Sensing stress responses in potato with whole-plant redox imaging

Nitrogen Assimilation Plays a Role in Balancing the Chloroplastic Glutathione Redox Potential Under High Light Conditions

Nitrate reduction requires reducing equivalents produced by the photosynthetic electron transport chain. Therefore, it has been suggested that nitrate assimilation provides a sink for electrons under high light conditions. We tested this hypothesis by monitoring photosynthetic efficiency and the chloroplastic glutathione redox potential (chl-EGSH) of plant lines with mutated glutamine synthetase 2 (GS2) and ferredoxin-dependent glutamate synthase 1 (GOGAT1). Mutant lines incorporated significantly less isotopically-labelled nitrate into amino acids than wild-type plants, demonstrating impaired nitrogen assimilation. When nitrate assimilation was compromised, photosystem II (PSII) proved more vulnerable to photodamage. The effect of the nitrate assimilation pathway on the chl- EGSH was monitored using the chloroplast-targeted roGFP2 biosensor (chl-roGFP2). Remarkably, while oxidation followed by reduction of chl-roGFP2 was detected in WT plants in response to high light, oxidation values were stable in the mutant lines, suggesting that chl-EGSH relaxation after high light-induced oxidation is achieved by diverting excess electrons to the nitrogen assimilation pathway. Importantly, similar ΦPSII and chl-roGFP2 patterns were observed at elevated CO2, suggesting that mutant phenotypes are not associated with photorespiration activity. Together, these findings indicate that the nitrogen assimilation pathway serves as a sustainable energy dissipation route, ensuring efficient photosynthetic activity and fine-tuning redox metabolism under light-saturated conditions.

An integrative overview of cold response and regulatory pathways in horticultural crops

Global climate change challenges agricultural production, as extreme temperature fluctuations negatively affect crop growth and yield. Low temperature (LT) stress impedes photosynthesis, disrupts metabolic processes, and compromises the integrity of cell membranes, ultimately resulting in diminished yield and quality. Notably, many tropical or subtropical horticultural plants are particularly susceptible to LT stress. To address these challenges, it is imperative to understand the mechanisms underlying cold tolerance in horticultural crops. This review summarizes recent advances in the physiological and molecular mechanisms that enable horticultural crops to withstand LT stress, emphasizing discrepancies between horticultural crops and model systems. These mechanisms include C-repeat binding factor-dependent transcriptional regulation, post-translational modifications, epigenetic control, and metabolic regulation. Reactive oxygen species, plant hormones, and light signaling pathways are integrated into the cold response network. Furthermore, technical advances for improving cold tolerance are highlighted, including genetic improvement, the application of light-emitting diodes, the utility of novel plant growth regulators, and grafting. Finally, prospective directions for fundamental research and practical applications to boost cold tolerance are discussed.

Enhancing Tolerance to Combined Heat and Drought Stress in Cool-Season Grain Legumes: Mechanisms, Genetic Insights, and Future Directions

The increasing frequency of concurrent heat and drought stress poses a significant challenge to agricultural productivity, particularly for cool-season grain legumes, including broad bean (Vicia Faba L.), lupin (Lupinus spp.), lentil (Lens culinaris Medik), chickpea (Cicer arietinum L.), grasspea (Lathyrus sativus L.), pea (Pisum sativum L.), and common vetch (Vicia sativa L.). These legumes play a vital role in sustainable agricultural systems due to their nitrogen-fixing ability and high nutritional value. This review synthesizes current knowledge of the impacts and tolerance mechanisms associated with combined heat and drought stresses in these crops. We evaluate physiological and biochemical responses to combined heat and drought stress, focusing on their detrimental effects on growth, development, and yield. Key genetic and molecular mechanisms, such as the roles of osmolytes, antioxidants, and stress-responsive genes, are explored. We also discuss the intricate interplay between heat and drought stress signaling pathways, including the involvement of Ca2+ ions, reactive oxygen species, transcription factor DREB2A, and the endoplasmic reticulum in mediating stress responses. This comprehensive analysis offers new insights into developing resilient legume varieties to enhance agricultural sustainability under climate change. Future research should prioritize integrating omics technologies to unravel plant responses to combined abiotic stresses.

Tools to understand hypoxia responses in plant tissues

Our understanding of how low oxygen (O2) conditions arise in plant tissues and how they shape specific responses has seen major advancement in recent years. Important drivers have been (1) the discovery of the molecular machinery that underpins plant O2 sensing; and (2) a growing set of dedicated tools to define experimental conditions and assess plant responses with increasing accuracy and resolution. While some of those tools, such as the Clark-type O2 electrode, were established decades ago, recent customization has set entirely new standards and enabled novel research avenues in plant hypoxia research. Other tools, such as optical hypoxia reporters and O2 biosensor systems, have been introduced more recently. Yet, their adoption into plant hypoxia research has started to generate novel insight into hypoxia physiology at the tissue and cellular levels. The aim of this update is to provide an overview of the currently available and emerging tools for O2 hypoxia measurements in plants, with an emphasis on high-resolution analyses in living plant tissues and cells. Furthermore, it offers directions for future development and deployment of tools to aid progress with the most pressing questions in plant hypoxia research.

Analysis of abiotic and biotic stress-induced Ca2+ transients in the crop species Solanum tuberosum

Secondary messengers, such as calcium ions (Ca2+), are integral parts of a system that transduces environmental stimuli into appropriate cellular responses. Different abiotic and biotic stresses as well as developmental processes trigger temporal increases in cytosolic free Ca2+ levels by an influx from external and internal stores. Stimulus-specificity is obtained by a certain amplitude, duration, oscillation and localisation of the response. Most knowledge on stress-specific Ca2+ transient, called calcium signatures, has been gained in the model plant Arabidopsis thaliana, while reports about stress-related Ca2+ signalling in crop plants are comparatively scarce. In this study, we introduced the Ca2+ biosensor apoaequorin into potato (Solanum tuberosum, Lcv. Désirée). We observed dose-dependent calcium signatures in response to a series of stress stimuli, including H2O2, NaCl, mannitol and pathogen-associated molecular patterns (PAMPs) with stimuli-specific kinetics. Direct comparison with Arabidopsis revealed differences in the kinetics and amplitude of Ca2+ transients between both species, implying species-specific sensitivity for different stress conditions. The potato line generated in this work provides a useful tool for further investigations on stress-induced signalling pathways, which could contribute to the generation of novel, stress-tolerant potato varieties.

Chloroplast thiol redox dynamics through the lens of genetically encoded biosensors

Chloroplasts fix carbon by using light energy and have evolved a complex redox network that supports plastid functions by (i) protecting against reactive oxygen species and (ii) metabolic regulation in response to environmental conditions. In thioredoxin- and glutathione/glutaredoxin-dependent redox cascades, protein cysteinyl redox steady states are set by varying oxidation and reduction rates. The specificity and interplay of these different redox-active proteins are still under investigation, for example to understand how plants cope with adverse environmental conditions by acclimation. Genetically encoded biosensors with distinct specificity can be targeted to subcellular compartments such as the chloroplast stroma, enabling in vivo real-time measurements of physiological parameters at different scales. These data have provided unique insights into dynamic behaviours of physiological parameters and redox-responsive proteins at several levels of the known redox cascades. This review summarizes current applications of different biosensor types as well as the dynamics of distinct protein cysteinyl redox steady states, with an emphasis on light responses.

Phosphorylated transcription factor PuHB40 mediates ROS-dependent anthocyanin biosynthesis in pear exposed to high light

Plants are increasingly vulnerable to environmental stresses because of global warming and climate change. Stress-induced reactive oxygen species (ROS) accumulation results in plant cell damage, even cell death. Anthocyanins are important antioxidants that scavenge ROS to maintain ROS homeostasis. However, the mechanism underlying ROS-induced anthocyanin accumulation is unclear. In this study, we determined that the HD-Zip I family member transcription factor PuHB40 mediates ROS-dependent anthocyanin biosynthesis under high-light stress in pear (Pyrus ussuriensis). Specifically, PuHB40 induces the PuMYB123-like-PubHLH3 transcription factor complex for anthocyanin biosynthesis. The PuHB40-mediated transcriptional activation depends on its phosphorylation level, which is regulated by protein phosphatase PP2A. Elevated ROS content maintains high PuHB40 phosphorylation levels while also enhancing the PuHB40-induced PuMYB123-like transcription by decreasing the PuPP2AA2 expression, ultimately leading to increased anthocyanin biosynthesis. Our study reveals a pathway regulating the ROS-induced anthocyanin biosynthesis in pears, further clarifying the mechanism underlying the abiotic stress-induced anthocyanin biosynthesis, which may have implications for improving plant stress tolerance.

Mechanistic Insights on Salicylic Acid-Induced Enhancement of Photosystem II Function in Basil Plants under Non-Stress or Mild Drought Stress

Photosystem II (PSII) functions were investigated in basil (Ocimum basilicum L.) plants sprayed with 1 mM salicylic acid (SA) under non-stress (NS) or mild drought-stress (MiDS) conditions. Under MiDS, SA-sprayed leaves retained significantly higher (+36%) chlorophyll content compared to NS, SA-sprayed leaves. PSII efficiency in SA-sprayed leaves under NS conditions, evaluated at both low light (LL, 200 μmol photons m-2 s-1) and high light (HL, 900 μmol photons m-2 s-1), increased significantly with a parallel significant decrease in the excitation pressure at PSII (1-qL) and the excess excitation energy (EXC). This enhancement of PSII efficiency under NS conditions was induced by the mechanism of non-photochemical quenching (NPQ) that reduced singlet oxygen (1O2) production, as indicated by the reduced quantum yield of non-regulated energy loss in PSII (ΦNO). Under MiDS, the thylakoid structure of water-sprayed leaves appeared slightly dilated, and the efficiency of PSII declined, compared to NS conditions. In contrast, the thylakoid structure of SA-sprayed leaves did not change under MiDS, while PSII functionality was retained, similar to NS plants at HL. This was due to the photoprotective heat dissipation by NPQ, which was sufficient to retain the same percentage of open PSII reaction centers (qp), as in NS conditions and HL. We suggest that the redox status of the plastoquinone pool (qp) under MiDS and HL initiated the acclimation response to MiDS in SA-sprayed leaves, which retained the same electron transport rate (ETR) with control plants. Foliar spray of SA could be considered as a method to improve PSII efficiency in basil plants under NS conditions, at both LL and HL, while under MiDS and HL conditions, basil plants could retain PSII efficiency similar to control plants.

Contrasting cytosolic glutathione redox dynamics under abiotic and biotic stress in barley as revealed by the biosensor Grx1-roGFP2

Barley is a staple crop of major global importance and relatively resilient to a wide range of stress factors in the field. Transgenic reporter lines to investigate physiological parameters during stress treatments remain scarce. We generated and characterized transgenic homozygous barley lines (cv. Golden Promise Fast) expressing the genetically encoded biosensor Grx1-roGFP2, which indicates the redox potential of the major antioxidant glutathione in the cytosol. Our results demonstrated functionality of the sensor in living barley plants. We determined the glutathione redox potential (EGSH) of the cytosol to be in the range of -308 mV to -320 mV. EGSH was robust against a combined NaCl (150 mM) and water deficit treatment (-0.8 MPa) but responded with oxidation to infiltration with the phytotoxic secretome of the necrotrophic fungus Botrytis cinerea. The generated reporter lines are a novel resource to study biotic and abiotic stress resilience in barley, pinpointing that even severe abiotic stress leading to a growth delay does not automatically induce cytosolic EGSH oxidation, while necrotrophic pathogens can undermine this robustness.

Complex plant responses to drought and heat stress under climate change

Global climate change is predicted to result in increased yield losses of agricultural crops caused by environmental conditions. In particular, heat and drought stress are major factors that negatively affect plant development and reproduction, and previous studies have revealed how these stresses induce plant responses at physiological and molecular levels. Here, we provide a comprehensive overview of current knowledge concerning how drought, heat, and combinations of these stress conditions affect the status of plants, including crops, by affecting factors such as stomatal conductance, photosynthetic activity, cellular oxidative conditions, metabolomic profiles, and molecular signaling mechanisms. We further discuss stress-responsive regulatory factors such as transcription factors and signaling factors, which play critical roles in adaptation to both drought and heat stress conditions and potentially function as 'hubs' in drought and/or heat stress responses. Additionally, we present recent findings based on forward genetic approaches that reveal natural variations in agricultural crops that play critical roles in agricultural traits under drought and/or heat conditions. Finally, we provide an overview of the application of decades of study results to actual agricultural fields as a strategy to increase drought and/or heat stress tolerance. This review summarizes our current understanding of plant responses to drought, heat, and combinations of these stress conditions.

Nanozyme-Based Regulation of Cellular Metabolism and Their Applications

Metabolism is the sum of the enzyme-dependent chemical reactions, which produces energy in catabolic process and synthesizes biomass in anabolic process, exhibiting high similarity in mammalian cell, microbial cell, and plant cell. Consequently, the loss or gain of metabolic enzyme activity greatly affects cellular metabolism. Nanozymes, as emerging enzyme mimics with diverse functions and adjustable catalytic activities, have shown attractive potential for metabolic regulation. Although the basic metabolic tasks are highly similar for the cells from different species, the concrete metabolic pathway varies with the intracellular structure of different species. Here, the basic metabolism in living organisms is described and the similarities and differences in the metabolic pathways among mammalian, microbial, and plant cells and the regulation mechanism are discussed. The recent progress on regulation of cellular metabolism mainly including nutrient uptake and utilization, energy production, and the accompanied redox reactions by different kinds of oxidoreductases and their applications in the field of disease therapy, antimicrobial therapy, and sustainable agriculture is systematically reviewed. Furthermore, the prospects and challenges of nanozymes in regulating cell metabolism are also discussed, which broaden their application scenarios.

Noninvasive In Planta Live Measurements of H2O2 and Glutathione Redox Potential with Fluorescent roGFPs-Based Sensors

In this protocol, we present a noninvasive in planta bioimaging technique for the analysis of hydrogen peroxide (H2O2) and glutathione redox potential in adult Arabidopsis thaliana plants. The technique is based on the use of stereo fluorescence microscopy to image A. thaliana plants expressing the two genetically encoded fluorescent sensors roGFP2-Orp1 and Grx1-roGFP2. We provide a detailed step-by-step protocol for performing low magnification imaging with mature plants grown in soil or hydroponic systems. This protocol aims to serve the scientific community by providing an accessible approach to noninvasive in planta bioimaging and data analysis.

Lighting the light reactions of photosynthesis by means of redox-responsive genetically encoded biosensors for photosynthetic intermediates

Oxygenic photosynthesis involves light and dark phases. In the light phase, photosynthetic electron transport provides reducing power and energy to support the carbon assimilation process. It also contributes signals to defensive, repair, and metabolic pathways critical for plant growth and survival. The redox state of components of the photosynthetic machinery and associated routes determines the extent and direction of plant responses to environmental and developmental stimuli, and therefore, their space- and time-resolved detection in planta becomes critical to understand and engineer plant metabolism. Until recently, studies in living systems have been hampered by the inadequacy of disruptive analytical methods. Genetically encoded indicators based on fluorescent proteins provide new opportunities to illuminate these important issues. We summarize here information about available biosensors designed to monitor the levels and redox state of various components of the light reactions, including NADP(H), glutathione, thioredoxin, and reactive oxygen species. Comparatively few probes have been used in plants, and their application to chloroplasts poses still additional challenges. We discuss advantages and limitations of biosensors based on different principles and propose rationales for the design of novel probes to estimate the NADP(H) and ferredoxin/flavodoxin redox poise, as examples of the exciting questions that could be addressed by further development of these tools. Genetically encoded fluorescent biosensors are remarkable tools to monitor the levels and/or redox state of components of the photosynthetic light reactions and accessory pathways. Reducing equivalents generated at the photosynthetic electron transport chain in the form of NADPH and reduced ferredoxin (FD) are used in central metabolism, regulation, and detoxification of reactive oxygen species (ROS). Redox components of these pathways whose levels and/or redox status have been imaged in plants using biosensors are highlighted in green (NADPH, glutathione, H2O2, thioredoxins). Analytes with available biosensors not tried in plants are shown in pink (NADP+). Finally, redox shuttles with no existing biosensors are circled in light blue. APX, ASC peroxidase; ASC, ascorbate; DHA, dehydroascorbate; DHAR, DHA reductase; FNR, FD-NADP+ reductase; FTR, FD-TRX reductase; GPX, glutathione peroxidase; GR, glutathione reductase; GSH, reduced glutathione; GSSG, oxidized glutathione; MDA, monodehydroascorbate; MDAR, MDA reductase; NTRC, NADPH-TRX reductase C; OAA, oxaloacetate; PRX, peroxiredoxin; PSI, photosystem I; PSII: photosystem II; SOD, superoxide dismutase; TRX, thioredoxin.

NERNST: a genetically-encoded ratiometric non-destructive sensing tool to estimate NADP(H) redox status in bacterial, plant and animal systems

NADP(H) is a central metabolic hub providing reducing equivalents to multiple biosynthetic, regulatory and antioxidative pathways in all living organisms. While biosensors are available to determine NADP⁺ or NADPH levels in vivo, no probe exists to estimate the NADP(H) redox status, a determinant of the cell energy availability. We describe herein the design and characterization of a genetically-encoded ratiometric biosensor, termed NERNST, able to interact with NADP(H) and estimate E_NADP(H). NERNST consists of a redox-sensitive green fluorescent protein (roGFP2) fused to an NADPH-thioredoxin reductase C module which selectively monitors NADP(H) redox states via oxido-reduction of the roGFP2 moiety. NERNST is functional in bacterial, plant and animal cells, and organelles such as chloroplasts and mitochondria. Using NERNST, we monitor NADP(H) dynamics during bacterial growth, environmental stresses in plants, metabolic challenges to mammalian cells, and wounding in zebrafish. NERNST estimates the NADP(H) redox poise in living organisms, with various potential applications in biochemical, biotechnological and biomedical research.

RNA-seq profiling in leaf tissues of two soybean (Glycine max [L.] Merr.) cultivars that show contrasting responses to drought stress during early developmental stages

Drought stress is the major environment constraint on soybean yield, and a variety of pathways underlie drought tolerance mechanisms. Transcriptomic profiling of two soybean cultivars, drought-tolerant SS2-2 and drought-sensitive Taekwang, was performed under normal and drought conditions to identify genes involved in drought tolerance. This revealed large differences in water loss during drought treatment. Genes involved in signaling, lipid metabolism, phosphorylation, and gene regulation were overrepresented among genes that were differentially expressed between cultivars and between treatments in each cultivar. The analysis revealed transcription factors from six families, including WRKYs and NACs, showed significant SS2-2-specific upregulation. Genes involved in stress defense pathways, including MAPK signaling, Ca2+ signaling, ROS scavenging, and NBS-LRR, were also identified. Expression of non-specific phospholipases, phospholipase D, and PHOSPHATIDYL INOSITOL MONOPHOSPHATE 5 KINASE (PIP5K), which act in the lipid-signaling pathway, was greatly increased in SS2-2. The roles of PIP5K in drought stress tolerance were confirmed in Arabidopsis thaliana. Arabidopsis pip5k mutants had significantly lower survival rates under drought stress than wild-type plants. This study identified additional elements in the mechanisms used by plants to protect themselves from drought stress and provides valuable information for the development of drought-tolerant soybean cultivars.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-023-01385-1.

Redox-mediated responses to high temperature in plants

As sessile organisms, plants are particularly affected by climate change and will face more frequent and extreme temperature variations in the future. Plants have developed a diverse range of mechanisms allowing them to perceive and respond to these environmental constraints, which requires sophisticated signalling mechanisms. Reactive oxygen species (ROS) are generated in plants exposed to various stress conditions including high temperatures and are presumed to be involved in stress response reactions. The diversity of ROS-generating pathways and the ability of ROS to propagate from cell to cell and to diffuse through cellular compartments and even across membranes between subcellular compartments put them at the centre of signalling pathways. In addition, their capacity to modify the cellular redox status and to modulate functions of target proteins, notably through cysteine oxidation, show their involvement in major stress response transduction pathways. ROS scavenging and thiol reductase systems also participate in the transmission of oxidation-dependent stress signals. In this review, we summarize current knowledge on the functions of ROS and oxidoreductase systems in integrating high temperature signals, towards the activation of stress responses and developmental acclimation mechanisms.

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.

Organelle-specific localization of glutathione in plants grown under different light intensities and spectra

Plant ascorbate and glutathione metabolism counteracts oxidative stress mediated, for example, by excess light. In this review, we discuss the properties of immunocytochemistry and transmission electron microscopy, redox-sensitive dyes or probes and bright-field microscopy, confocal microscopy or fluorescence microscopy for the visualization and quantification of glutathione at the cellular or subcellular level in plants and the quantification of glutathione from isolated organelles. In previous studies, we showed that subcellular ascorbate and glutathione levels in Arabidopsis are affected by high light stress. The use of light-emitting diodes (LEDs) is gaining increasing importance in growing indoor crops and ornamental plants. A combination of different LED types allows custom-made combinations of wavelengths and prevents damage related to high photon flux rates. In this review we provide an overview on how different light spectra and light intensities affect glutathione metabolism at the cellular and subcellular levels in plants. Findings obtained in our most recent study demonstrate that both light intensity and spectrum significantly affected glutathione metabolism in wheat at the transcriptional level and caused genotype-specific reactions in the investigated Arabidopsis lines.

Simple Phenotypic Sensor for Visibly Tracking H2O2 Fluctuation to Detect Plant Health Status

Hydrogen peroxide (H₂O₂), as a main component of reactive oxygen species (ROS), serves as a key signaling molecule relevant to plant stress response and health status. Many strategies have been developed for detecting or quantifying H₂O₂ concentration. However, reports on simply, visibly tracking H₂O₂ fluctuation in vivo are limited. Here, for visibly tracking the plant H₂O₂ wave, a green fluorescent phenotypic probe was designed by merging a H₂O₂-sensitive tertiary amine moiety with the core fluorophore tetraphenylethene skeleton. The green fluorescence emission is quenched up to 52% by H₂O₂ with good sensitivity, selectivity, and reversibility within the plant physiological range of 10-100 μM H₂O₂. In response to various abiotic stresses, including mechanical damage, high salt, strong light and drought, fluorescence fluctuations, response to H₂O₂ concentration alterations in vivo was visible to the naked eye under irradiation of commercially available UV light (365 nm) after simple injection of this H₂O₂ probe solution into seedling leaves. This phenotypic fluorescent H₂O₂ probe illustrates great potential as early sensors of plant health under stress without the aid of skillful operation and specialized equipment.

Light Intensity- and Spectrum-Dependent Redox Regulation of Plant Metabolism

Both light intensity and spectrum (280–800 nm) affect photosynthesis and, consequently, the formation of reactive oxygen species (ROS) during photosynthetic electron transport. ROS, together with antioxidants, determine the redox environment in tissues and cells, which in turn has a major role in the adjustment of metabolism to changes in environmental conditions. This process is very important since there are great spatial (latitude, altitude) and temporal (daily, seasonal) changes in light conditions which are accompanied by fluctuations in temperature, water supply, and biotic stresses. The blue and red spectral regimens are decisive in the regulation of metabolism because of the absorption maximums of chlorophylls and the sensitivity of photoreceptors. Based on recent publications, photoreceptor-controlled transcription factors such as ELONGATED HYPOCOTYL5 (HY5) and changes in the cellular redox environment may have a major role in the coordinated fine-tuning of metabolic processes during changes in light conditions. This review gives an overview of the current knowledge of the light-associated redox control of basic metabolic pathways (carbon, nitrogen, amino acid, sulphur, lipid, and nucleic acid metabolism), secondary metabolism (terpenoids, flavonoids, and alkaloids), and related molecular mechanisms. Light condition-related reprogramming of metabolism is the basis for proper growth and development of plants; therefore, its better understanding can contribute to more efficient crop production in the future.

Reactive oxygen species signalling in plant stress responses

Reactive oxygen species (ROS) are key signalling molecules that enable cells to rapidly respond to different stimuli. In plants, ROS play a crucial role in abiotic and biotic stress sensing, integration of different environmental signals and activation of stress-response networks, thus contributing to the establishment of defence mechanisms and plant resilience. Recent advances in the study of ROS signalling in plants include the identification of ROS receptors and key regulatory hubs that connect ROS signalling with other important stress-response signal transduction pathways and hormones, as well as new roles for ROS in organelle-to-organelle and cell-to-cell signalling. Our understanding of how ROS are regulated in cells by balancing production, scavenging and transport has also increased. In this Review, we discuss these promising developments and how they might be used to increase plant resilience to environmental stress.

Systematic monitoring of 2-Cys peroxiredoxin-derived redox signals unveiled its role in attenuating carbon assimilation rate

SignificanceIdentifying the intrinsic factors that regulate leaf photosynthetic rate may pave the way toward developing new strategies to enhance carbon assimilation. While the dependence of photosynthesis on the reductive activation of the Calvin-Benson cycle enzymes is well established, the role of oxidative signals in counterbalancing the reductive activity is just beginning to be explored. By developing 2-Cys peroxiredoxin-based genetically encoded biosensors, we demonstrated the induction of photosynthetically derived oxidative signals under habitual light conditions, a phenomenon typically masked by the dominance of the reductive power. Moreover, we unraveled the simultaneous activation of reductive and oxidative signals during photosynthesis induction phase and showed that 2-Cys peroxiredoxin activity attenuates carbon assimilation rates, demonstrating the restrictions imposed on photosynthetic performance by oxidative signals.

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.

Metabolic signatures of Arabidopsis thaliana abiotic stress responses elucidate patterns in stress priming, acclimation, and recovery

Temperature, water, and light are three abiotic stress factors that have major influences on plant growth, development, and reproduction. Plants can be primed by a prior mild stress to enhance their resistance to future stress. We used an untargeted metabolomics approach to examine Arabidopsis thaliana 11-day-old seedling's abiotic stress responses including heat (with and without priming), cold (with and without priming), water-deficit and high-light before and after a 2-day-recovery period. Analysis of the physiological phenotypes showed that seedlings with stress treatment resulted in a reduction in fresh weight, hypocotyl and root length but remained viable. Several stress responsive metabolites were identified, confirmed with reference standards, quantified, and clustered. We identified shared and specific stress signatures for cold, heat, water-deficit, and high-light treatments. Central metabolism including amino acid metabolism, sugar metabolism, glycolysis, TCA cycle, GABA shunt, glutathione metabolism, purine metabolism, and urea cycle were found to undergo changes that are fundamentally different, although some shared commonalities in response to different treatments. Large increases in cysteine abundance and decreases in reduced glutathione were observed following multiple stress treatments highlighting the importance of oxidative stress as a general phenomenon in abiotic stress. Large fold increases in low-turnover amino acids and maltose demonstrate the critical role of protein and starch autolysis in early abiotic stress responses.

Designs, applications, and limitations of genetically encoded fluorescent sensors to explore plant biology

The understanding of signaling and metabolic processes in multicellular organisms requires knowledge of the spatial dynamics of small molecules and the activities of enzymes, transporters, and other proteins in vivo, as well as biophysical parameters inside cells and across tissues. The cellular distribution of receptors, ligands, and activation state must be integrated with information about the cellular distribution of metabolites in relation to metabolic fluxes and signaling dynamics in order to achieve the promise of in vivo biochemistry. Genetically encoded sensors are engineered fluorescent proteins that have been developed for a wide range of small molecules, such as ions and metabolites, or to report biophysical processes, such as transmembrane voltage or tension. First steps have been taken to monitor the activity of transporters in vivo. Advancements in imaging technologies and specimen handling and stimulation have enabled researchers in plant sciences to implement sensor technologies in intact plants. Here, we provide a brief history of the development of genetically encoded sensors and an overview of the types of sensors available for quantifying and visualizing ion and metabolite distribution and dynamics. We further discuss the pros and cons of specific sensor designs, imaging systems, and sample manipulations, provide advice on the choice of technology, and give an outlook into future developments.

Glutathione Metabolism in Plants under Stress: Beyond Reactive Oxygen Species Detoxification

Glutathione is an essential metabolite for plant life best known for its role in the control of reactive oxygen species (ROS). Glutathione is also involved in the detoxification of methylglyoxal (MG) which, much like ROS, is produced at low levels by aerobic metabolism under normal conditions. While several physiological processes depend on ROS and MG, a variety of stresses can dramatically increase their concentration leading to potentially deleterious effects. In this review, we examine the structure and the stress regulation of the pathways involved in glutathione synthesis and degradation. We provide a synthesis of the current knowledge on the glutathione-dependent glyoxalase pathway responsible for MG detoxification. We present recent developments on the organization of the glyoxalase pathway in which alternative splicing generate a number of isoforms targeted to various subcellular compartments. Stress regulation of enzymes involved in MG detoxification occurs at multiple levels. A growing number of studies show that oxidative stress promotes the covalent modification of proteins by glutathione. This post-translational modification is called S-glutathionylation. It affects the function of several target proteins and is relevant to stress adaptation. We address this regulatory function in an analysis of the enzymes and pathways targeted by S-glutathionylation.

Deciphering the Role of Ion Channels in Early Defense Signaling against Herbivorous Insects

Plants and insect herbivores are in a relentless battle to outwit each other. Plants have evolved various strategies to detect herbivores and mount an effective defense system against them. These defenses include physical and structural barriers such as spines, trichomes, cuticle, or chemical compounds, including secondary metabolites such as phenolics and terpenes. Plants perceive herbivory by both mechanical and chemical means. Mechanical sensing can occur through the perception of insect biting, piercing, or chewing, while chemical signaling occurs through the perception of various herbivore-derived compounds such as oral secretions (OS) or regurgitant, insect excreta (frass), or oviposition fluids. Interestingly, ion channels or transporters are the first responders for the perception of these mechanical and chemical cues. These transmembrane pore proteins can play an important role in plant defense through the induction of early signaling components such as plasma transmembrane potential (V_m) fluctuation, intracellular calcium (Ca²⁺), and reactive oxygen species (ROS) generation, followed by defense gene expression, and, ultimately, plant defense responses. In recent years, studies on early plant defense signaling in response to herbivory have been gaining momentum with the application of genetically encoded GFP-based sensors for real-time monitoring of early signaling events and genetic tools to manipulate ion channels involved in plant-herbivore interactions. In this review, we provide an update on recent developments and advances on early signaling events in plant-herbivore interactions, with an emphasis on the role of ion channels in early plant defense signaling.

Live monitoring of plant redox and energy physiology with genetically encoded biosensors

Genetically encoded biosensors pave the way for understanding plant redox dynamics and energy metabolism on cellular and subcellular levels.

Biosensors: A Sneak Peek into Plant Cell's Immunity

Biosensors are indispensable tools to understand a plant’s immunity as its spatiotemporal dimension is key in withstanding complex plant immune signaling. The diversity of genetically encoded biosensors in plants is expanding, covering new analytes with ever higher sensitivity and robustness, but their assortment is limited in some respects, such as their use in following biotic stress response, employing more than one biosensor in the same chassis, and their implementation into crops. In this review, we focused on the available biosensors that encompass these aspects. We show that in vivo imaging of calcium and reactive oxygen species is satisfactorily covered with the available genetically encoded biosensors, while on the other hand they are still underrepresented when it comes to imaging of the main three hormonal players in the immune response: salicylic acid, ethylene and jasmonic acid. Following more than one analyte in the same chassis, upon one or more conditions, has so far been possible by using the most advanced genetically encoded biosensors in plants which allow the monitoring of calcium and the two main hormonal pathways involved in plant development, auxin and cytokinin. These kinds of biosensor are also the most evolved in crops. In the last section, we examine the challenges in the use of biosensors and demonstrate some strategies to overcome them.