A hit-and-run heat shock factor governs sustained histone methylation and transcriptional stress memory

Cause or effect: Probing the roles of epigenetics in plant development and environmental responses

Epigenetic modifications are inheritable, reversible changes that control gene expression without altering the DNA sequence itself. Recent advances in epigenetic and sequencing technologies have revealed key regulatory regions in genes with multiple epigenetic changes. However, causal associations between epigenetic changes and physiological events have rarely been examined. Epigenome editing enables alterations to the epigenome without changing the underlying DNA sequence. Modifying epigenetic information in plants has important implications for causality assessment of the epigenome. Here, we briefly review tools for selectively interrogating the epigenome. We highlight promising research on site-specific DNA methylation and histone modifications and propose future research directions to more deeply investigate epigenetic regulation, including cause-and-effect relationships between epigenetic modifications and the development/environmental responses of Arabidopsis thaliana.

Decoding histone 3 lysine methylation: Insights into seed germination and flowering

Histone lysine methylation is a highly conserved epigenetic modification across eukaryotes that contributes to creating different dynamic chromatin states, which may result in transcriptional changes. Over the years, an accumulated set of evidence has shown that histone methylation allows plants to align their development with their surroundings, enabling them to respond and memorize past events due to changes in the environment. In this review, we discuss the molecular mechanisms of histone methylation in plants. Writers, readers, and erasers of Arabidopsis histone methylation marks are described with an emphasis on their role in two of the most important developmental transition phases in plants, seed germination and flowering. Further, the crosstalk between different methylation marks is also discussed. An overview of the mechanisms of histone methylation modifications and their biological outcomes will shed light on existing research gaps and may provide novel perspectives to increase crop yield and resistance in the era of global climate change.

Mechanisms of heat stress-induced transcriptional memory

Transcriptional memory allows organisms to store information about transcriptional reprogramming in response to a stimulus. In plants, this often involves the response to an abiotic stress, which in nature may be cyclical or recurring. Such transcriptional memory confers sustained induction or enhanced re-activation in response to a recurrent stimulus, which may increase chances of survival and fitness. Heat stress (HS) has emerged as an excellent model system to study transcriptional memory in plants, and much progress has been made in elucidating the molecular mechanisms underlying this phenomenon. Here, we review how histone turnover and transcriptional co-regulator complexes contribute to reprogramming of transcriptional responses.

Heat stress transcription factors as the central molecular rheostat to optimize plant survival and recovery from heat stress

Heat stress transcription factors (HSFs) are the core regulators of the heat stress (HS) response in plants. HSFs are considered as a molecular rheostat: their activities define the response intensity, incorporating information about the environmental temperature through a network of partner proteins. A prompted activation of HSFs is required for survival, for example the de novo synthesis of heat shock proteins. Furthermore, a timely attenuation of the stress response is necessary for the restoration of cellular functions and recovery from stress. In an ever-changing environment, the balance between thermotolerance and developmental processes such as reproductive fitness highlights the importance of a tightly tuned response. In many cases, the response is described as an ON/OFF mode, while in reality, it is very dynamic. This review compiles recent findings to update existing models about the HSF-regulated HS response and address two timely questions: How do plants adjust the intensity of cellular HS response corresponding to the temperature they experience? How does this adjustment contribute to the fine-tuning of the HS and developmental networks? Understanding these processes is crucial not only for enhancing our basic understanding of plant biology but also for developing strategies to improve crop resilience and productivity under stressful conditions.

Transgenerational plasticity in salinity tolerance of rice: unraveling non-genetic phenotypic modifications and environmental influences

Transgenerational plasticity in plants enables rapid adaptation to environmental changes, allowing organisms and their offspring to adapt to the environment without altering their underlying DNA. In this study, we investigated the transgenerational plasticity in salinity tolerance of rice plants using a reciprocal transplant experimental strategy. Our aim was to assess whether non-genetic environment-induced phenotypic modifications and transgenerational salinity affect the salinity tolerance of progeny while excluding nuclear genomic factors for two generations. Using salt-tolerant and salt-sensitive rice genotypes, we observed that the parentally salt-stressed salt-sensitive genotype displayed greater growth performance, photosynthetic activity, yield performance, and transcriptional responses than the parentally non-stressed salt-sensitive plants under salt stress conditions. Surprisingly, salt stress-exposed salt-tolerant progeny did not exhibit as much salinity tolerance as salt stress-exposed salt-sensitive progeny under salt stress. Our findings indicate that the phenotypes of offspring plants differed based on the environment experienced by their ancestors, resulting in heritable transgenerational phenotypic modifications in salt-sensitive genotypes via maternal effects. These results elucidated the mechanisms underlying transgenerational plasticity in salinity tolerance, providing valuable insights into how plants respond to changing environmental conditions.

Establishment and Maintenance of Heat-Stress Memory in Plants

Among the rich repertoire of strategies that allow plants to adapt to high-temperature stress is heat-stress memory. The mechanisms underlying the establishment and maintenance of heat-stress memory are poorly understood, although the chromatin opening state appears to be an important structural basis for maintaining heat-stress memory. The chromatin opening state is influenced by epigenetic modifications, making DNA and histone modifications important entry points for understanding heat-shock memory. Current research suggests that traditional heat-stress signaling pathway components might be involved in chromatin opening, thereby promoting the establishment of heat-stress memory in plants. In this review, we discuss the relationship between chromatin structure-based maintenance and the establishment of heat-stress memory. We also discuss the association between traditional heat-stress signals and epigenetic modifications. Finally, we discuss potential research ideas for exploring plant adaptation to high-temperature stress in the future.

To live or let die? Epigenetic adaptations to climate change-a review

Anthropogenic activities are responsible for a wide array of environmental disturbances that threaten biodiversity. Climate change, encompassing temperature increases, ocean acidification, increased salinity, droughts, and floods caused by frequent extreme weather events, represents one of the most significant environmental alterations. These drastic challenges pose ecological constraints, with over a million species expected to disappear in the coming years. Therefore, organisms must adapt or face potential extinctions. Adaptations can occur not only through genetic changes but also through non-genetic mechanisms, which often confer faster acclimatization and wider variability ranges than their genetic counterparts. Among these non-genetic mechanisms are epigenetics defined as the study of molecules and mechanisms that can perpetuate alternative gene activity states in the context of the same DNA sequence. Epigenetics has received increased attention in the past decades, as epigenetic mechanisms are sensitive to a wide array of environmental cues, and epimutations spread faster through populations than genetic mutations. Epimutations can be neutral, deleterious, or adaptative and can be transmitted to subsequent generations, making them crucial factors in both long- and short-term responses to environmental fluctuations, such as climate change. In this review, we compile existing evidence of epigenetic involvement in acclimatization and adaptation to climate change and discuss derived perspectives and remaining challenges in the field of environmental epigenetics. Graphical Abstract.

Thermo-priming triggers species-specific physiological and transcriptome responses in Mediterranean seagrasses

Heat-priming improves plants' tolerance to a recurring heat stress event. The underlying molecular mechanisms of heat-priming are largely unknown in seagrasses. Here, ad hoc mesocosm experiments were conducted with two Mediterranean seagrass species, Posidonia oceanica and Cymodocea nodosa. Plants were first exposed to heat-priming, followed by a heat-triggering event. A comprehensive assessment of plant stress response across different levels of biological organization was performed at the end of the triggering event. Morphological and physiological results showed an improved response of heat-primed P. oceanica plants while in C. nodosa both heat- and non-primed plants enhanced their growth rates at the end of the triggering event. As resulting from whole transcriptome sequencing, molecular functions related to several cellular compartments and processes were involved in the response to warming of non-primed plants, while the response of heat-primed plants involved a limited group of processes. Our results suggest that seagrasses acquire a primed state during the priming event, that eventually gives plants the ability to induce a more energy-effective response when the thermal stress event recurs. Different species may differ in their ability to perform an improved heat stress response after priming. This study provides pioneer molecular insights into the emerging topic of seagrass stress priming and may benefit future studies in the field.

The complex transcriptional regulation of heat stress response in maize

As one of the most important food and feed crops worldwide, maize suffers much more tremendous damages under heat stress compared to other plants, which seriously inhibits plant growth and reduces productivity. To mitigate the heat-induced damages and adapt to high temperature environment, plants have evolved a series of molecular mechanisms to sense, respond and adapt high temperatures and heat stress. In this review, we summarized recent advances in molecular regulations underlying high temperature sensing, heat stress response and memory in maize, especially focusing on several important pathways and signals in high temperature sensing, and the complex transcriptional regulation of ZmHSFs (Heat Shock Factors) in heat stress response. In addition, we highlighted interactions between ZmHSFs and several epigenetic regulation factors in coordinately regulating heat stress response and memory. Finally, we laid out strategies to systematically elucidate the regulatory network of maize heat stress response, and discussed approaches for breeding future heat-tolerance maize.

Autophagy: a key player in the recovery of plants from heat stress

Plants can be primed to withstand otherwise lethal heat stress (HS) through exposure to a preceding temporary and mild HS, commonly known as the 'thermopriming stimulus'. Plants have also evolved mechanisms to establish 'memories' of a previous stress encounter, or to reset their physiology to the original cellular state once the stress has ended. The priming stimulus triggers a widespread change of transcripts, proteins, and metabolites, which is crucial for maintaining the memory state but may not be required for growth and development under optimal conditions or may even be harmful. In such a scenario, recycling mechanisms such as autophagy are crucial for re-establishing cellular homeostasis and optimizing resource use for post-stress growth. While pivotal for eliminating heat-induced protein aggregates and protecting plants from the harmful impact of HS, recent evidence implies that autophagy also breaks down heat-induced protective macromolecules, including heat shock proteins, functioning as a resetting mechanism during the recovery from mild HS. This review provides an overview of the latest advances in understanding the multifaceted functions of autophagy in HS responses, with a specific emphasis on its roles in recovery from mild HS, and the modulation of HS memory.

A review of the potential involvement of small RNAs in transgenerational abiotic stress memory in plants

Crop production is increasingly threatened by the escalating weather events and rising temperatures associated with global climate change. Plants have evolved adaptive mechanisms, including stress memory, to cope with abiotic stresses such as heat, drought, and salinity. Stress memory involves priming, where plants remember prior stress exposures, providing enhanced responses to subsequent stress events. Stress memory can manifest as somatic, intergenerational, or transgenerational memory, persisting for different durations. The chromatin, a central regulator of gene expression, undergoes modifications like DNA acetylation, methylation, and histone variations in response to abiotic stress. Histone modifications, such as H3K4me3 and acetylation, play crucial roles in regulating gene expression. Abiotic stresses like drought and salinity are significant challenges to crop production, leading to yield reductions. Plant responses to stress involve strategies like escape, avoidance, and tolerance, each influencing growth stages differently. Soil salinity affects plant growth by disrupting water potential, causing ion toxicity, and inhibiting nutrient uptake. Understanding plant responses to these stresses requires insights into histone-mediated modifications, chromatin remodeling, and the role of small RNAs in stress memory. Histone-mediated modifications, including acetylation and methylation, contribute to epigenetic stress memory, influencing plant adaptation to environmental stressors. Chromatin remodeling play a crucial role in abiotic stress responses, affecting the expression of stress-related genes. Small RNAs; miRNAs and siRNAs, participate in stress memory pathways by guiding DNA methylation and histone modifications. The interplay of these epigenetic mechanisms helps plants adapt to recurring stress events and enhance their resilience. In conclusion, unraveling the epigenetic mechanisms in plant responses to abiotic stresses provides valuable insights for developing resilient agricultural techniques. Understanding how plants utilize stress memory, histone modifications, chromatin remodeling, and small RNAs is crucial for designing strategies to mitigate the impact of climate change on crop production and global food security.

Beyond heat waves: Unlocking epigenetic heat stress memory in Arabidopsis

Plants remember their exposure to environmental changes and respond more effectively the next time they encounter a similar change by flexibly altering gene expression. Epigenetic mechanisms play a crucial role in establishing such memory of environmental changes and fine-tuning gene expression. With the recent advancements in biochemistry and sequencing technologies, it has become possible to characterize the dynamics of epigenetic changes on scales ranging from short term (minutes) to long term (generations). Here, our main focus is on describing the current understanding of the temporal regulation of histone modifications and chromatin changes during exposure to short-term recurring high temperatures and reevaluating them in the context of natural environments. Investigations of the dynamics of histone modifications and chromatin structural changes in Arabidopsis after repeated exposure to heat at short intervals have revealed the detailed molecular mechanisms of short-term heat stress memory, which include histone modification enzymes, chromatin remodelers, and key transcription factors. In addition, we summarize the spatial regulation of heat responses. Based on the natural temperature patterns during summer, we discuss how plants cope with recurring heat stress occurring at various time intervals by utilizing 2 distinct types of heat stress memory mechanisms. We also explore future research directions to provide a more precise understanding of the epigenetic regulation of heat stress memory.

The transcription factor HSFA7b controls thermomemory at the shoot apical meristem by regulating ethylene biosynthesis and signaling in Arabidopsis

The shoot apical meristem (SAM) is responsible for overall shoot growth by generating all aboveground structures. Recent research has revealed that the SAM displays an autonomous heat stress (HS) memory of a previous non-lethal HS event. Considering the importance of the SAM for plant growth, it is essential to determine how its thermomemory is mechanistically controlled. Here, we report that HEAT SHOCK TRANSCRIPTION FACTOR A7b (HSFA7b) plays a crucial role in this process in Arabidopsis, as the absence of functional HSFA7b results in the temporal suppression of SAM activity after thermopriming. We found that HSFA7b directly regulates ethylene response at the SAM by binding to the promoter of the key ethylene signaling gene ETHYLENE-INSENSITIVE 3 to establish thermotolerance. Moreover, we demonstrated that HSFA7b regulates the expression of ETHYLENE OVERPRODUCER 1 (ETO1) and ETO1-LIKE 1, both of which encode ethylene biosynthesis repressors, thereby ensuring ethylene homeostasis at the SAM. Taken together, these results reveal a crucial and tissue-specific role for HSFA7b in thermomemory at the Arabidopsis SAM.

Understanding plant stress memory traits can provide a way for sustainable agriculture

Being sessile, plants encounter various biotic and abiotic threats in their life cycle. To minimize the damages caused by such threats, plants have acquired sophisticated response mechanisms. One major such response includes memorizing the encountered stimuli in the form of a metabolite, hormone, protein, or epigenetic marks. All of these individually as well as together, facilitate effective transcriptional and post-transcriptional responses upon encountering the stress episode for a second time during the life cycle and in some instances even in the future generations. This review attempts to highlight the recent advances in the area of plant memory. A detailed understanding of plant memory has the potential to offer solutions for developing climate-resilient crops for sustainable agriculture.

Lily LlHSFC2 coordinates with HSFAs to balance heat stress response and improve thermotolerance

Heat stress transcription factors (HSFs) are core regulators of plant heat stress response. Much research has focused on class A and B HSFs, leaving those of class C relatively understudied. Here, we reported a lily (Lilium longiflorum) heat-inducible HSFC2 homology involved in thermotolerance. LlHSFC2 was located in the nucleus and cytoplasm and exhibited a repression ability by binding heat stress element. Overexpression of LlHSFC2 in Arabidopsis, tobacco (Nicotiana benthamiana), and lily, all increased the thermotolerance. Conversely, silencing of LlHSFC2 in lily reduced its thermotolerance. LlHSFC2 could interact with itself, or interact with LlHSFA1, LlHSFA2, LlHSFA3A, and LlHSFA3B of lily, AtHSFA1e and AtHSFA2 of Arabidopsis, and NbHSFA2 of tobacco. LlHSFC2 interacted with HSFAs to accelerate their transactivation ability and act as a transcriptional coactivator. Notably, compared with the separate LlHSFA3A overexpression, co-overexpression of LlHSFC2/LlHSFA3A further enhanced thermotolerance of transgenic plants. In addition, after suffering HS, the homologous interaction of LlHSFC2 was repressed, but its heterologous interaction with the heat-inducible HSFAs was promoted, enabling it to exert its co-activation effect for thermotolerance establishment and maintenance. Taken together, we identified that LlHSFC2 plays an active role in the general balance and maintenance of heat stress response by cooperating with HSFAs, and provided an important candidate for the enhanced thermotolerance breeding of crops and horticulture plants.

Revisiting plant stress memory: mechanisms and contribution to stress adaptation

Highly repetitive adverse environmental conditions are encountered by plants multiple times during their lifecycle. These repetitive encounters with stresses provide plants an opportunity to remember and recall the experiences of past stress-associated responses, resulting in better adaptation towards those stresses. In general, this phenomenon is known as plant stress memory. According to our current understanding, epigenetic mechanisms play a major role in plants stress memory through DNA methylation, histone, and chromatin remodeling, and modulating non-coding RNAs. In addition, transcriptional, hormonal, and metabolic-based regulations of stress memory establishment also exist for various biotic and abiotic stresses. Plant memory can also be generated by priming the plants using various stressors that improve plants' tolerance towards unfavorable conditions. Additionally, the application of priming agents has been demonstrated to successfully establish stress memory. However, the interconnection of all aspects of the underlying mechanisms of plant stress memory is not yet fully understood, which limits their proper utilization to improve the stress adaptations in plants. This review summarizes the recent understanding of plant stress memory and its potential applications in improving plant tolerance towards biotic and abiotic stresses.

The Mediator kinase module enhances polymerase activity to regulate transcriptional memory after heat stress in Arabidopsis

Plants are often exposed to recurring adverse environmental conditions in the wild. Acclimation to high temperatures entails transcriptional responses, which prime plants to better withstand subsequent stress events. Heat stress (HS)-induced transcriptional memory results in more efficient re-induction of transcription upon recurrence of heat stress. Here, we identified CDK8 and MED12, two subunits of the kinase module of the transcription co-regulator complex, Mediator, as promoters of heat stress memory and associated histone modifications in Arabidopsis. CDK8 is recruited to heat-stress memory genes by HEAT SHOCK TRANSCRIPTION FACTOR A2 (HSFA2). Like HSFA2, CDK8 is largely dispensable for the initial gene induction upon HS, and its function in transcriptional memory is thus independent of primary gene activation. In addition to the promoter and transcriptional start region of target genes, CDK8 also binds their 3'-region, where it may promote elongation, termination, or rapid re-initiation of RNA polymerase II (Pol II) complexes during transcriptional memory bursts. Our work presents a complex role for the Mediator kinase module during transcriptional memory in multicellular eukaryotes, through interactions with transcription factors, chromatin modifications, and promotion of Pol II efficiency.

METTL4-mediated N6-methyladenine DNA modification regulates thermotolerance in Arabidopsis thaliana

DNA N6-methyladenine (6 mA) is an evolutionarily conserved DNA modification in procaryotes and eukaryotes. The DNA 6 mA methylation is tightly controlled by 6 mA regulatory proteins. DNA N6-adenine methyltransferase 1 (DAMT-1) has been identified as a DNA 6 mA methyltransferase in animals. In plants, DNA 6 mA methylation has been found, however, the DNA 6 mA methyltransferases and their function in plants are largely unknown. In our study, we find METTL4 is a DNA 6 mA methyltransferase in Arabidopsis thaliana. Both in vitro and in vivo evidences support the DNA 6 mA methyltransferase activity of METTL4. mettl4 mutant is hypersensitive to heat stress, suggesting DNA 6 mA methylation plays important role in heat stress adaption. RNA-seq and 6 mA IP-qPCR analysis show that METTL4 participates in heat stress tolerance by regulating expression of heat responsive genes. Our study find METTL4 is a plant DNA 6 mA methyltransferase and illustrates its function in regulating heat stress response.

Physiological and transcriptional analyses reveal formation of memory under recurring drought stresses in seedlings of cotton (Gossypium hirsutum)

Plants are frequently subjected to a range of environmental stresses, including drought, salinity, cold, pathogens, and herbivore attacks. To survive in such conditions, plants have evolved a novel adaptive mechanism known as 'stress memory'. The formation of stress memories necessitates coordinated responses at the cellular, genetic/genomic, and epigenetic levels, involving altered physiological responses, gene activation, hyper-induction and chromatin modification. Cotton (Gossypium spp.) is an important economic crop with numerous applications and high economic value. In this study, we establish G. hirsutum drought memory following cycles of mild drought and re-watering treatments and analyzed memory gene expression patterns. Our findings reveal the physiological, biochemical, and molecular mechanisms underlying drought stress memory formation in G. hirsutum. Specifically, H3K4me3, a histone modification, plays a crucial role in regulating [+ /+ ] transcriptional memory. Moreover, we investigated the intergenerational inheritance of drought stress memory in G. hirsutum. Collectively, our data provides theoretical guidance for cotton breeding.

Meta-analysis of Arabidopsis thaliana microarray data in relation to heat stress response

Introduction

Increasing global warming has made heat stress a serious threat to crop productivity and global food security in recent years. One of the most promising solutions to address this issue is developing heat-stress-tolerant plants. Hence, a thorough understanding of heat stress response mechanisms, particularly molecular ones, is crucial.

Methods

Although numerous studies have used microarray expression profiling technology to explore this area, these experiments often face limitations, leading to inconsistent results. To overcome these limitations, a random effects meta-analysis was employed using advanced statistical methods. A meta-analysis of 16 microarray datasets related to heat stress response in Arabidopsis thaliana was conducted.

Results

The analysis revealed 1,972 significant differentially expressed genes between control and heat-stressed plants (826 over-expressed and 1,146 down-expressed), including 128 differentially expressed transcription factors from different families. The most significantly enriched biological processes, molecular functions, and KEGG pathways for over-expressed genes included heat response, mRNA splicing via spliceosome pathways, unfolded protein binding, and heat shock protein binding. Conversely, for down-expressed genes, the most significantly enriched categories included cell wall organization or biogenesis, protein phosphorylation, transmembrane transporter activity, ion transmembrane transporter, biosynthesis of secondary metabolites, and metabolic pathways.

Discussion

Through our comprehensive meta-analysis of heat stress transcriptomics, we have identified pivotal genes integral to the heat stress response, offering profound insights into the molecular mechanisms by which plants counteract such stressors. Our findings elucidate that heat stress influences gene expression both at the transcriptional phase and post-transcriptionally, thereby substantially augmenting our comprehension of plant adaptive strategies to heat stress.

Histone retention preserves epigenetic marks during heat stress-induced transcriptional memory in plants

Plants often experience recurrent stressful events, for example, during heat waves. They can be primed by heat stress (HS) to improve the survival of more severe heat stress conditions. At certain genes, sustained expression is induced for several days beyond the initial heat stress. This transcriptional memory is associated with hyper-methylation of histone H3 lysine 4 (H3K4me3), but it is unclear how this is maintained for extended periods. Here, we determined histone turnover by measuring the chromatin association of HS-induced histone H3.3. Genome-wide histone turnover was not homogenous; in particular, H3.3 was retained longer at heat stress memory genes compared to HS-induced non-memory genes during the memory phase. While low nucleosome turnover retained H3K4 methylation, methylation loss did not affect turnover, suggesting that low nucleosome turnover sustains H3K4 methylation, but not vice versa. Together, our results unveil the modulation of histone turnover as a mechanism to retain environmentally mediated epigenetic modifications.

Histone H3 lysine 27 trimethylation suppresses jasmonate biosynthesis and signaling to affect male fertility under high temperature in cotton

High-temperature (HT) stress causes male sterility in crops, thus decreasing yields. To explore the possible contribution of histone modifications to male fertility under HT conditions, we defined the histone methylation landscape for the marks histone H3 lysine 27 trimethylation (H3K27me3) and histone H3 lysine 4 trimethylation (H3K4me3) by chromatin immunoprecipitation sequencing (ChIP-seq) in two differing upland cotton (Gossypium hirsutum) varieties. We observed a global disruption in H3K4me3 and H3K27me3 modifications, especially H3K27me3, in cotton anthers subjected to HT. HT affected the bivalent H3K4me3-H3K27me3 modification more than either monovalent modification. We determined that removal of H3K27me3 at the promoters of jasmonate-related genes increased their expression, maintaining male fertility under HT in the HT-tolerant variety at the anther dehiscence stage. Modulating jasmonate homeostasis or signaling resulted in an anther indehiscence phenotype under HT. Chemical suppression of H3K27me3 deposition increased jasmonic acid contents and maintained male fertility under HT. In summary, our study provides new insights into the regulation of male fertility by histone modifications under HT and suggests a potential strategy for improving cotton HT tolerance.

Is winter coming? Impact of the changing climate on plant responses to cold temperature

Climate change is causing alterations in annual temperature regimes worldwide. Important aspects of this include the reduction of winter chilling temperatures as well as the occurrence of unpredicted frosts, both significantly affecting plant growth and yields. Recent studies advanced the knowledge of the mechanisms underlying cold responses and tolerance in the model plant Arabidopsis thaliana. However, how these cold-responsive pathways will readjust to ongoing seasonal temperature variation caused by global warming remains an open question. In this review, we highlight the plant developmental programmes that depend on cold temperature. We focus on the molecular mechanisms that plants have evolved to adjust their development and stress responses upon exposure to cold. Covering both genetic and epigenetic aspects, we present the latest insights into how alternative splicing, noncoding RNAs and the formation of biomolecular condensates play key roles in the regulation of cold responses. We conclude by commenting on attractive targets to accelerate the breeding of increased cold tolerance, bringing up biotechnological tools that might assist in overcoming current limitations. Our aim is to guide the reflection on the current agricultural challenges imposed by a changing climate and to provide useful information for improving plant resilience to unpredictable cold regimes.

The transcriptome of soybean reproductive tissues subjected to water deficit, heat stress, and a combination of water deficit and heat stress

Global warming and climate change are driving an alarming increase in the frequency and intensity of extreme climate events, such as droughts, heat waves, and their combination, inflicting heavy losses to agricultural production. Recent studies revealed that the transcriptomic responses of different crops to water deficit (WD) or heat stress (HS) are very different from that to a combination of WD + HS. In addition, it was found that the effects of WD, HS, and WD + HS are significantly more devastating when these stresses occur during the reproductive growth phase of crops, compared to vegetative growth. As the molecular responses of different reproductive and vegetative tissues of plants to WD, HS, or WD + HS could be different from each other and these differences could impact many current and future attempts to enhance the resilience of crops to climate change through breeding and/or engineering, we conducted a transcriptomic analysis of different soybean (Glycine max) tissues to WD, HS, and WD + HS. Here we present a reference transcriptomic dataset that includes the response of soybean leaf, pod, anther, stigma, ovary, and sepal to WD, HS, and WD + HS conditions. Mining this dataset for the expression pattern of different stress response transcripts revealed that each tissue had a unique transcriptomic response to each of the different stress conditions. This finding is important as it suggests that enhancing the overall resilience of crops to climate change could require a coordinated approach that simultaneously alters the expression of different groups of transcripts in different tissues in a stress-specific manner.

Epigenetic processes in plant stress priming: Open questions and new approaches

Priming reflects the capacity of plants to memorise environmental stress experience and improve their response to recurring stress. Epigenetic modifications in DNA and associated histone proteins may carry short-term and long-term memory in the same plant or mediate transgenerational effects, but the evidence is still largely circumstantial. New experimental tools now enable scientists to perform targeted manipulations that either prevent or generate a particular epigenetic modification in a particular location of the genome. Such 'reverse epigenetics' approaches allow for the interrogation of causality between individual priming-induced modifications and their role for altering gene expression and plant performance under recurring stress. Furthermore, combining site-directed epigenetic manipulation with conditional and cell-type specific promoters creates novel opportunities to test and engineer spatiotemporal patterns of priming.

The Role of Histone Modifications in Heat Signal Transduction in Plants

Global warming and the more frequent occurrence of extremly high temperatures seriously affect crop yields. Heat stress (HS) has become a major environmental factor threatening food security worldwide. Understanding how plants sense and respond to HS is of clear interest to plant scientists and crop breeders. However, it is not trivial to elucidate the underlying signaling cascade, as specific cellular responses (ranging from detrimental to systemic effects) must be disentangled. Plants respond and adapt to high temperatures in many ways. In this review, recent progress in understanding heat signal transduction and the role of histone modifications in regulating the expression of genes involved in HS responses are discussed. The outstanding issues that are crucial for understanding the interactions between plants and HS are also discussed. The study of heat signal transduction mechanisms in plants is essential to facilitate the cultivation of heat-resistant crop varieties.

Regulation of plant epigenetic memory in response to cold and heat stress: towards climate resilient agriculture

Plants have evolved to adapt and grow in hot and cold climatic conditions. Some also adapt to daily and seasonal temperature changes. Epigenetic modifications play an important role in regulating plant tolerance under such conditions. DNA methylation and post-translational modifications of histone proteins influence gene expression during plant developmental stages and under stress conditions, including cold and heat stress. While short-term modifications are common, some modifications may persist and result in stress memory that can be inherited by subsequent generations. Understanding the mechanisms of epigenomes responding to stress and the factors that trigger stress memory is crucial for developing climate-resilient agriculture, but such an integrated view is currently limited. This review focuses on the plant epigenetic stress memory during cold and heat stress. It also discusses the potential of machine learning to modify stress memory through epigenetics to develop climate-resilient crops.

Epigenetic and transcription factors synergistically promote the high temperature response in plants

Temperature is one of the main environmental cues affecting plant growth and development, and plants have evolved multiple mechanisms to sense and acclimate to high temperature. Emerging research has shown that transcription factors, epigenetic factors, and their coordination are essential for plant temperature responses and the resulting phenological adaptation. Here, we summarize recent advances in molecular and cellular mechanisms to understand how plants acclimate to high temperature and describe how plant meristems sense and integrate environmental signals. Furthermore, we lay out future directions for new technologies to reveal heterogeneous responses in different cell types thus improving plant environmental plasticity.

Transcriptional Regulators of Plant Adaptation to Heat Stress

Heat stress (HS) is becoming an increasingly large problem for food security as global warming progresses. As sessile species, plants have evolved different mechanisms to cope with the disruption of cellular homeostasis, which can impede plant growth and development. Here, we summarize the mechanisms underlying transcriptional regulation mediated by transcription factors, epigenetic regulators, and regulatory RNAs in response to HS. Additionally, cellular activities for adaptation to HS are discussed, including maintenance of protein homeostasis through protein quality control machinery, and autophagy, as well as the regulation of ROS homeostasis via a ROS-scavenging system. Plant cells harmoniously regulate their activities to adapt to unfavorable environments. Lastly, we will discuss perspectives on future studies for improving urban agriculture by increasing crop resilience to HS.

Remembering foods and foes: emerging principles of transcriptional memory

Transcriptional memory is characterized by a primed cellular state, induced by an external stimulus that results in an altered expression of primed genes upon re-exposure to the inducing signal. Intriguingly, the primed state is heritably maintained across somatic cell divisions even after the initial stimulus and target gene transcription cease. This phenomenon is widely observed across various organisms and appears to enable cells to retain a memory of external signals, thereby adapting to environmental changes. Signals range from nutrient supplies (food) to a variety of stress signals, including exposure to pathogens (foes), leading to long-term memory such as in the case of trained immunity in plants and mammals. Here, we review these priming phenomena and our current understanding of transcriptional memory. We consider different mechanistic models for how memory can work and discuss existing evidence for potential carriers of memory. Key molecular signatures include: the poising of RNA polymerase II machinery, maintenance of histone marks, as well as alterations in nuclear positioning and long-range chromatin interactions. Finally, we discuss the potential adaptive roles of transcriptional memory in the organismal response to its environment from nutrient sensing to trained immunity.

Inheritance of epigenetic transcriptional memory through read-write replication of a histone modification

Epigenetic transcriptional regulation frequently requires histone modifications. Some, but not all, of these modifications are able to template their own inheritance. Here, I discuss the molecular mechanisms by which histone modifications can be inherited and relate these ideas to new results about epigenetic transcriptional memory, a phenomenon that poises recently repressed genes for faster reactivation and has been observed in diverse organisms. Recently, we found that the histone H3 lysine 4 dimethylation that is associated with this phenomenon plays a critical role in sustaining memory and, when factors critical for the establishment of memory are inactivated, can be stably maintained through multiple mitoses. This chromatin-mediated inheritance mechanism may involve a physical interaction between an H3K4me2 reader, SET3C, and an H3K4me2 writer, Spp1- COMPASS. This is the first example of a chromatin-mediated inheritance of a mark that promotes transcription.

Multiomics Reveals the Regulatory Mechanisms of Arabidopsis Tissues under Heat Stress

Understanding the mechanisms of responses to high temperatures in Arabidopsis will provide insights into how plants may mitigate heat stress under global climate change. And exploring the interconnections of different modification levels in heat stress response could help us to understand the molecular mechanism of heat stress response in Arabidopsis more comprehensively and precisely. In this paper, we combined multiomics analyses to explore the common heat stress-responsive genes and specific heat-responsive metabolic pathways in Arabidopsis leaf, seedling, and seed tissues. We found that genes such as AT1G54050 play a role in promoting proper protein folding in response to HS (Heat stress). In addition, it was revealed that the binding profile of A1B is altered under elevated temperature conditions. Finally, we also show that two microRNAs, ath-mir156h and ath-mir166b-5p, may be core regulatory molecules in HS. Also elucidated that under HS, plants can regulate specific regulatory mechanisms, such as oxygen levels, by altering the degree of CHH methylation.

Plant environmental memory: implications, mechanisms and opportunities for plant scientists and beyond

Plants are extremely plastic organisms. They continuously receive and integrate environmental information and adjust their growth and development to favour fitness and survival. When this integration of information affects subsequent life stages or the development of subsequent generations, it can be considered an environmental memory. Thus, plant memory is a relevant mechanism by which plants respond adaptively to different environments. If the cost of maintaining the response is offset by its benefits, it may influence evolutionary trajectories. As such, plant memory has a sophisticated underlying molecular mechanism with multiple components and layers. Nonetheless, when mathematical modelling is combined with knowledge of ecological, physiological, and developmental effects as well as molecular mechanisms as a tool for understanding plant memory, the combined potential becomes unfathomable for the management of plant communities in natural and agricultural ecosystems. In this review, we summarize recent advances in the understanding of plant memory, discuss the ecological requirements for its evolution, outline the multilayered molecular network and mechanisms required for accurate and fail-proof plant responses to variable environments, point out the direct involvement of the plant metabolism and discuss the tremendous potential of various types of models to further our understanding of the plant's environmental memory. Throughout, we emphasize the use of plant memory as a tool to unlock the secrets of the natural world.

Targeted single-cell gene induction by optimizing the dually regulated CRE/loxP system by a newly defined heat-shock promoter and the steroid hormone in Arabidopsis thaliana

Multicellular organisms rely on intercellular communication systems to organize their cellular functions. In studies focusing on intercellular communication, the key experimental techniques include the generation of chimeric tissue using transgenic DNA recombination systems represented by the CRE/loxP system. If an experimental system enables the induction of chimeras at highly targeted cell(s), it will facilitate the reproducibility and precision of experiments. However, multiple technical limitations have made this challenging. The stochastic nature of DNA recombination events, especially, hampers reproducible generation of intended chimeric patterns. Infrared laser-evoked gene operator (IR-LEGO), a microscopic system that irradiates targeted cells using an IR laser, can induce heat shock-mediated expression of transgenes, for example, CRE recombinase gene, in the cells. In this study, we developed a method that induces CRE/loxP recombination in the target cell(s) of plant roots and leaves in a highly specific manner. We combined IR-LEGO, an improved heat-shock-specific promoter, and dexamethasone-dependent regulation of CRE. The optimal IR-laser power and irradiation duration were estimated via exhaustive irradiation trials and subsequent statistical modeling. Under optimized conditions, CRE/loxP recombination was efficiently induced without cellular damage. We also found that the induction efficiency varied among tissue types and cellular sizes. The developed method offers an experimental system to generate a precisely designed chimeric tissue, and thus, will be useful for analyzing intercellular communication at high resolution in roots and leaves.

Genomic and epigenomic determinants of heat stress-induced transcriptional memory in Arabidopsis

BACKGROUND

Transcriptional regulation is a key aspect of environmental stress responses. Heat stress induces transcriptional memory, i.e., sustained induction or enhanced re-induction of transcription, that allows plants to respond more efficiently to a recurrent HS. In light of more frequent temperature extremes due to climate change, improving heat tolerance in crop plants is an important breeding goal. However, not all heat stress-inducible genes show transcriptional memory, and it is unclear what distinguishes memory from non-memory genes. To address this issue and understand the genome and epigenome architecture of transcriptional memory after heat stress, we identify the global target genes of two key memory heat shock transcription factors, HSFA2 and HSFA3, using time course ChIP-seq.

RESULTS

HSFA2 and HSFA3 show near identical binding patterns. In vitro and in vivo binding strength is highly correlated, indicating the importance of DNA sequence elements. In particular, genes with transcriptional memory are strongly enriched for a tripartite heat shock element, and are hallmarked by several features: low expression levels in the absence of heat stress, accessible chromatin environment, and heat stress-induced enrichment of H3K4 trimethylation. These results are confirmed by an orthogonal transcriptomic data set using both de novo clustering and an established definition of memory genes.

CONCLUSIONS

Our findings provide an integrated view of HSF-dependent transcriptional memory and shed light on its sequence and chromatin determinants, enabling the prediction and engineering of genes with transcriptional memory behavior.

Hello darkness, my old friend: 3-KETOACYL-COENZYME A SYNTHASE4 is a branch point in the regulation of triacylglycerol synthesis in Arabidopsis thaliana

Plant lipids are important as alternative sources of carbon and energy when sugars or starch are limited. Here, we applied combined heat and darkness or extended darkness to a panel of ∼300 Arabidopsis (Arabidopsis thaliana) accessions to study lipid remodeling under carbon starvation. Natural allelic variation at 3-KETOACYL-COENZYME A SYNTHASE4 (KCS4), a gene encoding an enzyme involved in very long chain fatty acid (VLCFA) synthesis, underlies the differential accumulation of polyunsaturated triacylglycerols (puTAGs) under stress. Ectopic expression of KCS4 in yeast and plants proved that KCS4 is a functional enzyme localized in the endoplasmic reticulum with specificity for C22 and C24 saturated acyl-CoA. Allelic mutants and transient overexpression in planta revealed the differential role of KCS4 alleles in VLCFA synthesis and leaf wax coverage, puTAG accumulation, and biomass. Moreover, the region harboring KCS4 is under high selective pressure and allelic variation at KCS4 correlates with environmental parameters from the locales of Arabidopsis accessions. Our results provide evidence that KCS4 plays a decisive role in the subsequent fate of fatty acids released from chloroplast membrane lipids under carbon starvation. This work sheds light on both plant response mechanisms and the evolutionary events shaping the lipidome under carbon starvation.

Highlight Induced Transcriptional Priming against a Subsequent Drought Stress in Arabidopsis thaliana

In plants, priming allows a more rapid and robust response to recurring stresses. However, while the nature of plant response to a single stress can affect the subsequent response to the same stress has been deeply studied, considerably less is known on how the priming effect due to one stress can help plants cope with subsequent different stresses, a situation that can be found in natural ecosystems. Here, we investigate the potential priming effects in Arabidopsis plants subjected to a high light (HL) stress followed by a drought (D) stress. The cross-stress tolerance was assessed at the physiological and molecular levels. Our data demonstrated that HL mediated transcriptional priming on the expression of specific stress response genes. Furthermore, this priming effect involves both ABA-dependent and ABA-independent responses, as also supported by reduced expression of these genes in the aba1-3 mutant compared to the wild type. We have also assessed several physiological parameters with the aim of seeing if gene expression coincides with any physiological changes. Overall, the results from the physiological measurements suggested that these physiological processes did not experience metabolic changes in response to the stresses. In addition, we show that the H3K4me3 epigenetic mark could be a good candidate as an epigenetic mark in priming response. Overall, our results help to elucidate how HL-mediated priming can limit D-stress and enhance plant responses to stress.

Seed priming can enhance and retain stress tolerance in ensuing generations by inducing epigenetic changes and trans-generational memory

The significance of priming in enhancing abiotic stress tolerance is well-established in several important crops. Priming positively impacts plant growth and improves stress tolerance at multiple developmental stages, and seed priming is one of the most used methods. Seed priming influences the pre-germinative metabolism that ensures proper germination, early seedling establishment, enhanced stress tolerance and yield, even under unfavourable environmental conditions. Seed priming involves pre-exposure of seeds to mild stress, and this pre-treatment induces specific changes at the physiological and molecular levels. Interestingly, priming can improve the efficiency of the DNA repair mechanism, along with activation of specific signalling proteins and transcription factors for rapid and efficient stress tolerance. Notably, such acquired stress tolerance may be retained for longer duration, namely, later developmental stages or even subsequent generations. Epigenetic and chromatin-based mechanisms such as DNA methylation, histone modifications, and nucleosome positioning are some of the key molecular changes involved in priming/stress memory. Further, the retention of induced epigenetic changes may influence the priming-induced trans-generational stress memory. This review discusses known and plausible seed priming-induced molecular mechanisms that govern germination and stress memory within and across generations, highlighting their role in regulating the plant response to abiotic stresses. Understanding the molecular mechanism for activation of stress-responsive genes and the epigenetic changes resulting from seed priming will help to improve the resiliency of the crops for enhanced productivity under extreme environments.

Differential regulation of mRNA stability modulates transcriptional memory and facilitates environmental adaptation

Transcriptional memory, by which cells respond faster to repeated stimuli, is key for cellular adaptation and organism survival. Chromatin organization has been shown to play a role in the faster response of primed cells. However, the contribution of post-transcriptional regulation is not yet explored. Here we perform a genome-wide screen to identify novel factors modulating transcriptional memory in S. cerevisiae in response to galactose. We find that depletion of the nuclear RNA exosome increases GAL1 expression in primed cells. Our work shows that gene-specific differences in intrinsic nuclear surveillance factor association can enhance both gene induction and repression in primed cells. Finally, we show that primed cells present altered levels of RNA degradation machinery and that both nuclear and cytoplasmic mRNA decay modulate transcriptional memory. Our results demonstrate that mRNA post-transcriptional regulation, and not only transcription regulation, should be considered when investigating gene expression memory.

Stress memory and its regulation in plants experiencing recurrent drought conditions

Developing stress-tolerant plants continues to be the goal of breeders due to their realized yields and stability. Plant responses to drought have been studied in many different plant species, but the occurrence of stress memory as well as the potential mechanisms for memory regulation is not yet well described. It has been observed that plants hold on to past events in a way that adjusts their response to new challenges without altering their genetic constitution. This ability could enable training of plants to face future challenges that increase in frequency and intensity. A better understanding of stress memory-associated mechanisms leading to alteration in gene expression and how they link to physiological, biochemical, metabolomic and morphological changes would initiate diverse opportunities to breed stress-tolerant genotypes through molecular breeding or biotechnological approaches. In this perspective, this review discusses different stress memory types and gives an overall view using general examples. Further, focusing on drought stress, we demonstrate coordinated changes in epigenetic and molecular gene expression control mechanisms, the associated transcription memory responses at the genome level and integrated biochemical and physiological responses at cellular level following recurrent drought stress exposures. Indeed, coordinated epigenetic and molecular alterations of expression of specific gene networks link to biochemical and physiological responses that facilitate acclimation and survival of an individual plant during repeated stress.

Transcriptome Analysis Points to BES1 as a Transducer of Strigolactone Effects on Drought Memory in Arabidopsis thaliana

Strigolactones (SLs) are carotenoid-derived phytohormones governing a wide range of physiological processes, including drought-associated stomatal closure. We have previously shown in tomato that SLs regulate the so-called after-effect of drought, whereby stomatal conductance is not completely restored for some time during recovery after a drought spell, irrespective of the water potential. To ease the elucidation of its molecular underpinnings, we investigated whether this SL effect is conserved in Arabidopsis thaliana by contrasting the physiological performances of the wild-type with SL-depleted (more axillary growth 4, max4) and insensitive (dwarf 14, d14) mutants in a drought and recovery protocol. Physiological analyses showed that SLs are important to achieve a complete after-effect in A. thaliana, while transcriptome results suggested that the SL-dependent modulation of drought responses extends to a large subset (about 4/5) of genes displaying memory transcription patterns. Among these, we show that the activation of over 30 genes related to abscisic acid metabolism and signaling strongly depends on SL signaling. Furthermore, by using promoter-enrichment tools, we identified putative cis- and trans-acting factors that may be important in the SL-dependent and SL-independent regulation of genes during drought and recovery. Finally, in order to test the accuracy of our bioinformatic prediction, we confirmed one of the most promising transcription factor candidates mediating SL signaling effects on transcriptional drought memory-BRI-EMS SUPPRESSOR1 (BES1). Our findings reveal that SLs are master regulators of Arabidopsis transcriptional memory upon drought and that this role is partially mediated by the BES1 transcription factor.

DNA methylation dynamics in a coastal foundation seagrass species under abiotic stressors

DNA methylation (DNAm) has been intensively studied in terrestrial plants in response to environmental changes, but its dynamic changes in a temporal scale remain unexplored in marine plants. The seagrass Posidonia oceanica ranks among the slowest-growing and longest-living plants on Earth, and is particularly vulnerable to sea warming and local anthropogenic pressures. Here, we analysed the dynamics of DNAm changes in plants collected from coastal areas differentially impacted by eutrophication (i.e. oligotrophic, Ol; eutrophic, Eu) and exposed to abiotic stressors (nutrients, temperature increase and their combination). Levels of global DNAm (% 5-mC) and the expression of key genes involved in DNAm were assessed after one, two and five weeks of exposure. Results revealed a clear differentiation between plants, depending on environmental stimuli, time of exposure and plants' origin. % 5-mC levels were higher during the initial stress exposure especially in Ol plants, which upregulated almost all genes involved in DNAm. Contrarily, Eu plants showed lower expression levels, which increased under chronic exposure to stressors, particularly to temperature. These findings show that DNAm is dynamic in P. oceanica during stress exposure and underlined that environmental epigenetic variations could be implicated in the regulation of acclimation and phenotypic differences depending on local conditions.

Marine heatwaves and upwelling shape stress responses in a keystone predator

Climate change increases the frequency and intensifies the magnitude and duration of extreme events in the sea, particularly so in coastal habitats. However, the interplay of multiple extremes and the consequences for species and ecosystems remain unknown. We experimentally tested the impacts of summer heatwaves of differing intensities and durations, and a subsequent upwelling event on a temperate keystone predator, the starfish Asterias rubens. We recorded mussel consumption throughout the experiment and assessed activity and growth at strategically chosen time points. The upwelling event overall impaired starfish feeding and activity, likely driven by the acidification and low oxygen concentrations in the upwelled seawater. Prior exposure to a present-day heatwave (+5°C above climatology) alleviated upwelling-induced stress, indicating cross-stress tolerance. Heatwaves of present-day intensity decreased starfish feeding and growth. While the imposed heatwaves of limited duration (9 days) caused slight impacts but allowed for recovery, the prolonged (13 days) heatwave impaired overall growth. Projected future heatwaves (+8°C above climatology) caused 100% mortality of starfish. Our findings indicate a positive ecological memory imposed by successive stress events. Yet, starfish populations may still suffer extensive mortality during intensified end-of-century heatwave conditions.

NTR1 is involved in heat stress tolerance through mediating expression regulation and alternative splicing of heat stress genes in Arabidopsis

As a common adverse environmental factor, heat stress (HS) not only drastically changes the plant transcriptome at the transcription level but also increases alternative splicing (AS), especially intron retention (IR) events. However, the exact mechanisms are not yet well understood. Here, we reported that NTC-related protein 1 (NTR1), which acts as an accessory component for spliceosome disassembly, is necessary for this process. The mutants of NTR1, both the T-DNA insertion and the point mutation identified through ethyl methanesulfonate (EMS) mutagenesis screening, are vulnerable to HS, indicating that NTR1 is essential for plant HS tolerance. At the molecular level, genes of response to heat and response to temperature stimulus are highly enriched among those of heat-induced but less-expressed ntr1 mutants. Moreover, a large portion of HS response (HSR) genes such as heat shock transcription factors (HSFs) and heat shock proteins (HSPs) are less induced by heat treatment, and more AS events, especially IR events, were found in heat-treated ntr1 mutants. Furthermore, HS suppressed the expression of NTR1 and NTR1-associated complex components. Thus, it is very likely that upon HS, the plant reduces the expression of the NTR1-associated complex to fulfill the fast demands for transcription of HSR genes such as HSFs and HSPs, which in turn results in the accumulation of improperly spliced especially IR products and eventually causes harm to plants.

Maintenance of abiotic stress memory in plants: Lessons learned from heat acclimation

Plants acquire enhanced tolerance to intermittent abiotic stress by employing information obtained during prior exposure to an environmental disturbance, a process known as acclimation or defense priming. The capacity for stress memory is a critical feature in this process. The number of reports related to plant stress memory (PSM) has recently increased, but few studies have focused on the mechanisms that maintain PSM. Identifying the components involved in maintaining PSM is difficult due in part to the lack of clear criteria to recognize these components. In this review, based on what has been learned from genetic studies on heat acclimation memory, we propose criteria for identifying components of the regulatory networks that maintain PSM. We provide examples of the regulatory circuits formed by effectors and regulators of PSM. We also highlight strategies for assessing PSMs, update the progress in understanding the mechanisms of PSM maintenance, and provide perspectives for the further development of this exciting research field.

Perspectives in Plant Abiotic Stress Signaling

State-of-the-art collections of strategies, approaches, and methods are immediately useful for ongoing characterizations or for novel discoveries in the scientific field of plant abiotic stress signaling. It must however be kept in mind that, in the future, these strategies, approaches, and methods will be facing a number of increasingly complex issues. The development of the necessary confrontation of laboratory-based knowledge on abiotic stress signaling mechanisms with real-life in natura situations of plant-stress interactions involves at least five levels of complexity: (i) plant biodiversity, (ii) the spatio-temporal heterogeneity of stress-related parameters, (iii) the unknowns of future stress-related constraints, (iv) the influence of biotic interactions, (v) the crosstalk between various signaling pathways and their final integration into physiological responses. These complexities are major bottlenecks for assessing the evolutionary, ecological, and agronomical relevance of abiotic stress signaling studies. All of the presently-described strategies, approaches, and methods will have to be gradually complemented with the development of real-time and in natura tools, with systematic application of mathematical modeling to complex interactions and with further research on the impact of stress memory mechanisms on long-term responses.

Seaweed as a Natural Source against Phytopathogenic Bacteria

Plant bacterial pathogens can be devastating and compromise entire crops of fruit and vegetables worldwide. The consequences of bacterial plant infections represent not only relevant economical losses, but also the reduction of food availability. Synthetic bactericides have been the most used tool to control bacterial diseases, representing an expensive investment for the producers, since cyclic applications are usually necessary, and are a potential threat to the environment. The development of greener methodologies is of paramount importance, and some options are already available in the market, usually related to genetic manipulation or plant community modulation, as in the case of biocontrol. Seaweeds are one of the richest sources of bioactive compounds, already being used in different industries such as cosmetics, food, medicine, pharmaceutical investigation, and agriculture, among others. They also arise as an eco-friendly alternative to synthetic bactericides. Several studies have already demonstrated their inhibitory activity over relevant bacterial phytopathogens, some of these compounds are known for their eliciting ability to trigger priming defense mechanisms. The present work aims to gather the available information regarding seaweed extracts/compounds with antibacterial activity and eliciting potential to control bacterial phytopathogens, highlighting the extracts from brown algae with protective properties against microbial attack.

Ethylene: A Master Regulator of Plant-Microbe Interactions under Abiotic Stresses

The plant phytohormone ethylene regulates numerous physiological processes and contributes to plant-microbe interactions. Plants induce ethylene production to ward off pathogens after recognition of conserved microbe-associated molecular patterns (MAMPs). However, plant immune responses against pathogens are essentially not different from those triggered by neutral and beneficial microbes. Recent studies indicate that ethylene is an important factor for beneficial plant-microbial association under abiotic stress such as salt and heat stress. The association of beneficial microbes with plants under abiotic stresses modulates ethylene levels which control the expression of ethylene-responsive genes (ERF), and ERFs further regulate the plant transcriptome, epi-transcriptome, Na⁺/K⁺ homeostasis and antioxidant defense mechanisms against reactive oxygen species (ROS). Understanding ethylene-dependent plant-microbe interactions is crucial for the development of new strategies aimed at enhancing plant tolerance to harsh environmental conditions. In this review, we underline the importance of ethylene in beneficial plant-microbe interaction under abiotic stresses.

Epigenetic stress memory: A new approach to study cold and heat stress responses in plants

Understanding plant stress memory under extreme temperatures such as cold and heat could contribute to plant development. Plants employ different types of stress memories, such as somatic, intergenerational and transgenerational, regulated by epigenetic changes such as DNA and histone modifications and microRNAs (miRNA), playing a key role in gene regulation from early development to maturity. In most cases, cold and heat stresses result in short-term epigenetic modifications that can return to baseline modification levels after stress cessation. Nevertheless, some of the modifications may be stable and passed on as stress memory, potentially allowing them to be inherited across generations, whereas some of the modifications are reactivated during sexual reproduction or embryogenesis. Several stress-related genes are involved in stress memory inheritance by turning on and off transcription profiles and epigenetic changes. Vernalization is the best example of somatic stress memory. Changes in the chromatin structure of the Flowering Locus C (FLC) gene, a MADS-box transcription factor (TF), maintain cold stress memory during mitosis. FLC expression suppresses flowering at high levels during winter; and during vernalization, B3 TFs, cold memory cis-acting element and polycomb repressive complex 1 and 2 (PRC1 and 2) silence FLC activation. In contrast, the repression of SQUAMOSA promoter-binding protein-like (SPL) TF and the activation of Heat Shock TF (HSFA2) are required for heat stress memory. However, it is still unclear how stress memory is inherited by offspring, and the integrated view of the regulatory mechanisms of stress memory and mitotic and meiotic heritable changes in plants is still scarce. Thus, in this review, we focus on the epigenetic regulation of stress memory and discuss the application of new technologies in developing epigenetic modifications to improve stress memory.