidopsis

N 6-methyladenosine (m6A) modification on messenger RNAs (mRNAs) is deposited by evolutionarily conserved methyltransferases (writers). How individual m6A writers sculpt the overall landscape of the m6A methylome and the resulting biological impact in multicellular organisms remains unknown. Here, we systematically surveyed the quantitative m6A methylomes at single-nucleotide resolution and their corresponding transcriptomes in Arabidopsis (Arabidopsis thaliana) bearing respective impaired m6A writers. The m6A sites associated with the five Arabidopsis writers were located mostly within 3' untranslated regions with peaks at around 100 bp downstream of stop codons. m6A predominantly promoted the usage of distal poly(A) sites but had little effect on RNA splicing. Notably, impaired m6A writers resulted in hypomethylation and downregulation of transcripts encoding ribosomal proteins, indicating a possible correlation between m6A and protein translation. Besides the common effects on mRNA metabolism and biological functions uniquely exerted by different Arabidopsis m6A writers compared with their counterparts in human cell lines, our analyses also revealed the functional specificity of individual Arabidopsis m6A writers in plant development and response to stresses. Our findings thus reveal insights into the biological roles of various Arabidopsis m6A writers and their cognate counterparts in other multicellular m6A methyltransferase complexes.

Recent Development of Advanced Fluorescent Molecular Probes for Organelle-Targeted Cell Imaging

Fluorescent molecular probes are very powerful tools that have been generally applied in cell imaging in the research fields of biology, pathology, pharmacology, biochemistry, and medical science. In the last couple of decades, numerous molecular probes endowed with high specificity to particular organelles have been designed to illustrate intracellular images in more detail at the subcellular level. Nowadays, the development of cell biology has enabled the investigation process to go deeply into cells, even at the molecular level. Therefore, probes that can sketch a particular organelle's location while responding to certain parameters to evaluate intracellular bioprocesses are under urgent demand. It is significant to understand the basic ideas of organelle properties, as well as the vital substances related to each unique organelle, for the design of probes with high specificity and efficiency. In this review, we summarize representative multifunctional fluorescent molecular probes developed in the last decade. We focus on probes that can specially target nuclei, mitochondria, endoplasmic reticulums, and lysosomes. In each section, we first briefly introduce the significance and properties of different organelles. We then discuss how probes are designed to make them highly organelle-specific. Finally, we also consider how probes are constructed to endow them with additional functions to recognize particular physical/chemical signals of targeted organelles. Moreover, a perspective on the challenges in future applications of highly specific molecular probes in cell imaging is also proposed. We hope that this review can provide researchers with additional conceptual information about developing probes for cell imaging, assisting scientists interested in molecular biology, cell biology, and biochemistry to accelerate their scientific studies.

Redox Signaling in Plant Heat Stress Response

The increase in environmental temperature due to global warming is a critical threat to plant growth and productivity. Heat stress can cause impairment in several biochemical and physiological processes. Plants sense and respond to this adverse environmental condition by activating a plethora of defense systems. Among them, the heat stress response (HSR) involves an intricate network of heat shock factors (HSFs) and heat shock proteins (HSPs). However, a growing amount of evidence suggests that reactive oxygen species (ROS), besides potentially being responsible for cellular oxidative damage, can act as signal molecules in HSR, leading to adaptative responses. The role of ROS as toxic or signal molecules depends on the fine balance between their production and scavenging. Enzymatic and non-enzymatic antioxidants represent the first line of defense against oxidative damage and their activity is critical to maintaining an optimal redox environment. However, the HS-dependent ROS burst temporarily oxidizes the cellular environment, triggering redox-dependent signaling cascades. This review provides an overview of the redox-activated mechanisms that participate in the HSR.

Research progress on maintaining chloroplast homeostasis under stress conditions: a review

On a global scale, drought, salinity, extreme temperature, and other abiotic stressors severely limit the quality and yield of crops. Therefore, it is crucial to clarify the adaptation strategies of plants to harsh environments. Chloroplasts are important environmental sensors in plant cells. For plants to thrive in different habitats, chloroplast homeostasis must be strictly regulated, which is necessary to maintain efficient plant photosynthesis and other metabolic reactions under stressful environments. To maintain normal chloroplast physiology, two important biological processes are needed: the import and degradation of chloroplast proteins. The orderly import of chloroplast proteins and the timely degradation of damaged chloroplast components play a key role in adapting plants to their environment. In this review, we briefly described the mechanism of chloroplast TOC-TIC protein transport. The importance and recent progress of chloroplast protein turnover, retrograde signaling, and chloroplast protein degradation under stress are summarized. Furthermore, the potential of targeted regulation of chloroplast homeostasis is emphasized to improve plant adaptation to environmental stresses.

Early detection of late blight in potato by whole-plant redox imaging

Late blight caused by the oomycete Phytophthora infestans is a most devastating disease of potatoes (Solanum tuberosum). Its early detection is crucial for suppressing disease spread. Necrotic lesions are normally seen in leaves at 4 days post-inoculation (dpi) when colonized cells are dead, but early detection of the initial biotrophic growth stage, when the pathogen feeds on living cells, is challenging. Here, the biotrophic growth phase of P. infestans was detected by whole-plant redox imaging of potato plants expressing chloroplast-targeted reduction-oxidation sensitive green fluorescent protein (chl-roGFP2). Clear spots on potato leaves with a lower chl-roGFP2 oxidation state were detected as early as 2 dpi, before any visual symptoms were recorded. These spots were particularly evident during light-to-dark transitions, and reflected the mislocalization of chl-roGFP2 outside the chloroplasts. Image analysis based on machine learning enabled systematic identification and quantification of spots, and unbiased classification of infected and uninfected leaves in inoculated plants. Comparing redox with chlorophyll fluorescence imaging showed that infected leaf areas that exhibit mislocalized chl-roGFP2 also showed reduced non-photochemical quenching and enhanced quantum PSII yield (ΦPSII) compared with the surrounding leaf areas. The data suggest that mislocalization of chloroplast-targeted proteins is an efficient marker of late blight infection, and demonstrate how it can be utilized for non-destructive monitoring of the disease biotrophic stage using whole-plant redox imaging.

Co-regulation of mitochondrial and chloroplast function: Molecular components and mechanisms

The metabolic interdependence, interactions, and coordination of functions between chloroplasts and mitochondria are established and intensively studied. However, less is known about the regulatory components that control these interactions and their responses to external stimuli. Here, we outline how chloroplastic and mitochondrial activities are coordinated via common components involved in signal transduction pathways, gene regulatory events, and post-transcriptional processes. The endoplasmic reticulum emerges as a point of convergence for both transcriptional and post-transcriptional pathways that coordinate chloroplast and mitochondrial functions. Although the identification of molecular components and mechanisms of chloroplast and mitochondrial signaling increasingly suggests common players, this raises the question of how these allow for distinct organelle-specific downstream pathways. Outstanding questions with respect to the regulation of post-transcriptional pathways and the cell and/or tissue specificity of organelle signaling are crucial for understanding how these pathways are integrated at a whole-plant level to optimize plant growth and its response to changing environmental conditions.

Chloroplast redox state changes mark cell-to-cell signaling in the hypersensitive response

Hypersensitive response (HR)-conferred resistance is associated with induction of programmed cell death and pathogen spread restriction in its proximity. The exact role of chloroplastic reactive oxygen species and its link with salicylic acid (SA) signaling in HR remain unexplained. To unravel this, we performed a detailed spatiotemporal analysis of chloroplast redox response in palisade mesophyll and upper epidermis to potato virus Y (PVY) infection in a resistant potato genotype and its transgenic counterpart with impaired SA accumulation and compromised resistance. Besides the cells close to the cell death zone, we detected individual cells with oxidized chloroplasts further from the cell death zone. These are rare in SA-deficient plants, suggesting their role in signaling for resistance. We confirmed that chloroplast redox changes play important roles in signaling for resistance, as blocking chloroplast redox changes affected spatial responses at the transcriptional level. Through spatiotemporal study of stromule induction after PVY infection, we show that stromules are induced by cell death and also as a response to PVY multiplication at the front of infection. Overall induction of stromules is attenuated in SA-deficient plants.

Chloroplast-localized GUN1 contributes to the acquisition of basal thermotolerance in Arabidopsis thaliana

Heat stress (HS) severely affects different cellular compartments operating in metabolic processes and represents a critical threat to plant growth and yield. Chloroplasts are crucial for heat stress response (HSR), signaling to the nucleus the environmental challenge and adjusting metabolic and biosynthetic functions accordingly. GENOMES UNCOUPLED 1 (GUN1), a chloroplast-localized protein, has been recognized as one of the main players of chloroplast retrograde signaling. Here, we investigate HSR in Arabidopsis wild-type and gun1 plantlets subjected to 2 hours of HS at 45°C. In wild-type plants, Reactive Oxygen Species (ROS) accumulate promptly after HS, contributing to transiently oxidize the cellular environment and acting as signaling molecules. After 3 hours of physiological recovery at growth temperature (22°C), the induction of enzymatic and non-enzymatic antioxidants prevents oxidative damage. On the other hand, gun1 mutants fail to induce the oxidative burst immediately after HS and accumulate ROS and oxidative damage after 3 hours of recovery at 22°C, thus resulting in enhanced sensitivity to HS. These data suggest that GUN1 is required to oxidize the cellular environment, participating in the acquisition of basal thermotolerance through the redox-dependent plastid-to-nucleus communication.

Retrograde and anterograde signaling in the crosstalk between chloroplast and nucleus

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

Dehydroascorbate induces plant resistance in rice against root-knot nematode Meloidogyne graminicola

Ascorbic acid (AsA) is an important antioxidant in plants and regulates various physiological processes. In this study, we show that exogenous treatments with the oxidized form of AsA, that is, dehydroascorbate (DHA), activates induced systemic resistance in rice against the root-knot nematode Meloidogyne graminicola, and investigate the molecular and biochemical mechanisms underlying this phenotype. Detailed transcriptome analysis on roots of rice plants showed an early and robust transcriptional response on foliar DHA treatment, with induction of several genes related to plant stress responses, immunity, antioxidant activity, and secondary metabolism already at 1 day after treatment. Quantitative and qualitative evaluation of H₂ O₂ levels confirmed the appearance of a reactive oxygen species (ROS) burst on DHA treatment, both at the site of treatment and systemically. Experiments using chemical ROS inhibitors or scavengers confirmed that H₂ O₂ accumulation contributes to DHA-based induced resistance. Furthermore, hormone measurements in DHA-treated plants showed a significant systemic accumulation of the defence hormone salicylic acid (SA). The role of the SA pathway in DHA-based induced resistance was confirmed by nematode infection experiments using an SA-signalling deficient WRKY45-RNAi line and reverse transcription-quantitative PCR on SA marker genes. Our results collectively reveal that DHA activates induced systemic resistance in rice against the root-knot nematode M. graminicola, mediated through the production of ROS and activation of the SA pathway.

Iron Source and Medium pH Affect Nutrient Uptake and Pigment Content in Petunia hybrida ‘Madness Red’ Cultured In Vitro

Deficiency or excess of iron (Fe) and improper medium pH will inhibit the growth and development of plants, reduce the transfer and utilization of energy from the root to the leaf, and affect the utilization efficiency of inorganic nutrients. The most common symptom of Fe deficiency in plants is chlorosis of the young leaves. In this study, the effects of the iron source, in combination with the medium pH, on plant growth and development, plant pigment synthesis, and nutrient uptake in a model plant Petunia hybrida cultured in vitro were investigated. Iron sulfate (FeSO4·7H2O) or iron chelated with ethylenediaminetetraacetic acid (Fe-EDTA) were supplemented to the MNS (a multipurpose nutrient solution) medium at a concentration of 2.78 mg·L−1 Fe, and the treatment without any Fe was used as the control. The pH of the agar-solidified medium was adjusted to either 4.70, 5.70, or 6.70 before autoclaving. The experiment was carried out in an environmentally controlled culture room with a temperature of 24 °C with 100 µmol·m−2·s−1 photosynthetic photon flux density (PPFD) supplied by white light emitting diodes (LEDs) during a photoperiod of 16 h a day, 18 °C for 8 h a day in the dark, and 70% relative humidity. Regardless of the Fe source including the control, the greatest number of leaves was observed at pH 4.70. However, the greatest lengths of the leaf and root were observed in the treatment with Fe-EDTA combined with pH 5.70. The contents of the chlorophyll, carotenoid, and anthocyanin decreased with increasing medium pH, and contents of these plant pigments were positively correlated with the leaf color. The highest soluble protein content and activities of APX and CAT were observed in the Fe-EDTA under pH 5.70. However, the GPX activity was the highest in the control under pH 4.70. In addition, the highest contents of ammonium (NH4+) and nitrate (NO3−) were measured in the FeSO4-4.7 and EDTA-5.7, respectively. More than that, the treatment of Fe-EDTA combined with pH 5.70 (EDTA-5.7) enhanced nutrient absorption, as proven by the highest tissue contents of P, K, Ca, Mg, Fe, and Mn. The genes’ ferric reduction oxidase 1 and 8 (PhFRO1 and PhFRO8), iron-regulated transporter 1 (PhIRT1), nitrate transporter 2.5 (PhNRT2.5), and deoxyhypusine synthase (PhDHS) were expressed at the highest levels in this treatment as well. In the treatment of EDTA-5.7, the reduction and transport of chelated iron in P. hybrida leaves were enhanced, which also affected the transport of nitrate and catalyzed chlorophyll level in leaves. In conclusion, when the medium pH was adjusted to 5.70, supplementation of chelated Fe-EDTA was more conducive to promoting the growth and development of, and absorption of mineral nutrients by, the plant and the expression of related genes in the leaves.

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.

GUN1 involvement in the redox changes occurring during biogenic retrograde signaling

Chloroplast biogenesis requires a tight communication between nucleus and plastids. By retrograde signals, plastids transmit information about their functional and developmental state to adjust nuclear gene expression, accordingly. GENOMES UNCOUPLED 1 (GUN1), a chloroplast-localized protein integrating several developmental and stress-related signals, is one of the main players of retrograde signaling. Here, we focused on the interplay between GUN1 and redox regulation during biogenic retrograde signaling, by investigating redox parameters in Arabidopsis wild type and gun1 seedlings. Our data highlight that during biogenic retrograde signaling superoxide anion (O₂^-) and hydrogen peroxide (H₂O₂) play a different role in response to GUN1. Under physiological conditions, even in the absence of a visible phenotype, gun1 mutants show low activity of superoxide dismutase (SOD) and ascorbate peroxidase (APX), with an increase in O₂^- accumulation and lipid peroxidation, suggesting that GUN1 indirectly protects chloroplasts from oxidative damage. In wild type seedlings, perturbation of chloroplast development with lincomycin causes H₂O₂ accumulation, in parallel with the decrease of ROS-removal metabolites and enzymes. These redox changes do not take place in gun1 mutants which, in contrast, enhance SOD, APX and catalase activities. Our results indicate that in response to lincomycin, GUN1 is necessary for the H₂O₂-dependent oxidation of cellular environment, which might contribute to the redox-dependent plastid-to nucleus communication.

Cold Exposure Memory Reduces Pathogen Susceptibility in Arabidopsis Based on a Functional Plastid Peroxidase System

Chloroplasts serve as cold priming hubs modulating the transcriptional response of Arabidopsis thaliana to a second cold stimulus for several days by postcold accumulation of thylakoid ascorbate peroxidases (tAPX). In an attempt to investigate cross-priming effects of cold on plant pathogen protection, we show here that such a single 24-h cold treatment at 4°C decreased the susceptibility of Arabidopsis to virulent Pseudomonas syringae pv. tomato DC3000 but did not alter resistance against the avirulent P. syringae pv. tomato avRPM1 and P. syringae pv. tomato avrRPS4 strains or the effector-deficient P. syringae pv. tomato strain hrcC^-. The effect of cold priming against P. syringae pv. tomato was active immediately after cold exposure and memorized for at least 5 days. The priming benefit was established independent of the immune regulator Enhanced Disease Susceptibility 1 (EDS1) or activation of the immune-related genes NHL10, FRK1, ICS1 and PR1 but required thylakoid-bound as well as stromal ascorbate peroxidase activities because the effect was absent or weak in corresponding knock-out-lines. Suppression of tAPX postcold regulation in a conditional-inducible tAPX-RNAi line led to increased bacterial growth numbers. This highlights that the plant immune system benefits from postcold regeneration of the protective chloroplast peroxidase system.[Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Intertwined Roles of Reactive Oxygen Species and Salicylic Acid Signaling Are Crucial for the Plant Response to Biotic Stress

One of the earliest hallmarks of plant immune response is production of reactive oxygen species (ROS) in different subcellular compartments, which regulate plant immunity. A suitable equilibrium, which is crucial to prevent ROS overaccumulation leading to oxidative stress, is maintained by salicylic acid (SA), a chief regulator of ROS. However, ROS not only act downstream of SA signaling, but are also proposed to be a central component of a self-amplifying loop that regulates SA signaling as well as the interaction balance between different phytohormones. The exact role of this crosstalk, the position where SA interferes with ROS signaling and ROS interferes with SA signaling and the outcome of this regulation, depend on the origin of ROS but also on the pathosystem. The precise spatiotemporal regulation of organelle-specific ROS and SA levels determine the effectiveness of pathogen arrest and is therefore crucial for a successful immune response. However, the regulatory interplay behind still remains poorly understood, as up until now, the role of organelle-specific ROS and SA in hypersensitive response (HR)-conferred resistance has mostly been studied by altering the level of a single component. In order to address these aspects, a sophisticated combination of research methods for monitoring the spatiotemporal dynamics of key players and transcriptional activity in plants is needed and will most probably consist of biosensors and precision transcriptomics.

Transcriptomic Data Meta-Analysis Sheds Light on High Light Response in Arabidopsis thaliana L

The availability and intensity of sunlight are among the major factors of growth, development and metabolism in plants. However, excessive illumination disrupts the electronic balance of photosystems and leads to the accumulation of reactive oxygen species in chloroplasts, further mediating several regulatory mechanisms at the subcellular, genetic, and molecular levels. We carried out a comprehensive bioinformatic analysis that aimed to identify genetic systems and candidate transcription factors involved in the response to high light stress in Arabidopsis thaliana L. using resources GEO NCBI, string-db, ShinyGO, STREME, and Tomtom, as well as programs metaRE, CisCross, and Cytoscape. Through the meta-analysis of five transcriptomic experiments, we selected a set of 1151 differentially expressed genes, including 453 genes that compose the gene network. Ten significantly enriched regulatory motifs for TFs families ZF-HD, HB, C2H2, NAC, BZR, and ARID were found in the promoter regions of differentially expressed genes. In addition, we predicted families of transcription factors associated with the duration of exposure (RAV, HSF), intensity of high light treatment (MYB, REM), and the direction of gene expression change (HSF, S1Fa-like). We predicted genetic components systems involved in a high light response and their expression changes, potential transcriptional regulators, and associated processes.

Emerging Roles of Motile Epidermal Chloroplasts in Plant Immunity

Plant epidermis contains atypical small chloroplasts. However, the physiological role of this organelle is unclear compared to that of large mesophyll chloroplasts, the well-known function of which is photosynthesis. Although knowledge of the involvement of chloroplasts in the plant immunity has been expanded to date, the differences between the epidermal and mesophyll chloroplasts are beyond the scope of this study. Given the role of the plant epidermis as a barrier to environmental stresses, including pathogen attacks, and the immune-related function of chloroplasts, plant defense research on epidermal chloroplasts is an emerging field. Recent studies have revealed the dynamic movements of epidermal chloroplasts in response to fungal and oomycete pathogens. Furthermore, epidermal chloroplast-associated proteins and cellular events that are tightly linked to epidermal resistance against pathogens have been reported. In this review, I have focused on the recent progress in epidermal chloroplast-mediated plant immunity.

Photosynthesis of winter wheat effectively reflected multiple physiological responses under short-term drought-rewatering conditions

BACKGROUND

Based on the interrelationship among photosynthesis (Pn), water consumption and drought resistance physiology under water changes, this study aimed to explore whether easily measured Pn could be used to reflect the physiological state of winter wheat and soil moisture. The study was a greenhouse pot experiment, with three growth periods and four gradients of moisture.

RESULTS

The instantaneous water use efficiency of wheat improved significantly under short-term regulated deficit irrigation conditions. The photosynthetic parameters could effectively reflect the level of soil moisture (receiver operating characteristic curve analysis, area under the curve = 0.683-0.988). There was a significant correlation between Pn and yield under drought and rewatering (P < 0.05). The water consumption of winter wheat was significantly reduced by 15.5% to 47.6% (P < 0.05) during drought owing to the reduction of stomatal conductance and transpiration rate (Tr). There was a significant linear relationship between Tr and daily water consumption (R²  > 0.745, P< 0.05). There was a significant quadratic linear relationship (R²  > 0.600, P < 0.05) between Pn and the drought resistance indicators. The protective effect of drought resistance physiology on Pn was more significant during drought than during rewatering. Among the four physiological indicators of drought resistance, the relationship between peroxidase activity and Pn was relatively close (grey relational analysis, GRO = 1).

CONCLUSIONS

The photosynthetic parameters during conditions of short-term water changes could effectively reflect the status of soil moisture, water consumption, yield and drought resistance. A focus on Pn and the rational use of related relationships are conducive to the selection of drought-resistant varieties and developing refined agricultural management. © 2021 Society of Chemical Industry.

Chloroplast Thylakoidal Ascorbate Peroxidase, PtotAPX, Has Enhanced Resistance to Oxidative Stress in Populus tomentosa

Chloroplasts are the most major producers of reactive oxygen species (ROS) during photosynthesis. However, the function of thylakoid ascorbate peroxidase (tAPX) in response to oxidative stress in wood trees is largely unknown. Our results showed that PtotAPX of Populus tomentosa could effectively utilize ascorbic acid (AsA) to hydrolyze hydrogen peroxide (H2O2) in vitro. The overexpression or antisense of PtotAPX (OX-PtotAPX or anti-PtotAPX, respectively) in Populus tomentosa plants did not significantly affect plant morphology during plant growth. When treated with methyl viologen (MV), the OX-PtotAPX plants exhibited less morphological damage under stress conditions compared to WT plants. OX-PtotAPX plants maintained lower H2O2 levels and malondialdehyde (MDA) contents, but more reduced AsA levels, a higher photosynthetic rate (Pn), and the maximal photochemical efficiency of PSII (Fv/Fm), whereas anti-PtotAPX plants showed the opposite phenotype. Furthermore, the activity of APX was slightly higher in OX-PtotAPX under normal growth conditions, and this activity significantly decreased after stress treatment, which was the lowest in anti-P. Based on these results, we propose that PtotAPX is important for protecting the photosynthetic machinery under severe oxidative stress conditions in P. tomentosa, and is a potential genetic resource for regulating the stress tolerance of woody plants.

How to Cope with the Challenges of Environmental Stresses in the Era of Global Climate Change: An Update on ROS Stave off in Plants

With the advent of human civilization and anthropogenic activities in the shade of urbanization and global climate change, plants are exposed to a complex set of abiotic stresses. These stresses affect plants’ growth, development, and yield and cause enormous crop losses worldwide. In this alarming scenario of global climate conditions, plants respond to such stresses through a highly balanced and finely tuned interaction between signaling molecules. The abiotic stresses initiate the quick release of reactive oxygen species (ROS) as toxic by-products of altered aerobic metabolism during different stress conditions at the cellular level. ROS includes both free oxygen radicals {superoxide (O₂^•−) and hydroxyl (OH⁻)} as well as non-radicals [hydrogen peroxide (H₂O₂) and singlet oxygen (¹O₂)]. ROS can be generated and scavenged in different cell organelles and cytoplasm depending on the type of stimulus. At high concentrations, ROS cause lipid peroxidation, DNA damage, protein oxidation, and necrosis, but at low to moderate concentrations, they play a crucial role as secondary messengers in intracellular signaling cascades. Because of their concentration-dependent dual role, a huge number of molecules tightly control the level of ROS in cells. The plants have evolved antioxidants and scavenging machinery equipped with different enzymes to maintain the equilibrium between the production and detoxification of ROS generated during stress. In this present article, we have focused on current insights on generation and scavenging of ROS during abiotic stresses. Moreover, the article will act as a knowledge base for new and pivotal studies on ROS generation and scavenging.

Analysis of Ascorbate Metabolism in Arabidopsis Under High-Light Stress

Ascorbate is the most abundant soluble antioxidant in plants, and its concentration is enhanced under high-light and other abiotic stresses. One of the main functions of ascorbate is the detoxification of reactive oxygen species, as ascorbate-deficient plants are highly sensitive to high-light-induced photooxidative stress. Its antioxidative role in plants is further complemented by the presence of ascorbate peroxidases, as well as enzymes that recycle ascorbate from its oxidized forms. In parallel with ascorbate biosynthesis, the expression and activity of these enzymes are enhanced by photooxidative stress. Thus, ascorbate metabolism plays a key role in photooxidative stress acclimation. Herein, the present authors' preferred protocols for the application of high-light stress and the measurement of ascorbate and the activity of related enzymes are described.

Function of Chloroplasts in Plant Stress Responses

The chloroplast has a central position in oxygenic photosynthesis and primary metabolism. In addition to these functions, the chloroplast has recently emerged as a pivotal regulator of plant responses to abiotic and biotic stress conditions. Chloroplasts have their own independent genomes and gene-expression machinery and synthesize phytohormones and a diverse range of secondary metabolites, a significant portion of which contribute the plant response to adverse conditions. Furthermore, chloroplasts communicate with the nucleus through retrograde signaling, for instance, reactive oxygen signaling. All of the above facilitate the chloroplast’s exquisite flexibility in responding to environmental stresses. In this review, we summarize recent findings on the involvement of chloroplasts in plant regulatory responses to various abiotic and biotic stresses including heat, chilling, salinity, drought, high light environmental stress conditions, and pathogen invasions. This review will enrich the better understanding of interactions between chloroplast and environmental stresses, and will lay the foundation for genetically enhancing plant-stress acclimatization.

Photosystem I Inhibition, Protection and Signalling: Knowns and Unknowns

Photosynthesis is the process that harnesses, converts and stores light energy in the form of chemical energy in bonds of organic compounds. Oxygenic photosynthetic organisms (i.e., plants, algae and cyanobacteria) employ an efficient apparatus to split water and transport electrons to high-energy electron acceptors. The photosynthetic system must be finely balanced between energy harvesting and energy utilisation, in order to limit generation of dangerous compounds that can damage the integrity of cells. Insight into how the photosynthetic components are protected, regulated, damaged, and repaired during changing environmental conditions is crucial for improving photosynthetic efficiency in crop species. Photosystem I (PSI) is an integral component of the photosynthetic system located at the juncture between energy-harnessing and energy consumption through metabolism. Although the main site of photoinhibition is the photosystem II (PSII), PSI is also known to be inactivated by photosynthetic energy imbalance, with slower reactivation compared to PSII; however, several outstanding questions remain about the mechanisms of damage and repair, and about the impact of PSI photoinhibition on signalling and metabolism. In this review, we address the knowns and unknowns about PSI activity, inhibition, protection, and repair in plants. We also discuss the role of PSI in retrograde signalling pathways and highlight putative signals triggered by the functional status of the PSI pool.

Iron Supplement-Enhanced Growth and Development of Hydrangea macrophylla In Vitro under Normal and High pH

Hydrangea macrophylla is a popular perennial ornamental shrub commercially grown as potted plants, landscape plants, and cut flowers. In the process of reproduction and production of ornamental plants, the absorption of nutrients directly determines the value of the ornamental plants. Hydrangea macrophylla is very sensitive to the content and absorption of the micronutrient iron (Fe) that affects growth of its shoots. However, the physiological activity of Fe as affected by deficiency or supplementation is unknown. This work aimed at preliminary exploring the relationship between Fe and photosynthesis, and also to find the most favorable iron source and level of pH for the growth of H. macrophylla. Two Fe sources, non-chelated iron sulfate (FeSO₄) and iron ethylenediaminetetraacetic acid (Fe-EDTA), were supplemented to the multipurpose medium with a final Fe concentration of 2.78 mg·L^-1. The medium without any Fe supplementation was used as the control. The pH of the agar-solidified medium was adjusted to either 4.70, 5.70, or 6.70, before autoclaving. The experiment was conducted in a culture room for 60 days with 25/18 °C day and night temperatures, and a 16-hour photoperiod provided at a light intensity of 50 mmol·m^-2·s^-1 photosynthetic photon flux density (PPFD) from white light-emitting diodes. Supplementary Fe increased the tissue Fe content, and leaves were greener with the medium pH of 4.70, regardless of the Fe source. Compared to the control, the number of leaves for plantlets treated with FeSO₄ and Fe-EDTA were 2.0 and 1.5 times greater, respectively. The chlorophyll, macronutrient, and micronutrient contents were the greatest with Fe-EDTA at pH 4.70. Furthermore, the Fe in the leaf affected the photosynthesis by regulating stomata development, pigment content, and antioxidant system, and also by adjusting the expression of genes related to Fe absorption, transport, and redistribution. Supplementation of Fe in a form chelated with EDTA along with a medium pH of 4.70 was found to be the best for the growth and development of H. macrophylla plantlets cultured in vitro.

Cold-priming causes dampening of oxylipin biosynthesis and signalling during the early cold- and light-triggering response of Arabidopsis thaliana

Cold-priming uncouples cold and light regulation of otherwise tightly co-regulated genes. In this study, we focused on the early regulatory processes in Arabidopsis within the first 2 h in cold and in high light after a 5-d lag-phase at 20 °C and 24 h cold-priming at 4 °C. Priming quickly modified gene expression in a trigger-specific manner. In the early stress-response phase during cold and high-light triggering, it reduced the regulatory amplitudes of many up- and down-regulated genes. A third of the priming-regulated genes were jasmonate-sensitive, including the full set of genes required for oxylipin biosynthesis. Analysis of wild-type and mutant plants based on qPCR demonstrated that biosynthesis of the jasmonic acid (JA) precursor 12-oxo phytenoic acid (OPDA) relative to the availability of JA dampened the response of the genes for oxylipin biosynthesis. In oxylipin biosynthetic mutants, cold-priming more strongly affected genes involved in the biosynthesis of OPDA than in its conversion to JA. In addition, priming-dependent dampening of the triggering response was more linked to OPDA than to regulation of the JA concentration. Spray application of OPDA prior to triggering counteracted the priming effect. Regulation of the oxylipin hub was controlled by modulation of the oxylipin-sensitivity of the genes for OPDA biosynthesis, but it was insensitive to priming-induced accumulation of thylakoid ascorbate peroxidase, thus identifying a parallel-acting cold-priming pathway.

Earthworm casts restrained the accumulation and phytotoxicity of soil glyphosate to cowpea (Vigna unguiculata (L.) Walp.) plants

The heavy use of glyphosate during the cultivation of glyphosate-resistant crops, would trigger the so called "pseudo-persistent" glyphosate in soil, thereby threatening agricultural environment, crop production, and human health through food safety. Such that, there is pressing need for the development of strategies for the effective management of glyphosate contamination in soil to promote cleaner agricultural production. In this study, as witnessed via characterizing the bioavailability and phytotoxicity of glyphosate to cowpea plants in soils applied with or without earthworm casts (EWCs), EWCs could significantly facilitate glyphosate desorption from soil, thus enhance its bioaccessibility, nonetheless, sharply decreased rather than increased the accumulation of glyphosate in cowpea plants via reducing the residue pool of glyphosate in the soil. Consequently, in comparison with the glyphosate-alone group, EWCs involvement triggered the increase of chlorophyll content, alleviation of ROS accumulation and lipid peroxidation of membrane, and in turn reduced the activity of a series of stress-tolerance enzymes by means of down-regulating the expression of the corresponding mRNA; ultimately, helped plants to reverse the glyphosate-induced growth suppression. Our findings demonstrated that, EWCs were promising candidate for the cost-effective and easy-to-operate remediation and reuse of glyphosate-contaminated soil, while also being able to improve the quality of the cultivated land and promote crop growth and resistance as a nutrients supplier.

Cooperation of chloroplast ascorbate peroxidases and proton gradient regulation 5 is critical for protecting Arabidopsis plants from photo-oxidative stress

High-light (HL) stress enhances the production of H₂ O₂ from the photosynthetic electron transport chain in chloroplasts, potentially causing photo-oxidative damage. Although stromal and thylakoid membrane-bound ascorbate peroxidases (sAPX and tAPX, respectively) are major H₂ O₂ -scavenging enzymes in chloroplasts, their knockout mutants do not exhibit a visible phenotype under HL stress. Trans-thylakoid proton gradient (∆pH)-dependent mechanisms exist for controlling H₂ O₂ production from photosynthesis, such as thermal dissipation of light energy and downregulation of electron transfer between photosystems II and I, and these may compensate for the lack of APXs. To test this hypothesis, we focused on a proton gradient regulation 5 (pgr5) mutant, wherein both ∆pH-dependent mechanisms are impaired, and an Arabidopsis sapx tapx double mutant was crossed with the pgr5 single mutant. The sapx tapx pgr5 triple mutant exhibited extreme sensitivity to HL compared with its parental lines. This phenotype was consistent with cellular redox perturbations and enhanced expression of many oxidative stress-responsive genes. These findings demonstrate that the PGR5-dependent mechanisms compensate for chloroplast APXs, and vice versa. An intriguing finding was that the failure of induction of non-photochemical quenching in pgr5 (because of the limitation in ∆pH formation) was partially recovered in sapx tapx pgr5. Further genetic studies suggested that this recovery was dependent on the NADH dehydrogenase-like complex-dependent pathway for cyclic electron flow around photosystem I. Together with data from the sapx tapx npq4 mutant, we discuss the interrelationship between APXs and ∆pH-dependent mechanisms under HL stress.

FERONIA Confers Resistance to Photooxidative Stress in Arabidopsis

Plants absorb light energy required for photosynthesis, but excess light can damage plant cells. To protect themselves, plants have developed diverse signaling pathways which are activated under high-intensity light. Plant photoprotection mechanisms have been mainly investigated under conditions of extremely high amount of light; thus, it is largely unknown how plants manage photooxidative damage under moderate light intensities. In the present study, we found that FERONIA (FER) is a key protein that confers resistance to photooxidative stress in plants under moderate light intensity. FER-deficient mutants were highly susceptible to increasing light intensity and exhibited photobleaching even under moderately elevated light intensity (ML). Light-induced expression of stress genes was largely diminished by the fer-4 mutation. In addition, excitation pressure on Photosystem II was significantly increased in fer-4 mutants under ML. Consistently, reactive oxygen species, particularly singlet oxygen, accumulated in fer-4 mutants grown under ML. FER protein abundance was found to be elevated after exposure to ML, which is indirectly affected by the ubiquitin-proteasome pathway. Altogether, our findings showed that plants require FER-mediated photoprotection to maintain their photosystems even under moderate light intensity.

Combined Proteomic and Physiological Analysis of Chloroplasts Reveals Drought and Recovery Response Mechanisms in Nicotiana benthamiana

Chloroplasts play essential roles in plant metabolic processes and stress responses by functioning as environmental sensors. Understanding chloroplast responses to drought stress and subsequent recovery will help the ability to improve stress tolerance in plants. Here, a combined proteomic and physiological approach was used to investigate the response mechanisms of Nicotiana benthamiana chloroplasts to drought stress and subsequent recovery. Early in the stress response, changes in stomatal movement were accompanied by immediate changes in protein synthesis to sustain the photosynthetic process. Thereafter, increasing drought stress seriously affected photosynthetic efficiency and led to altered expression of photosynthesis- and carbon-fixation-related proteins to protect the plants against photo-oxidative damage. Additional repair mechanisms were activated at the early stage of recovery to restore physiological functions and repair drought-induced damages, even while the negative effects of drought stress were still ongoing. Prolonging the re-watering period led to the gradual recovery of photosynthesis at both physiological and protein levels, indicating that a long repair process is required to restore plant function. Our findings provide a precise view of drought and recovery response mechanisms in N. benthamiana and serve as a reference for further investigation into the physiological and molecular mechanisms underlying plant drought tolerance.

Redox responses of Arabidopsis thaliana to the green peach aphid, Myzus persicae

The green peach aphid (Myzus persicae) is a phloem-feeding insect that causes economic damage on a wide array of crops. Using a luminol-based assay, a superoxide-responsive reporter gene (Zat12::luciferase), and a probe specific to hydrogen peroxide (HyPer), we demonstrated that this aphid induces accumulation of reactive oxygen species (ROS) in Arabidopsis thaliana. Similar to the apoplastic oxidative burst induced by pathogens, this response to aphids was rapid and transient, with two peaks occurring within 1 and 4 hr after infestation. Aphid infestation also induced an oxidative response in the cytosol and peroxisomes, as measured using a redox-sensitive variant of green fluorescent protein (roGFP2). This intracellular response began within minutes of infestation but persisted 20 hr or more after inoculation, and the response of the peroxisomes appeared stronger than the response in the cytosol. Our results suggest that the oxidative response to aphids involves both apoplastic and intracellular sources of ROS, including ROS generation in the peroxisomes, and these different sources of ROS may potentially differ in their impacts on host suitability for aphids.

Selective redox signaling shapes plant-pathogen interactions

A review of recent progress in understanding the mechanisms whereby plants utilize selective and reversible redox signaling to establish immunity.

A Holistic Approach to Study Photosynthetic Acclimation Responses of Plants to Fluctuating Light

Plants in natural environments receive light through sunflecks, the duration and distribution of these being highly variable across the day. Consequently, plants need to adjust their photosynthetic processes to avoid photoinhibition and maximize yield. Changes in the composition of the photosynthetic apparatus in response to sustained changes in the environment are referred to as photosynthetic acclimation, a process that involves changes in protein content and composition. Considering this definition, acclimation differs from regulation, which involves processes that alter the activity of individual proteins over short-time periods, without changing the abundance of those proteins. The interconnection and overlapping of the short- and long-term photosynthetic responses, which can occur simultaneously or/and sequentially over time, make the study of long-term acclimation to fluctuating light in plants challenging. In this review we identify short-term responses of plants to fluctuating light that could act as sensors and signals for acclimation responses, with the aim of understanding how plants integrate environmental fluctuations over time and tailor their responses accordingly. Mathematical modeling has the potential to integrate physiological processes over different timescales and to help disentangle short-term regulatory responses from long-term acclimation responses. We review existing mathematical modeling techniques for studying photosynthetic responses to fluctuating light and propose new methods for addressing the topic from a holistic point of view.

Profiling thimet oligopeptidase-mediated proteolysis in Arabidopsis thaliana

Protein homeostasis (proteostasis) is crucial for proper cellular function, including the production of peptides with biological functions through controlled proteolysis. Proteostasis has roles in maintenance of cellular functions and plant interactions with the environment under physiological conditions. Plant stress continues to reduce agricultural yields causing substantial economic losses; thus, it is critical to understand how plants perceive stress signals to elicit responses for survival. As previously shown in Arabidopsis thaliana, thimet oligopeptidases (TOPs) TOP1 (also referred to as organellar oligopeptidase) and TOP2 (also referred to as cytosolic oligopeptidase) are essential components in plant response to pathogens, but further characterization of TOPs and their peptide substrates is required to understand their contributions to stress perception and defense signaling. Herein, label-free peptidomics via liquid chromatography-tandem mass spectrometry was used to differentially quantify 1111 peptides, originating from 369 proteins, between the Arabidopsis Col-0 wild type and top1top2 knock-out mutant. This revealed 350 peptides as significantly more abundant in the mutant, representing accumulation of these potential TOP substrates. Ten direct substrates were validated using in vitro enzyme assays with recombinant TOPs and synthetic candidate peptides. These TOP substrates are derived from proteins involved in photosynthesis, glycolysis, protein folding, biogenesis, and antioxidant defense, implicating TOP involvement in processes aside from defense signaling. Sequence motif analysis revealed TOP cleavage preference for non-polar residues in the positions surrounding the cleavage site. Identification of these substrates provides a framework for TOP signaling networks, through which the interplay between proteolytic pathways and defense signaling can be further characterized.

Chloroplast immunity illuminated

The chloroplast has recently emerged as pivotal to co-ordinating plant defence responses and as a target of plant pathogens. Beyond its central position in oxygenic photosynthesis and primary metabolism - key targets in the complex virulence strategies of diverse pathogens - the chloroplast integrates, decodes and responds to environmental signals. The capacity of chloroplasts to synthesize phytohormones and a diverse range of secondary metabolites, combined with retrograde and reactive oxygen signalling, provides exquisite flexibility to both perceive and respond to biotic stresses. These processes also represent a plethora of opportunities for pathogens to evolve strategies to directly or indirectly target 'chloroplast immunity'. This review covers the contribution of the chloroplast to pathogen associated molecular pattern and effector triggered immunity as well as systemic acquired immunity. We address phytohormone modulation of immunity and surmise how chloroplast-derived reactive oxygen species underpin chloroplast immunity through indirect evidence inferred from genetic modification of core chloroplast components and direct pathogen targeting of the chloroplast. We assess the impact of transcriptional reprogramming of nuclear-encoded chloroplast genes during disease and defence and look at future research challenges.