Chloroplast Retrograde Regulation of Heat Stress Responses in Plants

Multi-scale analysis of heat stress acclimation in Arabidopsis seedlings highlights the primordial contribution of energy-transducing organelles

Much progress has been made in understanding the molecular mechanisms of plant adaptation to heat stress. However, the great diversity of models and stress conditions, and the fact that analyses are often limited to a small number of approaches, complicate the picture. We took advantage of a liquid culture system in which Arabidopsis seedlings are arrested in their development, thus avoiding interference with development and drought stress responses, to investigate through an integrative approach seedlings' global response to heat stress and acclimation. Seedlings perfectly tolerate a noxious heat shock (43°C) when subjected to a heat priming treatment at a lower temperature (38°C) the day before, displaying a thermotolerance comparable to that previously observed for Arabidopsis. A major effect of the pre-treatment was to partially protect energy metabolism under heat shock and favor its subsequent rapid recovery, which was correlated with the survival of seedlings. Rapid recovery of actin cytoskeleton and mitochondrial dynamics were another landmark of heat shock tolerance. The omics confirmed the role of the ubiquitous heat shock response actors but also revealed specific or overlapping responses to priming, heat shock, and their combination. Since only a few components or functions of chloroplast and mitochondria were highlighted in these analyses, the preservation and rapid recovery of their bioenergetic roles upon acute heat stress do not require extensive remodeling of the organelles. Protection of these organelles is rather integrated into the overall heat shock response, thus allowing them to provide the energy required to elaborate other cellular responses toward acclimation.

Multiomics analyses reveal the central role of the nucleolus and its machinery during heat stress acclimation in Pinus radiata

Global warming is causing rapid changes in mean annual temperature and more severe drought periods. These are major contributors of forest dieback, which is becoming more frequent and widespread. In this work, we investigated how the transcriptome of Pinus radiata changed during initial heat stress response and acclimation. To this end, we generated a high-density dataset employing Illumina technology. This approach allowed us to reconstruct a needle transcriptome, defining 12 164 and 13 590 transcripts as down- and up-regulated, respectively, during a time course stress acclimation experiment. Additionally, the combination of transcriptome data with other available omics layers allowed us to determine the complex inter-related processes involved in the heat stress response from the molecular to the physiological level. Nucleolus and nucleoid activities seem to be a central core in the acclimating process, producing specific RNA isoforms and other essential elements for anterograde-retrograde stress signaling such as NAC proteins (Pra_vml_051671_1 and Pra_vml_055001_5) or helicase RVB. These mechanisms are connected by elements already known in heat stress response (redox, heat-shock proteins, or abscisic acid-related) and with others whose involvement is not so well defined such as shikimate-related, brassinosteriods, or proline proteases together with their potential regulatory elements. This work provides a first in-depth overview about molecular mechanisms underlying the heat stress response and acclimation in P. radiata.

Systematic identification and characterization of microRNAs with target genes involved in high night temperature stress at the filling stage of rice

High night temperature stress is one of the main environmental factors affecting rice yield and quality. More and more evidence shows that microRNA (miRNA) plays an important role in various abiotic stresses. However, the molecular network of miRNA regulation on rice tolerance to high night temperatures remains unclear. Here, small RNA, transcriptome and degradome sequencing were integrated to identify differentially expressed miRNAs, genes, and key miRNA-target gene pairs in rice heat-sensitive and heat-tolerant lines at the filling stage suffering from high night temperature stress. It was discovered that there were notable differences in the relative expression of 102 miRNAs between the two rice lines under stress. Meanwhile, 5263 and 5405 mRNAs were differentially expressed in the heat-sensitive line and heat-tolerant line, and functional enrichment analysis revealed that these genes were involved in heat-related processes and pathways. The miRNAs-mRNAs target relationship was further verified by degradome sequencing. Eventually, 49 miRNAs-222 mRNAs target pairs with reverse expression patterns showed significant relative expression changes between the heat-tolerant and the heat-sensitive line, being suggested to be responsible for the heat tolerance difference of these two rice lines. Functional analysis of these 222 mRNA transcripts showed that high night temperature-responsive miRNAs targeted these mRNAs involved in many heat-related biological processes, such as transcription regulation, chloroplast regulation, mitochondrion regulation, protein folding, hormone regulation and redox process. This study identified possible miRNA-mRNA regulation relationships in response to high night temperature stress in rice and potentially contributed to heat resistance breeding of rice in the future.

Expression of maize OXS2a in Arabidopsis stunts plant growth but enhances heat tolerance

As plants encounter various environmental stresses, judicial allocation of resources to stress response is crucial for plant fitness. The plant OXS2 (OXIDATIVE STRESS 2) family has been reported to play important roles in growth regulation and stress response. Here, we report that the maize OXS2 family member ZmOXS2a when expressed in Arabidopsis retards growth including delayed flowering, but improves heat tolerance. ZmOXS2a can be found in the cytoplasm, nucleus and PBs/P bodies (mRNA processing bodies), but heat treatment induces higher accumulation in the PBs. Deletion of ARR (arginine rich region) and TZF (tandem zinc finger) domains for high-affinity RNA-binding reduced PBs accumulation of ZmOXS2a; and unlike ZmOXS2a, expression of this deletion mutant gene affected neither Arabidopsis growth nor heat tolerance. This suggests that ZmOXS2a might be involved in RNA degradation, which would also account for the larger amount of down-regulated genes found in ZmOXS2a expressing lines. Furthermore, 240 of 890 down-regulated genes contain ARE (AU-rich elements) in the mRNA 3'UTR that might be potential targets of ZmOXS2a. Expression of ZmOXS2a also disturbs the response to ABA (abscisic acid) and cytokinin, as GO (gene ontology) analysis shows that 50 and 15 DEGs (differentially expressed genes) are enriched in the GO term for ABA and cytokinin responses, respectively. ZmOXS2a expression lines are more sensitive to ABA, but less sensitive to cytokinin. It is likely that ZmOXS2a promotes the degradation of the mRNA of down-regulated genes containing ARE, which consequently perturbs the hormone pathways that affect stress response-related plant growth.

Acclimation response and management strategies to combat heat stress in wheat for sustainable agriculture: A state-of-the-art review

Unpredicted variability in climate change on the planet is associated with frequent extreme high-temperature events impacting crop yield globally. Wheat is an economically and nutritionally important crop that fulfils global food requirements and each degree rise in temperature results in ∼6% of its yield reduction. Thus, understanding the impact of climate change, especially the terminal heat stress on global wheat production, becomes critically important for policymakers, crop breeders, researchers and scientists to ensure global food security. This review describes how wheat perceives heat stress and induces stress adaptation events by its morpho-physiological, phenological, molecular, and biochemical makeup. Temperature above a threshold level in crop vicinity leads to irreversible injuries, viz. destruction of cellular membranes and enzymes, generation of active oxygen species, redox imbalance, etc. To cope with these changes, wheat activates its heat tolerance mechanisms characterized by hoarding up soluble carbohydrates, signalling molecules, and heat tolerance gene expressions. Being vulnerable to heat stress, increasing wheat production without delay seeks strategies to mitigate the detrimental effects and provoke the methods for its sustainable development. Thus, to ensure the crop's resilience to stress and increasing food demand, this article circumscribes the integrated management approaches to enhance wheat's performance and adaptive capacity besides its alleviating risks of increasing temperature anticipated with climate change. Implementing these integrated strategies in the face of risks from rising temperatures will assist us in producing sustainable wheat with improved yield.

Extra- and intranuclear heat perception and triggering mechanisms in plants

The escalating impact of global warming on crop yield and quality poses a significant threat to future food supplies. Breeding heat-resistant crop varieties holds promise, but necessitates a deeper understanding of the molecular mechanisms underlying plant heat tolerance. Recent studies have shed light on the initial events of heat perception in plants. In this review, we provide a comprehensive summary of the recent progress made in unraveling the mechanisms of heat perception and response in plants. Calcium ion (Ca2+), hydrogen peroxide (H2O2), and nitric oxide (NO) have emerged as key participants in heat perception. Furthermore, we discuss the potential roles of the NAC transcription factor NTL3, thermo-tolerance 3.1 (TT3.1), and Target of temperature 3 (TOT3) as thermosensors associated with the plasma membrane. Additionally, we explore the involvement of cytoplasmic HISTONE DEACETYLASE 9 (HDA9), mRNA encoding the phytochrome-interacting factor 7 (PIF7), and chloroplasts in mediating heat perception. This review also highlights the role of intranuclear transcriptional condensates formed by phytochrome B (phyB), EARLY FLOWERING 3 (ELF3), and guanylate-binding protein (GBP)-like GTPase 3 (GBPL3) in heat perception. Finally, we raise the unresolved questions in the field of heat perception that require further investigation in the future.

The molecular basis of heat stress responses in plants

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

Salt Stress-Induced Modulation of Porphyrin Biosynthesis, Photoprotection, and Antioxidant Properties in Rice Plants (Oryza sativa)

Salt stress disrupts cellular ion homeostasis and adversely impacts plant growth and productivity. We examined the regulatory mechanisms of porphyrin biosynthesis, photoprotection, and antioxidant properties in salt-stressed rice seedlings. In response to 150 mM NaCl, the rice seedlings exhibited dehydration, reduced relative water content, and increased levels of conductivity, malondialdehyde, and H2O2. The expression levels of the salt-stress-responsive genes NHX1, SOS1, and MYB drastically increased after NaCl treatment. The seedlings grown under NaCl stress displayed declines in Fv/Fm, ΦPSII, rETRmax, and photochemical quenching but increases in nonphotochemical quenching (NPQ) and the expression of genes involved in zeaxanthin formation, BCH, and VDE. Under salt stress conditions, levels of chlorophyll precursors significantly decreased compared to controls, matching the downregulation of CHLD, CHLH, CHLI, and PORB. By contrast, NaCl treatment led to increased heme content at 24 h of treatment and significant upregulations of FC2, HO1, and HO2 compared to controls. Salt-stressed seedlings also increased their expression of CATs (catalases) and APXs (ascorbate peroxidases) as well as the activities of superoxide dismutase, CAT, APX, and peroxidase. Our results indicate that chlorophyll and heme biosynthesis involve the protective strategies for salt stress alleviation through photoprotection by the scavenging of chlorophyll precursors and NPQ as well as activating antioxidant enzymes.

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.

Photosynthetic acclimation to changing environments

Plants are exposed to environments that fluctuate of timescales varying from seconds to months. Leaves that develop in one set of conditions optimise their metabolism to the conditions experienced, in a process called developmental acclimation. However, when plants experience a sustained change in conditions, existing leaves will also acclimate dynamically to the new conditions. Typically this process takes several days. In this review, we discuss this dynamic acclimation process, focussing on the responses of the photosynthetic apparatus to light and temperature. We briefly discuss the principal changes occurring in the chloroplast, before examining what is known, and not known, about the sensing and signalling processes that underlie acclimation, identifying likely regulators of acclimation.

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.

The role of nitric oxide and hydrogen sulfide in regulation of redox homeostasis at extreme temperatures in plants

Nitric oxide and hydrogen sulfide, as important signaling molecules (gasotransmitters), are involved in many functions of plant organism, including adaptation to stress factors of various natures. As redox-active molecules, NO and H₂S are involved in redox regulation of functional activity of many proteins. They are also involved in maintaining cell redox homeostasis due to their ability to interact directly and indirectly (functionally) with ROS, thiols, and other molecules. The review considers the involvement of nitric oxide and hydrogen sulfide in plant responses to low and high temperatures. Particular attention is paid to the role of gasotransmitters interaction with other signaling mediators (in particular, with Ca²⁺ ions and ROS) in the formation of adaptive responses to extreme temperatures. Pathways of stress-induced enhancement of NO and H₂S synthesis in plants are considered. Mechanisms of the NO and H₂S effect on the activity of some proteins of the signaling system, as well as on the state of antioxidant and osmoprotective systems during adaptation to stress temperatures, were analyzed. Possibilities of practical use of nitric oxide and hydrogen sulfide donors as inductors of plant adaptive responses are discussed.

Fine Tuning of ROS, Redox and Energy Regulatory Systems Associated with the Functions of Chloroplasts and Mitochondria in Plants under Heat Stress

Heat stress severely affects plant growth and crop production. It is therefore urgent to uncover the mechanisms underlying heat stress responses of plants and establish the strategies to enhance heat tolerance of crops. The chloroplasts and mitochondria are known to be highly sensitive to heat stress. Heat stress negatively impacts on the electron transport chains, leading to increased production of reactive oxygen species (ROS) that can cause damages on the chloroplasts and mitochondria. Disruptions of photosynthetic and respiratory metabolisms under heat stress also trigger increase in ROS and alterations in redox status in the chloroplasts and mitochondria. However, ROS and altered redox status in these organelles also activate important mechanisms that maintain functions of these organelles under heat stress, which include HSP-dependent pathways, ROS scavenging systems and retrograde signaling. To discuss heat responses associated with energy regulating organelles, we should not neglect the energy regulatory hub involving TARGET OF RAPAMYCIN (TOR) and SNF-RELATED PROTEIN KINASE 1 (SnRK1). Although roles of TOR and SnRK1 in the regulation of heat responses are still unknown, contributions of these proteins to the regulation of the functions of energy producing organelles implicate the possible involvement of this energy regulatory hub in heat acclimation of plants.

Maize cytolines as models to study the impact of different cytoplasms on gene expression under heat stress conditions

BACKGROUND

Crops are under constant pressure due to global warming, which unfolds at a much faster pace than their ability to adapt through evolution. Agronomic traits are linked to cytoplasmic-nuclear genome interactions. It thus becomes important to understand the influence exerted by the organelles on gene expression under heat stress conditions and profit from the available genetic diversity. Maize (Zea mays) cytolines allow us to investigate how the gene expression changes under heat stress conditions in three different cytoplasmic environments, but each having the same nucleus. Analyzing retrograde signaling in such an experimental set-up has never been done before. Here, we quantified the response of three cytolines to heat stress as differentially expressed genes (DEGs), and studied gene expression patterns in the context of existing polymorphism in their organellar genomes.

RESULTS

Our study unveils a plethora of new genes and GO terms that are differentially expressed or enriched, respectively, in response to heat stress. We report 19,600 DEGs as responding to heat stress (out of 30,331 analyzed), which significantly enrich 164 GO biological processes, 30 GO molecular functions, and 83 GO cell components. Our approach allowed for the discovery of a significant number of DEGs and GO terms that are not common in the three cytolines and could therefore be linked to retrograde signaling. Filtering for DEGs with a fold regulation > 2 (absolute values) that are exclusive to just one of the cytolines, we find a total of 391 up- and down-DEGs. Similarly, there are 19 GO terms with a fold enrichment > 2 that are cytoline-specific. Using GBS data we report contrasting differences in the number of DEGs and GO terms in each cytoline, which correlate with the genetic distances between the mitochondrial genomes (but not chloroplast) and the original nuclei of the cytolines, respectively.

CONCLUSIONS

The experimental design used here adds a new facet to the paradigm used to explain how gene expression changes in response to heat stress, capturing the influence exerted by different organelles upon one nucleus rather than investigating the response of several nuclei in their innate cytoplasmic environments.

Comparative transcriptome analysis of the mechanism difference in heat stress response between indica rice cultivar "IR64" and japonica cultivar "Koshihikari" at the seedling stage

Heat stress (HS) has become a major abiotic stress in rice, considering the frequency and intensity of extreme hot weather. There is an urgent need to explore the differences in molecular mechanisms of HS tolerance in different cultivars, especially in indica and japonica. In this study, we investigated the transcriptome information of IR64 (indica, IR) and Koshihikari (japonica, Kos) in response to HS at the seedling stage. From the differentially expressed genes (DEGs) consistently expressed at six time points, 599 DEGs were identified that were co-expressed in both cultivars, as well as 945 and 1,180 DEGs that were specifically expressed in IR and Kos, respectively. The results of GO and KEGG analysis showed two different HS response pathways for IR and Kos. IR specifically expressed DEGs were mainly enriched in chloroplast-related pathways, whereas Kos specifically expressed DEGs were mainly enriched in endoplasmic reticulum and mitochondria-related pathways. Meanwhile, we highlighted the importance of NO biosynthesis genes, especially nitrate reductase genes, in the HS response of IR based on protein-protein interaction networks. In addition, we found that heat shock proteins and heat shock factors play very important roles in both cultivars. This study not only provides new insights into the differences in HS responses between different subspecies of rice, but also lays the foundation for future research on molecular mechanisms and breeding of heat-tolerant cultivars.

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.

Novel molecules and target genes for vegetative heat tolerance in wheat

To prevent yield losses caused by climate change, it is important to identify naturally tolerant genotypes with traits and related pathways that can be targeted for crop improvement. Here we report on the characterization of contrasting vegetative heat tolerance in two UK bread wheat varieties. Under chronic heat stress, the heat-tolerant cultivar Cadenza produced an excessive number of tillers which translated into more spikes and higher grain yield compared to heat-sensitive Paragon. RNAseq and metabolomics analyses revealed that over 5000 genotype-specific genes were differentially expressed, including photosynthesis-related genes, which might explain the observed ability of Cadenza to maintain photosynthetic rate under heat stress. Around 400 genes showed a similar heat-response in both genotypes. Only 71 genes showed a genotype × temperature interaction. As well as known heat-responsive genes such as heat shock proteins (HSPs), several genes that have not been previously linked to the heat response, particularly in wheat, have been identified, including dehydrins, ankyrin-repeat protein-encoding genes, and lipases. Contrary to primary metabolites, secondary metabolites showed a highly differentiated heat response and genotypic differences. These included benzoxazinoid (DIBOA, DIMBOA), and phenylpropanoids and flavonoids with known radical scavenging capacity, which was assessed via the DPPH assay. The most highly heat-induced metabolite was (glycosylated) propanediol, which is widely used in industry as an anti-freeze. To our knowledge, this is the first report on its response to stress in plants. The identified metabolites and candidate genes provide novel targets for the development of heat-tolerant wheat.

Modulating physiological and transcriptional regulatory mechanisms for enhanced climate resilience in cereal crops

Climate change adversely affects the yield and productivity of cereal crops, which consequently impacts food security. Therefore, studying stress acclimation, particularly transcriptional patterns and morpho-physiological responses of cereal crops to different stresses, will provide insights into the molecular determinants underlying climate resilience. The availability of advanced tools and approaches has enabled the characterization of plants at morphological, physiological, biochemical, and molecular levels, which will lead to the identification of genomic regions regulating the stress responses at these levels. This will further facilitate using transgenic, breeding, or genome editing approaches to manipulate the identified regions (genes, alleles, or QTLs) to enhance stress resilience. Next-generation sequencing approaches have advanced the identification of causal genes and markers in the genomes through forward or reverse genetics. In this context, the review enumerates the progress of dissecting the molecular mechanisms underlying transcriptional and physiological responses of major cereals to climate-induced stresses. The review systematically discusses different tools and approaches available to study the response of plants to various stresses and identify the molecular determinants regulating stress-resilience. Further, the application of genomics-assisted breeding, transgene-, and targeted editing-based approaches for modulating the genetic determinants for enhanced climate resilience has been elaborated.

The role of PAP4/FSD3 and PAP9/FSD2 in heat stress responses of chloroplast genes

Chloroplasts' mechanisms of adaptation to elevated temperatures are largely determined by the gene expression of the plastid transcription apparatus. Gene disruption of iron-containing superoxide dismutase PAP4/FSD3 and PAP9/FSD2, which are parts of the DNA-RNA polymerase complex of plastids, contributed to a decrease in resistance to oxidative stress caused by the prolonged action of elevated temperatures (5 days, 30 °C). Under heat stress conditions, pap4/fsd3 and pap9/fsd2 mutants showed a decline in chlorophyll content and photosynthesis level, as measured by photosynthetic parameters, and a different amplitude of HSP gene response to heat stress. The expression of nuclear- and plastid-encoded photosynthesis genes and corresponding proteins was strongly inhibited in the mutants as compared with wild-type plants and was further suppressed or displayed no additional changes at 30 °C. NEP-dependent plastid genes, as well as NEP genes RPOTp and RPOTmp, were also downregulated in the mutants by high temperature or remained stable, unlike in wild-type seedlings where these genes were strongly upregulated. The results obtained correspond to the concept of the complex effect of various forms of reactive oxygen species under all types of stresses, including heat stress, and confirm the hypothesis of a new regulatory function in plastid transcription acquired by enzymatic proteins during evolution.

Mechanisms Regulating Energy Homeostasis in Plant Cells and Their Potential to Inspire Electrical Microgrids Models

In this paper, the main features of systems that are required to flexibly modulate energy states of plant cells in response to environmental fluctuations are surveyed and summarized. Plant cells possess multiple sources (chloroplasts and mitochondria) to produce energy that is consumed to drive many processes, as well as mechanisms that adequately provide energy to the processes with high priority depending on the conditions. Such energy-providing systems are tightly linked to sensors that monitor the status of the environment and inside the cell. In addition, plants possess the ability to efficiently store and transport energy both at the cell level and at a higher level. Furthermore, these systems can finely tune the various mechanisms of energy homeostasis in plant cells in response to the changes in environment, also assuring the plant survival under adverse environmental conditions. Electrical power systems are prone to the effects of environmental changes as well; furthermore, they are required to be increasingly resilient to the threats of extreme natural events caused, for example, by climate changes, outages, and/or external deliberate attacks. Starting from this consideration, similarities between energy-related processes in plant cells and electrical power grids are identified, and the potential of mechanisms regulating energy homeostasis in plant cells to inspire the definition of new models of flexible and resilient electrical power grids, particularly microgrids, is delineated. The main contribution of this review is surveying energy regulatory mechanisms in detail as a reference and helping readers to find useful information for their work in this research field.

Membrane-Fluidization-Dependent and -Independent Pathways Are Involved in Heat-Stress-Inducible Gene Expression in the Marine Red Alga Neopyropia yezoensis

Heat stress responses are complex regulatory processes, including sensing, signal transduction, and gene expression. However, the exact mechanisms of these processes in seaweeds are not well known. We explored the relationship between membrane physical states and gene expression in the red alga Neopyropia yezoensis. To analyze heat-stress-induced gene expression, we identified two homologs of the heat-inducible high temperature response 2 (HTR2) gene in Neopyropia seriata, named NyHTR2 and NyHTR2L. We found conservation of HTR2 homologs only within the order Bangiales; their products contained a novel conserved cysteine repeat which we designated the Bangiales cysteine-rich motif. A quantitative mRNA analysis showed that expression of NyHTR2 and NyHTR2L was induced by heat stress. However, the membrane fluidizer benzyl alcohol (BA) did not induce expression of these genes, indicating that the effect of heat was not due to membrane fluidization. In contrast, expression of genes encoding multiprotein-bridging factor 1 (NyMBF1) and HSP70s (NyHSP70-1 and NyHSP70-2) was induced by heat stress and by BA, indicating that it involved a membrane-fluidization-dependent pathway. In addition, dark treatment under heat stress promoted expression of NyHTR2, NyHTR2L, NyMBF1, and NyHSP70-2, but not NyHSP70-1; expression of NyHTR2 and NyHTR2L was membrane-fluidization-independent, and that of other genes was membrane-fluidization-dependent. These findings indicate that the heat stress response in N. yezoensis involves membrane-fluidization-dependent and -independent pathways.

Ferroptosis in plants: regulation of lipid peroxidation and redox status

Regulated cell death (RCD) is an essential process that plays key roles along the plant life cycle. Unlike accidental cell death, which is an uncontrolled biological process, RCD involves integrated signaling cascades and precise molecular-mediated mechanisms that are triggered in response to specific exogenous or endogenous stimuli. Ferroptosis is a cell death pathway characterized by the iron-dependent accumulation of lipid reactive oxygen species. Although first described in animals, ferroptosis in plants shares all the main core mechanisms observed for ferroptosis in other systems. In plants as in animals, oxidant and antioxidant systems outline the process of lipid peroxidation during ferroptosis. In plants, cellular compartments such as mitochondria, chloroplasts and cytosol act cooperatively and coordinately to respond to changing redox environments. This particular context makes plants a unique model to study redox status regulation and cell death. In this review, we focus on our most recent understanding of the regulation of redox state and lipid peroxidation in plants and their role during ferroptosis.

Genetic and molecular mechanisms underlying root architecture and function under heat stress-A hidden story

Heat stress events are resulting in a significant negative impact on global food production. The dynamics of cellular, molecular and physiological homoeostasis in aboveground parts under heat stress are extensively deciphered. However, root responses to higher soil/air temperature or stress signalling from shoot to root are limited. Therefore, this review presents a holistic view of root physio-morphological and molecular responses to adapt under hotter environments. Heat stress reprogrammes root cellular machinery, including crosstalk between genes, phytohormones, reactive oxygen species (ROS) and antioxidants. Spatio-temporal regulation and long-distance transport of phytohormones, such as auxin, cytokinin and abscisic acid (ABA) determine the root growth and development under heat stress. ABA cardinally integrates a signalling pathway involving heat shock factors, heat shock proteins and ROS to govern heat stress responses. Additionally, epigenetic modifications by transposable elements, DNA methylation and acetylation also regulate root growth under heat stress. Exogenous application of chemical compounds or biological agents such as ascorbic acid, metal ion chelators, fungi and bacteria can alleviate heat stress-induced reduction in root biomass. Future research should focus on the systemic effect of heat stress from shoot to root with more detailed investigations to decipher the molecular cues underlying the roots architecture and function.

Sugar transporters and their molecular tradeoffs during abiotic stress responses in plants

Sugars as photosynthates are well known as energy providers and as building blocks of various structural components of plant cells, tissues and organs. Additionally, as a part of various sugar signaling pathways, they interact with other cellular machinery and influence many important cellular decisions in plants. Sugar signaling is further reliant on the differential distribution of sugars throughout the plant system. The distribution of sugars from source to sink tissues or within organelles of plant cells is a highly regulated process facilitated by various sugar transporters located in plasma membranes and organelle membranes, respectively. Sugar distribution, as well as signaling, is impacted during unfavorable environments such as extreme temperatures, salt, nutrient scarcity, or drought. Here, we have discussed the mechanism of sugar transport via various types of sugar transporters as well as their differential response during environmental stress exposure. The functional involvement of sugar transporters in plant's abiotic stress tolerance is also discussed. Besides, we have also highlighted the challenges in engineering sugar transporter proteins as well as the undeciphered modules associated with sugar transporters in plants. Thus, this review provides a comprehensive discussion on the role and regulation of sugar transporters during abiotic stresses and enables us to target the candidate sugar transporter(s) for crop improvement to develop climate-resilient crops.

Physiological Responses to Drought, Salinity, and Heat Stress in Plants: A Review

On the world stage, the increase in temperatures due to global warming is already a reality that has become one of the main challenges faced by the scientific community. Since agriculture is highly dependent on climatic conditions, it may suffer a great impact in the short term if no measures are taken to adapt and mitigate the agricultural system. Plant responses to abiotic stresses have been the subject of research by numerous groups worldwide. Initially, these studies were concentrated on model plants, and, later, they expanded their studies in several economically important crops such as rice, corn, soybeans, coffee, and others. However, agronomic evaluations for the launching of cultivars and the classical genetic improvement process focus, above all, on productivity, historically leaving factors such as tolerance to abiotic stresses in the background. Considering the importance of the impact that abiotic stresses can have on agriculture in the short term, new strategies are currently being sought and adopted in breeding programs to understand the physiological, biochemical, and molecular responses to environmental disturbances in plants of agronomic interest, thus ensuring the world food security. Moreover, integration of these approaches is bringing new insights on breeding. We will discuss how water deficit, high temperatures, and salinity exert effects on plants.

Genomics Associated Interventions for Heat Stress Tolerance in Cool Season Adapted Grain Legumes

Cool season grain legumes occupy an important place among the agricultural crops and essentially provide multiple benefits including food supply, nutrition security, soil fertility improvement and revenue for farmers all over the world. However, owing to climate change, the average temperature is steadily rising, which negatively affects crop performance and limits their yield. Terminal heat stress that mainly occurred during grain development phases severely harms grain quality and weight in legumes adapted to the cool season, such as lentils, faba beans, chickpeas, field peas, etc. Although, traditional breeding approaches with advanced screening procedures have been employed to identify heat tolerant legume cultivars. Unfortunately, traditional breeding pipelines alone are no longer enough to meet global demands. Genomics-assisted interventions including new-generation sequencing technologies and genotyping platforms have facilitated the development of high-resolution molecular maps, QTL/gene discovery and marker-assisted introgression, thereby improving the efficiency in legumes breeding to develop stress-resilient varieties. Based on the current scenario, we attempted to review the intervention of genomics to decipher different components of tolerance to heat stress and future possibilities of using newly developed genomics-based interventions in cool season adapted grain legumes.

Transcriptome Analysis of Jujube (Ziziphus jujuba Mill.) Response to Heat Stress

Heat stress (HS) is a common stress influencing the growth and reproduction of plant species. Jujube (Ziziphus jujuba Mill.) is an economically important tree with strong abiotic stress resistance, but the molecular mechanism of its response to HS remains elusive. In this study, we subjected seedlings of Z. jujuba cultivar "Hqing1-HR" to HS (45°C) for 0, 1, 3, 5, and 7 days, respectively, and collected the leaf samples (HR0, HR1, HR3, HR5, and HR7) accordingly. Fifteen cDNA libraries from leaves were constructed for transcriptomics assays. RNA sequencing and transcriptomics identified 1,642, 4,080, 5,160, and 2,119 differentially expressed genes (DEGs) in comparisons of HR1 vs. HR0, HR3 vs. HR0, HR5 vs. HR0, and HR7 vs. HR0, respectively. Gene ontology analyses of the DEGs from these comparisons revealed enrichment in a series of biological processes involved in stress responses, photosynthesis, and metabolism, suggesting that lowering or upregulating expression of these genes might play important roles in the response to HS. This study contributed to our understanding of the molecular mechanism of jujube response to HS and will be beneficial for developing jujube cultivars with improved heat resistance.

Mechanisms of elevated CO2-induced thermotolerance in plants: the role of phytohormones

Rising atmospheric CO₂ is a key driver of climate change, intensifying drastic changes in meteorological parameters. Plants can sense and respond to changes in environmental parameters including atmospheric CO₂ and temperatures. High temperatures beyond the physiological threshold can significantly affect plant growth and development and thus attenuate crop productivity. However, elevated atmospheric CO₂ can mitigate the deleterious effects of heat stress on plants. Despite a large body of literature supporting the positive impact of elevated CO₂ on thermotolerance, the underlying biological mechanisms and precise molecular pathways that lead to enhanced tolerance to heat stress remain largely unclear. Under heat stress, elevated CO₂-induced expression of respiratory burst oxidase homologs (RBOHs) and reactive oxygen species (ROS) signaling play a critical role in stomatal movement, which optimizes gas exchange to enhance photosynthesis and water use efficiency. Notably, elevated CO₂ also fortifies antioxidant defense and redox homeostasis to alleviate heat-induced oxidative damage. Both hormone-dependent and independent pathways have been shown to mediate high CO₂-induced thermotolerance. The activation of heat-shock factors and subsequent expression of heat-shock proteins are thought to be the essential mechanism downstream of hormone and ROS signaling. Here we review the role of phytohormones in plant response to high atmospheric CO₂ and temperatures. We also discuss the potential mechanisms of elevated CO₂-induced thermotolerance by focusing on several key phytohormones such as ethylene. Finally, we address some limitations of our current understanding and the need for further research to unveil the yet-unknown crosstalk between plant hormones in mediating high CO₂-induced thermotolerance in plants.

Molecular mechanisms of plant tolerance to heat stress: current landscape and future perspectives

We summarize recent studies focusing on the molecular basis of plant heat stress response (HSR), how HSR leads to thermotolerance, and promote plant adaptation to recurring heat stress events. The global crop productivity is facing unprecedented threats due to climate change as high temperature negatively influences plant growth and metabolism. Owing to their sessile nature, plants have developed complex signaling networks which enable them to perceive changes in ambient temperature. This in turn activates a suite of molecular changes that promote plant survival and reproduction under adverse conditions. Deciphering these mechanisms is an important task, as this could facilitate development of molecular markers, which could be ultimately used to breed thermotolerant crop cultivars. In current article, we summarize mechanisms involve in plant heat stress acclimation with special emphasis on advances related to heat stress perception, heat-induced signaling, heat stress-responsive gene expression and thermomemory that promote plant adaptation to short- and long-term-recurring heat-stress events. In the end, we will discuss impact of emerging technologies that could facilitate the development of heat stress-tolerant crop cultivars.

The Adaptation and Tolerance of Major Cereals and Legumes to Important Abiotic Stresses

Abiotic stresses, including drought, extreme temperatures, salinity, and waterlogging, are the major constraints in crop production. These abiotic stresses are likely to be amplified by climate change with varying temporal and spatial dimensions across the globe. The knowledge about the effects of abiotic stressors on major cereal and legume crops is essential for effective management in unfavorable agro-ecologies. These crops are critical components of cropping systems and the daily diets of millions across the globe. Major cereals like rice, wheat, and maize are highly vulnerable to abiotic stresses, while many grain legumes are grown in abiotic stress-prone areas. Despite extensive investigations, abiotic stress tolerance in crop plants is not fully understood. Current insights into the abiotic stress responses of plants have shown the potential to improve crop tolerance to abiotic stresses. Studies aimed at stress tolerance mechanisms have resulted in the elucidation of traits associated with tolerance in plants, in addition to the molecular control of stress-responsive genes. Some of these studies have paved the way for new opportunities to address the molecular basis of stress responses in plants and identify novel traits and associated genes for the genetic improvement of crop plants. The present review examines the responses of crops under abiotic stresses in terms of changes in morphology, physiology, and biochemistry, focusing on major cereals and legume crops. It also explores emerging opportunities to accelerate our efforts to identify desired traits and genes associated with stress tolerance.

A β-ketoacyl carrier protein reductase confers heat tolerance via the regulation of fatty acid biosynthesis and stress signaling in rice

Heat stress is a major environmental threat affecting crop growth and productivity. However, the molecular mechanisms associated with plant responses to heat stress are poorly understood. Here, we identified a heat stress-sensitive mutant, hts1, in rice. HTS1 encodes a thylakoid membrane-localized β-ketoacyl carrier protein reductase (KAR) involved in de novo fatty acid biosynthesis. Phylogenetic and bioinformatic analysis showed that HTS1 probably originated from streptophyte algae and is evolutionarily conserved in land plants. Thermostable HTS1 is predominantly expressed in green tissues and strongly induced by heat stress, but is less responsive to salinity, cold and drought treatments. An amino acid substitution at A254T in HTS1 causes a significant decrease in KAR enzymatic activity and, consequently, impairs fatty acid synthesis and lipid metabolism in the hts1 mutant, especially under heat stress. Compared to the wild-type, the hts1 mutant exhibited heat-induced higher H₂ O₂ accumulation, a larger Ca²⁺ influx to mesophyll cells, and more damage to membranes and chloroplasts. Also, disrupted heat stress signaling in the hts1 mutant depresses the transcriptional activation of HsfA2s and the downstream target genes. We suggest that HTS1 is critical for underpinning membrane stability, chloroplast integrity and stress signaling for heat tolerance in rice.

Real-time monitoring of subcellular H2O2 distribution in Chlamydomonas reinhardtii

Hydrogen peroxide (H2O2) is recognized as an important signaling molecule in plants. We sought to establish a genetically encoded, fluorescent H2O2 sensor that allows H2O2 monitoring in all major subcompartments of a Chlamydomonas cell. To this end, we used the Chlamydomonas Modular Cloning toolbox to target the hypersensitive H2O2 sensor reduction-oxidation sensitive green fluorescent protein2-Tsa2ΔCR to the cytosol, nucleus, mitochondrial matrix, chloroplast stroma, thylakoid lumen, and endoplasmic reticulum (ER). The sensor was functional in all compartments, except for the ER where it was fully oxidized. Employing our novel sensors, we show that H2O2 produced by photosynthetic linear electron transport (PET) in the stroma leaks into the cytosol but only reaches other subcellular compartments if produced under nonphysiological conditions. Furthermore, in heat-stressed cells, we show that cytosolic H2O2 levels closely mirror temperature up- and downshifts and are independent from PET. Heat stress led to similar up- and downshifts of H2O2 levels in the nucleus and, more mildly, in mitochondria but not in the chloroplast. Our results thus suggest the establishment of steep intracellular H2O2 gradients under normal physiological conditions with limited diffusion into other compartments. We anticipate that these sensors will greatly facilitate future investigations of H2O2 biology in plant cells.

Progress in Research on the Mechanisms Underlying Chloroplast-Involved Heat Tolerance in Plants

Global warming is a serious challenge plant production has to face. Heat stress not only affects plant growth and development but also reduces crop yield and quality. Studying the response mechanisms of plants to heat stress will help humans use these mechanisms to improve the heat tolerance of plants, thereby reducing the harm of global warming to plant production. Research on plant heat tolerance has gradually become a hotspot in plant molecular biology research in recent years. In view of the special role of chloroplasts in the response to heat stress in plants, this review is focusing on three perspectives related to chloroplasts and their function in the response of heat stress in plants: the role of chloroplasts in sensing high temperatures, the transmission of heat signals, and the improvement of heat tolerance in plants. We also present our views on the future direction of research on chloroplast related heat tolerance in plants.

Heat Shock Signaling in Land Plants: From Plasma Membrane Sensing to the Transcription of Small Heat Shock Proteins

Heat stress events are major factors limiting crop productivity. During summer days, land plants must anticipate in a timely manner upcoming mild and severe temperature. They respond by accumulating protective heat-shock proteins (HSPs), conferring acquired thermotolerance. All organisms synthetize HSPs; many of which are members of the conserved chaperones families. This review describes recent advances in plant temperature sensing, signaling, and response. We highlight the pathway from heat perception by the plasma membrane through calcium channels, such as cyclic nucleotide-gated channels, to the activation of the heat-shock transcription factors (HSFs). An unclear cellular signal activates HSFs, which act as essential regulators. In particular, the HSFA subfamily can bind heat shock elements in HSP promoters and could mediate the dissociation of bound histones, leading to HSPs transcription. Although plants can modulate their transcriptome, proteome, and metabolome to protect the cellular machinery, HSP chaperones prevent, use, and revert the formation of misfolded proteins, thereby avoiding heat-induced cell death. Remarkably, the HSP20 family is mostly tightly repressed at low temperature, suggesting that a costly mechanism can become detrimental under unnecessary conditions. Here, the role of HSP20s in response to HS and their possible deleterious expression at non-HS temperatures is discussed.

Safeguarding genome integrity under heat stress in plants

Heat stress adversely affects an array of molecular and cellular events in plant cells, such as denaturation of protein and lipid molecules and malformation of cellular membranes and cytoskeleton networks. Genome organization and DNA integrity are also disturbed under heat stress, and accordingly, plants have evolved sophisticated adaptive mechanisms that either protect their genomes from deleterious heat-induced damages or stimulate genome restoration responses. In particular, it is emerging that DNA damage responses are a critical defense process that underlies the acquirement of thermotolerance in plants, during which molecular players constituting the DNA repair machinery are rapidly activated. In recent years, thermotolerance genes that mediate the maintenance of genome integrity or trigger DNA repair responses have been functionally characterized in various plant species. Furthermore, accumulating evidence supports that genome integrity is safeguarded through multiple layers of thermoinduced protection routes in plant cells, including transcriptome adjustment, orchestration of RNA metabolism, protein homeostasis, and chromatin reorganization. In this review, we summarize topical progresses and research trends in understanding how plants cope with heat stress to secure genome intactness. We focus on molecular regulatory mechanisms by which plant genomes are secured against the DNA-damaging effects of heat stress and DNA damages are effectively repaired. We will also explore the practical interface between heat stress response and securing genome integrity in view of developing biotechnological ways of improving thermotolerance in crop species under global climate changes, a worldwide ecological concern in agriculture.

Hot topic: Thermosensing in plants

Plants alter their morphology and cellular homeostasis to promote resilience under a variety of heat regimes. Molecular processes that underlie these responses have been intensively studied and found to encompass diverse mechanisms operating across a broad range of cellular components, timescales and temperatures. This review explores recent progress throughout this landscape with a particular focus on thermosensing in the model plant Arabidopsis. Direct temperature sensors include the photosensors phytochrome B and phototropin, the clock component ELF3 and an RNA switch. In addition, there are heat-regulated processes mediated by ion channels, lipids and lipid-modifying enzymes, taking place at the plasma membrane and the chloroplast. In some cases, the mechanism of temperature perception is well understood but in others, this remains an open question. Potential novel thermosensing mechanisms are based on lipid and liquid-liquid phase separation. Finally, future research directions of high temperature perception and signalling pathways are discussed.

Small RNA, Transcriptome and Degradome Analysis of the Transgenerational Heat Stress Response Network in Durum Wheat

Heat stress is a major limiting factor of grain yield and quality in crops. Abiotic stresses have a transgenerational impact and the mechanistic basis is associated with epigenetic regulation. The current study presents the first systematic analysis of the transgenerational effects of post-anthesis heat stress in tetraploid wheat. Leaf physiological traits, harvest components and grain quality traits were characterized under the impact of parental and progeny heat stress. The parental heat stress treatment had a positive influence on the offspring for traits including chlorophyll content, grain weight, grain number and grain total starch content. Integrated sequencing analysis of the small RNAome, mRNA transcriptome and degradome provided the first description of the molecular networks mediating heat stress adaptation under transgenerational influence. The expression profile of 1771 microRNAs (733 being novel) and 66,559 genes was provided, with differentially expressed microRNAs and genes characterized subject to the progeny treatment, parental treatment and tissue-type factors. Gene Ontology and KEGG pathway analysis of stress responsive microRNAs-mRNA modules provided further information on their functional roles in biological processes such as hormone homeostasis, signal transduction and protein stabilization. Our results provide new insights on the molecular basis of transgenerational heat stress adaptation, which can be used for improving thermo-tolerance in breeding.

Distinctive in-planta acclimation responses to basal growth and acute heat stress were induced in Arabidopsis by cattle manure biochar

In-planta mechanisms of biochar (BC)-mediated improved growth were evaluated by examining oxidative stress, metabolic, and hormonal changes of Arabidopsis wild-type plants under basal or acute heat stress (-HS/ + HS) conditions with or without BC (+ BC/-BC). The oxidative stress was evaluated by using Arabidopsis expressing redox-sensitive green fluorescent protein in the plastids (pla-roGFP2). Fresh biomass and inflorescence height were greater in + BC(‒HS) plants than in the -BC(‒HS) plants, despite similar leaf nutrient levels, photosystem II (PSII) maximal efficiencies and similar oxidative poise. Endogenous levels of jasmonic and abscisic acids were higher in the + BC(‒HS) treatment, suggesting their role in growth improvement. HS in ‒BC plants caused reductions in inflorescence height and PSII maximum quantum yield, as well as significant oxidative stress symptoms manifested by increased lipid peroxidation, greater chloroplast redox poise (oxidized form of roGFP), increased expression of DNAJ heat shock proteins and Zn-finger genes, and reduced expression of glutathione-S-transferase gene in addition to higher abscisic acid and salicylic acid levels. Oxidative stress symptoms were significantly reduced by BC. Results suggest that growth improvements by BC occurring under basal and HS conditions are induced by acclimation mechanisms to 'microstresses' associated with basal growth and to oxidative stress of HS, respectively.

The calcium-dependent protein kinase ZmCDPK7 functions in heat-stress tolerance in maize

Global warming poses a serious threat to crops. Calcium-dependent protein kinases (CDPKs)/CPKs play vital roles in plant stress responses, but their exact roles in plant thermotolerance remains elusive. Here, we explored the roles of heat-induced ZmCDPK7 in thermotolerance in maize. ZmCDPK7-overexpressing maize plants displayed higher thermotolerance, photosynthetic rates, and antioxidant enzyme activity but lower H₂ O₂ and malondialdehyde (MDA) contents than wild-type plants under heat stress. ZmCDPK7-knockdown plants displayed the opposite patterns. ZmCDPK7 is attached to the plasma membrane but can translocate to the cytosol under heat stress. ZmCDPK7 interacts with the small heat shock protein sHSP17.4, phosphorylates sHSP17.4 at Ser-44 and the respiratory burst oxidase homolog RBOHB at Ser-99, and upregulates their expression. Site-directed mutagenesis of sHSP17.4 to generate a Ser-44-Ala substitution reduced ZmCDPK7's enhancement of catalase activity but enhanced ZmCDPK7's suppression of MDA accumulation in heat-stressed maize protoplasts. sHSP17.4, ZmCDPK7, and RBOHB were less strongly upregulated in response to heat stress in the abscisic acid-deficient mutant vp5 versus the wild type. Pretreatment with an RBOH inhibitor suppressed sHSP17.4 and ZmCDPK7 expression. Therefore, abscisic acid-induced ZmCDPK7 functions both upstream and downstream of RBOH and participates in thermotolerance in maize by mediating the phosphorylation of sHSP17.4, which might be essential for its chaperone function.

Heat Stress Targeting Individual Organs Reveals the Central Role of Roots and Crowns in Rice Stress Responses

Inter-organ communication and the heat stress (HS; 45°C, 6 h) responses of organs exposed and not directly exposed to HS were evaluated in rice (Oryza sativa) by comparing the impact of HS applied either to whole plants, or only to shoots or roots. Whole-plant HS reduced photosynthetic activity (F _v /F _m and QY_{_Lss} ), but this effect was alleviated by prior acclimation (37°C, 2 h). Dynamics of HSFA2d, HSP90.2, HSP90.3, and SIG5 expression revealed high protection of crowns and roots. Additionally, HSP26.2 was strongly expressed in leaves. Whole-plant HS increased levels of jasmonic acid (JA) and cytokinin cis-zeatin in leaves, while up-regulating auxin indole-3-acetic acid and down-regulating trans-zeatin in leaves and crowns. Ascorbate peroxidase activity and expression of alternative oxidases (AOX) increased in leaves and crowns. HS targeted to leaves elevated levels of JA in roots, cis-zeatin in crowns, and ascorbate peroxidase activity in crowns and roots. HS targeted to roots increased levels of abscisic acid and auxin in leaves and crowns, cis-zeatin in leaves, and JA in crowns, while reducing trans-zeatin levels. The weaker protection of leaves reflects the growth strategy of rice. HS treatment of individual organs induced changes in phytohormone levels and antioxidant enzyme activity in non-exposed organs, in order to enhance plant stress tolerance.

Pas de Trois: An Overview of Penta-, Tetra-, and Octo-Tricopeptide Repeat Proteins From Chlamydomonas reinhardtii and Their Role in Chloroplast Gene Expression

Penta-, Tetra-, and Octo-tricopeptide repeat (PPR, TPR, and OPR) proteins are nucleus-encoded proteins composed of tandem repeats of 35, 34, and 38-40 amino acids, respectively. They form helix-turn-helix structures that interact with mRNA or other proteins and participate in RNA stabilization, processing, maturation, and act as translation enhancers of chloroplast and mitochondrial mRNAs. These helical repeat proteins are unevenly present in plants and algae. While PPR proteins are more abundant in plants than in algae, OPR proteins are more abundant in algae. In Arabidopsis, maize, and rice there have been 450, 661, and 477 PPR proteins identified, respectively, which contrasts with only 14 PPR proteins identified in Chlamydomonas reinhardtii. Likewise, more than 120 OPR proteins members have been predicted from the nuclear genome of C. reinhardtii and only one has been identified in Arabidopsis thaliana. Due to their abundance in land plants, PPR proteins have been largely characterized making it possible to elucidate their RNA-binding code. This has even allowed researchers to generate engineered PPR proteins with defined affinity to a particular target, which has served as the basis to develop tools for gene expression in biotechnological applications. However, fine elucidation of the helical repeat proteins code in Chlamydomonas is a pending task. In this review, we summarize the current knowledge on the role PPR, TPR, and OPR proteins play in chloroplast gene expression in the green algae C. reinhardtii, pointing to relevant similarities and differences with their counterparts in plants. We also recapitulate on how these proteins have been engineered and shown to serve as mRNA regulatory factors for biotechnological applications in plants and how this could be used as a starting point for applications in algae.

Cyclic Nucleotide-Gated Ion Channel 6 Mediates Thermotolerance in Arabidopsis Seedlings by Regulating Hydrogen Peroxide Production via Cytosolic Calcium Ions

We previously reported the involvement of cyclic nucleotide-gated ion channel 6 (CNGC6) and hydrogen peroxide (H₂O₂) in plant responses to heat shock (HS). To demonstrate their relationship with plant thermotolerance, we assessed the effect of HS on several groups of Arabidopsis (Arabidopsis thaliana) seedlings: wild-type, cngc6 mutant, and its complementation line. Under exposure to HS, the level of H₂O₂ was lower in the cngc6 mutant seedlings than in the wild-type (WT) seedlings but obviously increased in the complementation line. The treatment of Arabidopsis seeds with calcium ions (Ca²⁺) increased the H₂O₂ levels in the seedlings under HS treatment, whereas treatment with a Ca²⁺ chelator (EGTA) inhibited it, indicating that CNGC6 may stimulate the accumulation of H₂O₂ in a manner dependent on an increase in cytosolic Ca²⁺ ([Ca²⁺]_cyt). This point was verified by phenotypic observations and thermotolerance testing with transgenic plants overexpressing AtRbohB and AtRbohD (two genes involved in HS-responsive H₂O₂ production), respectively, in a cngc6 background. Real-time reverse transcription-polymerase chain reactions and Western blotting suggested that CNGC6 enhanced the gene transcription of HS factors (HSFs) and the accumulation of HS proteins (HSPs) via H₂O₂. These upon results indicate that H₂O₂ acts downstream of CNGC6 in the HS signaling pathway, increasing our understanding of the initiation of plants responses to high temperatures.

Co-expression network analysis of protein phosphatase 2A (PP2A) genes with stress-responsive genes in Arabidopsis thaliana reveals 13 key regulators

Abiotic and biotic stresses adversely affect plant growth and development and eventually result in less yield and threaten food security worldwide. In plants, several studies have been carried out to understand molecular responses to abiotic and biotic stresses. However, the complete circuitry of stress-responsive genes that plants utilise in response to those environmental stresses are still unknown. The protein phosphatase 2A (PP2A) gene has been known to have a crucial role in abiotic and biotic stresses; but how it regulates the stress response in plants is still not known completely. In this study, we constructed gene co-expression networks of PP2A genes with stress-responsive gene datasets from cold, drought, heat, osmotic, genotoxic, salt, and wounding stresses to unveil their relationships with the PP2A under different conditions of stress. The graph analysis identified 13 hub genes and several influential genes based on closeness centrality score (CCS). Our findings also revealed the count of unique genes present in different settings of stresses and subunits. We also formed clusters of influential genes based on the stress, CCS, and co-expression value. Analysis of cis-regulatory elements (CREs), recurring in promoters of these genes was also performed. Our study has led to the identification of 16 conserved CREs.

Heat stress response in the closest algal relatives of land plants reveals conserved stress signaling circuits

All land plants (embryophytes) share a common ancestor that likely evolved from a filamentous freshwater alga. Elucidating the transition from algae to embryophytes - and the eventual conquering of Earth's surface - is one of the most fundamental questions in plant evolutionary biology. Here, we investigated one of the organismal properties that might have enabled this transition: resistance to drastic temperature shifts. We explored the effect of heat stress in Mougeotia and Spirogyra, two representatives of Zygnematophyceae - the closest known algal sister lineage to land plants. Heat stress induced pronounced phenotypic alterations in their plastids, and high-performance liquid chromatography-tandem mass spectroscopy-based profiling of 565 transitions for the analysis of main central metabolites revealed significant shifts in 43 compounds. We also analyzed the global differential gene expression responses triggered by heat, generating 92.8 Gbp of sequence data and assembling a combined set of 8905 well-expressed genes. Each organism had its own distinct gene expression profile; less than one-half of their shared genes showed concordant gene expression trends. We nevertheless detected common signature responses to heat such as elevated transcript levels for molecular chaperones, thylakoid components, and - corroborating our metabolomic data - amino acid metabolism. We also uncovered the heat-stress responsiveness of genes for phosphorelay-based signal transduction that links environmental cues, calcium signatures and plastid biology. Our data allow us to infer the molecular heat stress response that the earliest land plants might have used when facing the rapidly shifting temperature conditions of the terrestrial habitat.

Structural and Functional Heat Stress Responses of Chloroplasts of Arabidopsis thaliana

Temperature elevations constitute a major threat to plant performance. In recent years, much was learned about the general molecular mode of heat stress reaction of plants. The current research focuses on the integration of the knowledge into more global networks, including the reactions of cellular compartments. For instance, chloroplast function is central for plant growth and survival, and the performance of chloroplasts is tightly linked to the general status of the cell and vice versa. We examined the changes in photosynthesis, chloroplast morphology and proteomic composition posed in Arabidopsis thaliana chloroplasts after a single or repetitive heat stress treatment over a period of two weeks. We observed that the acclimation is potent in the case of repetitive application of heat stress, while a single stress results in lasting alterations. Moreover, the physiological capacity and its adjustment are dependent on the efficiency of the protein translocation process as judged from the analysis of mutants of the two receptor units of the chloroplast translocon, TOC64, and TOC33. In response to repetitive heat stress, plants without TOC33 accumulate Hsp70 proteins and plants without TOC64 have a higher content of proteins involved in thylakoid structure determination when compared to wild-type plants.

Unique and contrasting effects of light and temperature cues on plant transcriptional programs

Plants have adapted to tolerate and survive constantly changing environmental conditions by reprogramming gene expression in response to stress or to drive developmental transitions. Among the many signals that plants perceive, light and temperature are of particular interest due to their intensely fluctuating nature which is combined with a long-term seasonal trend. Whereas specific receptors are key in the light-sensing mechanism, the identity of plant thermosensors for high and low temperatures remains far from fully addressed. This review aims at discussing common as well as divergent characteristics of gene expression regulation in plants, controlled by light and temperature. Light and temperature signaling control the abundance of specific transcription factors, as well as the dynamics of co-transcriptional processes such as RNA polymerase elongation rate and alternative splicing patterns. Additionally, sensing both types of cues modulates gene expression by altering the chromatin landscape and through the induction of long non-coding RNAs (lncRNAs). However, while light sensing is channeled through dedicated receptors, temperature can broadly affect chemical reactions inside plant cells. Thus, direct thermal modifications of the transcriptional machinery add another level of complexity to plant transcriptional regulation. Besides the rapid transcriptome changes that follow perception of environmental signals, plant developmental transitions and acquisition of stress tolerance depend on long-term maintenance of transcriptional states (active or silenced genes). Thus, the rapid transcriptional response to the signal (Phase I) can be distinguished from the long-term memory of the acquired transcriptional state (Phase II - remembering the signal). In this review we discuss recent advances in light and temperature signal perception, integration and memory in Arabidopsis thaliana, focusing on transcriptional regulation and highlighting the contrasting and unique features of each type of cue in the process.

Nuclear-encoded synthesis of the D1 subunit of photosystem II increases photosynthetic efficiency and crop yield

In photosynthetic organisms, the photosystem II (PSII) complex is the primary target of thermal damage. Plants have evolved a repair process to prevent the accumulation of damaged PSII. The repair of PSII largely involves de novo synthesis of proteins, particularly the D1 subunit protein encoded by the chloroplast gene psbA. Here we report that the allotropic expression of the psbA complementary DNA driven by a heat-responsive promoter in the nuclear genome sufficiently protects PSII from severe loss of D1 protein and dramatically enhances survival rates of the transgenic plants of Arabidopsis, tobacco and rice under heat stress. Unexpectedly, we found that the nuclear origin supplementation of the D1 protein significantly stimulates transgenic plant growth by enhancing net CO2 assimilation rates with increases in biomass and grain yield. These findings represent a breakthrough in bioengineering plants to achieve efficient photosynthesis and increase crop productivity under normal and heat-stress conditions.

Sensitivity and Responses of Chloroplasts to Heat Stress in Plants

Increased temperatures caused by global warming threaten agricultural production, as warmer conditions can inhibit plant growth and development or even destroy crops in extreme circumstances. Extensive research over the past several decades has revealed that chloroplasts, the photosynthetic organelles of plants, are highly sensitive to heat stress, which affects a variety of photosynthetic processes including chlorophyll biosynthesis, photochemical reactions, electron transport, and CO2 assimilation. Important mechanisms by which plant cells respond to heat stress to protect these photosynthetic organelles have been identified and analyzed. More recent studies have made it clear that chloroplasts play an important role in inducing the expression of nuclear heat-response genes during the heat stress response. In this review, we summarize these important advances in plant-based research and discuss how the sensitivity, responses, and signaling roles of chloroplasts contribute to plant heat sensitivity and tolerance.

Differential physiological, transcriptomic and metabolomic responses of Arabidopsis leaves under prolonged warming and heat shock

BACKGROUND

Elevated temperature as a result of global climate warming, either in form of sudden heatwave (heat shock) or prolonged warming, has profound effects on the growth and development of plants. However, how plants differentially respond to these two forms of elevated temperatures is largely unknown. Here we have therefore performed a comprehensive comparison of multi-level responses of Arabidopsis leaves to heat shock and prolonged warming.

RESULTS

The plant responded to prolonged warming through decreased stomatal conductance, and to heat shock by increased transpiration. In carbon metabolism, the glycolysis pathway was enhanced while the tricarboxylic acid (TCA) cycle was inhibited under prolonged warming, and heat shock significantly limited the conversion of pyruvate into acetyl coenzyme A. The cellular concentration of hydrogen peroxide (H₂O₂) and the activities of antioxidant enzymes were increased under both conditions but exhibited a higher induction under heat shock. Interestingly, the transcription factors, class A1 heat shock factors (HSFA1s) and dehydration responsive element-binding proteins (DREBs), were up-regulated under heat shock, whereas with prolonged warming, other abiotic stress response pathways, especially basic leucine zipper factors (bZIPs) were up-regulated instead.

CONCLUSIONS

Our findings reveal that Arabidopsis exhibits different response patterns under heat shock versus prolonged warming, and plants employ distinctly different response strategies to combat these two types of thermal stress.